Interaction between Epsin/Yap180 adaptors and the scaffolds Ede1/Pan1 is required for endocytosis

The spatial and temporal regulation of the interactions among the approximately 60 proteins required for endocytosis is under active investigation in many laboratories. We have identified the interaction between monomeric clathrin adaptors and endocytic scaffold proteins as a critical prerequisite for the recruitment and/or spatiotemporal dynamics of endocytic proteins at early and late stages of internalization. Quadruple deletion yeast cells (DeltaDeltaDeltaDelta) lacking four putative adaptors, Ent1/2 and Yap1801/2 (homologues of epsin and AP180/CALM proteins), with a plasmid encoding Ent1 or Yap1802 mutants, have defects in endocytosis and growth at 37 degrees C. Live-cell imaging revealed that the dynamics of the early- and late-acting scaffold proteins Ede1 and Pan1, respectively, depend upon adaptor interactions mediated by adaptor asparagine-proline-phenylalanine motifs binding to scaffold Eps15 homology domains. These results suggest that adaptor/scaffold interactions regulate transitions from early to late events and that clathrin adaptor/scaffold protein interaction is essential for clathrin-mediated endocytosis.     

Yeast P4-ATPases Drs2p and Dnf1p are essential cargos of the NPFXD/Sla1p endocytic pathway

Drs2p family P-type ATPases (P4-ATPases) are required in multiple vesicle-mediated protein transport steps and are proposed to be phospholipid translocases (flippases). The P4-ATPases Drs2p and Dnf1p cycle between the exocytic and endocytic pathways, and here we define endocytosis signals required by these proteins to maintain a steady-state localization to internal organelles. Internalization of Dnf1p from the plasma membrane uses an NPFXD endocytosis signal and its recognition by Sla1p, part of an endocytic coat/adaptor complex with clathrin, Pan1p, Sla2p/End4p, and End3p. Drs2p has multiple endocytosis signals, including two NPFXDs near the C terminus and PEST-like sequences near the N terminus that may mediate ubiquitin (Ub)-dependent endocytosis. Drs2p localizes to the trans-Golgi network in wild-type cells and accumulates on the plasma membrane when both the Ub- and NPFXD-dependent endocytic mechanisms are inactivated. Surprisingly, the pan1-20 temperature-sensitive mutant is constitutively defective for Ub-dependent endocytosis but is not defective for NPFXD-dependent endocytosis at the permissive growth temperature. To sustain viability of pan1-20, Drs2p must be endocytosed through the NPFXD/Sla1p pathway. Thus, Drs2p is an essential endocytic cargo in cells compromised for Ub-dependent endocytosis. These results demonstrate an essential role for endocytosis in retrieving proteins back to the Golgi, and they define critical cargos of the NPFXD/Sla1p system.     

An Abp1-Dependent Route of Endocytosis Functions when the Classical Endocytic Pathway in Yeast Is Inhibited

Clathrin-mediated endocytosis (CME) is a well characterized pathway in both yeast and mammalian cells. An increasing number of alternative endocytic pathways have now been described in mammalian cells that can be both clathrin, actin, and Arf6- dependent or independent. In yeast, a single clathrin-mediated pathway has been characterized in detail. However, disruption of this pathway in many mutant strains indicates that other uptake pathways might exist, at least for bulk lipid and fluid internalization. Using a combination of genetics and live cell imaging, here we show evidence for a novel endocytic pathway in S. cerevisiae that does not involve several of the proteins previously shown to be associated with the ‘classic’ pathway of endocytosis. This alternative pathway functions in the presence of low levels of the actin-disrupting drug latrunculin-A which inhibits movement of the proteins Sla1, Sla2, and Sac6, and is independent of dynamin function. We reveal that in the absence of the ‘classic’ pathway, the actin binding protein Abp1 is now essential for bulk endocytosis. This novel pathway appears to be distinct from another described alternative endocytic route in S. cerevisiae as it involves at least some proteins known to be associated with cortical actin patches rather than being mediated at formin-dependent endocytic sites. These data indicate that cells have the capacity to use overlapping sets of components to facilitate endocytosis under a range of conditions.     

