EpsinR: an ENTH domain-containing protein that interacts with AP-1

Type II phosphatidylinositol 4-kinase regulates trafficking of secretory granule proteins in Drosophila

Type II phosphatidylinositol 4-kinase (PI4KII) produces the lipid phosphatidylinositol 4-phosphate (PI4P), a key regulator of membrane trafficking. Here, we generated genetic models of the sole Drosophila melanogaster PI4KII gene. A specific requirement for PI4KII emerged in larval salivary glands. In PI4KII mutants, mucin-containing glue granules failed to reach normal size, with glue protein aberrantly accumulating in enlarged Rab7-positive late endosomes. Presence of PI4KII at the Golgi and on dynamic tubular endosomes indicated two distinct foci for its function. First, consistent with the established role of PI4P in the Golgi, PI4KII is required for sorting of glue granule cargo and the granule-associated SNARE Snap24. Second, PI4KII also has an unforeseen function in late endosomes, where it is required for normal retromer dynamics and for formation of tubular endosomes that are likely to be involved in retrieving Snap24 and Lysosomal enzyme receptor protein (Lerp) from late endosomes to the trans-Golgi network. Our genetic analysis of PI4KII in flies thus reveals a novel role for PI4KII in regulating the fidelity of granule protein trafficking in secretory tissues.

Multivariate proteomic profiling identifies novel accessory proteins of coated vesicles

A multivariate proteomics approach identified numerous new clathrin-coated vesicle proteins as well as the first AP-4 accessory protein, and also revealed how auxilin depletion causes mitotic arrest through sequestration of spindle proteins in clathrin cages.

Despite recent advances in mass spectrometry, proteomic characterization of transport vesicles remains challenging. Here, we describe a multivariate proteomics approach to analyzing clathrin-coated vesicles (CCVs) from HeLa cells. siRNA knockdown of coat components and different fractionation protocols were used to obtain modified coated vesicle-enriched fractions, which were compared by stable isotope labeling of amino acids in cell culture (SILAC)-based quantitative mass spectrometry. 10 datasets were combined through principal component analysis into a “profiling” cluster analysis. Overall, 136 CCV-associated proteins were predicted, including 36 new proteins. The method identified >93% of established CCV coat proteins and assigned >91% correctly to intracellular or endocytic CCVs. Furthermore, the profiling analysis extends to less well characterized types of coated vesicles, and we identify and characterize the first AP-4 accessory protein, which we have named tepsin. Finally, our data explain how sequestration of TACC3 in cytosolic clathrin cages causes the severe mitotic defects observed in auxilin-depleted cells. The profiling approach can be adapted to address related cell and systems biological questions.

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.

The epsin protein family: coordinators of endocytosis and signaling

The epsins are a conserved family of endocytic adaptors essential for cell viability in yeast and for embryo development in higher eukaryotes. Epsins function as adaptors by recognizing ubiquitinated cargo and as endocytic accessory proteins by contributing to endocytic network stability/regulation and membrane bending. Importantly, epsins play a critical role in signaling by contributing to epidermal growth factor receptor downregulation and the activation of notch and RhoGTPase pathways. In this review, we present an overview of the epsins and emphasize their functional importance as coordinators of endocytosis and signaling.

Phosphoinositides in Golgi complex function

The Golgi complex is a ribbon-like organelle composed of stacks of flat cisternae interconnected by tubular junctions. It occupies a central position in the endomembrane system as proteins and lipids that are synthesized in the endoplasmic reticulum (ER) pass through the Golgi complex to undergo biosynthetic modification (mainly glycosylation) and to be sorted to their final destinations. In addition the Golgi complex possesses a number of activities, apparently not directly connected with its main role in trafficking and sorting, which have been recently reviewed in Wilson et al. 2011. In spite of the constant massive flux of material the Golgi complex maintains its identity and phosphoinositides (PIs), among other factors, play a central role in this process. The active metabolism of PIs at the Golgi is necessary for the proper functioning of the organelle both in terms of membrane trafficking/sorting and its manifold metabolic and signalling activities. Phosphatidylinositol 4-phosphate (PtdIns4P), in particular, is responsible for the recruitment of numerous cytosolic proteins that recognise and bind PtdIns4P via specific lipid-binding domains. In this chapter we will summarize the findings that have contributed to our current understanding of the role of PIs in the biology of the Golgi complex in terms of the regulation of PI metabolism and the functional roles and regulation of PtdIns4P effectors.

Liquid facets-related (lqfR) is required for egg chamber morphogenesis during Drosophila oogenesis

Clathrin interactor 1 [CLINT1] (also called enthoprotin/EpsinR) is an Epsin N-terminal homology (ENTH) domain-containing adaptor protein that functions in anterograde and retrograde clathrin-mediated trafficking between the trans-Golgi network and the endosome. Removal of both Saccharomyces cerevisiae homologs, Ent3p and Ent5p, result in yeast that are viable, but that display a cold-sensitive growth phenotype and mistrafficking of various vacuolar proteins. Similarly, either knock-down or overexpression of vertebrate CLINT1 in cell culture causes mistrafficking of proteins. Here, we have characterized Drosophila CLINT1, liquid-facets Related (lqfR). LqfR is ubiquitously expressed throughout development and is localized to the Golgi and endosome. Strong hypomorphic mutants generated by imprecise P-element excision exhibit extra macrochaetae, rough eyes and are female sterile. Although essentially no eggs are laid, the ovaries do contain late-stage egg chambers that exhibit abnormal morphology. Germline clones reveal that LqfR expression in the somatic follicle cells is sufficient to rescue the oogenesis defects. Clones of mutant lqfR follicle cells have a decreased cell size consistent with a downregulation of Akt1. We find that while total Akt1 levels are increased there is also a significant decrease in activated phosphorylated Akt1. Taken together, these results show that LqfR function is required to regulate follicle cell size and signaling during Drosophila oogenesis.

