ADP-ribosylation factor (ARF) interaction is not sufficient for yeast GGA protein function or localization

Fusarium graminearum GGA protein is critical for fungal development, virulence and ascospore discharge through its involvement in vesicular trafficking

Vesicular trafficking is a conserved material transport process in eukaryotic cells. The GGA family proteins are clathrin adaptors that are involved in eukaryotic vesicle transport, but their functions in phytopathogenic filamentous fungi remain unexplored. Here, we examined the only GGA family protein in Fusarium graminearum, FgGga1, which localizes to both the late Golgi and endosomes. In the absence of FgGga1, the fungal mutant exhibited defects in vegetative growth, DON biosynthesis, ascospore discharge and virulence. Fluorescence microscopy analysis revealed that FgGga1 is associated with trans-Golgi network (TGN)-to-plasma membrane, endosome-to-TGN and endosome-to-vacuole transport. Mutational analysis on the five domains of FgGga1 showed that the VHS domain was required for endosome-to-TGN transport while the GAT^167-248 and the hinge domains were required for both endosome-to-TGN and endosome-to-vacuole transport. Importantly, the deletion of the FgGga1 domains that are required in vesicular trafficking also inhibited vegetative growth and virulence of F. graminearum. In addition, FgGga1 interacted with the ascospore discharge regulator Ca²⁺ ATPase FgNeo1, whose transport to the vacuole is dependent on FgGga1-mediated endosome-to-vacuole transport. Our results suggest that FgGga1 is required for fungal development and virulence via FgGga1-mediated vesicular trafficking, and FgGga1-mediated endosome-to-vacuole transport facilitates ascospore discharge in F. graminearum.

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.

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.

Pathogenicity-associated protein domains: The fiercely-conserved evolutionary signatures

Proteins have highly conserved domains that determine their functionality. Out of the thousands of domains discovered so far across all living forms, some of the predominant clinically-relevant domains include IENR1, HNHc, HELICc, Pro-kuma_activ, Tryp_SPc, Lactamase_B, PbH1, ChtBD3, CBM49, acidPPc, G3P_acyltransf, RPOL8c, KbaA, HAMP, HisKA, Hr1, Dak2, APC2, Citrate_ly_lig, DALR, VKc, YARHG, WR1, PWI, ZnF_BED, TUDOR, MHC_II_beta, Integrin_B_tail, Excalibur, DISIN, Cadherin, ACTIN, PROF, Robl_LC7, MIT, Kelch, GAS2, B41, Cyclin_C, Connexin_CCC, OmpH, Bac_rhodopsin, AAA, Knot1, NH, Galanin, IB, Elicitin, ACTH, Cache_2, CHASE, AgrB, PRP, IGR, and Antimicrobial21. These domains are distributed in nucleases/helicases, proteases, esterases, lipases, glycosylase, GTPases, phosphatases, methyltransferases, acyltransferase, acetyltransferase, polymerase, kinase, ligase, synthetase, oxidoreductase, protease inhibitors, nucleic acid binding proteins, adhesion and immunity-related proteins, cytoskeletal component-manipulating proteins, lipid biosynthesis and metabolism proteins, membrane-associated proteins, hormone-like and signaling proteins, etc. These domains are ubiquitous stretches or folds of the proteins in pathogens and allergens. Pathogenesis alleviation efforts can benefit enormously if the characteristics of these domains are known. Hence, this review catalogs and discusses the role of such pivotal domains, suggesting hypotheses for better understanding of pathogenesis at molecular level.

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Proteins have highly conserved regions or domains across pathogens and allergens.

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Knowledge on these critical domains can facilitate our understanding of pathogenesis mechanisms.

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Such immune manipulation-related domains include IENR1, HNHc, HELICc, ACTIN, PROF, Robl_LC7, OmpH etc.

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These domains are presnt in enzyme, transcription regulators, adhesion proteins, and hormones.

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This review discusses and hypothesizes on these domains.

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.

