Yeast Gga coat proteins function with clathrin in Golgi to endosome transport

The role of the AP-1 adaptor complex in outgoing and incoming membrane traffic

The AP-1 adaptor complex is found in all eukaryotes, but it has been implicated in different pathways in different organisms. To look directly at AP-1 function, we generated stably transduced HeLa cells coexpressing tagged AP-1 and various tagged membrane proteins. Live cell imaging showed that AP-1 is recruited onto tubular carriers trafficking from the Golgi apparatus to the plasma membrane, as well as onto transferrin-containing early/recycling endosomes. Analysis of single AP-1 vesicles showed that they are a heterogeneous population, which starts to sequester cargo 30 min after exit from the ER. Vesicle capture showed that AP-1 vesicles contain transmembrane proteins found at the TGN and early/recycling endosomes, as well as lysosomal hydrolases, but very little of the anterograde adaptor GGA2. Together, our results support a model in which AP-1 retrieves proteins from post-Golgi compartments back to the TGN, analogous to COPI's role in the early secretory pathway. We propose that this is the function of AP-1 in all eukaryotes.

Distinct role of TGN-resident clathrin adaptors for Vps21p activation in the TGN-endosome trafficking pathway

Clathrin-mediated vesicle trafficking plays central roles in post-Golgi transport. In yeast (Saccharomyces cerevisiae), the AP-1 complex and GGA adaptors are predicted to generate distinct transport vesicles at the trans-Golgi network (TGN), and the epsin-related proteins Ent3p and Ent5p (collectively Ent3p/5p) act as accessories for these adaptors. Recently, we showed that vesicle transport from the TGN is crucial for yeast Rab5 (Vps21p)-mediated endosome formation, and that Ent3p/5p are crucial for this process, whereas AP-1 and GGA adaptors are dispensable. However, these observations were incompatible with previous studies showing that these adaptors are required for Ent3p/5p recruitment to the TGN, and thus the overall mechanism responsible for regulation of Vps21p activity remains ambiguous. Here, we investigated the functional relationships between clathrin adaptors in post-Golgi-mediated Vps21p activation. We show that AP-1 disruption in the ent3Δ5Δ mutant impaired transport of the Vps21p guanine nucleotide exchange factor Vps9p transport to the Vps21p compartment and severely reduced Vps21p activity. Additionally, GGA adaptors, the phosphatidylinositol-4-kinase Pik1p and Rab11 GTPases Ypt31p and Ypt32p were found to have partially overlapping functions for recruitment of AP-1 and Ent3p/5p to the TGN. These findings suggest a distinct role of clathrin adaptors for Vps21p activation in the TGN-endosome trafficking pathway.

Acute COG complex inactivation unveiled its immediate impact on Golgi and illuminated the nature of intra-Golgi recycling vesicles

Conserved Oligomeric Golgi (COG) complex controls Golgi trafficking and glycosylation, but the precise COG mechanism is unknown. The auxin-inducible acute degradation system was employed to investigate initial defects resulting from COG dysfunction. We found that acute COG inactivation caused a massive accumulation of COG-dependent (CCD) vesicles that carry the bulk of Golgi enzymes and resident proteins. v-SNAREs (GS15, GS28) and v-tethers (giantin, golgin84, and TMF1) were relocalized into CCD vesicles, while t-SNAREs (STX5, YKT6), t-tethers (GM130, p115), and most of Rab proteins remained Golgi-associated. Airyscan microscopy and velocity gradient analysis revealed that different Golgi residents are segregated into different populations of CCD vesicles. Acute COG depletion significantly affected three Golgi-based vesicular coats-COPI, AP1, and GGA, suggesting that COG uniquely orchestrates tethering of multiple types of intra-Golgi CCD vesicles produced by different coat machineries. This study provided the first detailed view of primary cellular defects associated with COG dysfunction in human cells.

Arf1 directly recruits the Pik1-Frq1 PI4K complex to regulate the final stages of Golgi maturation

Proper Golgi complex function depends on the activity of Arf1, a GTPase whose effectors assemble and transport outgoing vesicles. Phosphatidylinositol 4-phosphate (PI4P) generated at the Golgi by the conserved PI 4-kinase Pik1 (PI4KIIIβ) is also essential for Golgi function, although its precise roles in vesicle formation are less clear. Arf1 has been reported to regulate PI4P production, but whether Pik1 is a direct Arf1 effector is not established. Using a combination of live-cell time-lapse imaging analyses, acute PI4P depletion experiments, and in vitro protein-protein interaction assays on Golgi-mimetic membranes, we present evidence for a model in which Arf1 initiates the final stages of Golgi maturation by tightly controlling PI4P production through direct recruitment of the Pik1-Frq1 PI4-kinase complex. This PI4P serves as a critical signal for AP-1 and secretory vesicle formation, the final events at maturing Golgi compartments. This work therefore establishes the regulatory and temporal context surrounding Golgi PI4P production and its precise roles in Golgi maturation.

Lipid transfer proteins and instructive regulation of lipid kinase activities: Implications for inositol lipid signaling and disease

Cellular membranes are critical platforms for intracellular signaling that involve complex interfaces between lipids and proteins, and a web of interactions between a multitude of lipid metabolic pathways. Membrane lipids impart structural and functional information in this regulatory circuit that encompass biophysical parameters such as membrane thickness and fluidity, as well as chaperoning the interactions of protein binding partners. Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play key roles in intracellular membrane signaling, and these involvements are translated into an impressively diverse set of biological outcomes. The phosphatidylinositol transfer proteins (PITPs) are key regulators of phosphoinositide signaling. Found in a diverse array of organisms from plants, yeast and apicomplexan parasites to mammals, PITPs were initially proposed to be simple transporters of lipids between intracellular membranes. It now appears increasingly unlikely that the soluble versions of these proteins perform such functions within the cell. Rather, these serve to facilitate the activity of intrinsically biologically insufficient inositol lipid kinases and, in so doing, promote diversification of the biological outcomes of phosphoinositide signaling. The central engine for execution of such functions is the lipid exchange cycle that is a fundamental property of PITPs. How PITPs execute lipid exchange remains very poorly understood. Molecular dynamics simulation approaches are now providing the first atomistic insights into how PITPs, and potentially other lipid-exchange/transfer proteins, operate.

Plasma membrane to vacuole traffic induced by glucose starvation requires Gga2-dependent sorting at the trans-Golgi network

BACKGROUND INFORMATION

In the yeast Saccharomyces cerevisiae, acute glucose starvation induces rapid endocytosis followed by vacuolar degradation of many plasma membrane proteins. This process is essential for cell viability, but the regulatory mechanisms that control it remain poorly understood. Under normal growth conditions, a major regulatory decision for endocytic cargo occurs at the trans-Golgi network (TGN) where proteins can recycle back to the plasma membrane or can be recognized by TGN-localised clathrin adaptors that direct them towards the vacuole. However, glucose starvation reduces recycling and alters the localization and post-translational modification of TGN-localised clathrin adaptors. This raises the possibility that during glucose starvation endocytosed proteins are routed to the vacuole by a novel mechanism that bypasses the TGN or does not require TGN-localised clathrin adaptors.