Synaptotagmin‐11 inhibits clathrin‐mediated and bulk endocytosis

Precise and efficient endocytosis is essential for vesicle recycling during a sustained neurotransmission. The regulation of endocytosis has been extensively studied, but inhibitors have rarely been found. Here, we show that synaptotagmin‐11 (Syt11), a non‐Ca²⁺‐binding Syt implicated in schizophrenia and Parkinson's disease, inhibits clathrin‐mediated endocytosis (CME) and bulk endocytosis in dorsal root ganglion neurons. The frequency of both types of endocytic event increases in Syt11 knockdown neurons, while the sizes of endocytosed vesicles and the kinetics of individual bulk endocytotic events remain unaffected. Specifically, clathrin‐coated pits and bulk endocytosis‐like structures increase on the plasma membrane in Syt11‐knockdown neurons. Structural–functional analysis reveals distinct domain requirements for Syt11 function in CME and bulk endocytosis. Importantly, Syt11 also inhibits endocytosis in hippocampal neurons, implying a general role of Syt11 in neurons. Taken together, we propose that Syt11 functions to ensure precision in vesicle retrieval, mainly by limiting the sites of membrane invagination at the early stage of endocytosis.     

Cooperative regulation of endocytic vesicle transport by yeast Eps15-like protein Pan1p and epsins

Dynamic actin filaments are required for the formation and internalization of endocytic vesicles. Yeast actin cables serve as a track for the translocation of endocytic vesicles to early endosomes, but the molecular mechanisms regulating the interaction between vesicles and the actin cables remain ambiguous. Previous studies have demonstrated that the yeast Eps15-like protein Pan1p plays an important role in this interaction, and that interaction is not completely lost even after deletion of the Pan1p actin-binding domain, suggesting that additional proteins mediate association of the vesicle with the actin cable. Other candidates for mediating the interaction are endocytic coat proteins Sla2p (yeast Hip1R) and Ent1p/2p (yeast epsins), as these proteins can bind to both the plasma membrane and the actin filament. Here, we investigated the degree of redundancy in the actin-binding activities of Pan1p, Sla2p, and Ent1p/2p involved in the internalization and transport of endocytic vesicles. Expression of the nonphosphorylatable form of Pan1p, Pan1-18TA, caused abnormal accumulation of both actin cables and endocytic vesicles, and this accumulation was additively suppressed by deletion of the actin-binding domains of both Pan1p and Ent1p. Interestingly, deletion of the actin-binding domains of Pan1p and Ent1p in cells lacking the ENT2 gene resulted in severely defective internalization of endocytic vesicles and recruitment of actin cables to the site of endocytosis. These results suggest that Pan1p and Ent1p/2p cooperatively regulate the interaction between the endocytic vesicle and the actin cable.     

The function of the endocytic scaffold protein Pan1p depends on multiple domains

Pan1p is an essential protein of the yeast Saccharomyces cerevisiae that is required for the internalization step of endocytosis and organization of the actin cytoskeleton. Pan1p, which binds several other endocytic proteins, is composed of multiple protein-protein interaction domains including two Eps15 Homology (EH) domains, a coiled-coil domain, an acidic Arp2/3-activating region, and a proline-rich domain. In this study, we have induced high-level expression of various domains of Pan1p in wild-type cells to assess the dominant consequences on viability, endocytosis, and actin organization. We found that the most severe phenotypes, with blocked endocytosis and aggregated actin, required expression of nearly full length Pan1p, and also required the endocytic regulatory protein kinase Prk1p. The central coiled-coil domain was the smallest fragment whose overexpression caused any dominant effects; these effects were more pronounced by inclusion of the second EH domain. Co-overexpressing nonoverlapping amino- and carboxy-terminal fragments did not mimic the effects of the intact protein, whereas fragments that overlapped within the coiled-coil region could. Yeast two-hybrid and in vivo coimmunoprecipitation analyses suggest that Pan1 may form dimers or higher order oligomers. Collectively, our data support a view of Pan1p as a dimeric/oligomeric scaffold whose functions require both the amino- and carboxy-termini, linked by the central region.     

Negative regulation of yeast Eps15-like Arp2/3 complex activator, Pan1p, by the Hip1R-related protein, Sla2p, during endocytosis