Cinderella story: PI4P goes from precursor to key signaling molecule

Phosphatidylinositol lipids are signaling molecules involved in nearly all aspects of cellular regulation. Production of phosphatidylinositol 4-phosphate (PI4P) has long been recognized as one of the first steps in generating poly-phosphatidylinositol phosphates involved in actin organization, cell migration, and signal transduction. In addition, progress over the last decade has brought to light independent roles for PI4P in membrane trafficking and lipid homeostasis. Here, we describe recent advances that reveal the breadth of processes regulated by PI4P, the spectrum of PI4P effectors, and the mechanisms of spatiotemporal control that coordinate crosstalk between PI4P and cellular signaling pathways.

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.

Structural basis of Streptococcus pyogenes immunity to its NAD+ glycohydrolase toxin

The virulence of Gram-positive bacteria is enhanced by toxins like the Streptococcus pyogenes β-NAD(+) glycohydrolase known as SPN. SPN-producing strains of S. pyogenes additionally express the protein immunity factor for SPN (IFS), which forms an inhibitory complex with SPN. We have determined crystal structures of the SPN-IFS complex and IFS alone, revealing that SPN is structurally related to ADP-ribosyl transferases but lacks the canonical binding site for protein substrates. SPN is instead a highly efficient glycohydrolase with the potential to deplete cellular levels of β-NAD(+). The protective effect of IFS involves an extensive interaction with the SPN active site that blocks access to β-NAD(+). The conformation of IFS changes upon binding to SPN, with repacking of an extended C-terminal α helix into a compact shape. IFS is an attractive target for the development of novel bacteriocidal compounds functioning by blocking the bacterium's self-immunity to the SPN toxin.

Arf3 is activated uniquely at the trans-Golgi network by brefeldin A-inhibited guanine nucleotide exchange factors

Arf3 associates with the TGN in a manner that is both temperature-sensitive and uniquely dependent on BIGs. TGN localization and release at 20°C are readily separated and depend on pairs of residues absolutely conserved and unique to Arf3 present at opposite ends of the protein. These results suggest that Arf3 plays a unique function at the TGN.

It is widely assumed that class I and II Arfs function interchangeably throughout the Golgi complex. However, we report here that in vivo, Arf3 displays several unexpected properties. Unlike other Golgi-localized Arfs, Arf3 associates selectively with membranes of the trans-Golgi network (TGN) in a manner that is both temperature-sensitive and uniquely dependent on guanine nucleotide exchange factors of the BIGs family. For example, BIGs knockdown redistributed Arf3 but not Arf1 from Golgi membranes. Furthermore, shifting temperature to 20°C, a temperature known to block cargo in the TGN, selectively redistributed Arf3 from Golgi membranes. Arf3 redistribution occurred slowly, suggesting it resulted from a change in membrane composition. Arf3 knockdown and overexpression experiments suggest that redistribution is not responsible for the 20°C block. To investigate in more detail the mechanism for Arf3 recruitment and temperature-dependent release, we characterized several mutant forms of Arf3. This analysis demonstrated that those properties are readily separated and depend on pairs of residues present at opposite ends of the protein. Furthermore, phylogenetic analysis established that all four critical residues were absolutely conserved and unique to Arf3. These results suggest that Arf3 plays a unique function at the TGN that likely involves recruitment by a specific receptor.

Gga2 mediates sequential ubiquitin-independent and ubiquitin-dependent steps in the trafficking of ARN1 from the trans-Golgi network to the vacuole

In Saccharomyces cerevisiae, ARN1 encodes a transporter for the uptake of ferrichrome, an important nutritional source of iron. In the absence of ferrichrome, Arn1p is sorted directly from the trans-Golgi network (TGN) to the vacuolar lumen via the vacuolar protein-sorting pathway. Arn1p is mis-sorted to the plasma membrane in cells lacking Gga2p, a monomeric clathrin-adaptor protein involved in vesicular transport from the TGN. Although Ggas have been characterized as ubiquitin receptors, we show here that ubiquitin binding by Gga2 was not required for the TGN-to-endosome trafficking of Arn1, but it was required for subsequent sorting of Arn1 into the multivesicular body. In a ubiquitin-binding mutant of Gga2, Arn1p accumulated on the vacuolar membrane in a ubiquitinated form. The yeast epsins Ent3p and Ent4p were also involved in TGN-to-vacuole sorting of Arn1p. Amino-terminal sequences of Arn1p were required for vacuolar protein sorting, as mutation of ubiquitinatable lysine residues resulted in accumulation on the vacuolar membrane, and mutation of either a THN or YGL sequence resulted in mis-sorting to the plasma membrane. These studies suggest that Gga2 is involved in sorting at both the TGN and multivesicular body and that the first step can occur without ubiquitin binding.

