Origins of amino acid transporter loci in trypanosomatid parasites

Background

In membrane trafficking, the mechanisms ensuring vesicle fusion specificity remain to be fully elucidated. Early models proposed that specificity was encoded entirely by SNARE proteins; more recent models include contributions from Rab proteins, Syntaxin-binding (SM) proteins and tethering factors. Most information on membrane trafficking derives from an evolutionarily narrow sampling of model organisms. However, considering factors from a wider diversity of eukaryotes can provide both functional information on core systems and insight into the evolutionary history of the trafficking machinery. For example, the major Qa/syntaxin SNARE families are present in most eukaryotic genomes and likely each evolved via gene duplication from a single ancestral syntaxin before the existing eukaryotic groups diversified. This pattern is also likely for Rabs and various other components of the membrane trafficking machinery.

Results

We performed comparative genomic and phylogenetic analyses, when relevant, on the SM proteins and components of the tethering complexes, both thought to contribute to vesicle fusion specificity. Despite evidence suggestive of secondary losses amongst many lineages, the tethering complexes are well represented across the eukaryotes, suggesting an origin predating the radiation of eukaryotic lineages. Further, whilst we detect distant sequence relations between GARP, COG, exocyst and DSL1 components, these similarities most likely reflect convergent evolution of similar secondary structural elements. No similarity is found between the TRAPP and HOPS complexes and the other tethering factors. Overall, our data favour independent origins for the various tethering complexes. The taxa examined possess at least one homologue of each of the four SM protein families; since the four monophyletic families each encompass a wide diversity of eukaryotes, the SM protein families very likely evolved before the last common eukaryotic ancestor (LCEA).

Conclusion

These data further support a highly complex LCEA and indicate that the basic architecture of the trafficking system is remarkably conserved and ancient, with the SM proteins and tethering factors having originated very early in eukaryotic evolution. However, the independent origin of the tethering complexes suggests a novel pattern for increasing complexity in the membrane trafficking system, in addition to the pattern of paralogous machinery elaboration seen thus far.     

Is host-schistosome coevolution going anywhere?

Background

Rab proteins are regulators of vesicular trafficking, requiring a lipid modification for proper function, prenylation of C-terminal cysteines. This is catalysed by a complex of a catalytic heterodimer (Rab Geranylgeranyl Transferase – RabGGTase) and an accessory protein (Rab Escort Protein. REP). Components of this complex display domain insertions relative to paralogous proteins. The function of these inserted domains is unclear.

Results

We profiled the domain architecture of the components of the Rab prenylation complex in evolution. We identified the orthologues of the components of the Rab prenylation machinery in 43 organisms, representing the crown eukaryotic groups. We characterize in detail the domain structure of all these components and the phylogenetic relationships between the individual domains.

Conclusion

We found different domain insertions in different taxa, in α-subunits of RGGTase and REP. Our results suggest that there were multiple insertions, expansions and contractions in the evolution of this prenylation complex.     

Rab GTPases and membrane identity: Causal or inconsequential?

Rab GTPases are highly conserved components of vesicle trafficking pathways that help to ensure the fusion of a vesicle with a specific target organelle membrane. Specific regulatory pathways promote kinetic proofreading of membrane surfaces by Rab GTPases, and permit accumulation of active Rabs only at the required sites. Emerging evidence indicates that Rab activation and inactivation are under complex feedback control, suggesting that ultrasensitivity and bistability, principles established for other cellular regulatory networks, may also apply to Rab regulation. Such systems can promote the rapid membrane accumulation and removal of Rabs to create time-limited membrane domains with a unique composition, and can explain how Rabs define the identity of vesicle and organelle membranes.

Rab GTPases regulate membrane tethering and vesicle fusion

Eukaryotic cells are defined in part by their complex membrane organelles. This organization permits the coexistence of different chemical environments within the same cell. For example, the endoplasmic reticulum (ER) is a neutral pH, reducing environment containing chaperones conducive to protein folding and the formation of disulfide bonds, whereas the lysosomes are ∼pH 5 and contain catabolic enzymes maximally active at acidic pH. Though valuable, this organization requires some form of active transport machinery for the exchange of material between these compartments because large hydrophilic molecules such as proteins cannot easily cross membranes. This transfer of molecules between compartments is achieved by vesicular transport systems that use cytosolic coat protein complexes to select small regions of membrane and shape these into defined 40–80-nm-diameter transport vesicles (Bonifacino and Glick, 2004; Faini et al., 2013). Vesicle coats contain binding sites for specific transport sequences, and thus only transfer a subset of proteins into the vesicle. Once produced, these vesicles have to identify, tether to, and then fuse with a specific target organelle (Zerial and McBride, 2001). Research over many years has defined small transmembrane proteins (SNAREs) and a set of accessory factors as the minimal machinery for membrane fusion (McNew et al., 2000; Shi et al., 2012). Tethering is a less well-defined event involving the Rab GTPases and effector protein complexes, typically large extended molecules thought to bridge the space between two approaching membranes (Gillingham and Munro, 2003).Rab GTPases were first linked to vesicle transport by groundbreaking genetic screens for mutants defective in protein secretion (Novick et al., 1980; Salminen and Novick, 1987). Sec4, Rab8 in humans, was found to function in the terminal step of the secretory pathway, delivery of Golgi-derived transport vesicles to the cell surface (Salminen and Novick, 1987; Goud et al., 1988). Ypt1, Rab1 in humans, was then shown to regulate secretion at the Golgi apparatus (Segev et al., 1988; Bacon et al., 1989). These findings led to an influential model for Rab function in which the cycle of GTPase activation and inactivation is coupled to recognition events in vesicle docking (Bourne, 1988). Consistent with the idea that they control vesicle targeting, work in mammalian cells then showed that there is a large family of highly conserved Rab GTPases, each with a specific subcellular localization (Chavrier et al., 1990). A series of seminal studies has since provided direct evidence that Rab1 and Rab5 promote membrane fusion (Gorvel et al., 1991; Segev, 1991) by regulating the activation and engagement of SNAREs (Lian et al., 1994; Søgaard et al., 1994), as a consequence of recruiting tethering factors to membrane surfaces (Segev, 1991; Sapperstein et al., 1996; Cao et al., 1998; Christoforidis et al., 1999; McBride et al., 1999; Allan et al., 2000; Shorter et al., 2002). Similar findings were also made for the Rab Ypt7, which functions in vacuole docking in yeast (Price et al., 2000; Ungermann et al., 2000), a system that allows direct visualization of docked or tethered intermediates due to the large size of the membrane structures (Wang et al., 2002).The evidence that Rabs function upstream of SNARE protein in vesicle trafficking pathways has led to the notion that Rabs help to define the identity of vesicle and organelle membranes (Pfeffer, 2001; Zerial and McBride, 2001). This is best exemplified by the early endocytic pathway, where the identity of early and late endosomes is thought to be determined by Rab5 and Rab7, respectively (Rink et al., 2005). However, in most other cases it remains unclear if this is a causal relationship, where the Rab directly defines the identity of the membrane rather than acting as an upstream regulator of vesicle targeting before the SNARE-mediated membrane fusion event. In addition to Rabs, GTPases of the Arf/Arl family and specific phosphoinositide lipids have also been proposed to act in specifying membrane identity (Munro, 2002; Di Paolo and De Camilli, 2006). It therefore seems likely that no single factor can explain how membrane identity is achieved in vesicle transport, and that Rabs, phosphoinositides, and other factors act in concert.