Control of actin assembly nucleated by the Arp2/3 complex plays a crucial role during budding yeast endocytosis. The yeast Eps15-related Arp2/3 complex activator, Pan1p, is essential for endocytic internalization and proper actin organization. Pan1p activity is negatively regulated by Prk1 kinase phosphorylation after endocytic internalization. Phosphorylated Pan1p is probably then dephosphorylated in the cytosol. Pan1p is recruited to endocytic sites approximately 25 s before initiation of actin polymerization, suggesting that its Arp2/3 complex activation activity is kept inactive during early stages of endocytosis by a yet-to-be-identified mechanism. However, how Pan1p is maintained in an inactive state is not clear. Using tandem affinity purification-tagged Pan1p, we identified End3p as a stoichiometric component of the Pan1p complex, and Sla2p, a yeast Hip1R-related protein, as a novel binding partner of Pan1p. Interestingly, Sla2p specifically inhibited Pan1p Arp2/3 complex activation activity in vitro. The coiled-coil region of Sla2p was important for Pan1p inhibition, and a pan1 partial loss-of-function mutant suppressed the temperature sensitivity, endocytic phenotypes, and actin phenotypes observed in sla2DeltaCC mutant cells that lack the coiled-coil region. Overall, our results establish that Sla2p's regulation of Pan1p plays an important role in controlling Pan1p-stimulated actin polymerization during endocytosis.     

Scd5p mediates phosphoregulation of actin and endocytosis by the type 1 phosphatase Glc7p in yeast

Pan1p plays essential roles in both actin and endocytosis in yeast. It interacts with, and regulates the function of, multiple endocytic proteins and actin assembly machinery. Phosphorylation of Pan1p by the kinase Prk1p down-regulates its activity, resulting in disassembly of the endocytic vesicle coat complex and termination of vesicle-associated actin polymerization. In this study, we focus on the mechanism that acts to release Pan1p from phosphorylation inhibition. We show that Pan1p is dephosphorylated by the phosphatase Glc7p, and the dephosphorylation is dependent on the Glc7p-targeting protein Scd5p, which itself is a phosphorylation target of Prk1p. Scd5p links Glc7p to Pan1p in two ways: directly by interacting with Pan1p and indirectly by interacting with the Pan1p-binding protein End3p. Depletion of Glc7p from the cells causes defects in cell growth, actin organization, and endocytosis, all of which can be partially suppressed by deletion of the PRK1 gene. These results suggest that Glc7p antagonizes the activity of the Prk1p kinase in regulating the functions of Pan1p and possibly other actin- and endocytosis-related proteins.     

Pan1p: an actin director of endocytosis in yeast

The yeast protein Pan1p plays a key role in actin-driven endocytosis. The molecular architecture enables the protein to perform multivalent tasks. First, Pan1p acts as a central scaffold for assembly of coat complex at the endocytic sites through its binding to multiple endocytic proteins. Secondly, Pan1p is also required for normal actin cytoskeleton organization and dynamics at the cell cortex. It is capable of F-actin binding and promoting the Arp2/3-mediated actin nucleation via its WH2 and acid domains. Pan1p, therefore, is responsible for the mechanism of coupling the vesicle coat to actin network in the early steps of internalization. The function of Pan1p is under a negative regulation by the kinase Prk1p. Phosphorylation of Pan1p by Prk1p results in disassembly of the coat complex and dissociation of the vesicle from actin meshwork after internalization. The phosphorylation of Pan1p is possibly reversed by the type 1 phosphatase Glc7p, which will allow Pan1p to be reused for coat assembly in the next round of endocytosis.     

Targeted Disruption of an EH‐domain Protein Endocytic Complex,Pan1‐End3

Pan1 is a multi‐domain scaffold that enables dynamic interactions with both structural and regulatory components of the endocytic pathway. Pan1 is composed of Eps15 Homology (EH) domains which interact with adaptor proteins, a central region that is responsible for its oligomerization and C‐terminal binding sites for Arp2/3, F‐actin, and type‐I myosin motors. In this study, we have characterized the binding sites between Pan1 and its constitutive binding partner End3, another EH domain containing endocytic protein. The C‐terminal End3 Repeats of End3 associate with the N‐terminal part of Pan1's central coiled‐coil region. These repeats appear to act independently of one another as tandem, redundant binding sites for Pan1. The end3‐1 allele was sequenced, and corresponds to a C‐terminal truncation lacking the End3 Repeats. Mutations of the End3 Repeats highlight that those residues which are identical between these repeats serve as contact sites for the interaction with Pan1.      