RESULTS

Here, we investigate the role of TGN-localised clathrin adaptors in the traffic of several amino acid permeases, including Can1, during glucose starvation. We find that Can1 transits through the TGN after endocytosis in both starved and normal conditions. Can1 and other amino acid permeases require TGN-localised clathrin adaptors for maximal delivery to the vacuole. Furthermore, these permeases are actively sorted to the vacuole, because ectopically forced de-ubiquitination at the TGN results in the recycling of the Tat1 permase in starved cells. Finally, we report that the Mup1 permease requires the clathrin adaptor Gga2 for vacuolar delivery. In contrast, the clathrin adaptor protein complex AP-1 plays a minor role, potentially in retaining permeases in the TGN, but it is otherwise dispensable for vacuolar delivery.

CONCLUSIONS AND SIGNIFICANCE

This work elucidates one membrane trafficking pathway needed for yeast to respond to acute glucose starvation. It also reveals the functions of TGNlocalised clathrin adaptors in this process. Our results indicate that the same machinery is needed for vacuolar protein sorting at the GN in glucose starved cells as is needed in the presence of glucose. In addition, our findings provide further support for the model that the TGN is a transit point for many endocytosed proteins, and that Gga2 and AP-1 function in distinct pathways at the TGN.

System-Wide Characterization of MoArf GTPase Family Proteins and Adaptor Protein MoGga1 Involved in the Development and Pathogenicity of Magnaporthe oryzae

ADP ribosylation factor (Arf) small GTPase family members are involved in vesicle trafficking and organelle maintenance in organisms ranging from Saccharomyces cerevisiae to humans. A previous study identified Magnaporthe oryzae Arf6 (MoArf6) as one of the Arf proteins that regulates growth and conidiation in the rice blast fungus M. oryzae, but the remaining family proteins remain unknown. Here, we identified six additional Arf proteins, including MoArf1, MoArl1, MoArl3, MoArl8, MoCin4, and MoSar1, as well as their sole adaptor protein, MoGga1, and determined their shared and specific functions. We showed that the majority of these proteins exhibit positive regulatory functions, most notably, in growth. Importantly, MoArl1, MoCin4, and MoGga1 are involved in pathogenicity through the regulation of host penetration and invasive hyphal growth. MoArl1 and MoCin4 also regulate normal vesicle trafficking, and MoCin4 further controls the formation of the biotrophic interfacial complex (BIC). Moreover, we showed that Golgi-cytoplasm cycling of MoArl1 is required for its function. Finally, we demonstrated that interactions between MoArf1 and MoArl1 with MoGga1 are important for Golgi localization and pathogenicity. Collectively, our findings revealed the shared and specific functions of Arf family members in M. oryzae and shed light on how these proteins function through conserved mechanisms to govern growth, transport, and virulence of the blast fungus.IMPORTANCEMagnaporthe oryzae is the causal agent of rice blast, representing the most devastating diseases of rice worldwide, which results in losses of amounts of rice that could feed more than 60 million people each year. Arf (ADP ribosylation factor) small GTPase family proteins are involved in vesicle trafficking and organelle maintenance in eukaryotic cells. To investigate the function of Arf family proteins in M. oryzae, we systematically characterized all seven Arf proteins and found that they have shared and specific functions in governing the growth, development, and pathogenicity of the blast fungus. We have also identified the pathogenicity-related protein MoGga1 as the common adaptor of MoArf1 and MoArl1. Our findings are important because they provide the first comprehensive characterization of the Arf GTPase family proteins and their adaptor protein MoGga1 functioning in a plant-pathogenic fungus, which could help to reveal new fungicide targets to control this devastating disease.

Ent3 and GGA adaptors facilitate diverse anterograde and retrograde trafficking events to and from the prevacuolar endosome

Carboxypeptidases Y (Cpy1) and S (Cps1), the receptor Vps10, and the ATPase subunit Vph1 follow the carboxypeptidase Y (CPY) pathway from the trans-Golgi network (TGN) to the prevacuolar endosome (PVE). Using Schizosaccharomyces pombe quantitative live-cell imaging, biochemical and genetic analyses, we extended the previous knowledge and showed that collaboration between Gga22, the dominant Golgi-localized Gamma-ear-containing ARF-binding (GGA) protein, and Gga21, and between Gga22 and the endosomal epsin Ent3, was required for efficient: i) Vps10 anterograde trafficking from the TGN to the PVE; ii) Vps10 retrograde trafficking from the PVE to the TGN; iii) Cps1 exit from the TGN, and its sorting in the PVE en route to the vacuole; and iv) Syb1/Snc1 recycling to the plasma membrane through the PVE. Therefore, monomeric clathrin adaptors facilitated the trafficking of Vps10 in both directions of the CPY pathway, and facilitated trafficking events of Cps1 in different organelles. By contrast, they were dispensable for Vph1 trafficking. Thus, these adaptors regulated the traffic of some, but not all, of the cargo of the CPY pathway, and regulated the traffic of cargoes that do not follow this pathway. Additionally, this collaboration was required for PVE organization and efficient growth under stress.

Investigation of Ldb19/Art1 localization and function at the late Golgi

The arrestin-related family of proteins (ARTs) are potent regulators of membrane traffic at multiple cellular locations in the yeast Saccharomyces cerevisiae. Several ARTs act at multiple locations, suggesting that ARTs with well-established functions at one location may have additional, as of yet, uncharacterized roles at other locations in the cell. To more fully understand the spectrum of cellular functions regulated by ART proteins, we explored the localization and function of Ldb19/Art1, which has previously been shown to function at the plasma membrane, yet is reported to localize to the trans-Golgi network (TGN). We report that the C-terminal fusion of Ldb19 with GFP is functional and, as previously reported, localizes to the TGN. We further establish that Ldb19 associates with late stages of TGN maturation that are enriched in the clathrin adaptor protein complex-1 (AP-1). Additionally, we present genetic interaction assays that suggest Ldb19 acts at the late TGN in a mechanism related to that of AP-1. However, Ldb19 and AP-1 have dissimilar phenotypes in a subset of assays of membrane traffic, suggesting Ldb19 functions at the TGN are distinct from those of AP-1. Together these results indicate Ldb19 functions at the TGN, in addition to its well-established role in endocytosis.

The Golgi-localized, gamma ear-containing, ARF-binding (GGA) protein family alters alpha synuclein (α-syn) oligomerization and secretion

Several age-related neurodegenerative disorders are associated with protein misfolding and aggregation of toxic peptides. α-synuclein (α-syn) aggregation and the resulting cytotoxicity is a hallmark of Parkinson's disease (PD) as well as dementia with Lewy bodies. Rising evidence points to oligomeric and pre-fibrillar forms as the pathogenic species, and oligomer secretion seems to be crucial for the spreading and progression of PD pathology. Recent studies implicate that dysfunctions in endolysosomal/autophagosomal pathways increase α-syn secretion. Mutation in the retromer-complex protein VPS35, which is involved in endosome to Golgi transport, was suggested to cause familial PD. GGA proteins regulate vesicular traffic between Golgi and endosomes and might work as antagonists for retromer complex mediated transport. To investigate the role of the GGAs in the α-syn oligomerization and/or secretion process we utilized protein-fragment complementation assays (PCA). We here demonstrate that GGAs alter α-syn oligomer secretion and α-syn oligomer-mediated toxicity. Specifically, we determined that GGA3 modifies extracellular α-syn species in an exosome-independent manner. Our data suggest that GGA3 drives α-syn oligomerization in endosomal compartments and thus facilitates α-syn oligomer secretion. Preventing the early events in α-syn oligomer release may be a novel approach to halt disease spreading in PD and other synucleinopathies.

