The p115-interactive proteins GM130 and giantin participate in endoplasmic reticulum-Golgi traffic

The transport factor p115 is essential for endoplasmic reticulum (ER) to Golgi traffic. P115 interacts with two Golgi proteins, GM130 and giantin, suggesting that they might also participate in ER-Golgi traffic. Here, we show that peptides containing the GM130 or the giantin p115 binding domain and anti-GM130 and anti-giantin antibodies inhibit transport of vesicular stomatitis virus (VSV)-G protein to a mannosidase II-containing Golgi compartment. To determine whether p115, GM130, and giantin act together or sequentially during transport, we compared kinetics of traffic inhibition. Anti-p115, anti-GM130, and anti-giantin antibodies inhibited transport at temporally distinct steps, with the p115-requiring step before the GM130-requiring stage, and both preceding the giantin-requiring stage. Examination of the distribution of the arrested VSV-G protein showed that anti-p115 antibodies inhibited transport at the level of vesicular-tubular clusters, whereas anti-GM130 and anti-giantin antibodies inhibited after the VSV-G protein moved to the Golgi complex. Our results provide the first evidence that GM130 and giantin are required for the delivery of a cargo protein to the mannosidase II-containing Golgi compartment. These data are most consistent with a model where transport from the ER to the cis/medial-Golgi compartments requires the action of p115, GM130, and giantin in a sequential rather than coordinate mechanism.     

Coordination of golgin tethering and SNARE assembly: GM130 binds syntaxin 5 in a p115-regulated manner

During membrane traffic, transport carriers are first tethered to the target membrane prior to undergoing fusion. Mechanisms exist to connect tethering with fusion, but in most cases, the details remain poorly understood. GM130 is a member of the golgin family of coiled-coil proteins tat is involved in membrane tethering at the endoplasmic reticulum (ER) to Golgi intermediate compartment and cis-Golgi. Here, we demonstrate that GM130 interacts with syntaxin 5, a t-SNARE also localized to the early secretory pathway. Binding to syntaxin 5 is specific, direct, and mediated by the membrane-proximal region of GM130. Interestingly, interaction with syntaxin 5 is inhibited by the binding of the vesicle docking protein p115 to a distal binding site in GM130. The interaction between GM130 and the small GTPase Rab1 is also inhibited by p115 binding. Our findings suggest a mechanism for coupling membrane tethering and fusion at the ER to Golgi intermediate compartment and cis-Golgi, with GM130 playing a central role in linking these processes. Consistent with this hypothesis, we find that depletion of GM130 by RNA interference slows the rate of ER to Golgi trafficking in vivo. The interactions of GM130 with syntaxin 5 and Rab1 are also regulated by mitotic phosphorylation, which is likely to contribute to the inhibition of ER to Golgi trafficking that occurs when mammalian cells enter mitosis.     

Binding relationships of membrane tethering components. The giantin N terminus and the GM130 N terminus compete for binding to the p115 C terminus

By forming a molecular tether between two membranes, p115, giantin, and GM130 may mediate multiple Golgi-related processes including vesicle transport, cisternae formation, and cisternal stacking. The tether is proposed to involve the simultaneous binding of p115 to giantin on one membrane and to GM130 on another membrane. To explore this model, we tested for the presence of the putative giantin-p115-GM130 ternary complex. We first mapped p115-binding site in giantin to a 70-amino acid coiled-coil domain at the extreme N terminus, a position that may exist up to 400 nm away from the Golgi membrane. We then generated glutathione S-transferase (GST) fusion proteins containing either giantin's or GM130's p115 binding site and tested whether such proteins could bind p115 and GM130 or bind p115 and giantin, respectively. Unexpectedly, GST fusions containing either the giantin or the GM130 p115 binding site efficiently bound p115, but the p115 bound to GST-giantin did not bind GM130, and the p115 bound to GST-GM130 did not bind giantin. To explain this result, we mapped the giantin binding site in p115 and found that it is located at the C-terminal acidic domain, the same domain involved in binding GM130. The presence of a single binding site in p115 for giantin and GM130 was confirmed by demonstration that giantin and GM130 compete for binding to p115. These results question a simple tethering model involving a ternary giantin-p115-GM130 complex and suggest that p115-giantin and p115-GM130 interactions might mediate independent membrane tethering events.     