Rab GEFs provide the minimal machinery for targeting and activation

Despite the progress in defining Rab function, the claim that Rab GTPases define organelle identity therefore remains premature due to crucial unanswered questions. In particular, the issue of how Rabs are targeted to specific organelles, or even restricted to subdomains of these organelles, has remained problematic. Initial work using chimeric GTPases suggested that the variable C-terminal region of the different Rabs provided a targeting mechanism (Chavrier et al., 1991). However, subsequent work indicated that this failed to provide a general mechanism to explain specific Rab targeting, and that multiple regions of the Rab including C-terminal prenylation contribute to membrane recruitment (Ali et al., 2004). Emerging evidence based on the improved understanding of the family of Rab guanine nucleotide exchange factors (GEFs) now provides an alternative view for Rab activation at specific membrane surfaces. Mechanistic details of how Rab GEFs activate Rabs have been discussed elsewhere (Barr and Lambright, 2010), and are not directly relevant for this discussion so won’t be detailed further. Two studies now show that Rab GEFs can provide the minimal machinery needed to target a Rab to a specific membrane within the cell (Gerondopoulos et al., 2012; Blümer et al., 2013). In both cases, Rab GEFs were fused to mitochondrial outer membrane targeting sequences, and the effects on different Rabs observed. Using this strategy it was possible to specifically target Rab1, Rab5, Rab8, Rab35, and Rab32/38 to mitochondria with biochemically defined cognate GEFs (Gerondopoulos et al., 2012; Blümer et al., 2013). Mutants that either reduced the nucleotide exchange activity of the GEF or the target GTPase gave a correspondingly reduced rate of Rab targeting (Blümer et al., 2013). Alone this does not provide a full explanation for Rab targeting; for this an understanding of the interaction of prenylated Rabs with the chaperone GDI (guanine nucleotide displacement inhibitor) is needed. Structural and biophysical analysis of the Ypt1–GDI complex has revealed two components of this interaction relevant for Rab targeting (Pylypenko et al., 2006). Domain I of GDI interacts with the switch II region of Ypt1 only when this is in the GDP-bound inactive form. The doubly prenylated C terminus of Ypt1 occupies a hydrophobic cavity created by domain II of GDI. Simulation of this system and direct biophysical measurements suggests that in the absence of other factors GDI will rapidly deliver Rabs to and extract them from a lipid bilayer (Pylypenko et al., 2006; Wu et al., 2010). These ideas can be combined into a simple model for Rab activation at specific membrane surfaces (Fig. 1 A). In simple terms this model is a form of molecular speed-dating in which the Rab spends a short time sampling each membrane surface it encounters before finally meeting its cognate GEF partner, triggering a period of longer residence at that site (Fig. 1 A). In this model, GEF-mediated nucleotide exchange renders the Rab resistant to extraction by GDI, and thus drives accumulation of the active GTP-bound form of the Rab. This active Rab can then recruit effector proteins to the membrane surface and promote the desired recognition event. Such a system is analogous to the rapid proofreading of amino-acyl tRNAs during protein synthesis by the ribosome (Ibba and Söll, 1999). All amino-acyl tRNAs can enter the so-called acceptor site, but only if stable codon recognition occurs is the peptidyltransferase reaction initiated, otherwise the tRNA is rejected (Steitz, 2008). The two-stage kinetic proofreading of membrane surfaces by Rabs may similarly increase fidelity at little overall cost to the rate of vesicular traffic.Open in a separate windowFigure 1.The Rab activation and inactivation cycle. (A) Prenylated Rabs (black wavy lines) are bound by the chaperone GDI in the cytosol. Partitioning of the prenylated tail moiety between the hydrophobic pocket in GDI and the membrane bilayer allows Rabs to rapidly and reversibly sample membrane surfaces. When the GDP-bound inactive Rab encounters a cognate GEF nucleotide exchange occurs. This GTP-bound active Rab species does not interact with GDI and can therefore accumulate on the membrane surface, where it may further recruit effector proteins with specific biological functions. This cycle is reset when a GTP-bound Rab encounters a GAP (GTPase-activating protein) and the bound GTP is hydrolyzed to generate GDP and inorganic phosphate. (B) Additional specification of membrane domains within complex organelles, such as tubular domains of endosomes, or the fenestrated rims and different cisternae of the Golgi apparatus, may involve membrane receptors for Rabs (shown as light blue, dark blue, and green boxes). This could either involve (a) sequestration of the active Rab to a subdomain defined by the membrane receptor, or (b) direction of GDI unloading of an inactive Rab to specific sites on the organelle membrane also defined by a membrane receptor. Accumulation of a Rab at a specific site may be favored by GAPs opposing Rab activation at unwanted sites (Haas et al., 2007).Although this minimal Rab-targeting system does not require any additional factors, it is important to mention that this does not mean such factors do not exist. A family of membrane proteins with prenylated Rab-binding activity that can promote dissociation of some prenylated Rabs from GDI and favor retention of the GDP-bound form of the Rab downstream of membrane delivery by GDI has been identified (Dirac-Svejstrup et al., 1997; Martincic et al., 1997; Hutt et al., 2000; Sivars et al., 2003). These may therefore favor Rab activation, although recent data has suggested that such factors are not generally essential (Blümer et al., 2013). Intriguingly, other evidence links this family of proteins to factors involved in shaping subdomains of the ER and to the Golgi apparatus (Yang et al., 1998; Calero et al., 2001; Chen et al., 2004; Voeltz et al., 2006), perhaps suggesting that they may play roles in defining at which subdomain of an organelle an active Rab is enriched (Fig. 1 B).In addition to these regulatory factors, covalent modification can also be used to modulate the Rab activation cycle. Phosphorylation of Rab1 and Rab4 in mitosis alters the fraction of these GTPases that can associate with membranes (Bailly et al., 1991; van der Sluijs et al., 1992), although the exact mechanisms remain unclear. Furthermore, emerging evidence indicates that one Rab in yeast, Ypt11, is controlled by a phosphorylation-dependent mechanism regulating its activation and abundance (Lewandowska et al., 2013). A number of bacterial pathogens also encode enzymes that directly modify Rab GTPases and as a consequence alter the Rab regulatory cycle. During Legionella infection, Rab1 is modulated by a cycle of adenylylation and de-adenylylation by DrrA and SidA, respectively, and this modification of the conserved tyrosine residue in the switch II renders the protein constitutively active (Müller et al., 2010; Neunuebel et al., 2011; Tan and Luo, 2011). DrrA also has a GEF domain and can therefore directly activate and trap Rab1 in an active form independent of other cellular factors (Schoebel et al., 2009). A second bacterial protein, AnkX, mediates phosphocholination of an adjacent serine within the switch II region (Mukherjee et al., 2011; Campanacci et al., 2013). Pathogens such as Legionella use this covalent modification of Rabs to modulate their localization and activation (Stein et al., 2012). Although cellular enzymes that carry out related modification of Rabs are currently unknown, it would be premature to dismiss the possibility of their existence and use by cells to similarly control Rab activation and inactivation at specific sites.