Membrane-trafficking sorting hubs: cooperation between PI4P and small GTPases at the trans-Golgi network

Cell polarity in eukaryotes requires constant sorting, packaging and transport of membrane-bound cargo within the cell. These processes occur in two sorting hubs: the recycling endosome for incoming material and the trans-Golgi network for outgoing material. Phosphatidylinositol 3-phosphate and phosphatidylinositol 4-phosphate are enriched at the endocytic and exocytic sorting hubs, respectively, where they act together with small GTPases to recruit factors to segregate cargo and regulate carrier formation and transport. In this review, we summarize the current understanding of how these lipids and GTPases regulate membrane trafficking directly, emphasizing the recent discoveries of phosphatidylinositol 4-phosphate functions at the trans-Golgi network.

Gadkin: A novel link between endosomal vesicles and microtubule tracks

Different types of endosomal vesicles show distinct distribution patterns within cells. While early endosomes can be found throughout the cell, recycling endosomal vesicles and tubules tend to cluster near the microtubule organizing center in the perinuclear region in most cell types. The molecular mechanisms underlying the steady-state distribution and dynamics of various types of endosomal vesicles has long remained enigmatic. However, during the past decade it has become evident that microtubule-based motor proteins of the kinesin family play a pivotal role in the positioning of endosomes. Early endosomes were shown to cluster in the perinuclear area in the absence of KIF16B,1 KIF3A is required for the steady-state distribution of late endosomes/lysosomes,2 and KIF13A directs M6PR-containing vesicles from the TGN to the plasma membrane3 to name only a few examples. In the case of Tf-containing recycling endosomes antibody-injection experiments implicated kinesin-1, a heteromer comprised of KIF5 heavy and KLC light chains, as a motor for their transport towards the cell periphery.4 Indeed, KIF5B knockdown experiments confirmed that kinesin-1 is necessary to maintain the peripheral pool of recycling endosomes.5 But how is kinesin-1 linked to endosomal vesicles? Work from our own laboratory has identified the AP-1-binding protein Gadkin as a molecular link between AP-1-mediated traffic and kinesin-1-based transport along microtubules.5 This work as well as hypothetical models for kinesin-dependent endosomal membrane traffic will be discussed here.

Entry at the trans-face of the Golgi

The trans-Golgi network (TGN) receives a select set of proteins from the endocytic pathway-about 5% of total plasma membrane glycoproteins (Duncan and Kornfeld 1988). Proteins that are delivered include mannose 6-phosphate receptors (MPRs), TGN46, sortilin, and various toxins that hitchhike a ride backward through the secretory pathway to intoxicate cells after they exit into the cytoplasm from the endoplasmic reticulum (ER). This article will review work on the molecular players that drive protein transport from the endocytic pathway to the TGN. Distinct requirements have revealed multiple routes for retrograde transport; in addition, the existence of multiple, potential coat proteins and/or cargo adaptors imply that multiple vesicular transfers are likely involved. Several comprehensive reviews have appeared recently and should be sought for additional details (Bonifacino and Rojas 2006; Johannes and Popoff 2008).

ARFGAP2 and ARFGAP3 are essential for COPI coat assembly on the Golgi membrane of living cells

Coat protein complex I (COPI) vesicles play a central role in the recycling of proteins in the early secretory pathway and transport of proteins within the Golgi stack. Vesicle formation is initiated by the exchange of GDP for GTP on ARF1 (ADP-ribosylation factor 1), which, in turn, recruits the coat protein coatomer to the membrane for selection of cargo and membrane deformation. ARFGAP1 (ARF1 GTPase-activating protein 1) regulates the dynamic cycling of ARF1 on the membrane that results in both cargo concentration and uncoating for the generation of a fusion-competent vesicle. Two human orthologues of the yeast ARFGAP Glo3p, termed ARFGAP2 and ARFGAP3, have been demonstrated to be present on COPI vesicles generated in vitro in the presence of guanosine 5'-3-O-(thio)triphosphate. Here, we investigate the function of these two proteins in living cells and compare it with that of ARFGAP1. We find that ARFGAP2 and ARFGAP3 follow the dynamic behavior of coatomer upon stimulation of vesicle budding in vivo more closely than does ARFGAP1. Electron microscopy of ARFGAP2 and ARFGAP3 knockdowns indicated Golgi unstacking and cisternal shortening similarly to conditions where vesicle uncoating was blocked. Furthermore, the knockdown of both ARFGAP2 and ARFGAP3 prevents proper assembly of the COPI coat lattice for which ARFGAP1 does not seem to play a major role. This suggests that ARFGAP2 and ARFGAP3 are key components of the COPI coat lattice and are necessary for proper vesicle formation.

Phosphoinositides: lipid regulators of membrane proteins

Phosphoinositides are a family of minority acidic phospholipids in cell membranes. Their principal role is instructional: they interact with proteins. Each cellular membrane compartment uses a characteristic species of phosphoinositide. This signature phosphoinositide attracts a specific complement of functionally important, loosely attached peripheral proteins to that membrane. For example, the phosphatidylinositol 4,5-bisphosphate (PIP(2)) of the plasma membrane attracts phospholipase C, protein kinase C, proteins involved in membrane budding and fusion, proteins regulating the actin cytoskeleton, and others. Phosphoinositides also regulate the activity level of the integral membrane proteins. Many ion channels of the plasma membrane need the plasma-membrane-specific PIP(2) to function. Their activity decreases when the abundance of this lipid falls, as for example after activation of phospholipase C. This behaviour is illustrated by the suppression of KCNQ K(+) channel current by activation of M(1) muscarinic receptors; KCNQ channels require PIP(2) for their activity. In summary, phosphoinositides contribute to the selection of peripheral proteins for each membrane and regulate the activity of the integral proteins.

Structural basis of wedging the Golgi membrane by FAPP pleckstrin homology domains

Overduin and colleagues present the NMR structures of free, micelle and PtdIns(4)P-bound FAPP1-PH domain. The micelle-bound structure reveals how its prominent wedge independently tubulates Golgi membranes by leaflet penetration. A hydrophobic element inserts into and bends membranes, and is conserved in pleckstrin homology domains of CERT and OSBP proteins.