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.

The role of cargo proteins in GGA recruitment

Coat proteins are recruited onto membranes to form vesicles that transport cargo from one compartment to another, but the extent to which the cargo helps to recruit the coat proteins is still unclear. Here we have examined the role of cargo in the recruitment of Golgi-localized, γ-ear-containing, ADP ribosylation factor (ARF)-binding proteins (GGAs) onto membranes in HeLa cells. Moderate overexpression of CD8 chimeras with cytoplasmic tails containing DXXLL-sorting signals, which bind to GGAs, increased the localization of all three GGAs to perinuclear membranes, as observed by immunofluorescence. GGA2 was also expressed at approximately twofold higher levels in these cells because it was degraded more slowly. However, this difference only partially accounted for the increase in membrane localization because there was a approximately fivefold increase in GGA2 associated with crude membranes and a ∼12-fold increase in GGA2 associated with clathrin-coated vesicles (CCVs) in cells expressing CD8-DXXLL chimeras. The effect of cargo proteins on GGA recruitment was reconstituted in vitro using permeabilized control and CD8-DXXLL-expressing cells incubated with cytosol containing recombinant GGA2 constructs. Together, these results demonstrate that cargo proteins contribute to the recruitment of GGAs onto membranes and to the formation of GGA-positive CCVs.

Ent3p and Ent5p exhibit cargo-specific functions in trafficking proteins between the trans-Golgi network and the endosomes in yeast

The phosphoinositide-binding proteins Ent3p and Ent5p are required for protein transport from the trans-Golgi network (TGN) to the vacuole in Saccharomyces cerevisiae. Both proteins interact with the monomeric clathrin adaptor Gga2p, but Ent5p also interacts with the clathrin adaptor protein 1 (AP-1) complex, which facilitates retention of proteins such as Chs3p at the TGN. When both ENT3 and ENT5 are mutated, Chs3p is diverted from an intracellular reservoir to the cell surface. However, Ent3p and Ent5p are not required for the function of AP-1, but rather they seem to act in parallel with AP-1 to retain proteins such as Chs3p at the TGN. They have all the properties of clathrin adaptors, because they can both bind to clathrin and to cargo proteins. Like AP-1, Ent5p binds to Chs3p, whereas Ent3p facilitates the interaction between Gga2p and the endosomal syntaxin Pep12p. Thus, Ent3p has an additional function in Gga-dependent transport to the late endosome. Ent3p also facilitates the association between Gga2p and clathrin; however, Ent5p can partially substitute for this function. We conclude that the clathrin adaptors AP-1, Ent3p, Ent5p, and the Ggas cooperate in different ways to sort proteins between the TGN and the endosomes.

Distinct roles for TGN/endosome epsin-like adaptors Ent3p and Ent5p

Clathrin adaptors are key factors in clathrin-coated vesicle formation, coupling clathrin to cargo and/or the lipid bilayer. A physically interacting network of three classes of adaptors participate in clathrin-mediated traffic between the trans-Golgi network (TGN) and endosomes: AP-1, Gga proteins, and epsin-like proteins. Here we investigate functional relationships within this network through transport assays and protein localization analysis in living yeast cells. We observed that epsin-like protein Ent3p preferentially localized with Gga2p, whereas Ent5p distributed equally between AP-1 and Gga2p. Ent3p was mislocalized in Gga-deficient but not in AP-1-deficient cells. In contrast, Ent5p retained localization in cells lacking either or both AP-1 and Gga proteins. The Ent proteins were dispensable for AP-1 or Gga localization. Synthetic genetic growth and alpha-factor maturation defects were observed when ent5Delta but not ent3Delta was introduced together with deletions of the GGA genes. In AP-1-deficient cells, ent3Delta and to a lesser extent ent5Delta caused minor alpha-factor maturation defects, but together resulted in a near-lethal phenotype. Deletions of ENT3 and ENT5 also displayed synthetic defects similar to, but less severe than, synthetic effects of AP-1 and Gga inactivation. These results differentiate Ent3p and Ent5p function in vivo, suggesting that Ent3p acts primarily with Gga proteins, whereas Ent5p acts with both AP-1 and Gga proteins but is more critical for AP-1-mediated transport. The data also support a model in which the Ent adaptors provide important accessory functions to AP-1 and Gga proteins in TGN/endosome traffic.