Distinct Actin and Lipid Binding Sites in Ysc84 Are Required during Early Stages of Yeast Endocytosis

During endocytosis in S. cerevisiae, actin polymerization is proposed to provide the driving force for invagination against the effects of turgor pressure. In previous studies, Ysc84 was demonstrated to bind actin through a conserved N-terminal domain. However, full length Ysc84 could only bind actin when its C-terminal SH3 domain also bound to the yeast WASP homologue Las17. Live cell-imaging has revealed that Ysc84 localizes to endocytic sites after Las17/WASP but before other known actin binding proteins, suggesting it is likely to function at an early stage of membrane invagination. While there are homologues of Ysc84 in other organisms, including its human homologue SH3yl-1, little is known of its mode of interaction with actin or how this interaction affects actin filament dynamics. Here we identify key residues involved both in Ysc84 actin and lipid binding, and demonstrate that its actin binding activity is negatively regulated by PI(4,5)P₂. Ysc84 mutants defective in their lipid or actin-binding interaction were characterized in vivo. The abilities of Ysc84 to bind Las17 through its C-terminal SH3 domain, or to actin and lipid through the N-terminal domain were all shown to be essential in order to rescue temperature sensitive growth in a strain requiring YSC84 expression. Live cell imaging in strains with fluorescently tagged endocytic reporter proteins revealed distinct phenotypes for the mutants indicating the importance of these interactions for regulating key stages of endocytosis.     

Unconventional endocytic mechanisms

Endocytosis mediates the uptake of extracellular proteins, micronutrients and transmembrane cell surface proteins. Importantly, many viruses, toxins and bacteria hijack endocytosis to infect cells. The canonical pathway is clathrin-mediated endocytosis (CME) and is active in all eukaryotic cells to support critical house-keeping functions. Unconventional mechanisms of endocytosis exit in parallel of CME, to internalize specific cargoes and support various cellular functions. These clathrin-independent endocytic (CIE) routes use three distinct mechanisms: acute signaling-induced membrane remodeling drives macropinocytosis, activity-dependent bulk endocytosis (ADBE), massive endocytosis (MEND) and EGFR non-clathrin endocytosis (EGFR-NCE). Cargo capture and local membrane deformation by cytosolic proteins is used by fast endophilin-mediated endocytosis (FEME), IL-2Rβ endocytosis and ultrafast endocytosis at synapses. Finally, the formation of endocytic pits by clustering of extracellular lipids or cargoes according to the Glycolipid-Lectin (GL-Lect) hypothesis mediates the uptake of SV40 virus, Shiga and cholera toxins, and galectin-clustered receptors by the CLIC/GEEC and the endophilin-A3-mediated CIE.     

A novel fluorescence-activated cell sorter-based screen for yeast endocytosis mutants identifies a yeast homologue of mammalian eps15

A complete understanding of the molecular mechanisms of endocytosis requires the discovery and characterization of the protein machinery that mediates this aspect of membrane trafficking. A novel genetic screen was used to identify yeast mutants defective in internalization of bulk lipid. The fluorescent lipophilic styryl dye FM4-64 was used in conjunction with FACS to enrich for yeast mutants that exhibit internalization defects. Detailed characterization of two of these mutants, dim1-1 and dim2-1, revealed defects in the endocytic pathway. Like other yeast endocytosis mutants, the temperature-sensitive dim mutant were unable to endocytose FM4-64 or radiolabeled alpha-factor as efficiently as wild-type cells. In addition, double mutants with either dim1-delta or dim2-1 and the endocytosis mutants end4-1 or act1-1 displayed synthetic growth defects, indicating that the DIM gene products function in a common or parallel endocytic pathway. Complementation cloning of the DIM genes revealed identity of DIM1 to SHE4 and DIM2 to PAN1. Pan1p shares homology with the mammalian clathrin adaptor-associated protein, eps15. Both proteins contain multiple EH (eps15 homology) domains, a motif proposed to mediate protein-protein interactions. Phalloidin labeling of filamentous actin revealed profound defects in the actin cytoskeleton in both dim mutants. EM analysis revealed that the dim mutants accumulate vesicles and tubulo-vesicular structures reminiscent of mammalian early endosomes. In addition, the accumulation of novel plasma membrane invaginations where endocytosis is likely to occur were visualized in the mutants by electron microscopy using cationized ferritin as a marker for the endocytic pathway. This new screening strategy demonstrates a role for She4p and Pan1p in endocytosis, and provides a new general method for the identification of additional endocytosis mutants.     