Evidence that Golgi structure depends on a p115 activity that is independent of the vesicle tether components giantin and GM130

Inhibition of the putative coatomer protein I (COPI) vesicle tethering complex, giantin-p115-GM130, may contribute to mitotic Golgi breakdown. However, neither this, nor the role of the giantin-p115-GM130 complex in the maintenance of Golgi structure has been demonstrated in vivo. Therefore, we generated antibodies directed against the mapped binding sites in each protein of the complex and injected these into mammalian tissue culture cells. Surprisingly, the injected anti-p115 and antigiantin antibodies caused proteasome-mediated degradation of the corresponding antigens. Reduction of p115 levels below detection led to COPI-dependent Golgi fragmentation and apparent accumulation of Golgi-derived vesicles. In contrast, neither reduction of giantin below detectable levels, nor inhibition of p115 binding to GM130, had any detectable effect on Golgi structure or Golgi reassembly after cell division or brefeldin A washout. These observations indicate that inhibition of p115 can induce a mitotic-like Golgi disassembly, but its essential role in Golgi structure is independent of its Golgi-localized binding partners giantin and GM130.     

Retrograde transport of protein toxins through the Golgi apparatus

A number of protein toxins from plants and bacteria take advantage of transport through the Golgi apparatus to gain entry into the cytosol where they exert their action. These toxins include the plant toxin ricin, the bacterial Shiga toxins, and cholera toxin. Such toxins bind to lipids or proteins at the cell surface, and they are endocytosed both by clathrin-dependent and clathrin-independent mechanisms. Sorting to the Golgi and retrograde transport to the endoplasmic reticulum (ER) are common to these toxins, but the exact mechanisms turn out to be toxin and cell-type dependent. In the ER, the enzymatically active part is released and then transported into the cytosol, exploiting components of the ER-associated degradation system. In this review, we will discuss transport of different protein toxins, but we will focus on factors involved in entry and sorting of ricin and Shiga toxin into and through the Golgi apparatus.     

The interaction of two tethering factors, p115 and COG complex, is required for Golgi integrity

The vesicle-tethering protein p115 functions in endoplasmic reticulum-Golgi trafficking. We explored the function of homologous region 2 (HR2) of the p115 head domain that is highly homologous with the yeast counterpart, Uso1p. By expression of p115 mutants in p115 knockdown (KD) cells, we found that deletion of HR2 caused an irregular assembly of the Golgi, which consisted of a cluster of mini-stacked Golgi fragments, and gathered around microtubule-organizing center in a microtubule-dependent manner. Protein interaction analyses revealed that p115 HR2 interacted with Cog2, a subunit of the conserved oligomeric Golgi (COG) complex that is known another putative cis-Golgi vesicle-tethering factor. The interaction between p115 and Cog2 was found to be essential for Golgi ribbon reformation after the disruption of the ribbon by p115 KD or brefeldin A treatment and recovery by re-expression of p115 or drug wash out, respectively. The interaction occurred only in interphase cells and not in mitotic cells. These results strongly suggested that p115 plays an important role in the biogenesis and maintenance of the Golgi by interacting with the COG complex on the cis-Golgi in vesicular trafficking.     

On and off membrane dynamics of the endoplasmic reticulum-golgi tethering factor p115 in vivo

The mechanisms regulating membrane recruitment of the p115 tethering factor in vivo are unknown. Here, we describe cycling of p115 between membranes and cytosol and document the effects of Golgi matrix proteins, Rab1, and soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors (SNAREs) on this process. Rapid membrane/cytosol exchange is shown by swift (t1/2 approximately 20 s) loss of Golgi-localized p115-green fluorescent protein (GFP) after repeated photobleaching of cell periphery and rapid (t1/2 approximately 13 s) fluorescence recovery after photobleaching Golgi-localized p115-GFP. p115 mutant missing the GM130/giantin binding site exhibits analogous fluorescence recovery after photobleaching (FRAP) (t1/2 approximately 13 s), suggesting that GM130 and giantin are not major determinants of p115 membrane dynamics. In contrast, p115-GFP exchanges more rapidly (t1/2 approximately 8 s) in cells expressing the inactive Rab1/N121I mutant, indicating that p115 cycling is influenced by Rab1. p115-GFP dynamics is also influenced by the assembly status of SNAREs. In cells expressing an ATPase-deficient NSF/E329Q mutant that inhibits SNARE complex disassembly, the cycling kinetics of p115-GFP are significantly slower (t1/2 approximately 21 s). In contrast, in cells incubated at reduced temperature (10 degrees C) that inhibits vesicular traffic, the cycling kinetics of p115-GFP are faster (t1/2 approximately 7 s). These data suggest that p115-binding sites on the membrane are provided by unassembled SNAREs. In agreement, biochemical studies show increased p115 recruitment to membranes in the presence of NSF and alpha-SNAP. Our data support a model in which recruitment of tethers is directly regulated by the assembly status of SNAREs.     