A phosphatidylinositol-3-kinase-dependent signal transition regulates ARF1 and ARF6 during Fcgamma receptor-mediated phagocytosis

Fcgamma receptor (FcgammaR)-mediated phagocytosis of IgG-coated particles is regulated by 3'-phosphoinositides (3'PIs) and several classes of small GTPases, including ARF6 from the ADP Ribosylation Factor subfamily. The insensitivity of phagocytosis to brefeldin A (BFA), an inhibitor of certain ARF guanine nucleotide exchange factors (GEFs), previously indicated that ARF1 did not participate in phagocytosis. In this study, we show that ARF1 was activated during FcgammaR-mediated phagocytosis and that blocking normal ARF1 cycling inhibited phagosome closure. We examined the distributions and activation patterns of ARF6 and ARF1 during FcgammaR-mediated phagocytosis using fluorescence resonance energy transfer (FRET) stoichiometric microscopy of macrophages expressing CFP- or YFP-chimeras of ARF1, ARF6, and a GTP-ARF-binding protein domain. Both GTPases were activated by BFA-insensitive factors at sites of phagocytosis. ARF6 activation was restricted to the leading edge of the phagocytic cup, while ARF1 activation was delayed and delocalized over the phagosome. Phagocytic cups formed after inhibition of PI 3-kinase (PI-3K) contained persistently activated ARF6 and minimally activated ARF1. This indicates that a PI-3K-dependent signal transition defines the sequence of ARF GTPase activation during phagocytosis and that ARF6 and ARF1 coordinate different functions at the forming phagosome.

Laa1p, a conserved AP-1 accessory protein important for AP-1 localization in yeast

AP-1 and Gga adaptors participate in clathrin-mediated protein transport between the trans-Golgi network and endosomes. Both adaptors contain homologous domains that act to recruit accessory proteins involved in clathrin-coated vesicle formation, but the spectrum of known adaptor-binding partners is limited. This study describes an evolutionarily conserved protein of Saccharomyces cerevisiae, Laa1p (Yjl207cp), that interacts and functions specifically with AP-1. Deletion of LAA1, when combined with a conditional mutation in clathrin heavy chain or deletion of GGA genes, accentuated growth defects and increased disruption of clathrin-dependent alpha-factor maturation and transport of carboxypeptidase Y to the vacuole. In contrast, such genetic interactions were not observed between deletions of LAA1 and AP-1 subunit genes. Laa1p preferentially interacted with AP-1 compared with Gga proteins by glutathione S-transferase-fusion affinity binding and coimmunoprecipitations. Localization of AP-1 and Laa1p, but not Gga proteins, was highly sensitive to brefeldin A, an inhibitor of ADP-ribosylation factor (Arf) activation. Importantly, deletion of LAA1 caused mislocalization of AP-1, especially in cells at high density (postdiauxic shift), but it did not affect Gga protein distribution. Our results identify Laa1p as a new determinant of AP-1 localization, suggesting a model in which Laa1p and Arf cooperate to direct stable association of AP-1 with appropriate intracellular membranes.