The mechanisms underlying Golgi targeting and vesiculation are unknown, although the responsible phosphatidylinositol 4-phosphate (PtdIns(4)P) ligand and four-phosphate-adaptor protein (FAPP) modules have been defined. The micelle-bound structure of the FAPP1 pleckstrin homology domain reveals how its prominent wedge independently tubulates Golgi membranes by leaflet penetration. Mutations compromising the exposed hydrophobicity of full-length FAPP2 abolish lipid monolayer binding and compression. The trafficking process begins with an electrostatic approach, phosphoinositide sampling and perpendicular penetration. Extensive protein contacts with PtdIns(4)P and neighbouring phospholipids reshape the bilayer and initiate tubulation through a conserved wedge with features shared by diverse protein modules.

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.

A novel subtype of AP-1-binding motif within the palmitoylated trans-Golgi network/endosomal accessory protein Gadkin/gamma-BAR

Membrane traffic between the trans-Golgi network (TGN) and endosomes is mediated in part by the assembly of clathrin-AP-1 adaptor complex-coated vesicles. This process involves multiple accessory proteins that directly bind to the ear domain of AP-1gamma via degenerate peptide motifs that conform to the consensus sequence diameterG(P/D/E)(diameter/L/M) (with diameter being a large hydrophobic amino acid). Recently, gamma-BAR (hereafter referred to as Gadkin for reasons explained below) has been identified as a novel AP-1 recruitment factor involved in AP-1-dependent endosomal trafficking of lysosomal enzymes. How precisely Gadkin interacts with membranes and with AP-1gamma has remained unclear. Here we show that Gadkin is an S-palmitoylated peripheral membrane protein that lacks stable tertiary structure. S-Palmitoylation is required for the recruitment of Gadkin to TGN/endosomal membranes but not for binding to AP-1. Furthermore, we identify a novel subtype of AP-1-binding motif within Gadkin that specifically associates with the gamma1-adaptin ear domain. Mutational inactivation of this novel type of motif, either alone or in combination with three more conventional AP-1gamma binding peptides, causes Gadkin to mislocalize to the plasma membrane and interferes with its ability to render AP-1 brefeldin A-resistant, indicating its physiological importance. Our studies thus unravel the molecular basis for Gadkin-mediated AP-1 recruitment to TGN/endosomal membranes and identify a novel subtype of the AP-1-binding motif.

AP-1 and KIF13A coordinate endosomal sorting and positioning during melanosome biogenesis

Specialized cell types exploit endosomal trafficking to deliver protein cargoes to cell type-specific lysosome-related organelles (LROs), but how endosomes are specified for this function is not known. In this study, we show that the clathrin adaptor AP-1 and the kinesin motor KIF13A together create peripheral recycling endosomal subdomains in melanocytes required for cargo delivery to maturing melanosomes. In cells depleted of AP-1 or KIF13A, a subpopulation of recycling endosomes redistributes to pericentriolar clusters, resulting in sequestration of melanosomal enzymes like Tyrp1 in vacuolar endosomes and consequent inhibition of melanin synthesis and melanosome maturation. Immunocytochemistry, live cell imaging, and electron tomography reveal AP-1- and KIF13A-dependent dynamic close appositions and continuities between peripheral endosomal tubules and melanosomes. Our results reveal that LRO protein sorting is coupled to cell type-specific positioning of endosomes that facilitate endosome-LRO contacts and are required for organelle maturation.

The clavesin family, neuron-specific lipid- and clathrin-binding Sec14 proteins regulating lysosomal morphology

Clathrin-coated vesicles (CCVs) originating from the trans-Golgi network (TGN) provide a major transport pathway from the secretory system to endosomes/lysosomes. Herein we describe paralogous Sec14 domain-bearing proteins, clavesin 1/CRALBPL and clavesin 2, identified through a proteomic analysis of CCVs. Clavesins are enriched on CCVs and form a complex with clathrin heavy chain (CHC) and adaptor protein-1, major coat components of TGN-derived CCVs. The proteins co-localize with markers of endosomes and the TGN as well as with CHC and adaptor protein-1. A membrane mimic assay using the Sec14 domain of clavesin 1 reveals phosphatidylinositol 3,5-bisphosphate as a specific lipid partner. Phosphatidylinositol 3,5-bisphosphate is localized to late endosomes/lysosomes, and interestingly, isoform-specific knockdown of clavesins in neurons using lentiviral delivery of interfering RNA leads to enlargement of a lysosome-associated membrane protein 1-positive membrane compartment with no obvious influence on the CCV machinery at the TGN. Since clavesins are expressed exclusively in neurons, this new protein family appears to provide a unique neuron-specific regulation of late endosome/lysosome morphology.

Mechanisms of membrane deformation by lipid-binding domains

Among an increasing number of lipid-binding domains, a group that not only binds to membrane lipids but also changes the shape of the membrane has been found. These domains are characterized by their strong ability to transform globular liposomes as well as flat plasma membranes into elongated membrane tubules both in vitro and in vivo. Biochemical studies on the structures of these proteins have revealed the importance of the amphipathic helix, which potentially intercalates into the lipid bilayer to induce and/or sense membrane curvature. Among such membrane-deforming domains, BAR and F-BAR/EFC domains form crescent-shaped dimers, suggesting a preference for a curved membrane, which is important for curvature sensing. Bioinformatics in combination with structural analyses has been identifying an increasing number of novel families of lipid-binding domains. This review attempts to summarize the evidence obtained by recent studies in order to gain general insights into the roles of membrane-deforming domains in a variety of biological events.

Drosophila liquid facets-Related encodes Golgi epsin and is an essential gene required for cell proliferation, growth, and patterning

Epsin and epsin-Related (epsinR) are multi-modular proteins that stimulate clathrin-coated vesicle formation. Epsin promotes endocytosis at the plasma membrane, and epsinR functions at the Golgi and early endosomes for trans-Golgi network/endosome vesicle trafficking. In Drosophila, endocytic epsin is known as Liquid facets, and it is essential specifically for Notch signaling. Here, by generating and analyzing loss-of-function mutants in the liquid facets-Related (lqfR) gene of Drosophila, we investigated the function of Golgi epsin in a multicellular context. We found that LqfR is indeed a Golgi protein, and that like liquid facets, lqfR is essential for Drosophila viability. In addition, primarily by analyzing mutant eye discs, we found that lqfR is required for cell proliferation, insulin-independent cell growth, and cell patterning, consistent with a role in one or several signaling pathways. Epsins in all organisms share an ENTH (epsin N-terminal homology) domain, which binds phosphoinositides enriched at the plasma membrane or the Golgi membrane. The epsinR ENTH domain is also the recognition element for particular cargos. By generating wild-type and mutant lqfR transgenes, we found that all apparent LqfR functions are independent of its ENTH domain. These results suggest that LqfR transports specific cargo critical to one or more signaling pathways, and lays the foundation for identifying those proteins.