Evidence for Rab activation on vesicle and target membrane surfaces

Based on the model and discussion so far it seems obvious that Rabs accumulate on the same membrane as their cognate GEF. Indeed, there is evidence that Rab1 may be activated and recruit the p115 tethering factor during the COP II vesicle formation stage of ER-to-Golgi transport (Allan et al., 2000). This would have the advantage that identity would be created at an early stage in vesicle biogenesis, and the vesicle could therefore be tethered to the Golgi before completion of the vesicle, thus increasing targeting efficiency. However, there is also evidence that Rab activation can occur at the target membrane and not only on a vesicle surface. Careful analysis of cell-free ER–Golgi transport assays revealed that Ypt1–Rab1 is not always required on the vesicle fraction, but is essential on the target Golgi membranes (Cao and Barlowe, 2000). Furthermore, a Ypt1 mutant with reduced nucleotide hydrolysis (which prevents its recycling from the Golgi compartment; Richardson et al., 1998), or Golgi membrane-anchored forms of Ypt1 (Cao and Barlowe, 2000) both support apparently normal ER–Golgi transport and cell growth. Subsequently, it was found that the COP II coat required to form ER–Golgi transport vesicles is the membrane receptor for the Ypt1–Rab1 GEF TRAPP (transport protein particle; Jones et al., 2000; Wang et al., 2000; Cai et al., 2007), indicating that Rab1 activation may occur on the coated vesicle. This raises questions about how the cytosolic Rab–GDI complex can access the membrane surface of a still-coated vesicle. However, because the COP II coat has an open lattice structure (Faini et al., 2013), it may be possible in this case for Ypt1–Rab1 to approach the membrane and insert. A further possibility is that COP II vesicles recruit TRAPP and promote the activation of Ypt1–Rab1 at the adjacent Golgi membranes to signal that an ER-derived vesicle is in close proximity (Fig. 2 A). This Golgi pool of activated Rab would then recruit effector proteins such as Uso1/p115 that trap and tether the incoming vesicle by directly engaging with vesicle SNAREs (Cao et al., 1998; Shorter et al., 2002).Open in a separate windowFigure 2.Recruitment mechanisms for Rab GEFs. Rab GEFs can be divided into two groups according to the mechanism of membrane recruitment. (A) Discrete coat protein complexes (green) recruit the first group. For example, COP II recruits the Rab1 GEF TRAPP to ER-Golgi vesicles, while clathrin-AP2 recruits DENND1A, the Rab35 GEF, to endocytic sites at the cell surface. In the case of TRAPP, biochemical and genetic data suggest that Rab1 can be activated on the target membrane, before vesicle tethering and SNARE-mediated fusion. (B) The larger second group of Rab GEFs is recruited by Rab GTPases either alone or in combination with a second factor (Rabs/factors listed next to arrow). For example, the GEF Sec2 is recruited to late-Golgi vesicles trafficking to the bud in yeast by the activated Rab Ypt31/32 and phosphatidylinositol 4-phosphate (PI4P), where it activates the Rab Sec4 (Rab8 in humans). The Rabex5–rabaptin complex, which is a Rab5 GEF, interacts with activated Rab4 or Rab5 and ubiquitylated cargo proteins on endocytic vesicles and early endosomes. A number of other GEFs (some additional examples shown) have been found to interact with active Rabs. Whether or not these represent the sole mode of membrane interaction for these GEFs is not defined at this time. PM, plasma membrane. (C) In situations where the GEF for a second Rab in the pathway is an effector for the first, a cascade can develop, where Rab-A promotes the recruitment of GEF-B for this second Rab-B.

Rab GEF targeting and regulation

The mechanism of GEF targeting is of crucial importance for understanding how Rabs are activated at a particular membrane site. At present, two different solutions to the problem depending on the GEF are known. First, as already mentioned, is vesicle coat–dependent GEF targeting (Fig. 2 A). Three examples are known at present: COP II recruitment of the Rab1 GEF TRAPP-I (Cai et al., 2007), and clathrin-adaptor protein complex 2–dependent recruitment of either the Rab35 GEF DENND1A (Allaire et al., 2010; Yoshimura et al., 2010) or the Rab5 GEF RME-6 (Sato et al., 2005; Semerdjieva et al., 2008) during endocytic transport from the plasma membrane. In the latter cases the exact nature of the membrane on which the target Rab is activated is unclear, but it is tempting to speculate that like COP II, the coated vesicle promotes Rab activation on the target organelle to signal the presence of an incoming vesicle to be tethered. The second larger group of GEFs comprises those known to interact with active Rab GTPases (Fig. 2 B). The first of these Rab GEF effectors defined was the Rabex-5–rabaptin complex, which is both a Rab5 exchange factor and effector for Rab4 and Rab5 (Horiuchi et al., 1997). Rabex-5 also binds to ubiquitin via a specific domain and this is important for regulating its recruitment to early endosomes (Lee et al., 2006; Mattera et al., 2006; Mattera and Bonifacino, 2008) where it activates Rab5.Specific phosphatidylinositols play a key role in defining membrane identity (Di Paolo and De Camilli, 2006)), and this is in part due to a role in recruitment or regulation of Rab exchange factors. Sec2, the exchange factor for Sec4–Rab8, is recruited to post-Golgi vesicles by a combination of the Rab Ypt32 and phosphatidylinositol 4-phosphate generated by Pik1 (Ortiz et al., 2002; Sciorra et al., 2005; Mizuno-Yamasaki et al., 2010). Similarly, in mammalian cells the Rab GEF Sec2–Rabin8 is recruited by the Ypt31/32 orthologue Rab11 (Knödler et al., 2010), and phosphatidylinositol 4-phosphate generated by the Pik1 orthologue Fwd is important for Rab11 regulation in Drosophila (Polevoy et al., 2009). Although less is known about the targeting of other Rab GEFs, the clear theme is that many are effectors for a Rab other than the one they activate (Fig. 2 B). The Ric1–Rgp1 complex is a GEF for Rab6 and effector for Rab33B at the Golgi (Pusapati et al., 2012) and the Rab21 GEF VARP is an effector for Rab32/38 (Zhang et al., 2006; Tamura et al., 2009). Additionally, a GEF for Rab32/38 is an effector for Rab9 (Kloer et al., 2010; Gerondopoulos et al., 2012), and the DENND5A Rab39 GEF is an effector for Rab6 (Recacha et al., 2009; Yoshimura et al., 2010). In addition to these canonical trafficking functions there are specialized examples that indicate there is some plasticity to both GEF targeting and specificity. The Ypt1 GEF TRAPP exists in an alternate form (TRAPP-II) with additional subunits that promote late-Golgi targeting and may create additional GEF activity toward Ypt31/32 (Morozova et al., 2006). Interestingly, in higher eukaryotes there is evidence that TRAPP-II may regulate the Ypt31/32 orthologues Rab11 in male meiotic cytokinesis in flies (Robinett et al., 2009) and Rab-A in plant cell polarization and division (Qi et al., 2011), respectively. TRS85 in another alternate TRAPP complex (TRAPP-III) promotes localization to the forming autophagosome and activates Rab1 during autophagy (Lynch-Day et al., 2010).The counterpart to this interlinked network of Rab activation is an equally complex set of interactions between Rabs and Rab GAPs. The GAP Gyp1 is an effector for Ypt32 and promotes GTP hydrolysis by Ypt1 in budding yeast (Rivera-Molina and Novick, 2009). In the absence of Gyp1, Ypt1 spreads into the later compartments of the secretory pathway that should be occupied by Ypt32 (Rivera-Molina and Novick, 2009). Interestingly, one of the cellular GAPs for Ypt1–Rab1 is a transmembrane protein of the ER that may prevent Rab1 activity from spreading earlier in the pathway to the ER rather than act to terminate Rab1 activity at the Golgi (Haas et al., 2007; Sklan et al., 2007). Similarly, two related proteins, RUTBC1 and RUTBC2, bind to active Rab9 and are GAPs for Rab32 and Rab36, respectively (Nottingham et al., 2011, 2012).Together, these findings have led to the general idea that the order of trafficking events in a pathway can potentially be defined by a series of Rabs acting as a cascade (Fig. 2 C). In such models one Rab triggers the next in the pathway by recruiting its cognate GEF, and then feedback develops as a GTPase-activating protein (GAP) is recruited to terminate the action of the previous Rab in the series (Mizuno-Yamasaki et al., 2012; Pfeffer, 2013). In part, this simply passes the problem on because we are then left with the question of how the previous Rab in the pathway or a cofactor for recruitment such as phosphatidylinositol 4-phosphate or ubiquitin is localized and generated only when required. In the case of the secretory pathway the ER provides a defined starting point where activation of Rab1–Ypt1 will inevitably result in a defined and correctly timed wave of Rab activation through the secretory pathway. However, a note of caution is needed when considering these ideas because far more support from experimental data looking at the biochemical properties of these systems both in vitro and in vivo is required to come to any definitive conclusions.