The Functional Relationship between the Cdc50p-Drs2p Putative Aminophospholipid Translocase and the Arf GAP Gcs1p in Vesicle Formation in the Retrieval Pathway from Yeast Early Endosomes to the TGN

Drs2p, the catalytic subunit of the Cdc50p-Drs2p putative aminophospholipid translocase, has been implicated in conjunction with the Arf1 signaling pathway in the formation of clathrin-coated vesicles (CCVs) from the TGN. Herein, we searched for Arf regulator genes whose mutations were synthetically lethal with cdc50Delta, and identified the Arf GAP gene GCS1. Most of the examined transport pathways in the Cdc50p-depleted gcs1Delta mutant were nearly normal, including endocytic transport to vacuoles, carboxypeptidase Y sorting, and the processing and secretion of invertase. In contrast, this mutant exhibited severe defects in the early endosome-to-TGN transport pathway; proteins that are transported via this pathway, such as the v-SNARE Snc1p, the t-SNARE Tlg1p, and the chitin synthase III subunit Chs3p, accumulated in TGN-independent aberrant membrane structures. We extended our analyses to clathrin adaptors, and found that Gga1p/Gga2p and AP-1 were also involved in this pathway. The Cdc50p-depleted gga1Delta gga2Delta mutant and the gcs1Delta apl2Delta (the beta1 subunit of AP-1) mutant exhibited growth defects and intracellular Snc1p-containing membranes accumulated in these cells. These results suggest that Cdc50p-Drs2p plays an important role in the Arf1p-mediated formation of CCVs for the retrieval pathway from early endosomes to the TGN.

The GAT domains of clathrin-associated GGA proteins have two ubiquitin binding motifs

Ubiquitin (Ub) attachment to membrane proteins can serve as a sorting signal for lysosomal delivery. Recognition of Ub as a sorting signal can occur at the trans-Golgi network and is mediated in part by the clathrin-associated Golgi-localizing, gamma-adaptin ear domain homology, ARF-binding proteins (GGA). GGA proteins bind Ub via a three-helix bundle subdomain in their GAT (GGA and target of Myb1 protein) domain, which is also present in the Ub binding domain of target of Myb1 protein. Ubiquitin binding by yeast Ggas is required to direct sorting of ubiquitinated proteins such as general amino acid permease (Gap1) from the trans-Golgi network to endosomes. Using affinity chromatography and nuclear magnetic resonance spectroscopy, we have found that the human GGA3 GAT domain contains two Ub binding motifs that bind to the same surface of ubiquitin. These motifs are found within different helices within the three-helix GAT subdomain. When functionally analyzed in yeast, each motif was sufficient to mediate trans-Golgi network to endosomal sorting of Gap1, and mutation of both motifs resulted in defective Gap1 sorting without defects in other GGA-dependent processes.

The trihelical bundle subdomain of the GGA proteins interacts with multiple partners through overlapping but distinct sites

The Golgi-localized, gamma-adaptin ear-containing, ARF-binding (GGA) proteins are monomeric clathrin adaptors that mediate the sorting of cargo at the trans-Golgi network and endosomes. The GGAs contain four different domains named Vps27, Hrs, Stam (VHS); GGAs and TOM1 (GAT); hinge; and gamma-adaptin ear (GAE). The VHS domain recognizes transmembrane cargo, whereas the hinge and GAE regions bind clathrin and accessory proteins, respectively. The GAT domain is a polyfunctional module that interacts with various partners including the small GTPase ARF, the endosomal fusion regulator Rabaptin-5, ubiquitin, and the product of the tumor susceptibility gene 101 (TSG101). Previous x-ray crystallographic analyses showed that the GAT region is composed of two subdomains, an N-terminal helix-loop-helix containing the ARF binding site, and a C-terminal triple alpha-helical (trihelical) bundle. In this study, we define the Rabaptin-5 binding site on the GGA1-GAT domain and its relationship to the binding sites for ubiquitin and TSG101. Our observations show that Rabaptin-5, ubiquitin, and TSG101 bind to overlapping but distinct binding sites on the trihelical bundle. The different GAT binding partners engage in both competitive and cooperative interactions that may be important for the function of the GGAs in protein sorting.