Budding Yeast Has a Minimal Endomembrane System

The endomembrane system consists of the secretory and endocytic pathways, which communicate by transport to and from the trans-Golgi network (TGN). In mammalian cells, the endocytic pathway includes early, late, and recycling endosomes. In budding yeast, different types of endosomes have been described, but the organization of the endocytic pathway has remained unclear. We performed a spatial and temporal analysis of yeast endosomal markers and endocytic cargoes. Our results indicate that the yeast TGN also serves as an early and recycling endosome. In addition, as previously described, yeast contains a late or prevacuolar endosome (PVE). Endocytic cargoes localize to the TGN shortly after internalization, and manipulations that perturb export from the TGN can slow the passage of endocytic cargoes to the PVE. Yeast apparently lacks a distinct early endosome. Thus, yeast has a simple endocytic pathway that may reflect the ancestral organization of the endomembrane system.

Traffic Through the Trans-Golgi Network and the Endosomal System Requires Collaboration Between Exomer and Clathrin Adaptors in Fission Yeast

Despite its biological and medical relevance, traffic from the Golgi to the plasma membrane (PM) is one of the least understood steps of secretion. Exomer is a protein complex that mediates the trafficking of certain cargoes from the trans-Golgi network/early endosomes to the PM in budding yeast. Here, we show that in Schizosaccharomyces pombe the Cfr1 and Bch1 proteins constitute the simplest form of an exomer. Cfr1 co-immunoprecipitates with Assembly Polypeptide adaptor 1 (AP-1), AP-2, and Golgi-localized, gamma-adaptin ear domain homology, ARF-binding (GGA) subunits, and cfr1⁺ interacts genetically with AP-1 and GGA genes. Exomer-defective cells exhibit multiple mild defects, including alterations in the morphology of Golgi stacks and the distribution of the synaptobrevin-like Syb1 protein, carboxypeptidase missorting, and stress sensitivity. S. pombe apm1Δ cells exhibit a defect in trafficking through the early endosomes that is severely aggravated in the absence of exomer. apm1Δ cfr1Δ cells exhibit a dramatic disorganization of intracellular compartments, including massive accumulation of electron-dense tubulovesicular structures. While the trans-Golgi network/early endosomes are severely disorganized in the apm1Δ cfr1Δ strain, gga21Δ gga22Δ cfr1Δ cells exhibit a significant disturbance of the prevacuolar/vacuolar compartments. Our findings show that exomer collaborates with clathrin adaptors in trafficking through diverse cellular compartments, and that this collaboration is important to maintain their integrity. These results indicate that the effect of eliminating exomer is more pervasive than that described to date, and suggest that exomer complexes might participate in diverse steps of vesicle transport in other organisms.

Clathrin localization and dynamics in Aspergillus nidulans

Cell growth necessitates extensive membrane remodeling events including vesicle fusion or fission, processes that are regulated by coat proteins. The hyphal cells of filamentous fungi concentrate both exocytosis and endocytosis at the apex. This investigation focuses on clathrin in Aspergillus nidulans, with the aim of understanding its role in membrane remodeling in growing hyphae. We examined clathrin heavy chain (ClaH-GFP) which localized to three distinct subcellular structures: late Golgi (trans-Golgi equivalents of filamentous fungi), which are concentrated just behind the hyphal tip but are intermittently present throughout all hyphal cells; the region of concentrated endocytosis just behind the hyphal apex (the "endocytic collar"); and small, rapidly moving puncta that were seen trafficking long distances in nearly all hyphal compartments. ClaH localized to distinct domains on late Golgi, and these clathrin "hubs" dispersed in synchrony after the late Golgi marker PH^OSBP . Although clathrin was essential for growth, ClaH did not colocalize well with the endocytic patch marker fimbrin. Tests of FM4-64 internalization and repression of ClaH corroborated the observation that clathrin does not play an important role in endocytosis in A. nidulans. A minor portion of ClaH puncta exhibited bidirectional movement, likely along microtubules, but were generally distinct from early endosomes.

Proteomic Analysis of Rhizoctonia solani Identifies Infection-specific, Redox Associated Proteins and Insight into Adaptation to Different Plant Hosts

Rhizoctonia solani is an important root infecting pathogen of a range of food staples worldwide including wheat, rice, maize, soybean, potato and others. Conventional resistance breeding strategies are hindered by the absence of tractable genetic resistance in any crop host. Understanding the biology and pathogenicity mechanisms of this fungus is important for addressing these disease issues, however, little is known about how R. solani causes disease. This study capitalizes on recent genomic studies by applying mass spectrometry based proteomics to identify soluble, membrane-bound and culture filtrate proteins produced under wheat infection and vegetative growth conditions. Many of the proteins found in the culture filtrate had predicted functions relating to modification of the plant cell wall, a major activity required for pathogenesis on the plant host, including a number found only under infection conditions. Other infection related proteins included a high proportion of proteins with redox associated functions and many novel proteins without functional classification. The majority of infection only proteins tested were confirmed to show transcript up-regulation during infection including a thaumatin which increased susceptibility to R. solani when expressed in Nicotiana benthamiana. In addition, analysis of expression during infection of different plant hosts highlighted how the infection strategy of this broad host range pathogen can be adapted to the particular host being encountered. Data are available via ProteomeXchange with identifier PXD002806.

COPI selectively drives maturation of the early Golgi

COPI coated vesicles carry material between Golgi compartments, but the role of COPI in the secretory pathway has been ambiguous. Previous studies of thermosensitive yeast COPI mutants yielded the surprising conclusion that COPI was dispensable both for the secretion of certain proteins and for Golgi cisternal maturation. To revisit these issues, we optimized the anchor-away method, which allows peripheral membrane proteins such as COPI to be sequestered rapidly by adding rapamycin. Video fluorescence microscopy revealed that COPI inactivation causes an early Golgi protein to remain in place while late Golgi proteins undergo cycles of arrival and departure. These dynamics generate partially functional hybrid Golgi structures that contain both early and late Golgi proteins, explaining how secretion can persist when COPI has been inactivated. Our findings suggest that cisternal maturation involves a COPI-dependent pathway that recycles early Golgi proteins, followed by multiple COPI-independent pathways that recycle late Golgi proteins.

DOI:http://dx.doi.org/10.7554/eLife.13232.001

Proteins play many important roles for cells, and these roles often require the proteins to be in particular locations in or around the cells. A set of cell compartments called the Golgi packages certain proteins into bubble-like structures called vesicles to enable the proteins to be used elsewhere in the cell or released to the outside of the cell, in a process called the secretory pathway. The operation of the secretory pathway requires the Golgi compartments to be continually remodeled.

Proteins and other materials can be ferried between the compartments of the Golgi by another type of vesicle. These vesicles are coated with a group, or complex, of proteins called COPI, which forms a curved lattice around the vesicles and helps them to capture the materials they will transport. However, it is not clear whether COPI is also involved in remodeling of the Golgi compartments.