Ultrasensitive Rab activation switches

One of the key tenets of the membrane identity hypothesis is that Rabs should rapidly and accurately establish membrane identity and then be lost once the membrane recognition event is over. Although biochemical data on Rab GEFs clearly indicate these molecules generally have sufficiently high specificity to ensure activation of only one Rab or a set of closely related Rabs (Delprato et al., 2004; Yoshimura et al., 2010; Gerondopoulos et al., 2012), how rapid switch-like accumulation is ensured is less obvious. Similar issues exist for termination of the Rab cycle by Rab GAPs. As already mentioned, Rab cascade models give part of the solution to this problem, and provide features that can ensure vectorial flow in a membrane traffic pathway (Mizuno-Yamasaki et al., 2012; Pfeffer, 2013). However, they do not fully explain how switch-like transitions and defined compartmental boundaries are achieved (Del Conte-Zerial et al., 2008). A possible solution to this problem comes from studies on the regulation of other complex biological systems, exemplified by control of cell cycle transitions (Tyson et al., 2001). Rather than displaying the expected Michaelis-Menten kinetics (Fig. 3 A), Rab cycles may yield properties of ultrasensitivity (Goldbeter and Koshland, 1981, 1984). This would appear to be a valid proposal if the Rab cycle is treated as being analogous to a covalent modification (Rab and Rab-modified, for GDP and GTP forms, respectively) and because GEF activity is generally assumed to be limiting (Blümer et al., 2013). In such a situation, inputs activating the GEF, for example membrane recruitment requiring multiple or binding of an activator, would be amplified and give rise to very large changes in the amount of activated Rab (Fig. 3 A). When combined with feedback loops, this can create a bistable switch between two states as shown for cell cycle transitions (Novak and Tyson, 1993; Pomerening et al., 2003). In the case of GTPase regulation, as the input controlling the GEF increases then the system transitions to a Rab-active state that remains stable over a wide range of GEF activity. GAP activation could then trigger exit from this state. This is also useful for providing a potential explanation for the timing properties of a Rab cascade. Ultrasensitivity and bistability are therefore likely to be useful concepts when explaining the behavior of Rabs, especially when considering complex interlinked cycles (Fig. 3 B) because they avoid the futile cycles where GAPs and GEFs fight one another and thus don’t do any useful work.Open in a separate windowFigure 3.Ultrasensitivity and bistability in Rab regulatory networks. (A) A simplified schematic of a Rab activation cycle is shown treating GDP–GTP exchange as equivalent to a covalent modification cycle such as phosphorylation. Because the reaction can only occur at a membrane surface, membrane recruitment factors are treated as activating inputs. Assuming no feedback and normal first-order reaction kinetics, Rab recruitment would be expected to follow Michaelis-Menten behavior. In cases where substrate is saturating and the reaction becomes zero-order, Goldbeter and Koshland (1984) have shown that product formation becomes more sensitive to enzyme concentration. In this case, generation of GTP-bound Rab becomes ultrasensitive to GEF concentration at the membrane surface. If additional positive feedback controls exist as shown in the bottom panel, then bistability may develop. In this case a rapid switch-like transition in Rab activity develops as Rab GEF concentration increases. Once in the active state the system becomes less dependent on continued high GEF activity. (B) A model for an interlinked Rab cascade is shown. The GEF for Rab-B is an effector for activated Rab-A, while the GAP for Rab-A is regulated by Rab-B. An example of this latter situation is provided by the Ypt1–Yp32 system discussed in the main text and shown in the bottom panel, where a Ypt1 GAP Gyp1 is an effector for Ypt32 (Rivera-Molina and Novick, 2009) and inhibits Ypt1. This coupling of the two cycles can result in coupled ultrasensitive switch-like transitions or bistability.A groundbreaking study in this area has applied these ideas to the conversion of Rab5-positive early endosomes to Rab7-positive late endosomes and lysosomes (Del Conte-Zerial et al., 2008). This analysis has provided strong evidence that positive and negative feedback loops in this system mediated by Rab GEFs and GAPs result in bistability in the form of a cut-out switch, so that Rab5 accumulation is followed by an abrupt transition at which Rab5 is rapidly lost and Rab7 accumulates (Del Conte-Zerial et al., 2008). Underpinning this is a biochemical network in which the Mon1–Ccz1 Rab7 GEF complex displaces Rabex-5, thus breaking the positive feedback loop to Rab5 activation (Poteryaev et al., 2010) and simultaneously promoting recruitment and activation of Rab7 (Nordmann et al., 2010; Gerondopoulos et al., 2012). Although there are only few studies where these ideas have been considered, they can be experimentally tested and are likely to be of increasing importance in membrane traffic regulation.

