A role for epsin N-terminal homology/AP180 N-terminal homology (ENTH/ANTH) domains in tubulin binding

Pseudokinase NRP1 facilitates endocytosis of transferrin in the African trypanosome

Trypanosoma brucei causes human African trypanosomiasis (HAT) and nagana in cattle. During infection of a vertebrate, endocytosis of host transferrin (Tf) is important for viability of the parasite. The majority of proteins involved in trypanosome endocytosis of Tf are unknown. Here we identify pseudokinase NRP1 (Tb427tmp.160.4770) as a regulator of Tf endocytosis. Genetic knockdown of NRP1 inhibited endocytosis of Tf without blocking uptake of bovine serum albumin. Binding of Tf to the flagellar pocket was not affected by knockdown of NRP1. However the quantity of Tf per endosome dropped significantly, consistent with NRP1 promoting robust capture and/or retention of Tf in vesicles. NRP1 is involved in motility of Tf-laden vesicles since distances between endosomes and the kinetoplast were reduced after knockdown of the gene. In search of possible mediators of NRP1 modulation of Tf endocytosis, the gene was knocked down and the phosphoproteome analyzed. Phosphorylation of protein kinases forkhead, NEK6, and MAPK10 was altered, in addition to EpsinR, synaptobrevin and other vesicle-associated proteins predicted to be involved in endocytosis. These candidate proteins may link NRP1 functionally either to protein kinases or to vesicle-associated proteins.

Topological Adaptation of Transmembrane Domains to the Force-Modulated Lipid Bilayer Is a Basis of Sensing Mechanical Force

Cells can sense and respond to various mechanical stimuli from their surrounding environment. One of the explanations for mechanosensitivity, a lipid-bilayer model, suggests that a stretch of the membrane induced by mechanical force alters the physical state of the lipid bilayer, driving mechanosensors to assume conformations better matched to the altered membrane. However, mechanosensors of this class are restricted to ion channels. Here, we reveal that integrin αIIbβ3, a prototypic adhesion receptor, can be activated by various mechanical stimuli including stretch, shear stress, and osmotic pressure. The force-induced integrin activation was not dependent on its known intracellular activation signaling events and was even observed in reconstituted cell-free liposomes. Instead, these mechanical stimuli were found to alter the lipid embedding of the integrin β3 transmembrane domain (TMD) and subsequently weaken the αIIb-β3 TMD interaction, which results in activation of the receptor. Moreover, artificial modulation of the membrane curvature near integrin αIIbβ3 can induce its activation in cells as well as in lipid nanodiscs, suggesting that physical deformation of the lipid bilayer, either by mechanical force or curvature, can induce integrin activation. Thus, our results establish the adhesion receptor as a bona fide mechanosensor that directly senses and responds to the force-modulated lipid environment. Furthermore, this study expands the lipid-bilayer model by suggesting that the force-induced topological change of TMDs and subsequent alteration in the TMD interactome is a molecular basis of sensing mechanical force transmitted via the lipid bilayer.

Unusual proteins in Giardia duodenalis and their role in survival

The capacity of the parasite Giardia duodenalis to perform complex functions with minimal amounts of proteins and organelles has attracted increasing numbers of scientists worldwide, trying to explain how this parasite adapts to internal and external changes to survive. One explanation could be that G. duodenalis evolved from a structurally complex ancestor by reductive evolution, resulting in adaptation to its parasitic lifestyle. Reductive evolution involves the loss of genes, organelles, and functions that commonly occur in many parasites, by which the host renders some structures and functions redundant. However, there is increasing data that Giardia possesses proteins able to perform more than one function. During recent decades, the concept of moonlighting was described for multitasking proteins, which involves only proteins with an extra independent function(s). In this chapter, we provide an overview of unusual proteins in Giardia that present multifunctional properties depending on the location and/or parasite requirement. We also discuss experimental evidence that may allow some giardial proteins to be classified as moonlighting proteins by examining the properties of moonlighting proteins in general. Up to date, Giardia does not seem to require the numerous redundant proteins present in other organisms to accomplish its normal functions, and thus this parasite may be an appropriate model for understanding different aspects of moonlighting proteins, which may be helpful in the design of drug targets.