Papanikou, Day et al. addressed this question using a technique called the “anchor-away method” combined with microscopy to study COPI in yeast cells. The yeast were genetically engineered so that COPI activity was effectively shut down in the presence of a drug called rapamycin. The experiments show that COPI is involved in the early stages of remodeling the Golgi compartments, but not the later stages. This finding supports the emerging view of the Golgi as a self-organizing cellular machine, and it provides a framework for uncovering the engineering principles that underlie the secretory pathway.

DOI:http://dx.doi.org/10.7554/eLife.13232.002

Forty Years of Clathrin-coated Vesicles

The purification of coated vesicles and the discovery of clathrin by Barbara Pearse in 1975 was a landmark in cell biology. Over the past 40 years, work from many labs has uncovered the molecular details of clathrin and its associated proteins, including how they assemble into a coated vesicle and how they select cargo. Unexpected connections have been found with signalling, development, neuronal transmission, infection, immunity and genetic disorders. But there are still a number of unanswered questions, including how clathrin-mediated trafficking is regulated and how the machinery evolved.

BEACH-domain proteins act together in a cascade to mediate vacuolar protein trafficking and disease resistance in Arabidopsis

Membrane trafficking to the protein storage vacuole (PSV) is a specialized process in seed plants. However, this trafficking mechanism to PSV is poorly understood. Here, we show that three types of Beige and Chediak-Higashi (BEACH)-domain proteins contribute to both vacuolar protein transport and effector-triggered immunity (ETI). We screened a green fluorescent seed (GFS) library of Arabidopsis mutants with defects in vesicle trafficking and isolated two allelic mutants gfs3 and gfs12 with a defect in seed protein transport to PSV. The gene responsible for the mutant phenotype was found to encode a putative protein belonging to group D of BEACH-domain proteins, which possess kinase domains. Disruption of other BEACH-encoding loci in the gfs12 mutant showed that BEACH homologs acted in a cascading manner for PSV trafficking. The epistatic genetic interactions observed among BEACH homologs were also found in the ETI responses of the gfs12 and gfs12 bchb-1 mutants, which showed elevated avirulent bacterial growth. The GFS12 kinase domain interacted specifically with the pleckstrin homology domain of BchC1. These results suggest that a cascade of multiple BEACH-domain proteins contributes to vacuolar protein transport and plant defense.

Direct binding of the Kex2p cytosolic tail to the VHS domain of yeast Gga2p facilitates TGN to prevacuolar compartment transport and is regulated by phosphorylation

The VHS domains of yeast Gga1p and Gga2p bind sites (GBSs) in the Kex2p and Vps10p cytosolic tails. Phosphorylation of Ser-780 in the Kex2p GBS enhances Kex2p transport from the TGN to the PVC and is induced by cell wall damage. Kex2p GBS function is shown by direct binding, cell-free transport, and in vivo assays for Kex2 localization.

Human Golgi-localized, γ-ear–containing, ADP-ribosylation factor–binding proteins (Ggas) bind directly to acidic dileucine sorting motifs in the cytosolic tails (C-tails) of intracellular receptors. Despite evidence for a role in recruiting ubiquitinated cargo, it remains unclear whether yeast Ggas also function by binding peptide-sorting signals directly. Two-hybrid analysis shows that the Gga1p and Gga2p Vps27, Hrs, Stam (VHS) domains both bind a site in the Kex2p C-tail and that the Gga2p VHS domain binds a site in the Vps10p C-tail. Binding requires deletion of an apparently autoinhibitory sequence in the Gga2p hinge. Ser₇₈₀ in the Kex2p C-tail is crucial for binding: an Ala substitution blocks but an Asp substitution permits binding. Biochemical assays using purified Gga2p VHS–GGA and TOM1 (GAT) and glutathione S-transferase–Kex2p C-tail fusions show that Gga2p binds directly to the Kex2p C-tail, with relative affinities Asp₇₈₀ > Ser₇₈₀ > Ala₇₈₀. Affinity-purified antibody against a peptide containing phospho-Ser­₇₈₀ recognizes wild-type Kex2p but not S₇₈₀A Kex2p, showing that Ser₇₈₀ is phosphorylated in vivo; phosphorylation of Ser₇₈₀ is up-regulated by cell wall–damaging drugs. Finally, mutation of Ser₇₈₀ alters trafficking of Kex2p both in vivo and in cell-free trans-Golgi network (TGN)–prevacuolar compartment (PVC) transport. Thus yeast Gga adaptors facilitate TGN–PVC transport by direct binding of noncanonical phosphoregulated Gga-binding sites in cargo molecules.

Direct binding of the Kex2p cytosolic tail to the VHS domain of yeast Gga2p facilitates TGN to prevacuolar compartment transport and is regulated by phosphorylation

Human Golgi-localized, γ-ear–containing, ADP-ribosylation factor–binding proteins (Ggas) bind directly to acidic dileucine sorting motifs in the cytosolic tails (C-tails) of intracellular receptors. Despite evidence for a role in recruiting ubiquitinated cargo, it remains unclear whether yeast Ggas also function by binding peptide-sorting signals directly. Two-hybrid analysis shows that the Gga1p and Gga2p Vps27, Hrs, Stam (VHS) domains both bind a site in the Kex2p C-tail and that the Gga2p VHS domain binds a site in the Vps10p C-tail. Binding requires deletion of an apparently autoinhibitory sequence in the Gga2p hinge. Ser780in the Kex2p C-tail is crucial for binding: an Ala substitution blocks but an Asp substitution permits binding. Biochemical assays using purified Gga2p VHS–GGA and TOM1 (GAT) and glutathione S-transferase–Kex2p C-tail fusions show that Gga2p binds directly to the Kex2p C-tail, with relative affinities Asp780> Ser780> Ala780. Affinity-purified antibody against a peptide containing phospho-Ser­780recognizes wild-type Kex2p but not S780A Kex2p, showing that Ser780is phosphorylated in vivo; phosphorylation of Ser780is up-regulated by cell wall–damaging drugs. Finally, mutation of Ser780alters trafficking of Kex2p both in vivo and in cell-free trans-Golgi network (TGN)–prevacuolar compartment (PVC) transport. Thus yeast Gga adaptors facilitate TGN–PVC transport by direct binding of noncanonical phosphoregulated Gga-binding sites in cargo molecules.

Energy metabolism regulates clathrin adaptors at the trans-Golgi network and endosomes

Glucose is a master regulator of cell behavior in the yeast Saccharomyces cerevisiae. It acts as both a metabolic substrate and a potent regulator of intracellular signaling cascades. Glucose starvation induces the transient delocalization and then partial relocalization of clathrin adaptors at the trans-Golgi network and endosomes. Although these localization responses are known to depend on the protein kinase A (PKA) signaling pathway, the molecular mechanism of this regulation is unknown. Here we demonstrate that PKA and the AMP-regulated kinase regulate adaptor localization through changes in energy metabolism. We show that genetic and chemical manipulation of intracellular ATP levels cause corresponding changes in adaptor localization. In permeabilized cells, exogenous ATP is sufficient to induce adaptor localization. Furthermore, we reveal distinct energy-dependent steps in adaptor localization: a step that requires the ADP-ribosylation factor ARF, an ATP-dependent step that requires the phosphatidyl-inositol-4 kinase Pik1, and third ATP-dependent step for which we provide evidence but for which the mechanism is unknown. We propose that these energy-dependent mechanisms precisely synchronize membrane traffic with overall proliferation rates and contribute a crucial aspect of energy conservation during acute glucose starvation.