Origins of Rab GTPase control systems

One of the most difficult questions in membrane trafficking relates to the origins of complex internal membrane systems in eukaryotes. Analysis of Rab GTPases themselves suggests a pattern of evolution of Rabs consistent with the evolution of a core set of membrane organelles of the endocytic and secretory pathways (Diekmann et al., 2011; Klöpper et al., 2012). Yet, this provides little insight into how membrane organelles initially arose. Recent data on the structure of Rab GTPase regulators and coat protein complexes has identified common features with GTPase regulators in other systems including prokaryotes (Kinch and Grishin, 2006; Zhang et al., 2012; Levine et al., 2013). The conserved Longin–Roadblock fold has emerged as a structural feature of the large family of DENN-domain Rab GEFs in human cells (Yoshimura et al., 2010; Wu et al., 2011; Levine et al., 2013). Intriguingly, related domains are also present in the signal sequence receptor involved in protein translocation into the ER, vesicle coat protein complexes, and the MglA GTPase–MglB bacterial cell polarity regulator (Sun et al., 2007; Miertzschke et al., 2011; Levine et al., 2013). Although far from conclusive, these findings provide important pointers to the development of GTPase control systems, and more generally the early origins of membrane traffic pathways in eukaryotes from membrane-associated GTPases and their effector proteins.Are Rabs alone capable of triggering the pathways defining membrane identity? Multiple lines of evidence show Rab GTPases are clearly important and far from inconsequential regulators of vesicle traffic; however, further evidence is required before we should conclude that they are causal regulators of vesicle or organelle membrane identity. Neither of the studies using strategies to modulate the cellular localization of Rab GEFs reported that the mitochondria altered their identity or were converted into an endosome or Golgi because of the mistargeted Rabs (Gerondopoulos et al., 2012; Blümer et al., 2013). The picture emerging is therefore one in which Rabs cannot program membrane identity alone and must work in concert with other factors. Defining and reconstituting the systems needed to create membrane identity is therefore a major goal for membrane traffic research.     

Rabs grab motors: defining the connections between Rab GTPases and motor proteins.

Rab GTPases and their effectors regulate membrane traffic by determining, along with cognate SNAREs, the specificity of transport vesicle docking and fusion steps. Recent studies have also implicated Rabs in the movement of these transport vesicles from their site of formation to their site of fusion, and several Rabs have been linked to specific microtubule- or actin-based motor proteins. Analyses of Rab and motor protein mutants, coupled with advanced imaging techniques, have led to the suggestion that certain Rabs function as essential components of the vesicle receptor for specific motor proteins.     

An Overexpression Screen of Toxoplasma gondii Rab-GTPases Reveals Distinct Transport Routes to the Micronemes

The basic organisation of the endomembrane system is conserved in all eukaryotes and comparative genome analyses provides compelling evidence that the endomembrane system of the last common eukaryotic ancestor (LCEA) is complex with many genes required for regulated traffic being present. Although apicomplexan parasites, causative agents of severe human and animal diseases, appear to have only a basic set of trafficking factors such as Rab-GTPases, they evolved unique secretory organelles (micronemes, rhoptries and dense granules) that are sequentially secreted during invasion of the host cell. In order to define the secretory pathway of apicomplexans, we performed an overexpression screen of Rabs in Toxoplasma gondii and identified Rab5A and Rab5C as important regulators of traffic to micronemes and rhoptries. Intriguingly, we found that not all microneme proteins traffic depends on functional Rab5A and Rab5C, indicating the existence of redundant microneme targeting pathways. Using two-colour super-resolution stimulated emission depletion (STED) we verified distinct localisations of independent microneme proteins and demonstrate that micronemal organelles are organised in distinct subsets or subcompartments. Our results suggest that apicomplexan parasites modify classical regulators of the endocytic system to carryout essential parasite-specific roles in the biogenesis of their unique secretory organelles.     

Comparative modeling of Rab6 proteins: identification of key residues and their interactions with guanine nucleotides

The cytoplasm of a eukaryotic cell consists of a wide variety of membrane bound cell organelles and continuous flow of proteins amongst these organelles is a major challenge and must be stringently maintained in order to continue the correct biochemical functioning inside a cell. The transportation of various proteins amongst these organelles is facilitated by a vast Tubulo-vesicular network mediated by carrier proteins. The Rabs belong to small G proteins super family involved in the regulation and vesicle transport in between the organelles by shuttling between the active GTP and inactive GDP bound states. In this paper we put forth the homology modeling and docking studies of Rab6A proteins (Mus musculus, Gallus gallus and Caenorhabditis elegans) with GTP, GMP-PNP and GDP molecules and a comparative study between these proteins is done to identify key residues out of which serine of the phosphate binding loop (P – loop) and aspartic acid showed prominent interactions with the GTP, GDP and GMP-PNP nucleotides and cogitate that aspartic acid might also help in the stabilization of the switch I region of the Rab proteins besides serine.     

Overexpression of a Mutant Form of EhRabA,a Unique Rab GTPase of Entamoeba histolytica,Alters Endoplasmic Reticulum Morphology and Localization of the Gal/GalNAc Adherence Lectin

Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery and liver abscess. Vesicle trafficking events, such as phagocytosis and delivery of plasma membrane proteins, have been implicated in pathogenicity. Rab GTPases are proteins whose primary function is to regulate vesicle trafficking; therefore, understanding the function of Rabs in this organism may provide insight into virulence. E. histolytica possesses a number of unique Rabs that exhibit limited homology to host Rabs. In this study we examined the function of one such Rab, EhRabA, by characterizing a mutant overexpressing a constitutively GTP-bound version of the protein. Overexpression of mutant EhRabA resulted in decreased adhesion to and phagocytosis of human red blood cells and in the appearance of large tubular organelles that could be stained with endoplasmic reticulum (ER)-specific but not Golgi complex-specific antibodies. Consistent with the adhesion defect, two subunits of a cell surface adhesin, the galactose/N-acetylgalactosamine lectin, were mislocalized to the novel organelle. A cysteine protease, EhCP2, was also localized to the ER-like compartment in the mutant; however, the localization of two additional cell surface proteins, Igl and SREHP, remained unchanged in the mutant. The phenotype of the mutant could be recapitulated by treatment with brefeldin A, a cellular toxin that disrupts ER-to-Golgi apparatus vesicle traffic. This suggests that EhRabA influences vesicle trafficking pathways that are also sensitive to brefeldin A. Together, the data indicate that EhRabA directly or indirectly influences the morphology of secretory organelles and regulates trafficking of a subset of secretory proteins in E. histolytica.Entamoeba histolytica, a protozoan parasite, is the causative agent of amoebic dysentery and causes liver abscess. It is prevalent in developing countries that cannot prevent its fecal-oral spread and is responsible for considerable global morbidity and mortality (reviewed in reference 12). Infection is acquired by ingestion of the cyst form of the parasite. Excystation occurs in the small intestine, and released amoeboid trophozoites move to, and colonize, the bowel lumen. Here, the pathogen acquires nutrients via phagocytosis of colonic bacteria, host cells, and host cell debris. The ability of the pathogen to carry out phagocytosis has been correlated to its virulence potential. For example, phagocytosis-deficient mutants of E. histolytica exhibit reduced pathogenicity in vitro and in vivo (24, 33), and a noninvasive Entamoeba species, E. dispar, exhibits low rates of phagocytosis (26).Given the importance of phagocytosis to virulence, the identification of proteins that directly or indirectly regulate this process has been the focus of a considerable research effort (reviewed in references 16 and 23). Proteomic analyses of purified phagosomes have identified numerous proteins that may regulate phagocytosis in E. histolytica (2, 17, 22). Functional studies have identified several cell surface proteins as possible phagocytic receptors. One such receptor, the galactose/N-acetylgalactosamine-inhibitable lectin (Gal/GalNAc lectin) (6, 31), consists of a transmembrane heavy subunit (Hgl) which is covalently associated with a glycosylphosphatidylinositol (GPI)-linked light subunit (Lgl). This heterodimer is noncovalently associated with a GPI-anchored intermediate subunit (Igl). Other cell surface receptors include SREHP, a serine-rich protein that is proposed to be lipid anchored (42), and PATMK, a transmembrane kinase (2).The complement of receptors found on the cell surface is influenced both by anterograde and retrograde vesicle trafficking pathways. For example, newly synthesized proteins are incorporated into the membrane of the endoplasmic reticulum (ER), transported to the Golgi apparatus, and finally delivered to the plasma membrane via anterograde transport vesicles. On the other hand, cell surface receptors may be internalized and delivered to intracellular compartments or back to the plasma membrane via retrograde transport vesicles (44). While E. histolytica trophozoites have been shown to possess an ER and Golgi apparatus (8, 19, 41), insight into the molecular mechanisms regulating the morphological and functional integrity of these organelles is limited. Such insight would contribute to our understanding of phagocytosis, which relies on the proper functioning of these organelles for localization of cell surface receptors.In other systems, secretory vesicle trafficking events are regulated by Rab GTPases, a family of GTP-binding proteins involved in the docking and fusion of transport vesicles with target membranes. Substantial evidence suggests that Rab proteins carry out their function by cycling between a GDP-bound cytosolic form and a GTP-bound membrane form (reviewed in reference 39). In silico genome mining has shown that E. histolytica possesses greater than 90 Rab GTPases (34). A number of these exhibit limited homology to other known Rabs and thus are considered unique to E. histolytica (34). We previously reported that one of these unique Rabs, EhRabA, may be involved in the regulation of polarization, motility, and actin cytoskeletal dynamics (47, 48). In the current study, we demonstrate that overexpression of the putatively GTP-bound form of EhRabA results in the alteration of ER morphology, mislocalization of two subunits of the Gal/GalNAc lectin, and reduced phagocytosis, suggesting that this Rab plays a direct or indirect role in cellular functions that contribute to virulence.     