GGA proteins bind ubiquitin to facilitate sorting at the trans-Golgi network

Ubiquitination functions as a sorting signal for lysosomal degradation of cell-surface proteins by facilitating their internalization from the plasma membrane and incorporation into lumenal vesicles of multivesicular bodies (MVBs). Ubiquitin may also mediate sorting of proteins from the trans-Golgi network (TGN) to the endosome, thereby preventing their appearance on the cell surface and hastening their degradation in the lysosome-vacuole. Substantiation of a direct ubiquitin-dependent TGN sorting pathway relies in part on identifying candidate machinery that may function as a ubiquitin-sorting 'receptor'at the TGN. Members of the GGA family of coat proteins localize to the TGN and promote the incorporation of proteins into clathrin-coated vesicles destined for transport to endosomes. We show that the GGA coat proteins bind directly to ubiquitin through their GAT domain and demonstrate that this interaction is required for the ubiquitin-dependent sorting of the Gap1 amino acid transporter from the TGN to endosomes. Thus, GGA proteins fulfill the role of ubiquitin sorting receptors at the TGN.

The GGA proteins: adaptors on the move

The GGA proteins are a family of ubiquitously expressed, Arf-dependent clathrin adaptors that mediate the sorting of mannose-6-phosphate receptors between the trans-Golgi network and endosomes. Recent studies have elucidated the biochemical and structural bases for the interaction of the GGA proteins with many binding partners, and have shed light on the molecular and cellular mechanisms by which the GGA proteins participate in protein sorting.

Structure of the GAT domain of human GGA1: a syntaxin amino-terminal domain fold in an endosomal trafficking adaptor

The Golgi-associated, gamma-adaptin homologous, ADP-ribosylation factor (ARF)-interacting proteins (GGAs) are adaptors that sort receptors from the trans-Golgi network into the endosomallysosomal pathway. The GGAs and TOM1 (GAT) domains of the GGAs are responsible for their ARF-dependent localization. The 2.4-A crystal structure of the GAT domain of human GGA1 reveals a three-helix bundle, with a long N-terminal helical extension that is not conserved in GAT domains that do not bind ARF. The ARF binding site is located in the N-terminal extension and is separate from the core three-helix bundle. An unanticipated structural similarity to the N-terminal domain of syntaxin 1a was discovered, comprising the entire three-helix bundle. A conserved binding site on helices 2 and 3 of the GAT domain three-helix bundle is predicted to interact with coiled-coil-containing proteins. We propose that the GAT domain is descended from the same ancestor as the syntaxin 1a N-terminal domain, and that both protein families share a common function in binding coiled-coil domain proteins.

The structure of the GGA1-GAT domain reveals the molecular basis for ARF binding and membrane association of GGAs

The GGAs are a family of clathrin adaptor proteins involved in vesicular transport between the trans-Golgi network and endosomal system. Here we confirm reports that GGAs are targeted to the Golgi via interaction between the GGA-GAT domain and ARF-GTP, and we present the structure of the GAT domain of human GGA1, completing the structural description of the folded domains of GGA proteins. The GGA-GAT domain possesses an all alpha-helical fold with a "paper clip" topology comprising two independent subdomains. Structure-based mutagenesis demonstrates that ARF1-GTP binding by GGAs is exclusively governed by the N-terminal "hook" subdomain, and, using an in vitro recruitment assay, we show that ARF-GTP binding by this small structure is required and sufficient for Golgi targeting of GGAs.

The Structure and Function of GGAs, the Traffic Controllers at the TGN Sorting Crossroads

GGAs (Golgi-localizing, gamma-adaptin ear homology domain, ARF-binding proteins) are a family of monomeric clathrin adaptor proteins that are conserved from yeasts to humans. Data published during the past four years have provided detailed pictures of the localization, domain organization and structure-function relationships of GGAs. GGAs possess four conserved functional domains, each of which interacts with cargo proteins including mannose 6-phosphate receptors, the small GTPase ARF, clathrin, or accessory proteins including Rabaptin-5 and gamma-synergin. Together with or independent of the adaptor protein complex AP-1, GGAs regulate selective transport of cargo proteins, such as mannose 6-phosphate receptors, from the trans-Golgi network to endosomes mediated by clathrin-coated vesicles.