Efficient Golgi Forward Trafficking Requires GOLPH3-Driven, PI4P-Dependent Membrane Curvature

Vesicle budding for Golgi-to-plasma membrane trafficking is a key step in secretion. Proteins that induce curvature of the Golgi membrane are predicted to be required, by analogy to vesicle budding from other membranes. Here, we demonstrate that GOLPH3, upon binding to the phosphoinositide PI4P, induces curvature of synthetic membranes in vitro and the Golgi in cells. Moreover, efficient Golgi-to-plasma membrane trafficking critically depends on the ability of GOLPH3 to curve the Golgi membrane. Interestingly, uncoupling of GOLPH3 from its binding partner MYO18A results in extensive curvature of Golgi membranes, producing dramatic tubulation of the Golgi, but does not support forward trafficking. Thus, forward trafficking from the Golgi to the plasma membrane requires the ability of GOLPH3 both to induce Golgi membrane curvature and to recruit MYO18A. These data provide fundamental insight into the mechanism of Golgi trafficking and into the function of the unique Golgi secretory oncoproteins GOLPH3 and MYO18A.

Transcriptional analysis of apoptotic cerebellar granule neurons following rescue by gastric inhibitory polypeptide

Apoptosis triggered by exogenous or endogenous stimuli is a crucial phenomenon to determine the fate of neurons, both in physiological and in pathological conditions. Our previous study established that gastric inhibitory polypeptide (Gip) is a neurotrophic factor capable of preventing apoptosis of cerebellar granule neurons (CGNs), during its pre-commitment phase. In the present study, we conducted whole-genome expression profiling to obtain a comprehensive view of the transcriptional program underlying the rescue effect of Gip in CGNs. By using DNA microarray technology, we identified 65 genes, we named survival related genes, whose expression is significantly de-regulated following Gip treatment. The expression levels of six transcripts were confirmed by real-time quantitative polymerase chain reaction. The proteins encoded by the survival related genes are functionally grouped in the following categories: signal transduction, transcription, cell cycle, chromatin remodeling, cell death, antioxidant activity, ubiquitination, metabolism and cytoskeletal organization. Our data outline that Gip supports CGNs rescue via a molecular framework, orchestrated by a wide spectrum of gene actors, which propagate survival signals and support neuronal viability.

Phosphorylation by Dyrk1A of clathrin coated vesicle-associated proteins: identification of the substrate proteins and the effects of phosphorylation

Dyrk1A phosphorylated multiple proteins in the clathrin-coated vesicle (CCV) preparations obtained from rat brains. Mass spectrometric analysis identified MAP1A, MAP2, AP180, and α- and β-adaptins as the phosphorylated proteins in the CCVs. Each protein was subsequently confirmed by [(32)P]-labeling and immunological methods. The Dyrk1A-mediated phosphorylation released the majority of MAP1A and MAP2 and enhanced the release of AP180 and adaptin subunits from the CCVs. Furthermore, Dyrk1A displaced adaptor proteins physically from CCVs in a kinase-concentration dependent manner. The clathrin heavy chain release rate, in contrast, was not affected by Dyrk1A. Surprisingly, the Dyrk1A-mediated phosphorylation of α- and β-adaptins led to dissociation of the AP2 complex, and released only β-adaptin from the CCVs. AP180 was phosphorylated by Dyrk1A also in the membrane-free fractions, but α- and β-adaptins were not. Dyrk1A was detected in the isolated CCVs and was co-localized with clathrin in neurons from mouse brain sections and from primary cultured rat hippocampus. Previously, we proposed that Dyrk1A inhibits the onset of clathrin-mediated endocytosis in neurons by phosphorylating dynamin 1, amphiphysin 1, and synaptojanin 1. Current results suggest that besides the inhibition, Dyrk1A promotes the uncoating process of endocytosed CCVs.