Phospholipid flippases Lem3p-Dnf1p and Lem3p-Dnf2p are involved in the sorting of the tryptophan permease Tat2p in yeast

The type 4 P-type ATPases are flippases that generate phospholipid asymmetry in membranes. In budding yeast, heteromeric flippases, including Lem3p-Dnf1p and Lem3p-Dnf2p, translocate phospholipids to the cytoplasmic leaflet of membranes. Here, we report that Lem3p-Dnf1/2p are involved in transport of the tryptophan permease Tat2p to the plasma membrane. The lem3Δ mutant exhibited a tryptophan requirement due to the mislocalization of Tat2p to intracellular membranes. Tat2p was relocalized to the plasma membrane when trans-Golgi network (TGN)-to-endosome transport was inhibited. Inhibition of ubiquitination by mutations in ubiquitination machinery also rerouted Tat2p to the plasma membrane. Lem3p-Dnf1/2p are localized to endosomal/TGN membranes in addition to the plasma membrane. Endocytosis mutants, in which Lem3p-Dnf1/2p are sequestered to the plasma membrane, also exhibited the ubiquitination-dependent missorting of Tat2p. These results suggest that Tat2p is ubiquitinated at the TGN and missorted to the vacuolar pathway in the lem3Δ mutant. The NH(2)-terminal cytoplasmic region of Tat2p containing ubiquitination acceptor lysines interacted with liposomes containing acidic phospholipids, including phosphatidylserine. This interaction was abrogated by alanine substitution mutations in the basic amino acids downstream of the ubiquitination sites. Interestingly, a mutant Tat2p containing these substitutions was missorted in a ubiquitination-dependent manner. We propose the following model based on these results; Tat2p is not ubiquitinated when the NH(2)-terminal region is bound to membrane phospholipids, but if it dissociates from the membrane due to a low level of phosphatidylserine caused by perturbation of phospholipid asymmetry in the lem3Δ mutant, Tat2p is ubiquitinated and then transported from the TGN to the vacuole.

The unique GGA clathrin adaptor of Drosophila melanogaster is not essential

The Golgi-localized, γ-ear-containing, ARF binding proteins (GGAs) are a highly conserved family of monomeric clathrin adaptor proteins implicated in clathrin-mediated protein sorting between the trans-Golgi network and endosomes. GGA RNAi knockdowns in Drosophila have resulted in conflicting data concerning whether the Drosophila GGA (dGGA) is essential. The goal of this study was to define the null phenotype for the unique Drosophila GGA. We describe two independently derived dGGA mutations. Neither allele expresses detectable dGGA protein. Homozygous and hemizygous flies with each allele are viable and fertile. In contrast to a previous report using RNAi knockdown, GGA mutant flies show no evidence of age-dependent retinal degeneration or cathepsin missorting. Our results demonstrate that several of the previous RNAi knockdown phenotypes were the result of off-target effects. However, GGA null flies are hypersensitive to dietary chloroquine and to starvation, implicating GGA in lysosomal function and autophagy.

Yeast Irc6p is a novel type of conserved clathrin coat accessory factor related to small G proteins

Yeast Irc6p is a novel type of conserved clathrin coat accessory protein that functions in clathrin-mediated traffic between the trans-Golgi network and endosomes, linking clathrin adaptor complex AP-1 and the Rab GTPase Ypt31p. Irc6p and the mammalian homologue p34 are founding members of a new G protein–like family.

Clathrin coat accessory proteins play key roles in transport mediated by clathrin-coated vesicles. Yeast Irc6p and the related mammalian p34 are putative clathrin accessory proteins that interact with clathrin adaptor complexes. We present evidence that Irc6p functions in clathrin-mediated traffic between the trans-Golgi network and endosomes, linking clathrin adaptor complex AP-1 and the Rab GTPase Ypt31p. The crystal structure of the Irc6p N-terminal domain revealed a G-protein fold most related to small G proteins of the Rab and Arf families. However, Irc6p lacks G-protein signature motifs and high-affinity GTP binding. Also, mutant Irc6p lacking candidate GTP-binding residues retained function. Mammalian p34 rescued growth defects in irc6∆ cells, indicating functional conservation, and modeling predicted a similar N-terminal fold in p34. Irc6p and p34 also contain functionally conserved C-terminal regions. Irc6p/p34-related proteins with the same two-part architecture are encoded in genomes of species as diverse as plants and humans. Together these results define Irc6p/p34 as a novel type of conserved clathrin accessory protein and founding members of a new G protein–like family.

Vacuolar protein sorting receptor in Giardia lamblia

In Giardia, lysosome-like peripheral vacuoles (PVs) need to specifically coordinate their endosomal and lysosomal functions to be able to successfully perform endocytosis, protein degradation and protein delivery, but how cargo, ligands and molecular components generate specific routes to the PVs remains poorly understood. Recently, we found that delivering membrane Cathepsin C and the soluble acid phosphatase (AcPh) to the PVs is adaptin (AP1)-dependent. However, the receptor that links AcPh and AP1 was never described. We have studied protein-binding to AcPh by using H6-tagged AcPh, and found that a membrane protein interacted with AcPh. This protein, named GlVps (for Giardia lamblia Vacuolar protein sorting), mainly localized to the ER-nuclear envelope and in some PVs, probably functioning as the sorting receptor for AcPh. The tyrosine-binding motif found in the C-terminal cytoplasmic tail domain of GlVps was essential for its exit from the endoplasmic reticulum and transport to the vacuoles, with this motif being necessary for the interaction with the medium subunit of AP1. Thus, the mechanism by which soluble proteins, such as AcPh, reach the peripheral vacuoles in Giardia appears to be very similar to the mechanism of lysosomal protein-sorting in more evolved eukaryotic cells.

Trying to make sense of retromer

Retromer is a cytosolic protein complex which binds to post-Golgi organelles involved in the trafficking of proteins to the lytic compartment of the cell. In non-plant organisms, retromer mediates the recycling of acid hydrolase receptors from early endosomal (EE) compartments. In plants, retromer components are required for the targeting of vacuolar storage proteins, and for the recycling of endocytosed PIN proteins. However, there are contradictory reports as to the localization of the sorting nexins and the core subunit of retromer. There is also uncertainty as to the identity of the organelles from which vacuolar sorting receptors (VSRs) and endocytosed plasma membrane (PM) proteins are recycled. In this review we try to resolve some of these conflicting observations.