The Golgi protein GM130 regulates centrosome morphology and function

The Golgi apparatus (GA) of mammalian cells is positioned in the vicinity of the centrosome, the major microtubule organizing center of the cell. The significance of this physical proximity for organelle function and cell cycle progression is only beginning to being understood. We have identified a novel function for the GA protein, GM130, in the regulation of centrosome morphology, position and function during interphase. RNA interference-mediated depletion of GM130 from five human cell lines revealed abnormal interphase centrosomes that were mispositioned and defective with respect to microtubule organization and cell migration. When GM130-depleted cells entered mitosis, they formed multipolar spindles, arrested in metaphase, and died. We also detected aberrant centrosomes during interphase and multipolar spindles during mitosis in ldlG cells, which do not contain detectable GM130. Although GA proteins have been described to regulate mitotic centrosomes and spindle formation, this is the first report of a role for a GA protein in the regulation of centrosomes during interphase.     

A role for the vesicle tethering protein, p115, in the post-mitotic stacking of reassembling Golgi cisternae in a cell-free system.

During telophase, Golgi cisternae are regenerated and stacked from a heterogeneous population of tubulovesicular clusters. A cell-free system that reconstructs these events has revealed that cisternal regrowth requires interplay between soluble factors and soluble N-ethylmaleimide (NEM)-sensitive fusion protein (NSF) attachment protein receptors (SNAREs) via two intersecting pathways controlled by the ATPases, p97 and NSF. Golgi reassembly stacking protein 65 (GRASP65), an NEM-sensitive membrane-bound component, is required for the stacking process. NSF-mediated cisternal regrowth requires a vesicle tethering protein, p115, which we now show operates through its two Golgi receptors, GM130 and giantin. p97-mediated cisternal regrowth is p115-independent, but we now demonstrate a role for p115, in conjunction with its receptors, in stacking p97 generated cisternae. Temporal analysis suggests that p115 plays a transient role in stacking that may be upstream of GRASP65-mediated stacking. These results implicate p115 and its receptors in the initial alignment and docking of single cisternae that may be an important prerequisite for stack formation.     

The Golgi protein p115 associates with gamma-tubulin and plays a role in Golgi structure and mitosis progression

The Golgi apparatus is a network of polarized cisternae localized to the perinuclear region in mammalian cells. It undergoes extensive vesiculation at the onset of mitosis and its reassembly requires factors that are in part segregated via the mitotic spindle. Here we show that unlike typical Golgi markers, the Golgi-protein p115 partitioned with the spindle poles throughout mitosis. An armadillo-fold in its N terminus mediated a novel interaction between p115 and γ-tubulin and functioned in its centrosomal targeting. Both the N- and C-terminal regions of p115 were required to maintain Golgi structure. Strikingly, p115 was essential for mitotic spindle function and the resolution of the cytokinetic bridge because its depletion resulted in spindle collapse, chromosome missegregation, and failed cytokinesis. We demonstrate that p115 plays a critical role in mitosis progression, implicating it as the only known golgin to regulate both mitosis and apoptosis.     

A caspase cleavage fragment of p115 induces fragmentation of the Golgi apparatus and apoptosis

In mammalian cells, the Golgi apparatus undergoes extensive fragmentation during apoptosis. p115 is a key vesicle tethering protein required for maintaining the structural organization of the Golgi apparatus. Here, we demonstrate that p115 was cleaved during apoptosis by caspases 3 and 8. Compared with control cells expressing native p115, those expressing a cleavage-resistant form of p115 delayed Golgi fragmentation during apoptosis. Expression of cDNAs encoding full-length or an NH2-terminal caspase cleavage fragment of p115 had no effect on Golgi morphology. In contrast, expression of the COOH-terminal caspase cleavage product of p115 itself caused Golgi fragmentation. Furthermore, this fragment translocated to the nucleus and its expression was sufficient to induce apoptosis. Most significantly, in vivo expression of the COOH-terminal fragment in the presence of caspase inhibitors, or upon coexpression with a cleavage-resistant mutant of p115, showed that p115 degradation plays a key role in amplifying the apoptotic response independently of Golgi fragmentation.     