Rab proteins: The key regulators of intracellular vesicle transport

Vesicular/membrane trafficking essentially regulates the compartmentalization and abundance of proteins within the cells and contributes in many signalling pathways. This membrane transport in eukaryotic cells is a complex process regulated by a large and diverse array of proteins. A large group of monomeric small GTPases; the Rabs are essential components of this membrane trafficking route. Most of the Rabs are ubiquitously expressed proteins and have been implicated in vesicle formation, vesicle motility/delivery along cytoskeleton elements and docking/fusion at target membranes through the recruitment of effectors. Functional impairments of Rabs affecting transport pathways manifest different diseases. Rab functions are accompanied by cyclical activation and inactivation of GTP-bound and GDP-bound forms between the cytosol and membranes which is regulated by upstream regulators. Rab proteins are characterized by their distinct sub-cellular localization and regulate a wide variety of endocytic, transcytic and exocytic transport pathways. Mutations of Rabs affect cell growth, motility and other biological processes.     

Identification and phylogenetic analysis ofDictyostelium discoideumkinesin proteins

Background

Kinesins constitute a large superfamily of motor proteins in eukaryotic cells. They perform diverse tasks such as vesicle and organelle transport and chromosomal segregation in a microtubule- and ATP-dependent manner. In recent years, the genomes of a number of eukaryotic organisms have been completely sequenced. Subsequent studies revealed and classified the full set of members of the kinesin superfamily expressed by these organisms. ForDictyostelium discoideum, only five kinesin superfamily proteins (Kif's) have already been reported.

Results

Here, we report the identification of thirteen kinesin genes exploiting the information from the raw shotgun reads of theDictyostelium discoideumgenome project. A phylogenetic tree of 390 kinesin motor domain sequences was built, grouping theDictyosteliumkinesins into nine subfamilies. According to known cellular functions or strong homologies to kinesins of other organisms, four of theDictyosteliumkinesins are involved in organelle transport, six are implicated in cell division processes, two are predicted to perform multiple functions, and one kinesin may be the founder of a new subclass.

Conclusion

This analysis of theDictyosteliumgenome led to the identification of eight new kinesin motor proteins. According to an exhaustive phylogenetic comparison,Dictyosteliumcontains the same subset of kinesins that higher eukaryotes need to perform mitosis. Some of the kinesins are implicated in intracellular traffic and a small number have unpredictable functions.     

The C. elegans Rab Family: Identification,Classification and Toolkit Construction

Rab monomeric GTPases regulate specific aspects of vesicle transport in eukaryotes including coat recruitment, uncoating, fission, motility, target selection and fusion. Moreover, individual Rab proteins function at specific sites within the cell, for example the ER, golgi and early endosome. Importantly, the localization and function of individual Rab subfamily members are often conserved underscoring the significant contributions that model organisms such as Caenorhabditis elegans can make towards a better understanding of human disease caused by Rab and vesicle trafficking malfunction. With this in mind, a bioinformatics approach was first taken to identify and classify the complete C. elegans Rab family placing individual Rabs into specific subfamilies based on molecular phylogenetics. For genes that were difficult to classify by sequence similarity alone, we did a comparative analysis of intron position among specific subfamilies from yeast to humans. This two-pronged approach allowed the classification of 30 out of 31 C. elegans Rab proteins identified here including Rab31/Rab50, a likely member of the last eukaryotic common ancestor (LECA). Second, a molecular toolset was created to facilitate research on biological processes that involve Rab proteins. Specifically, we used Gateway-compatible C. elegans ORFeome clones as starting material to create 44 full-length, sequence-verified, dominant-negative (DN) and constitutive active (CA) rab open reading frames (ORFs). Development of this toolset provided independent research projects for students enrolled in a research-based molecular techniques course at California State University, East Bay (CSUEB).     

Rab GTPases: specifying and deciphering organelle identity and function

Ten years ago, 20 Rab proteins had been identified as organelle-specific GTPases, and two were known to be essential for vesicle targeting in yeast. Today, more than 60 mammalian Rab proteins have been identified. While Rabs were always viewed as key regulatory factors, no one could have anticipated their diversity of functions and multitude of effectors. Rabs organize distinct protein scaffolds within a single organelle and act in a combinatorial manner with their effectors to regulate all stages of membrane traffic.     

Rab-regulated interaction of early endosomes with lipid droplets

Recent studies indicate that lipid droplets isolated from a variety of different cells are rich in proteins known to regulate membrane traffic. Among these proteins are multiple Rab GTPases. Rabs are GTP switches that regulate intracellular membrane traffic through an ability to control membrane-membrane docking as well as vesicle motility. Here we present evidence that the multiple Rabs associated with droplets have a function in regulating membrane traffic. Droplet Rabs are removed by Rab GDP-dissociation inhibitor (RabGDI) in a GDP-dependent reaction, and are recruited to Rab-depleted droplets from cytosol in a GTP-dependent reaction. Rabs also control the recruitment of the early endosome (EE) marker EEA1 from cytosol. We use an in vitro reconstitution assay to show that transferrin receptor positive EEs bind to the droplet in a GTP/Rab-dependent reaction that appears not to lead to membrane fusion. This docking reaction is insensitive to ATP(gamma s) but is blocked by ATP. Finally, we show that when GTP bound active or GDP bound inactive Rab5 is targeted to the droplet, the active form recruits EEA1. We conclude that the Rabs associated with droplets may be capable of regulating the transient interaction of specific membrane systems, probably to transport lipids between membrane compartments.     