The single ENTH-domain protein of trypanosomes; endocytic functions and evolutionary relationship with epsin

Epsin N-terminal homology (ENTH) domains occur in proteins of either the epsin or epsin-related (epsinR) form. They principally function in clathrin-mediated trafficking and membrane deformation. Both epsin and epsinR possess clathrin-binding motifs, but only epsin incorporates a ubiquitin-interaction motif (UIM). To better understand the origins of ENTH-domain proteins and their functions, we performed detailed comparative genomics and phylogenetics on the epsin family. The epsin ENTH-UIM configuration is an architecture restricted to yeast and animals. Further, we undertook functional analysis in Trypanosoma brucei (T. brucei), a divergent organism possessing a single ENTH-domain protein (TbEpsinR). TbEpsinR has a cellular location similar to both epsin and epsinR at plasma membrane clathrin budding sites and endosomal compartments, and associates with clathrin, as demonstrated by coimmunoprecipitation. Knockdown of TbEpsinR leads to a significant decrease in the intracellular pools of multiple surface antigens, without affecting bulk membrane internalization. Therefore, despite lacking the UIM, TbEpsinR maintains a similar role to metazoan epsin in endocytosis and participates as a clathrin-associated adaptor. We suggest that recruitment of a UIM to the ENTH-domain proteins was not essential for participation in endocytosis of ubiquitylated molecules, and is presumably a specific innovation restricted to higher eukaryotes.

SNX9 - a prelude to vesicle release

The sorting nexin SNX9 has, in the past few years, been singled out as an important protein that participates in fundamental cellular activities. SNX9 binds strongly to dynamin and is partly responsible for the recruitment of this GTPase to sites of endocytosis. SNX9 also has a high capacity for modulation of the membrane and might therefore participate in the formation of the narrow neck of endocytic vesicles before scission occurs. Once assembled on the membrane, SNX9 stimulates the GTPase activity of dynamin to facilitate the scission reaction. It has also become clear that SNX9 has the ability to activate the actin regulator N-WASP in a membrane-dependent manner to coordinate actin polymerization with vesicle release. In this Commentary, we summarize several aspects of SNX9 structure and function in the context of membrane remodeling, discuss its interplay with various interaction partners and present a model of how SNX9 might work in endocytosis.

The multiple roles of PtdIns(4)P -- not just the precursor of PtdIns(4,5)P2

The phosphoinositides (PIs) are membrane phospholipids that actively operate at membrane-cytosol interfaces through the recruitment of a number of effector proteins. In this context, each of the seven different PI species represents a topological determinant that can establish the nature and the function of the membrane where it is located. Phosphatidylinositol 4-phosphate (PtdIns(4)P) is the most abundant of the monophosphorylated inositol phospholipids in mammalian cells, and it is produced by D-4 phosphorylation of the inositol ring of PtdIns. PtdIns(4)P can be further phosphorylated to PtdIns(4,5)P(2) by PtdIns(4)P 5-kinases and, indeed, PtdIns(4)P has for many years been considered to be just the precursor of PtdIns(4,5)P(2). Over the last decade, however, a large body of evidence has accumulated that shows that PtdIns(4)P is, in its own right, a direct regulator of important cell functions. The subcellular localisation of the PtdIns(4)P effectors initially led to the assumption that the bulk of this lipid is present in the membranes of the Golgi complex. However, the existence and physiological relevance of ;non-Golgi pools' of PtdIns(4)P have now begun to be addressed. The aim of this Commentary is to describe our present knowledge of PtdIns(4)P metabolism and the molecular machineries that are directly regulated by PtdIns(4)P within and outside of the Golgi complex.

Phosphoinositide signaling pathways: promising role as builders of epithelial cell polarity

Polarity is a prerequisite for proper development and function of epithelia in metazoa. The major feature of polarized epithelial cells is the presence of specialized domains with asymmetric distribution of macromolecular contents including proteins and lipids. The apical domain is involved in exchange with the organ lumen, and the basolateral membrane maintains contact with neighboring cells and the underlying extracellular matrix. The two domains are separated by tight junctions, which act as a diffusion barrier to prevent free mixing of domain-specific proteins and lipids. Extensive studies have shed light on the numerous protein families involved in cell polarization. However, many questions still remain regarding the molecular mechanisms of polarity regulation and in particular very little is known about the role of lipids in building polarity. In this chapter, essential determinants of epithelial polarity will be reviewed with a particular focus on metabolism and function of phosphoinositides.

Sorting of lysosomal proteins

Lysosomes are composed of soluble and transmembrane proteins that are targeted to lysosomes in a signal-dependent manner. The majority of soluble acid hydrolases are modified with mannose 6-phosphate (M6P) residues, allowing their recognition by M6P receptors in the Golgi complex and ensuing transport to the endosomal/lysosomal system. Other soluble enzymes and non-enzymatic proteins are transported to lysosomes in an M6P-independent manner mediated by alternative receptors such as the lysosomal integral membrane protein LIMP-2 or sortilin. Sorting of cargo receptors and lysosomal transmembrane proteins requires sorting signals present in their cytosolic domains. These signals include dileucine-based motifs, DXXLL or [DE]XXXL[LI], and tyrosine-based motifs, YXXØ, which interact with components of clathrin coats such as GGAs or adaptor protein complexes. In addition, phosphorylation and lipid modifications regulate signal recognition and trafficking of lysosomal membrane proteins. The complex interaction of both luminal and cytosolic signals with recognition proteins guarantees the specific and directed transport of proteins to lysosomes.