Transmembrane kinase 1-mediated auxin signal regulates membrane-associated clathrin in Arabidopsis roots

Clathrin-mediated endocytosis (CME) is the major endocytic pathway in eukaryotic cells that directly regulates abundance of plasma membrane proteins. Clathrin triskelia are composed of clathrin heavy chains (CHCs) and light chains (CLCs), and the phytohormone auxin differentially regulates membrane-associated CLCs and CHCs, modulating the endocytosis and therefore the distribution of auxin efflux transporter PIN-FORMED2 (PIN2). However, the molecular mechanisms by which auxin regulates clathrin are still poorly understood. Transmembrane kinase (TMKs) family proteins are considered to contribute to auxin signaling and plant development; it remains unclear whether they are involved in PIN transport by CME. We assessed TMKs involvement in the regulation of clathrin by auxin, using genetic, pharmacological, and cytological approaches including live-cell imaging and immunofluorescence. In tmk1 mutant seedlings, auxin failed to rapidly regulate abundance of both CHC and CLC and to inhibit PIN2 endocytosis, leading to an impaired asymmetric distribution of PIN2 and therefore auxin. Furthermore, TMK3 and TMK4 were shown not to be involved in regulation of clathrin by auxin. In summary, TMK1 is essential for auxin-regulated clathrin recruitment and CME. TMK1 therefore plays a critical role in the establishment of an asymmetric distribution of PIN2 and an auxin gradient during root gravitropism.     

Inhibition of clathrin‐mediated endocytosis by knockdown of AP‐2 leads to alterations in the plasma membrane proteome

In eukaryotic cells, clathrin‐mediated endocytosis (CME) is a central pathway for the internalization of proteins from the cell surface, thereby contributing to the maintenance of the plasma membrane protein composition. A key component for the formation of endocytic clathrin‐coated vesicles (CCVs) is AP‐2, as it sequesters cargo membrane proteins, recruits a multitude of other endocytic factors and initiates clathrin polymerization. Here, we inhibited CME by depletion of AP‐2 and explored the consequences for the plasma membrane proteome. Quantitative analysis revealed accumulation of major constituents of the endosomal‐lysosomal system reflecting a block in retrieval by compensatory CME. The noticeable enrichment of integrins and blockage of their turnover resulted in severely impaired cell migration. Rare proteins such as the anti‐cancer drug target CA9 and tumor markers (CD73, CD164, CD302) were significantly enriched. The AP‐2 knockdown attenuated the global endocytic capacity, but clathrin‐independent entry pathways were still operating, as indicated by persistent internalization of specific membrane‐spanning and GPI‐anchored receptors (PVR, IGF1R, CD55, TNAP). We hypothesize that blocking AP‐2 function and thus inhibiting CME may be a novel approach to identify new druggable targets, or to increase their residence time at the plasma membrane, thereby increasing the probability for efficient therapeutic intervention.     

A Pan1/End3/Sla1 complex links Arp2/3-mediated actin assembly to sites of clathrin-mediated endocytosis

More than 60 highly conserved proteins appear sequentially at sites of clathrin-mediated endocytosis in yeast and mammals. The yeast Eps15-related proteins Pan1 and End3 and the CIN85-related protein Sla1 are known to interact with each other in vitro, and they all appear after endocytic-site initiation but before endocytic actin assembly, which facilitates membrane invagination/scission. Here we used live-cell imaging in parallel with genetics and biochemistry to explore comprehensively the dynamic interactions and functions of Pan1, End3, and Sla1. Our results indicate that Pan1 and End3 associate in a stable manner and appear at endocytic sites before Sla1. The End3 C-terminus is necessary and sufficient for its cortical localization via interaction with Pan1, whereas the End3 N-terminus plays a crucial role in Sla1 recruitment. We systematically examined the dynamic behaviors of endocytic proteins in cells in which Pan1 and End3 were simultaneously eliminated, using the auxin-inducible degron system. The results lead us to propose that endocytic-site initiation and actin assembly are separable processes linked by a Pan1/End3/Sla1 complex. Finally, our study provides mechanistic insights into how Pan1 and End3 function with Sla1 to coordinate cargo capture with actin assembly.     

ESCRT‐dependent protein sorting is required for the viability of yeast clathrin‐mediated endocytosis mutants