Adaptor autoregulation promotes coordinated binding within clathrin coats

Membrane traffic is an essential process that allows protein and lipid exchange between the endocytic, lysosomal, and secretory compartments. Clathrin-mediated traffic between the trans-Golgi network and endosomes mediates responses to the environment through the sorting of biosynthetic and endocytic protein cargo. Traffic through this pathway is initiated by the controlled assembly of a clathrin-adaptor protein coat on the cytosolic surface of the originating organelle. In this process, clathrin is recruited by different adaptor proteins that act as a bridge between clathrin and the transmembrane cargo proteins to be transported. Interactions between adaptors and clathrin and between different types of adaptors lead to the formation of a densely packed protein network within the coat. A key unresolved issue is how the highly complex adaptor-clathrin interaction and adaptor-adaptor interaction landscape lead to the correct spatiotemporal assembly of the clathrin coat. Here we report the discovery of a new autoregulatory motif within the clathrin adaptor Gga2 that drives synergistic binding of Gga2 to clathrin and the adaptor Ent5. This autoregulation influences the temporal and/or spatial location of the Gga2-Ent5 interaction. We propose that this synergistic binding provides built-in regulation to ensure the correct assembly of clathrin coats.

Phosphoinositide-mediated clathrin adaptor progression at the trans-Golgi network

Clathrin coated vesicles mediate endocytosis and transport between the trans Golgi network (TGN) and endosomes in eukaryotic cells. Clathrin adaptors play central roles in coat assembly, interacting with clathrin, cargo, and membranes. Two major types of clathrin adaptors act in TGN-endosome traffic, Gga proteins and the AP-1 complex. Here we characterize the relationship between Gga proteins, AP-1, and other TGN clathrin adaptors using live cell and superresolution microscopy in yeast. We present evidence that Gga proteins and AP-1 are recruited sequentially in two waves of coat assembly at the TGN. Mutations that decrease phosphatidylinositol 4-phosphate (PI4P) levels at the TGN slow or uncouple AP-1 coat assembly from Gga coat assembly. Conversely, enhanced PI4P synthesis shortens the time between adaptor waves. Gga2p binds directly to the TGN PI4-kinase Pik1p and contributes to Pik1p recruitment. These results identify a PI4P-based mechanism for regulating progressive assembly of adaptor-specific clathrin coats at the TGN.

Analysis of Gga null mice demonstrates a non-redundant role for mammalian GGA2 during development

Numerous studies using cultured mammalian cells have shown that the three GGAs (Golgi-localized, gamma-ear containing, ADP-ribosylation factor- binding proteins) function in the transport of cargo proteins between the trans- Golgi network and endosomes. However, the in vivo role(s) of these adaptor proteins and their possible functional redundancy has not been analyzed. In this study, the genes encoding GGAs1-3 were disrupted in mice by insertional mutagenesis. Loss of GGA1 or GGA3 alone was well tolerated whereas the absence of GGA2 resulted in embryonic or neonatal lethality, depending on the genetic background of the mice. Thus, GGA2 mediates a vital function that cannot be compensated for by GGA1and/or GGA3. The combined loss of GGA1 and GGA3 also resulted in a high incidence of neonatal mortality but in this case the expression level of GGA2 may be inadequate to compensate for the loss of the other two GGAs. We conclude that the three mammalian GGAs are essential proteins that are not fully redundant.

Phosphoinositide signaling during membrane transport in Saccharomyces cerevisiae

Phosphatidylinositol (PI) is distinct from other phospholipids, possessing a head group that can be modified by phosphorylation at multiple positions to generate unique signaling molecules collectively known as phosphoinositides. The set of kinases and phosphatases that regulate PI metabolism are conserved throughout eukaryotic evolution, and numerous studies have demonstrated that phosphoinositides regulate a diverse spectrum of cellular processes, including vesicle transport, cell proliferation, and cytoskeleton organization. Over the past two decades, nearly all PI derivatives have been shown to interact directly with cellular proteins to affect their localization and/or activity. Additionally, there is growing evidence, which suggests that phosphoinositides may also affect local membrane topology. Here, we focus on the role of phosphoinositides in membrane trafficking and underscore the significant role that yeast has played in the field.

Drosophila GGA model: an ultimate gateway to GGA analysis

Golgi-localized, γ-ear-containing, ADP ribosylation factor-binding (GGA) proteins are monomeric adaptors implicated in clathrin-mediated vesicular transport between the trans Golgi network and endosomes, characterized mainly from cell culture analysis of lysosomal sorting. To provide the first demonstration of GGA's role in vivo, we used Drosophila which has a single GGA and a single lysosomal sorting receptor, lysosomal enzyme receptor protein (LERP). Using RNAi knockdowns, we show that the Drosophila GGA is required for lysosomal sorting. We further identified authentic components of the Drosophila lysosomal sorting system--the sorting receptor LERP, the sorting adaptor GGA and the lysosomal cargo cathepsins B1, D and L--to show that GGA depletion results in lysosomal dysfunction. Abnormal lysosomal morphology, missorting of lysosomal cathepsins and impaired lysosomal proteolysis show disturbed LERP trafficking after GGA depletion. GGA is highly expressed in the mushroom bodies and the pigment cells of the retina, and increasing or decreasing the levels of GGA in the eyes leads to retinal defects. Reduced GGA levels also enhance an eye defect caused by overexpression of the autophagy-associated protein Blue cheese (Bchs), implicating GGA in autophagic processes. This shows that Drosophila provides an excellent whole-animal model to gain new insights into the function of GGA in the physiological environment of a multicellular organism.

Glucose regulates clathrin adaptors at the trans-Golgi network and endosomes

Traffic at the trans-Golgi network (TGN) and endosomes is regulated by glucose via an unknown mechanism that depends on protein kinase A (PKA). TGN–endosomal clathrin adaptors exhibit specific responses to glucose starvation that likely are coordinated with other cell behaviors regulated by PKA.

Glucose is a rich source of energy and the raw material for biomass increase. Many eukaryotic cells remodel their physiology in the presence and absence of glucose. The yeast Saccharomyces cerevisiae undergoes changes in transcription, translation, metabolism, and cell polarity in response to glucose availability. Upon glucose starvation, translation initiation and cell polarity are immediately inhibited, and then gradually recover. In this paper, we provide evidence that, as in cell polarity and translation, traffic at the trans-Golgi network (TGN) and endosomes is regulated by glucose via an unknown mechanism that depends on protein kinase A (PKA). Upon glucose withdrawal, clathrin adaptors exhibit a biphasic change in localization: they initially delocalize from the membrane within minutes and later partially recover onto membranes. Additionally, the removal of glucose induces changes in posttranslational modifications of adaptors. Ras and Gpr1 signaling pathways, which converge on PKA, are required for changes in adaptor localization and changes in posttranslational modifications. Acute inhibition of PKA demonstrates that inhibition of PKA prior to glucose withdrawal prevents several adaptor responses to starvation. This study demonstrates that PKA activity prior to glucose starvation primes membrane traffic at the TGN and endosomes in response to glucose starvation.

Quantitative proteomics of yeast post-Golgi vesicles reveals a discriminating role for Sro7p in protein secretion

We here report the first comparative proteomics of purified yeast post-Golgi vesicles (PGVs). Vesicle samples isolated from PGV-accumulating sec6-4 mutants were treated with isobaric tags (iTRAQ) for subsequent quantitative tandem mass spectrometric analysis of protein content. After background subtraction, a total of 66 vesicle-associated proteins were identified, including known or assumed vesicle residents as well as a fraction not previously known to be PGV associated. Vesicles isolated from cells lacking the polarity protein Sro7p contained essentially the same catalogue of proteins but showed a reduced content of a subset of cargo proteins, in agreement with a previously shown selective role for Sro7p in cargo sorting.