The ENTH and C-terminal domains of Dictyostelium epsin cooperate to regulate the dynamic interaction with clathrin-coated pits

Epsin contains a phospholipid-binding ENTH domain coupled to C-terminal domain motifs that bind coated pit proteins. We examined how these domains interact to influence epsin function and localization in Dictyostelium. Although not required for global clathrin function, epsin was essential for constructing oval spores during development. Within the epsin protein, we found that features important for essential function were distinct from features targeting epsin to clathrin-coated pits. On its own, the phospholipid-binding ENTH domain could rescue the epsin-null phenotype. Although necessary and sufficient for function, the isolated ENTH domain was not targeted within clathrin-coated pits. The C-terminal domain containing the coated-pit motif was also insufficient, highlighting a requirement for both domains for targeting to coated pits. Replacement of the ENTH domain by an alternative membrane-binding domain resulted in epsin that sequestered clathrin and AP2 and ablated clathrin function, supporting a modulatory role for the ENTH domain. Within the ENTH domain, residues important for PtdIns(4,5)P2 binding were essential for both epsin localization and function, whereas residue T107 was essential for function but not coated pit localization. Our results support a model where the ENTH domain coordinates with the clathrin-binding C-terminal domain to allow a dynamic interaction of epsin with coated pits.

Aftiphilin and gamma-synergin are required for secretagogue sensitivity of Weibel-Palade bodies in endothelial cells

Formation of secretory organelles requires the coupling of cargo selection to targeting into the correct exocytic pathway. Although the assembly of regulated secretory granules is driven in part by selective aggregation and retention of content, we recently reported that adaptor protein-1 (AP-1) recruitment of clathrin is essential to the initial formation of Weibel-Palade bodies (WPBs) at the trans-Golgi network. A selective co-aggregation process might include recruitment of components required for targeting to the regulated secretory pathway. However, we find that acquisition of the regulated secretory phenotype by WPBs in endothelial cells is coupled to but can be separated from formation of the distinctive granule core by ablation of the AP-1 effectors aftiphilin and gamma-synergin. Their depletion by small interfering RNA leads to WPBs that fail to respond to secretagogue and release their content in an unregulated manner. We find that these non-responsive WPBs have density, markers of maturation, and highly multimerized von Willebrand factor similar to those of wild-type granules. Thus, by also recruiting aftiphilin/gamma-synergin in addition to clathrin, AP-1 coordinates formation of WPBs with their acquisition of a regulated secretory phenotype.

Function and dysfunction of the PI system in membrane trafficking

The phosphoinositides (PIs) function as efficient and finely tuned switches that control the assembly–disassembly cycles of complex molecular machineries with key roles in membrane trafficking. This important role of the PIs is mainly due to their versatile nature, which is in turn determined by their fast metabolic interconversions. PIs can be tightly regulated both spatially and temporally through the many PI kinases (PIKs) and phosphatases that are distributed throughout the different intracellular compartments. In spite of the enormous progress made in the past 20 years towards the definition of the molecular details of PI–protein interactions and of the regulatory mechanisms of the individual PIKs and phosphatases, important issues concerning the general principles of the organisation of the PI system and the coordination of the different PI-metabolising enzymes remain to be addressed. The answers should come from applying a systems biology approach to the study of the PI system, through the integration of analyses of the protein interaction data of the PI enzymes and the PI targets with those of the ‘phenomes' of the genetic diseases that involve these PI-metabolising enzymes.

Novel suppressors of alpha-synuclein toxicity identified using yeast

The mechanism by which the Parkinson's disease-related protein alpha-synuclein (alpha-syn) causes neurodegeneration has not been elucidated. To determine the genes that protect cells from alpha-syn, we used a genetic screen to identify suppressors of the super sensitivity of the yeast Saccharomyces cerevisiae expressing alpha-syn to killing by hydrogen peroxide. Forty genes in ubiquitin-dependent protein catabolism, protein biosynthesis, vesicle trafficking and the response to stress were identified. Five of the forty genes--ENT3, IDP3, JEM1, ARG2 and HSP82--ranked highest in their ability to block alpha-syn-induced reactive oxygen species accumulation, and these five genes were characterized in more detail. The deletion of any of these five genes enhanced the toxicity of alpha-syn as judged by growth defects compared with wild-type cells expressing alpha-syn, which indicates that these genes protect cells from alpha-syn. Strikingly, four of the five genes are specific for alpha-syn in that they fail to protect cells from the toxicity of the two inherited mutants A30P or A53T. This finding suggests that alpha-syn causes toxicity to cells through a different pathway than these two inherited mutants. Lastly, overexpression of Ent3p, which is a clathrin adapter protein involved in protein transport between the Golgi and the vacuole, causes alpha-syn to redistribute from the plasma membrane into cytoplasmic vesicular structures. Our interpretation is that Ent3p mediates the transport of alpha-syn to the vacuole for proteolytic degradation. A similar clathrin adaptor protein, epsinR, exists in humans.

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.

Auxilin depletion causes self-assembly of clathrin into membraneless cages in vivo

Auxilin is a cofactor for Hsc70-mediated uncoating of clathrin-coated vesicles (CCVs). However, small interfering RNA (siRNA) knockdown of the ubiquitous auxilin 2 in HeLa cells only moderately impairs clathrin-dependent trafficking. In this study, we show that HeLa cells also express auxilin 1, previously thought to be neuron specific, and that both auxilins need to be depleted for inhibition of clathrin-mediated endocytosis and intracellular sorting. Depleting both auxilins cause an ∼50% reduction in the number of clathrin-coated pits at the plasma membrane but enhances the association of clathrin and adaptors with intracellular membranes. CCV fractions isolated from auxilin-depleted cells have an ∼1.5-fold increase in clathrin content and more than fivefold increase in the amount of AP-2 adaptor complex and other endocytic machinery, with no concomitant increase in cargo. In addition, the structures isolated from auxilin-depleted cells are on average smaller than CCVs from control cells and are largely devoid of membrane, indicating that they are not CCVs but membraneless clathrin cages. Similar structures are observed by electron microscopy in intact auxilin-depleted HeLa cells. Together, these findings indicate that the two auxilins have overlapping functions and that they not only facilitate the uncoating of CCVs but also prevent the formation of nonproductive clathrin cages in the cytosol.