Rab蛋白在葡萄糖转运中的作用

在脂肪和骨骼肌细胞中,胰岛素可迅速刺激葡萄糖转运,即通常所说的GLUT4转运。 GLUT4转运是指Rabs与GTP结合时,促进囊泡与微管和微丝蛋白结合,并通过锚定和融合作用使GLUT4囊泡与目标膜结构融合。多数 Rab 家族成员广泛表达于各种组织细胞中,且在细胞内定位十分广泛,几乎存在于真核细胞所有的膜相关的细胞器的胞浆侧。 Rab 蛋白作为囊泡运输的分子开关,通过调节运输小泡的停泊和融合,在囊泡的形成、转运、粘附、锚定、融合等过程中起着重要的作用。 Rab蛋白受到多种上游调节蛋白的调节,同时调控着下游的多种效应蛋白,构成了复杂的调控网络：任何一个环节改变都可能会导致蛋白质转运的异常,进而引发疾病。本文系统阐述了Rab蛋白在葡萄糖转运过程中的作用及该领域的最新进展。     

Plant PRA plays an important role in intracellular vesicular trafficking between compartments as GDF

Rab GTPases like Ras-related monomeric GTPases are well known to regulate intracellular vesicle trafficking by cycling between membrane-bound and cytosolic states. The functions of these proteins are controlled by upstream regulators and downstream effectors. Ypt/Rabs transmit signals to downstream effectors in a GTP-dependent manner. GDP-bound Rab proteins are extracted from their target membrane by cytosolic proteins known as GDP dissociation inhibitors (GDIs), and the Rab GTPase is recruited to the membrane compartment following dissociation from the GDI by GDI displacement factor (GDF). Now, we''re going to discuss the role of plant PRA concerted with Rab and GDI proteins by recycling Rab between membrane and cytosol for intracellular trafficking of cargo proteins.Key words: GDF, GDI, PRA1, Rab, vacuolar trafficking, vesicle traffickingAlthough Rabs appear to undergo multiple cycles of GDI-mediated delivery to, and extraction from membranes,¹ the mechanisms underlying Rab membrane delivery and association by GDI and other factors remain unclear. GDP-GTP exchange occurs at the target membrane, catalyzed by a guanine nucleotide exchange factor (GEF),^2,3 and the GTP-bound Rab transmits signals to downstream effectors and associates with the membrane to ensure proper docking and fusion of transport vesicles.⁴ After vesicle fusion on its target membrane, subsequently hydrolysis of GTP by the Rab is facilitated by GTPase-activating proteins (GAPs).⁴ The resulting GDP-bound Rab is subsequently retrieved from the membrane by GDI, which then maintains Rab in the cytosol to complete the cycle.Many research groups isolated PRAs, a homolog of human YIP3, in several two-hybrid screenings as interacting with multiple Rabs in their GTP- or GDP-bound form.^5,6 PRA contains two extensive hydrophobic domains which may form a membrane-spanning domain or the inner hydrophobic core of the protein.⁷ PRA1 is localized to the Golgi and late endosomes,⁸ and the related PRA2 is present in the endoplasmic reticulum.⁹Recently it has been shown that the human YIP3 stimulates the rate of nucleotide binding to Rab9 when added to prenyl Rab9-GDI complex and catalyzes the dissociation of the endosomal Rab-GDI complex, indicating that YIP3 is a GDI displacement factor that recruits Rab to membranes.¹⁰ According to the Gougeon et al. report (2002),¹¹ PRA1 inhibits the extraction of membrane-bound Rab3A by GDI1, suggesting that recycling of Rab depends on the opposing actions of PRA and GDI, with PRA favoring membrane retention but GDI favoring solubilization.Moreover, mammalian PRA1 is required for vesicle formation from the Golgi complex and might influence the recruitment of Rab effectors during cargo sequestration as well as proteins required for subsequent vesicle docking and fusion.¹¹ This is consistent with its transport function based on interaction of yeast homologue Yip3p with proteins in the secretory pathway.⁷ Yip1-Yif1p complex binds to the ER and to the Golgi SNAREs, Bos1p and Sec22p, and is required for membrane fusion machinery.⁵ In addition, a role of Yip1p had also been proposed in COPII vesicle biogenesis.¹²To our current knowledge there is no report on the physiological role of a GDF in plants. Aims to enrich the understanding of the mechanism of Rab recycling and trafficking pathways in plant, we identified and characterized OsPRA1, a rice homolog of PRA. OsPRA1, isolated by yeast two-hybrid screening using OsRab7 as bait, localized to the prevacuolar compartment as a membrane protein.¹³ Additionally, through western blot and protoplast transient assays it was confirmed that OsPRA1 has GDF activity, which dissociates the Rab7-GDI2 complex and recruits dissociated Rab from the Rab7-GDI2 complex to the donor membrane (unpublished data). When yeast two-hybrid interaction assay between OsPRA1 and OsGDI2 was performed, OsPRA1 interacted with OsGDI2 weakly (unpublished data), supporting our proposition that OsPRA1 dissociates the OsRab7-OsGDI2 complex.Furthermore, by using yeast two-hybrid and co-immunoprecipitation assays it was demonstrated that OsPRA1 interacted with dominant negative OsRab7 (T22N) which has no GTP binding activity, but not the constitutively active OsRab7 (Q67L),¹³ indicating that OsPRA1 may interact with GDP-bound OsRab7 at the donor membrane, PVC. These results support that OsPRA1 is a GDF for OsRab7.Subsequently, in order to find its interacting proteins implicated in vesicular trafficking, such as t- or v-SNAREs, yeast two-hybrid screening using OsPRA1 as bait was performed. Interestingly, a t-SNARE, OsVam3p, homolgous to AtVam3p involved in vacuolar trafficking and localizing to both PVC and vacuole membranes in Arabidopsis,¹⁴ was isolated (unpublished data). This suggests that OsPRA1 may be a component of the vesicle fusion machinery. To further strengthen our hypothesis, we examined whether or not mutant OsPRA1 (Y94A) and OsRab7 interact. Mutant OsPRA1 (Y94A) showed weak and no interaction with OsRab7 and OsVam3p, respectively, indicating that mutant OsPRA1 (Y94A) may lose its activity for recruiting Rab GTPase and Rab effector proteins and fusing vesicles to the vacuolar membrane. Actually, when OsPRA1 was mutated, its GDF activity was reduced to less than 50%, and its localization was changed from the PVC to the cytosol. These results are consistent with the assigned transport function of OsPRA1. Besides, our data from transient expression assay using vacuole markers suggested a direct involvement of OsPRA1 in the trafficking of vacuolar proteins.In summary, OsPRA1, a Yip homologous protein, may function in regulating vacuolar trafficking as a GDF dissociating OsRab7-OsGDI2 complex in plant cells.     