Microtubule nucleation at the cis‐side of the Golgi apparatus requires AKAP450 and GM130

We report that microtubule (MT) nucleation at the Golgi apparatus requires AKAP450, a centrosomal γ‐TuRC‐interacting protein that also forms a distinct network associated with the Golgi. Depletion of AKAP450 abolished MT nucleation at the Golgi, whereas depletion of the cis‐Golgi protein GM130 led to the disorganisation of AKAP450 network and impairment of MT nucleation. Brefeldin‐A treatment induced relocalisation of AKAP450 to ER exit sites and concomitant redistribution of MT nucleation capacity to the ER. AKAP450 specifically binds the cis‐side of the Golgi in an MT‐independent, GM130‐dependent manner. Short AKAP450‐dependent growing MTs are covered by CLASP2. Like for centrosome, dynein/dynactin complexes are necessary to anchor MTs growing from the Golgi. We further show that Golgi‐associated AKAP450 has a role in cell migration rather than in cell polarisation of the centrosome–Golgi apparatus. We propose that the recruitment of AKAP450 on the Golgi membranes through GM130 allows centrosome‐associated nucleating activity to extend to the Golgi, to control the assembly of subsets of MTs ensuring specific functions within the Golgi or for transporting specific cargos to the cell periphery.     

Evidence that all newly synthesized proteins destined for fast axonal transport pass through the Golgi apparatus

Effects of the sodium ionophore, monensin, were examined on the passage from neuronal cell body to axon of materials undergoing fast intracellular transport. In vitro exposure of bullfrog dorsal root ganglia to concentrations of drug less than 1.0 micron led to a dose-dependent depression in the amount of fast-transported [3H]leucine- or [3H]glycerol-labeled material appearing in the nerve trunk. Incorporation of either precursor was unaffected. Exposure of a desheathed nerve trunk to similar concentrations of monensin, while ganglia were incubated in drug-free medium, had no effect on transport. With [3H]fucose as precursor, fast transport of labeled glycoproteins was depressed to the same extent as with [3H]leucine; synthesis, again, was unaffected. By contrast, with [3H]galactose as precursor, an apparent reduction in transport of labeled glycoproteins was accounted for by a marked depression in incorporation. The inference from these findings, that monensin acts to block fast transport at the level of the Golgi apparatus, was supported by ultrastructural examination of the drug-treated neurons. An extensive and selective disruption of Golgi saccules was observed, accompanied by an accumulation of clumped smooth membranous cisternae. Quantitative analyses of 48 individual fast-transported protein species, after separation by two-dimensional gel electrophoresis, revealed that monensin depresses all proteins to a similar extent. These results indicate that passage through the Golgi apparatus is an obligatory step in the intracellular routing of materials destined for fast axonal transport.     

The role of microtubules in transport between the endoplasmic reticulum and Golgi apparatus in mammalian cells

The organization of intracellular compartments and the transfer of components between them are central to the correct functioning of mammalian cells. Proteins and lipids are transferred between compartments by the formation, movement and subsequent specific fusion of transport intermediates. These vesicles and membrane clusters must be coupled to the cytoskeleton and to motor proteins that drive motility. Anterograde ER (endoplasmic reticulum)-to-Golgi transport, and the converse step of retrograde traffic from the Golgi to the ER, are now known to involve coupling of membranes to the microtubule cytoskeleton. Here we shall discuss our current understanding of the mechanisms that link membrane traffic in the early secretory pathway to the microtubule cytoskeleton in mammalian cells. Recent data have also provided molecular detail of functional co-ordination of motor proteins to specify directionality, as well as mechanisms for regulating motor activity by protein phosphorylation.     

The p24 family of transmembrane proteins at the interface between endoplasmic reticulum and Golgi apparatus

Summary The p24 family of small transmembrane proteins was discovered recently in yeast and mammalian cells, and some of its members have been implicated in biosynthetic protein transport. The p24 proteins are proposed to act on transport vesicles as receptors for coat and/or cargo, but their precise function(s) remain controversial. Here, we describe this protein family, and we review the available experimental data concerning their localization and function. Finally, we hypothesize about a possible role of p24 proteins in organelle morphogenesis.Abbreviations CGN cis-Golgi network - COP coat protein - ER endoplasmic reticulum - VSV-G vesicular stomatitis virus glycoprotein G