Formation and function of Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are secretory organelles used for post-synthesis storage in endothelial cells that can, very rapidly, be triggered to release their contents. They carry a variety of bioactive molecules that are needed to mount a rapid response to the complex environment of cells that line blood vessels. They store factors that are essential to haemostasis and inflammation, as well as factors that modulate vascular tonicity and angiogenesis. The number of WPBs and their precise content vary between endothelial tissues, reflecting their differing physiological circumstances. The particular functional demands of the highly multimerised haemostatic protein von Willebrand Factor (VWF), which is stored in WPBs as tubules until release, are responsible for the cigar shape of these granules. How VWF tubules drive the formation of these uniquely shaped organelles, and how WPB density increases during maturation, has recently been revealed by EM analysis using high-pressure freezing and freeze substitution. In addition, an AP1/clathrin coat has been found to be essential to WPB formation. Following recruitment of cargo at the TGN, there is a second wave of recruitment that delivers integral and peripheral membrane proteins to WPBs, some of which is AP3 dependent.

SNX18 is an SNX9 paralog that acts as a membrane tubulator in AP-1-positive endosomal trafficking

SNX9, SNX18 and SNX30 constitute a separate subfamily of PX-BAR-containing sorting nexin (SNX) proteins. We show here that most tissues express all three paralogs, and immunoprecipitation and immunofluorescence experiments demonstrated that the SNX9-family proteins act as individual entities in cells. Their SH3 domains displayed a high selectivity for dynamin 2, and the PX-BAR units had the capacity to tubulate membranes when expressed in HeLa cells. As previously described for the PX-BAR domain of SNX9 (SNX9-PX-BAR), purified SNX18-PX-BAR caused liposome tubulation in vitro and had a binding preference for PtdIns(4,5)P(2). However, contrary to SNX9, which primarily acts in clathrin-mediated endocytosis at the plasma membrane, endogenous SNX18 localized to AP-1- and PACS1-positive endosomal structures, which were devoid of clathrin and resistant to Brefeldin A. Moreover, a gamma-adaptin recognition motif was defined in a low-complexity region of SNX18, and a complex of endogenous SNX18 and AP-1 could be immunoprecipitated after Brefeldin A treatment. Overexpression of SNX18 sequestered AP-1 from peripheral endosomes and resulted in the formation of short SNX18-decorated tubes with distinct dynamin puncta. The results indicate that SNX9-family members make up discrete membrane-scission units together with dynamin, and suggest that SNX18 mediates budding of carriers for AP-1-positive endosomal trafficking.

ENTH domain proteins are cargo adaptors for multiple SNARE proteins at the TGN endosome

ENTH and ANTH domain proteins are involved in budding of clathrin-coated vesicles. SNAREs are fusogenic proteins that function in the targeting and fusion of transport vesicles. In mammalian and yeast cells, ENTH domain proteins (epsinR and Ent3p) interact with SNAREs of the vti1 family (Vti1b or Vti1p). This interaction indicates that ENTH proteins could function in cargo sorting, which prompted us to search for additional SNAREs as potential cargo for Ent3p and epsinR. We carried out specific yeast two-hybrid assays, which identified interactions between epsinR and the mammalian late endosomal SNAREs syntaxin 7 and syntaxin 8 as well as between Ent3p and the endosomal SNAREs Pep12p and Syn8p from yeast. Lack of Ent3p affected the trafficking of Pep12p. Ent3p binding to Pep12p required the FSD late endosomal sorting signal in Pep12p. Inactivation of the sorting signal had a similar effect to removal of Ent3p on Pep12p stability indicating that Ent3p acts as a cargo adaptor for Pep12p by binding to the sorting signal. As Vti1p, Pep12p and Syn8p participate in a SNARE complex whereas Vti1b, syntaxin 7 and syntaxin 8 are mammalian SNARE partners, we propose that ENTH domain proteins at the TGN-endosome are cargo adaptors for these endosomal SNAREs.

The tyrosine binding pocket in the adaptor protein 1 (AP-1) mu1 subunit is necessary for Nef to recruit AP-1 to the major histocompatibility complex class I cytoplasmic tail

To evade the anti-human immunodeficiency virus (HIV) immune response, the HIV Nef protein disrupts major histocompatibility complex class I (MHC-I) trafficking by recruiting the clathrin adaptor protein 1 (AP-1) to the MHC-I cytoplasmic tail. Under normal conditions AP-1 binds dileucine and tyrosine signals (YXX phi motifs) via physically separate binding sites. In the case of the Nef-MHC-I complex, a tyrosine in the human leukocyte antigen (HLA)-A2 cytoplasmic tail ((320)YSQA) and a methionine in Nef (Met(20)) are absolutely required for AP-1 binding. Also present in Nef is a dileucine motif, which does not normally affect MHC-I trafficking and is not needed to recruit AP-1 to the Nef-MHC-I-complex. However, evidence is presented here that this dileucine motif can be activated by fusing Nef to the HLA-A2 tail in cis. Thus, the inability of this motif to function in trans likely results from a structural change that occurs when Nef binds to the MHC-I cytoplasmic tail. The physiologically relevant tyrosine-dependent recruitment of AP-1 to MHC-I, which occurs whether Nef is present in cis or trans, was stabilized by the acidic and polyproline domains within Nef. Additionally, amino acids Ala(324) and Asp(327) in the cytoplasmic tails of HLA-A and (but not HLA-C and HLA-E) molecules also stabilized AP-1 binding. Finally, mutation of the tyrosine binding pocket in the mu subunit of AP-1 created a dominant negative inhibitor of Nef-induced down-modulation of HLA-A2 that disrupted binding of wild type AP-1 to the Nef-MHC-I complex. Thus, these data provide evidence that Nef binding to the MHC-I cytoplasmic tail stabilizes the interaction of a tyrosine in the MHC-I cytoplasmic tail with the natural tyrosine binding pocket in AP-1.