Regulation of clathrin adaptor function in endocytosis: novel role for the SAM domain

During clathrin-mediated endocytosis, adaptor proteins play central roles in coordinating the assembly of clathrin coats and cargo selection. Here we characterize the binding of the yeast endocytic adaptor Sla1p to clathrin through a variant clathrin-binding motif that is negatively regulated by the Sla1p SHD2 domain. The crystal structure of SHD2 identifies the domain as a sterile alpha-motif (SAM) domain and shows a propensity to oligomerize. By co-immunoprecipitation, Sla1p binds to clathrin and self-associates in vivo. Mutations in the clathrin-binding motif that abolish clathrin binding and structure-based mutations in SHD2 that impede self-association result in endocytosis defects and altered dynamics of Sla1p assembly at the sites of endocytosis. These results define a novel mechanism for negative regulation of clathrin binding by an adaptor and suggest a role for SAM domains in clathrin-mediated endocytosis.

Functional characterization of protein-sorting machineries at the trans-Golgi network in Drosophila melanogaster

Targeting of proteins to their final destination is a prerequisite for living cells to maintain their homeostasis. Clathrin functions as a coat that forms transport carriers called clathrin-coated vesicles (CCVs) at the plasma membrane and post-Golgi compartments. In this study, we established an experimental system using Schneider S2 cells derived from the fruit fly, Drosophila melanogaster, as a model system to study the physiological roles of clathrin adaptors, and to dissect the processes of CCV formation. We found that a clathrin adaptor Drosophila GGA (dGGA), a homolog of mammalian GGA proteins, localizes to the trans-Golgi network (TGN) and is capable of recruiting clathrin from the cytosol onto TGN membranes. dGGA itself is recruited from the cytosol to the TGN in an ARF1 small GTPase (dARF79F)-dependent manner. dGGA recognizes the cytoplasmic acidic-cluster-dileucine (ACLL) sorting signal of Lerp (lysosomal enzyme receptor protein), a homolog of mammalian mannose 6-phosphate receptors. Moreover, both dGGA and another type of TGN-localized clathrin adaptor, AP-1 (adaptor protein-1 complex), are shown to be involved in the trafficking of Lerp from the TGN to endosomes and/or lysosomes. Taken together, our findings indicate that the protein-sorting machinery in fly cells is well conserved relative to that in mammals, enabling the use of fly cells to dissect CCV biogenesis and clathrin-dependent protein trafficking at the TGN of higher eukaryotes.

Clathrin functions in the absence of the terminal domain binding site for adaptor-associated clathrin-box motifs

Clathrin is involved in vesicle formation in the trans-Golgi network (TGN)/endosomal system and during endocytosis. Clathrin recruitment to membranes is mediated by the clathrin heavy chain (HC) N-terminal domain (TD), which forms a seven-bladed beta-propeller. TD binds membrane-associated adaptors, which have short peptide motifs, either the clathrin-box (CBM) and/or the W-box; however, the importance of the TD binding sites for these motifs has not been tested in vivo. We investigated the importance of the TD in clathrin function by generating 1) mutations in the yeast HC gene (CHC1) to disrupt the binding sites for the CBM and W-box (chc1-box), and 2) four TD-specific temperature-sensitive alleles of CHC1. We found that TD is important for the retention of resident TGN enzymes and endocytosis of alpha-factor; however, the known adaptor binding sites are not necessary, because chc1-box caused little to no effect on trafficking pathways involving clathrin. The Chc1-box TD was able to interact with the endocytic adaptor Ent2 in a CBM-dependent manner, and HCs encoded by chc1-box formed clathrin-coated vesicles. These data suggest that additional or alternative binding sites exist on the TD propeller to help facilitate the recruitment of clathrin to sites of vesicle formation.

Control of protein and sterol trafficking by antagonistic activities of a type IV P-type ATPase and oxysterol binding protein homologue

The oxysterol binding protein homologue Kes1p has been implicated in nonvesicular sterol transport in Saccharomyces cerevisiae. Kes1p also represses formation of protein transport vesicles from the trans-Golgi network (TGN) through an unknown mechanism. Here, we show that potential phospholipid translocases in the Drs2/Dnf family (type IV P-type ATPases [P4-ATPases]) are downstream targets of Kes1p repression. Disruption of KES1 suppresses the cold-sensitive (cs) growth defect of drs2Delta, which correlates with an enhanced ability of Dnf P4-ATPases to functionally substitute for Drs2p. Loss of Kes1p also suppresses a drs2-ts allele in a strain deficient for Dnf P4-ATPases, suggesting that Kes1p antagonizes Drs2p activity in vivo. Indeed, Drs2-dependent phosphatidylserine translocase (flippase) activity is hyperactive in TGN membranes from kes1Delta cells and is potently attenuated by addition of recombinant Kes1p. Surprisingly, Drs2p also antagonizes Kes1p activity in vivo. Drs2p deficiency causes a markedly increased rate of cholesterol transport from the plasma membrane to the endoplasmic reticulum (ER) and redistribution of endogenous ergosterol to intracellular membranes, phenotypes that are Kes1p dependent. These data suggest a homeostatic feedback mechanism in which appropriately regulated flippase activity in the Golgi complex helps establish a plasma membrane phospholipid organization that resists sterol extraction by a sterol binding protein.

K63-linked ubiquitin chains as a specific signal for protein sorting into the multivesicular body pathway

A growing number of yeast and mammalian plasma membrane proteins are reported to be modified with K63-linked ubiquitin (Ub) chains. However, the relative importance of this modification versus monoubiquitylation in endocytosis, Golgi to endosome traffic, and sorting into the multivesicular body (MVB) pathway remains unclear. In this study, we show that K63-linked ubiquitylation of the Gap1 permease is essential for its entry into the MVB pathway. Carboxypeptidase S also requires modification with a K63-Ub chain for correct MVB sorting. In contrast, monoubiquitylation of a single target lysine of Gap1 is a sufficient signal for its internalization from the cell surface, and Golgi to endosome transport of the permease requires neither its ubiquitylation nor the Ub-binding GAT (Gga and Tom1) domain of Gga (Golgi localizing, gamma-ear containing, ARF binding) adapter proteins, the latter being crucial for subsequent MVB sorting of the permease. Our data reveal that K63-linked Ub chains act as a specific signal for MVB sorting, providing further insight into the Ub code of membrane protein trafficking.

Linking phospholipid flippases to vesicle-mediated protein transport

Type IV P-type ATPases (P4-ATPases) are a large family of putative phospholipid translocases (flippases) implicated in the generation of phospholipid asymmetry in biological membranes. P4-ATPases are typically the largest P-type ATPase subgroup found in eukaryotic cells, with five members in Saccharomyces cerevisiae, six members in Caenorhabditis elegans, 12 members in Arabidopsis thaliana and 14 members in humans. In addition, many of the P4-ATPases require interaction with a noncatalytic subunit from the CDC50 gene family for their transport out of the endoplasmic reticulum (ER). Deficiency of a P4-ATPase (Atp8b1) causes liver disease in humans, and studies in a variety of model systems indicate that P4-ATPases play diverse and essential roles in membrane biogenesis. In addition to their proposed role in establishing and maintaining plasma membrane asymmetry, P4-ATPases are linked to vesicle-mediated protein transport in the exocytic and endocytic pathways. Recent studies have also suggested a role for P4-ATPases in the nonvesicular intracellular trafficking of sterols. Here, we discuss the physiological requirements for yeast P4-ATPases in phospholipid translocase activity, transport vesicle budding and ergosterol metabolism, with an emphasis on Drs2p and its noncatalytic subunit, Cdc50p.