Endocytosis regulates many processes, including signaling pathways, nutrient uptake, and protein turnover. During clathrin‐mediated endocytosis (CME), adaptors bind to cytoplasmic regions of transmembrane cargo proteins, and many endocytic adaptors are also directly involved in the recruitment of clathrin. This clathrin‐associated sorting protein family includes the yeast epsins, Ent1/2, and AP180/PICALM homologs, Yap1801/2. Mutant strains lacking these four adaptors, but expressing an epsin N‐terminal homology (ENTH) domain necessary for viability (4Δ+ENTH), exhibit endocytic defects, such as cargo accumulation at the plasma membrane (PM). This CME‐deficient strain provides a sensitized background ideal for revealing cellular components that interact with clathrin adaptors. We performed a mutagenic screen to identify alleles that are lethal in 4Δ+ENTH cells using a colony‐sectoring reporter assay. After isolating candidate synthetic lethal genes by complementation, we confirmed that mutations in VPS4 led to inviability of a 4Δ+ENTH strain. Vps4 mediates the final step of endosomal sorting complex required for transport (ESCRT)‐dependent trafficking, and we found that multiple ESCRTs are also essential in 4Δ+ENTH cells, including Snf7, Snf8 and Vps36. Deletion of VPS4 from an end3Δ strain, another CME mutant, similarly resulted in inviability, and upregulation of a clathrin‐independent endocytosis pathway rescued 4Δ+ENTH vps4Δ cells. Loss of Vps4 from an otherwise wild‐type background caused multiple cargoes to accumulate at the PM because of an increase in Rcy1‐dependent recycling of internalized protein to the cell surface. Additionally, vps4Δ rcy1Δ mutants exhibited deleterious growth phenotypes. Together, our findings reveal previously unappreciated effects of disrupted ESCRT‐dependent trafficking on endocytic recycling and the PM.     

α-AP-2 Directs Myosin VI-dependent Endocytosis of Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channels in the Intestine

The actin motor myosin VI regulates endocytosis of cystic fibrosis transmembrane conductance regulator (CFTR) in the intestine, but the endocytic adaptor linking CFTR to myosin VI is unknown. Dab2 (Disabled 2) is the binding partner for myosin VI, clathrin, and α-AP-2 and directs endocytosis of low density lipoprotein receptor family members by recognizing a phosphotyrosine-binding domain. However, CFTR does not possess a phosphotyrosine-binding domain. We examined whether α-AP-2 and/or Dab2 were binding partners for CFTR and the role of myosin VI in localizing endocytic adaptors in the intestine. CFTR co-localized with α-AP-2, Dab2, and myosin VI and was identified in a complex with all three endocytic proteins in the intestine. Apical CFTR was increased in the intestines of Dab-2 KO mice, suggesting its involvement in regulating surface CFTR. Glutathione S-transferase pulldown assays revealed binding of CFTR to α-AP-2 (but not Dab2) in the intestine, whereas Dab-2 interacted with α-AP-2. siRNA silencing of α-AP-2 in cells significantly reduced CFTR endocytosis, further supporting α-AP-2 as the direct binding partner for CFTR. α-AP-2 and Dab2 localized to the terminal web regions of enterocytes, but Dab2 accumulated in this location in Snell''s Waltzer myosin VI^(sv/sv) intestine. Ultrastructural examination revealed that the accumulation of Dab2 correlated with prominent involution and the loss of normal positioning of the intermicrovillar membranes that resulted in expansion of the terminal web region in myosin VI^(sv/sv) enterocytes. The findings support α-AP-2 in directing myosin VI-dependent endocytosis of CFTR and a requirement for myosin VI in membrane invagination and coated pit formation in enterocytes.     

Actin and Endocytosis in Budding Yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.     

Control of lipid organization and actin assembly during clathrin-mediated endocytosis by the cytoplasmic tail of the rhomboid protein Rbd2

Clathrin-mediated endocytosis (CME) is facilitated by a precisely regulated burst of actin assembly. PtdIns(4,5)P2 is an important signaling lipid with conserved roles in CME and actin assembly regulation. Rhomboid family multipass transmembrane proteins regulate diverse cellular processes; however, rhomboid-mediated CME regulation has not been described. We report that yeast lacking the rhomboid protein Rbd2 exhibit accelerated endocytic-site dynamics and premature actin assembly during CME through a PtdIns(4,5)P2-dependent mechanism. Combined genetic and biochemical studies showed that the cytoplasmic tail of Rbd2 binds directly to PtdIns(4,5)P2 and is sufficient for Rbd2''s role in actin regulation. Analysis of an Rbd2 mutant with diminished PtdIns(4,5)P2-binding capacity indicates that this interaction is necessary for the temporal regulation of actin assembly during CME. The cytoplasmic tail of Rbd2 appears to modulate PtdIns(4,5)P2 distribution on the cell cortex. The syndapin-like F-BAR protein Bzz1 functions in a pathway with Rbd2 to control the timing of type 1 myosin recruitment and actin polymerization onset during CME. This work reveals that the previously unstudied rhomboid protein Rbd2 functions in vivo at the nexus of three highly conserved processes: lipid regulation, endocytic regulation, and cytoskeletal function.