Identification of a novel conserved sorting motif required for retromer-mediated endosome-to-TGN retrieval

The cation-independent mannose 6-phosphate receptor (CIMPR) cycles between the trans-Golgi network (TGN) and endosomes to mediate sorting of lysosomal hydrolases. The endosome-to-TGN retrieval of the CIMPR requires the retromer complex. Genetic, biochemical and structural data support the hypothesis that the retromer can directly bind to the tail of the CIMPR, to sort the CIMPR into vesicles and tubules for retrieval to the TGN. Presently, however, no known retromer sorting motif in the tail of the CIMPR has been identified. Using CD8-reporter proteins carrying the cytoplasmic tail of the CIMPR we have systematically dissected the CIMPR tail to identify a novel, conserved aromatic-containing sorting motif that is critical for the endosome-to-TGN retrieval of the CIMPR and for the interaction with retromer and the clathrin adaptor AP-1.

CVAK104 is a novel regulator of clathrin-mediated SNARE sorting

Clathrin-coated vesicles (CCVs) mediate transport between the plasma membrane, endosomes and the trans Golgi network. Using comparative proteomics, we have identified coated-vesicle-associated kinase of 104 kDa (CVAK104) as a candidate accessory protein for CCV-mediated trafficking. Here, we demonstrate that the protein colocalizes with clathrin and adaptor protein-1 (AP-1), and that it is associated with a transferrin-positive endosomal compartment. Consistent with these observations, clathrin as well as the cargo adaptors AP-1 and epsinR can be coimmunoprecipitated with CVAK104. Small interfering RNA (siRNA) knockdown of CVAK104 in HeLa cells results in selective loss of the SNARE proteins syntaxin 8 and vti1b from CCVs. Morpholino-mediated knockdown of CVAK104 in Xenopus tropicalis causes severe developmental defects, including a bent body axis and ventral oedema. Thus, CVAK104 is an evolutionarily conserved protein involved in SNARE sorting that is essential for normal embryonic development.

Arf GAPs and their interacting proteins

Membrane trafficking and remodeling of the actin cytoskeleton are critical activities contributing to cellular events that include cell growth, migration and tumor invasion. ADP-ribosylation factor (Arf)-directed GTPase activating proteins (GAPs) have crucial roles in these processes. The Arf GAPs function in part by regulating hydrolysis of GTP bound to Arf proteins. The Arf GAPs, which have multiple functional domains, also affect the actin cytoskeleton and membranes by specific interactions with lipids and proteins. A description of these interactions provides insights into the molecular mechanisms by which Arf GAPs regulate physiological and pathological cellular events. Here we describe the Arf GAP family and summarize the currently identified protein interactors in the context of known Arf GAP functions.

The trans-Golgi network accessory protein p56 promotes long-range movement of GGA/clathrin-containing transport carriers and lysosomal enzyme sorting

The sorting of acid hydrolase precursors at the trans-Golgi network (TGN) is mediated by binding to mannose 6-phosphate receptors (MPRs) and subsequent capture of the hydrolase-MPR complexes into clathrin-coated vesicles or transport carriers (TCs) destined for delivery to endosomes. This capture depends on the function of three monomeric clathrin adaptors named GGAs. The GGAs comprise a C-terminal "ear" domain that binds a specific set of accessory proteins. Herein we show that one of these accessory proteins, p56, colocalizes and physically interacts with the three GGAs at the TGN. Moreover, overexpression of the GGAs enhances the association of p56 with the TGN, and RNA interference (RNAi)-mediated depletion of the GGAs decreases the TGN association and total levels of p56. RNAi-mediated depletion of p56 or the GGAs causes various degrees of missorting of the precursor of the acid hydrolase, cathepsin D. In the case of p56 depletion, this missorting correlates with decreased mobility of GGA-containing TCs. Transfection with an RNAi-resistant p56 construct, but not with a p56 construct lacking the GGA-ear-interacting motif, restores the mobility of the TCs. We conclude that p56 tightly cooperates with the GGAs in the sorting of cathepsin D to lysosomes, probably by enabling the movement of GGA-containing TCs.

HIV-1 Nef-induced down-regulation of MHC class I requires AP-1 and clathrin but not PACS-1 and is impeded by AP-2

Major histocompatibility complex class I is down-regulated from the surface of human immunodeficiency virus (HIV)-1-infected cells by Nef, a virally encoded protein that is thought to reroute MHC-I to the trans-Golgi network (TGN) in a phosphofurin acidic cluster sorting protein (PACS) 1, adaptor protein (AP)-1, and clathrin-dependent manner. More recently, an alternative model has been proposed, in which Nef uses AP-1 to direct MHC-I to endosomes and lysosomes. Here, we show that knocking down either AP-1 or clathrin with small interfering RNA inhibits the down-regulation of HLA-A2 (an MHC-I isotype) by Nef in HeLa cells. However, knocking down PACS-1 has no effect, not only on Nef-induced down-regulation of HLA-A2 but also on the localization of other proteins containing acidic cluster motifs. Surprisingly, knocking down AP-2 actually enhances Nef activity. Immuno-electron microscopy labeling of Nef-expressing cells indicates that HLA-A2 is rerouted not to the TGN, but to endosomes. In AP-2-depleted cells, more of the HLA-A2 localizes to the inner vesicles of multivesicular bodies. We propose that depleting AP-2 potentiates Nef activity by altering the membrane composition and dynamics of endosomes and causing increased delivery of HLA-A2 to a prelysosomal compartment.

The mast cell: where endocytosis and regulated exocytosis meet

We have investigated whether Ca(2+)-binding proteins, which have been implicated in the control of neurons and neuroendocrine secretion, play a role in controlling mast cell function. These studies have identified synaptotagmins (Syts) II, III, and IX as well as neuronal Ca(2+) sensor 1 (NCS-1) as important regulators of mast cell function. Strikingly, we find that these Ca(2+)-binding proteins contribute to mast cell function by regulating specific endocytic pathways. Syt II, the most abundant Syt homologue in mast cells, resides in an amine-free lysosomal compartment. Studying the function of Syt II-knocked down rat basophilic leukemia cells has shown a dual function of this homologue. Syt II is required for the downregulation of protein kinase Calpha, but it negatively regulates lysosomal exocytosis. Syt III, the next most abundant homologue, localizes to early endosomes and is required for the formation of the endocytic recycling compartment (ERC). Syt IX and NCS-1 localize to the ERC and regulate ERC export, NCS-1 by activating phosphatidylinositol 4-kinase beta. Finally, we show that recycling through the ERC is needed for secretory granule protein sorting as well as for the activation of the mitogen-activated protein kinases, extracellular signal-regulated kinase 1 and 2. Accordingly, NCS-1 stimulates Fc epsilon RI-triggered exocytosis and release of arachidonic acid metabolites.