Yeast Golgi-localized, gamma-Ear-containing, ADP-ribosylation factor-binding proteins are but adaptor protein-1 is not required for cell-free transport of membrane proteins from the trans-Golgi network to the prevacuolar compartment

Golgi-localized, gamma-Ear-containing, ADP-ribosylation factor-binding proteins (GGAs) and adaptor protein-1 (AP-1) mediate clathrin-dependent trafficking of transmembrane proteins between the trans-Golgi network (TGN) and endosomes. In yeast, the vacuolar sorting receptor Vps10p follows a direct pathway from the TGN to the late endosome/prevacuolar compartment (PVC), whereas, the processing protease Kex2p partitions between the direct pathway and an indirect pathway through the early endosome. To examine the roles of the Ggas and AP-1 in TGN-PVC transport, we used a cell-free assay that measures delivery to the PVC of either Kex2p or a chimeric protein (K-V), in which the Vps10p cytosolic tail replaces the Kex2p tail. Either antibody inhibition or dominant-negative Gga2p completely blocked K-V transport but only partially blocked Kex2p transport. Deletion of APL2, encoding the beta subunit of AP-1, did not affect K-V transport but partially blocked Kex2p transport. Residual Kex2p transport seen with apl2Delta membranes was insensitive to dominant-negative Gga2p, suggesting that the apl2Delta mutation causes Kex2p to localize to a compartment that precludes Gga-dependent trafficking. These results suggest that yeast Ggas facilitate the specific and direct delivery of Vps10p and Kex2p from the TGN to the PVC and that AP-1 modulates Kex2p trafficking through a distinct pathway, presumably involving the early endosome.

P4-ATPase requirement for AP-1/clathrin function in protein transport from the trans-Golgi network and early endosomes

Drs2p is a resident type 4 P-type ATPase (P4-ATPase) and potential phospholipid translocase of the trans-Golgi network (TGN) where it has been implicated in clathrin function. However, precise protein transport pathways requiring Drs2p and how it contributes to clathrin-coated vesicle budding remain unclear. Here we show a functional codependence between Drs2p and the AP-1 clathrin adaptor in protein sorting at the TGN and early endosomes of Saccharomyces cerevisiae. Genetic criteria indicate that Drs2p and AP-1 operate in the same pathway and that AP-1 requires Drs2p for function. In addition, we show that loss of AP-1 markedly increases Drs2p trafficking to the plasma membrane, but does not perturb retrieval of Drs2p from the early endosome back to the TGN. Thus AP-1 is required at the TGN to sort Drs2p out of the exocytic pathway, presumably for delivery to the early endosome. Moreover, a conditional allele that inactivates Drs2p phospholipid translocase (flippase) activity disrupts its own transport in this AP-1 pathway. Drs2p physically interacts with AP-1; however, AP-1 and clathrin are both recruited normally to the TGN in drs2Delta cells. These results imply that Drs2p acts independently of coat recruitment to facilitate AP-1/clathrin-coated vesicle budding from the TGN.

Substrate- and ubiquitin-dependent trafficking of the yeast siderophore transporter Sit1

Eukaryotic plasma membrane transporters are subjected to a tightly regulated intracellular trafficking. The yeast siderophore iron transporter1 (Sit1) displays substrate-regulated trafficking. It is targeted to the plasma membrane or to a vacuolar degradative pathway when synthesized in the presence or absence of external substrate, respectively. Sorting of Sit1 to the vacuolar pathway is dependent on the clathrin adaptor Gga2, and more specifically on its C-GAT subdomain. Plasma membrane undergoes substrate-induced ubiquitylation dependent on the Rsp5 ubiquitin protein ligase. Sit1 is also ubiquitylated in an Rsp5-dependent manner in internal compartments when expressed in the absence of substrate. In several rsp5 mutants including cells deleted for RSP5, Sit1 expressed in the absence of substrate is correctly targeted to the endosomal pathway but its sorting to multivesicular bodies (MVBs) is impaired. Consequently, it displays endosome to plasma membrane targeting, with kinetics similar to those observed in vps mutants defective for MVB sorting. Plasma membrane Sit1 is modified by Lys63-linked ubiquitin chains. We also show for the first time in yeast that modification by this latter type of ubiquitin chains is required directly or indirectly for efficient MVB sorting, as it is for efficient internalization at the plasma membrane.

Yeast and human Ysl2p/hMon2 interact with Gga adaptors and mediate their subcellular distribution

The Gga proteins represent a family of ubiquitously expressed clathrin adaptors engaged in vesicle budding at the tubular endosomal network/trans Golgi network. Their membrane recruitment is commonly thought to involve interactions with Arf and signals in cargo through the so-called VHS domain. For yeast Gga proteins, however, partners binding to its VHS domain have remained elusive and Gga localization does not absolutely depend on Arf. Here, we demonstrate that yeast Gga recruitment relies on a network of interactions between the scaffold Ysl2p/Mon2p, the small GTPase Arl1p, and the flippase Neo1p. Deletion of either YSL2 or ARL1 causes mislocalization of Gga2p, whereas a neo1-69 mutant accumulates Gga2p on aberrant structures. Remarkably, Ysl2p directly interacts with human and yeast Ggas through the VHS domain, and binding to Gga proteins is also found for the human Ysl2p orthologue hMon2. Thus, Ysl2p represents an essential, evolutionarily conserved member of a network controlling direct binding and membrane docking of Ggas. Because activated Arl1p is part of the network that binds Gga2p, Arf and Arf-like GTPases may interact in a regulatory cascade.

The clathrin adaptor Gga2p is a phosphatidylinositol 4-phosphate effector at the Golgi exit

Phosphatidylinositol 4-phosphate (PI(4)P) is a key regulator of membrane transport required for the formation of transport carriers from the trans-Golgi network (TGN). The molecular mechanisms of PI(4)P signaling in this process are still poorly understood. In a search for PI(4)P effector molecules, we performed a screen for synthetic lethals in a background of reduced PI(4)P and found the gene GGA2. Our analysis uncovered a PI(4)P-dependent recruitment of the clathrin adaptor Gga2p to the TGN during Golgi-to-endosome trafficking. Gga2p recruitment to liposomes is stimulated both by PI(4)P and the small GTPase Arf1p in its active conformation, implicating these two molecules in the recruitment of Gga2p to the TGN, which ultimately controls the formation of clathrin-coated vesicles. PI(4)P binding occurs through a phosphoinositide-binding signature within the N-terminal VHS domain of Gga2p resembling a motif found in other clathrin interacting proteins. These data provide an explanation for the TGN-specific membrane recruitment of Gga2p.