Rab GTPases and microtubule motors

Rab proteins are a family of small GTPases which, since their initial identification in the late 1980s, have emerged as master regulators of all stages of intracellular trafficking processes in eukaryotic cells. Rabs cycle between distinct conformations that are dependent on their guanine-nucleotide-bound status. When active (GTP-bound), Rabs are distributed to the cytosolic face of specific membranous compartments where they recruit downstream effector proteins. Rab-effector complexes then execute precise intracellular trafficking steps, which, in many cases, include vesicle motility. Microtubule-based kinesin and cytoplasmic dynein motor complexes are prominent among the classes of known Rab effector proteins. Additionally, many Rabs associate with microtubule-based motors via effectors that act as adaptor molecules that can simultaneously associate with the GTP-bound Rab and specific motor complexes. Thus, through association with motor complexes, Rab proteins can allow for membrane association and directional movement of various vesicular cargos along the microtubule cytoskeleton. In this mini-review, we highlight the expanding repertoire of Rab/microtubule motor protein interactions, and, in doing so, present an outline of the multiplicity of transport processes which result from such interactions.     

Regulation of autophagy by the Rab GTPase network

Autophagy (macroautophagy) is a highly conserved intracellular and lysosome-dependent degradation process in which autophagic substrates are enclosed and degraded by a double-membrane vesicular structure in a continuous and dynamic vesicle transport process. The Rab protein is a small GTPase that belongs to the Ras-like GTPase superfamily and regulates the vesicle traffic process. Numerous Rab proteins have been shown to be involved in various stages of autophagy. Rab1, Rab5, Rab7, Rab9A, Rab11, Rab23, Rab32, and Rab33B participate in autophagosome formation, whereas Rab9 is required in non-canonical autophagy. Rab7, Rab8B, and Rab24 have a key role in autophagosome maturation. Rab8A and Rab25 are also involved in autophagy, but their role is unknown. Here, we summarize new findings regarding the involvement of Rabs in autophagy and provide insights regarding future research on the mechanisms of autophagy regulation.     

Rapid degradation of dominant-negative Rab27 proteins <Emphasis Type="Italic">in vivo</Emphasis> precludes their use in transgenic mouse models

Background

Transgenic mice have proven to be a powerful system to study normal and pathological gene functions. Here we describe an attempt to generate a transgenic mouse model for choroideremia (CHM), a slow-onset X-linked retinal degeneration caused by mutations in the Rab Escort Protein-1 (REP1) gene. REP1 is part of the Rab geranylgeranylation machinery, a modification that is essential for Rab function in membrane traffic. The loss of REP1 in CHM patients may trigger retinal degeneration through its effects on Rab proteins. We have previously reported that Rab27a is the Rab most affected in CHM lymphoblasts and hypothesised that the selective dysfunction of Rab27a (and possibly a few other Rab GTPases) plays an essential role in the retinal degenerative process.

Results

To investigate this hypothesis, we generated several lines of dominant-negative, constitutively-active and wild-type Rab27a (and Rab27b) transgenic mice whose expression was driven either by the pigment cell-specific tyrosinase promoter or the ubiquitous β-actin promoter. High levels of mRNA and protein were observed in transgenic lines expressing wild-type or constitutively active Rab27a and Rab27b. However, only modest levels of transgenic protein were expressed. Pulse-chase experiments suggest that the dominant-negative proteins, but not the constitutively-active or wild type proteins, are rapidly degraded. Consistently, no significant phenotype was observed in our transgenic lines. Coat-colour was normal, indicating normal Rab27a activity. Retinal function as determined by fundoscopy, angiography, electroretinography and histology was also normal.

Conclusions

We suggest that the instability of the dominant-negative mutant Rab27 proteins in vivo precludes the use of this approach to generate mouse models of disease caused by Rab27 GTPases.

     

植物Rab蛋白的分类与功能

Rab蛋白构成小G蛋白超家族中最大的1个家族,广泛存在于动物、植物和微生物中.Rab调控细胞内的囊泡形成、转运、锚定及囊泡与质膜的融合等过程,在细胞内吞和分泌途径中发挥分子开关的作用.不同生物中Rab的结构和作用机制非常保守,但Rab的分类和生理学功能存在差异.植物Rab不仅行使类似于动物或微生物同源Rab的细胞学功能,而且在植物生长发育、激素信号调节、生物或非生物胁迫应答等方面表现出功能特异性.本文结合近年的研究进展,对植物Rab的分类、结构、调节机制和功能进行了综述,并对当前植物Rab功能研究的难点和方向进行了
讨论.     

Saccharomyces cerevisiae Rab-GDI displacement factor ortholog Yip3p forms distinct complexes with the Ypt1 Rab GTPase and the reticulon Rtn1p

Rab GTPases are crucial regulators of organelle biogenesis, maintenance, and transport. Multiple Rabs are expressed in all cells, and each is localized to a distinct set of organelles, but little is known regarding the mechanisms by which Rabs are targeted to their resident organelles. Integral membrane proteins have been postulated to serve as receptors that recruit Rabs from the cytosol in a complex with the Rab chaperone, GDI, to facilitate the dissociation of Rab and GDI, hence facilitating loading of Rabs on membranes. We show here that the yeast (Saccharomyces cerevisiae) Golgi Rab GTPase Ypt1p can be copurified with the integral membrane protein Yip3p from detergent cell extracts. In addition, a member of the highly conserved reticulon protein family, Rtn1p, is also associated with Yip3p in vivo. However, Ypt1p did not copurify with Rtn1p, indicating that Yip3p is a component of at least two different protein complexes. Yip3p and Rtn1p are only partially colocalized in cells, with Yip3p localized predominantly to the Golgi and secondarily to the endoplasmic reticulum, whereas Rtn1p is localized predominantly to the endoplasmic reticulum and secondarily to the Golgi. Surprisingly, the intracellular localization of Rabs was not perturbed in yip3Delta or rtn1Delta mutants, suggesting that these proteins do not play a role in targeting Rabs to intracellular membranes. These data indicate that Yip3p may have multiple functions and that its interaction with Rabs is not critical for their recruitment to organelle membranes.     

Guanine Nucleotide Exchange Factors (GEFs) Have a Critical but Not Exclusive Role in Organelle Localization of Rab GTPases

Membrane fusion at eukaryotic organelles is initiated by Rab GTPases and tethering factors. Rabs in their GDP-bound form are kept soluble in the cytoplasm by the GDP dissociation inhibitor (GDI) chaperone. Guanine nucleotide exchange factors (GEFs) are found at organelles and are critical for Rab function. Here, we surveyed the overall role of GEFs in Rab localization. We show that GEFs, but none of the proposed GDI displacement factors, are essential for the correct membrane localization of yeast Rabs. In the absence of the GEF, Rabs lost their primary localization to the target organelle. Several Rabs, such as vacuolar Ypt7, were found at the endoplasmic reticulum and thus were still membrane-bound. Surprisingly, a Ypt7 mutant that undergoes facilitated nucleotide exchange localized to vacuoles independently of its GEF Mon1-Ccz1 and rescued vacuole morphology. In contrast, wild-type Ypt7 required its GEF for localization and to counteract the extraction by GDI. Our data agree with the emerging model that GEFs are critical for Rab localization but raise the possibility that additional factors can contribute to this process.