Canonical interaction of cyclin G associated kinase with adaptor protein 1 regulates lysosomal enzyme sorting

The adaptor protein 1 (AP1) complex is a heterotetramer that participates in cargo sorting into clathrin-coated vesicles at the trans-Golgi network (TGN) and endosomes. The gamma subunit of AP1 possesses a C-terminal "ear" domain that recruits a cohort of accessory proteins through recognition of a shared canonical motif, PsiG[PDE][PsiLM] (where Psi is an aromatic residue). The physiological relevance of these ear-motif interactions, however, remains to be demonstrated. Here we report that the cyclin G-associated kinase (GAK) has two sequences fitting this motif, FGPL and FGEF, which mediate binding to the AP1-gamma-ear domain in vitro. Mutation of both gamma-ear-binding sequences or depletion of AP1-gamma by RNA interference (RNAi) decreases the association of GAK with the TGN in vivo. Depletion of GAK by RNAi impairs the sorting of the acid hydrolase, cathepsin D, to lysosomes. Importantly, expression of RNAi-resistant GAK restores the lysosomal sorting of cathepsin D in cells depleted of endogenous GAK, whereas expression of a similar construct bearing mutations in both gamma-ear-binding sequences fails to correct the sorting defect. Thus, interactions between the PsiG[PDE][PsiLM]-motif sequences in GAK and the AP1-gamma-ear domain are critical for the recruitment of GAK to the TGN and the function of GAK in lysosomal enzyme sorting.

Molecular Basis for Autoregulatory Interaction Between GAE Domain and Hinge Region of GGA1

Golgi-localizing, gamma-adaptin ear domain homology, ADP ribosylation factor-binding (GGA) proteins and the adaptor protein (AP) complex, AP-1, are involved in membrane traffic between the trans Golgi network and the endosomes. The gamma-adaptin ear (GAE) domain of GGAs and the gamma1 ear domain of AP-1 interact with an acidic phenylalanine motif found in accessory proteins. The GAE domain of GGA1 (GGA1-GAE) interacts with a WNSF-containing peptide derived from its own hinge region, although the peptide sequence deviates from the standard acidic phenylalanine motif. We report here the structure of GGA1-GAE in complex with the GGA1 hinge peptide, which revealed that the two aromatic side chains of the WNSF sequence fit into a hydrophobic groove formed by aliphatic portions of the side chains of conserved arginine and lysine residues of GGA1-GAE, in a similar manner to the interaction between GGA-GAEs and acidic phenylalanine sequences from the accessory proteins. Fluorescence quenching experiments indicate that the GGA1 hinge region binds to GGA1-GAE and competes with accessory proteins for binding. Taken together with the previous observation that gamma1 ear binds to the GGA1 hinge region, the interaction between the hinge region and the GAE domain underlies the autoregulation of GGA function in clathrin-mediated trafficking through competing with the accessory proteins and the AP-1 complex.

PI4P promotes the recruitment of the GGA adaptor proteins to the trans-Golgi network and regulates their recognition of the ubiquitin sorting signal

Phosphatidylinositol 4 phosphate (PI4P) is highly enriched in the trans-Golgi network (TGN). Here we establish that PI4P is a key regulator of the recruitment of the GGA clathrin adaptor proteins to the TGN and that PI4P has a novel role in promoting their recognition of the ubiquitin (Ub) sorting signal. Knockdown of PI4KIIalpha by RNA interference (RNAi), which depletes the TGN's PI4P, impaired the recruitment of the GGAs to the TGN. GGAs bind PI4P primarily through their GAT domain, in a region called C-GAT, which also binds Ub but not Arf1. We identified two basic residues in the GAT domain that are essential for PI4P binding in vitro and for the recruitment of GGAs to the TGN in vivo. Unlike wild-type GGA, GGA with mutated GATs failed to rescue the abnormal TGN phenotype of the GGA RNAi-depleted cells. These residues partially overlap with those that bind Ub, and PI4P increased the affinity of the GAT domain for Ub. Because the recruitment of clathrin adaptors and their cargoes to the TGN is mediated through a web of low-affinity interactions, our results show that the dual roles of PI4P can promote specific GGA targeting and cargo recognition at the TGN.

EpsinR2 interacts with clathrin, adaptor protein-3, AtVTI12, and phosphatidylinositol-3-phosphate. Implications for EpsinR2 function in protein trafficking in plant cells

Members of the epsin family of proteins (epsins) are characterized by the presence of an epsin N-terminal homology (ENTH) domain. Epsins have been implicated in various protein-trafficking pathways in animal and yeast (Saccharomyces cerevisiae) cells. Plant cells also contain multiple epsin-related proteins. In Arabidopsis (Arabidopsis thaliana), EPSIN1 is involved in vacuolar trafficking of soluble proteins. In this study, we investigated the role of Arabidopsis EpsinR2 in protein trafficking in plant cells. EpsinR2 contains a highly conserved ENTH domain but a fairly divergent C-terminal sequence. We found that the N-terminal ENTH domain specifically binds to phosphatidylinositol-3-P in vitro and has a critical role in the targeting of EpsinR2. Upon transient expression in protoplasts, hemagglutinin epitope-tagged EpsinR2 was translocated primarily to a novel cellular compartment, while a minor portion localized to the Golgi complex. Protein-binding experiments showed that EpsinR2 interacts with clathrin, AtVTI12, and the Arabidopsis homologs of adaptor protein-3 delta-adaptin and adaptor protein-2 alpha-adaptin. Localization experiments revealed that hemagglutinin epitope-tagged EpsinR2 colocalizes primarily with delta-adaptin and partially colocalizes with clathrin and AtVTI12. Based on these findings, we propose that EpsinR2 plays an important role in protein trafficking through interactions with delta-adaptin, AtVTI12, clathrin, and phosphatidylinositol-3-P.

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.