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.

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.

Mitotic moonlighting functions for membrane trafficking proteins

In recent years, cell biologists have uncovered a number of new functions for proteins that were previously thought to operate solely in membrane trafficking. These alternative roles, termed moonlighting functions, can occur at distinct intracellular sites or at different stages of the cell cycle. Here, I evaluate the evidence for mitotic moonlighting functions of proteins that have membrane trafficking roles during interphase. The aim is to identify key issues facing the field and to outline important questions for future work.

A requirement for epsin in mitotic membrane and spindle organization

Eukaryotic cells possess a sophisticated membrane system to facilitate diverse functions. Whereas much is known about the nature of membrane systems in interphase, the organization and function of the mitotic membrane system are less well understood. In this study, we show that epsin, an endocytic adapter protein, regulates mitotic membrane morphology and spindle integrity in HeLa cells. Using epsin that harbors point mutations in the epsin NH2-terminal homology domain and spindle assembly assays in Xenopus laevis egg extracts, we show that epsin-induced membrane curvature is required for proper spindle morphogenesis, independent of its function in endocytosis during interphase. Although several other membrane-interacting proteins, including clathrin, AP2, autosomal recessive hypercholesterolemia, and GRASP65, are implicated in the regulation of mitosis, whether they participate through regulation of membrane organization is unclear. Our study of epsin provides evidence that mitotic membrane organization influences spindle integrity.

SGIP1α Is an Endocytic Protein That Directly Interacts with Phospholipids and Eps15

SGIP1 has been shown to be an endophilin-interacting protein that regulates energy balance, but its function is not fully understood. Here, we identified its splicing variant of SGIP1 and named it SGIP1alpha. SGIP1alpha bound to phosphatidylserine and phosphoinositides and deformed the plasma membrane and liposomes into narrow tubules, suggesting the involvement in vesicle formation during endocytosis. SGIP1alpha furthermore bound to Eps15, an important adaptor protein of clathrin-mediated endocytic machinery. SGIP1alpha was colocalized with Eps15 and the AP-2 complex. Upon epidermal growth factor (EGF) stimulation, SGIP1alpha was colocalized with EGF at the plasma membrane, indicating the localization of SGIP1alpha at clathrin-coated pits/vesicles. SGIP1alpha overexpression reduced transferrin and EGF endocytosis. SGIP1alpha knockdown reduced transferrin endocytosis but not EGF endocytosis; this difference may be due to the presence of redundant pathways in EGF endocytosis. These results suggest that SGIP1alpha plays an essential role in clathrin-mediated endocytosis by interacting with phospholipids and Eps15.

Phosphoinositide 3-kinase C2alpha links clathrin to microtubule-dependent movement

Phosphoinositide 3-kinase C2alpha (PI3K-C2alpha) is a type II PI-3-kinase that has been implicated in several important membrane transport and signaling processes. We previously found that overexpression of PI3K-C2alpha inhibits clathrin-mediated membrane trafficking and induces proliferation of novel clathrin-coated structures within the cytoplasm. Using fluorescently tagged fusions of PI3K-C2alpha and clathrin, we explored the behavior of these structures in intact cells. Both proteins are present in the structures, and using rapid image acquisition and fluorescence photoactivation probes, we find that they exhibit localized, rapid mobility (5-20 microm/s). The movement is micro-tubule-based as revealed by use of inhibitors, and PI3K-C2alpha accumulates on microtubules rapidly and reversibly following cytoplasmic acidification, which also blocks movement. Dynactin mediates the movement of these clathrin-PI3K-C2alpha structures, since disruption of dynactin function by overexpression of its p50 subunit also inhibits movement. Finally, immunoprecipitation experiments reveal an interaction between endogenous PI3K-C2alpha and dynactin subunits. Together, these results reveal a molecular linkage between PI3K-C2alpha and the microtubule motor machinery, with implications for membrane trafficking in intact cells.

Membrane traffic: catching the lysosome express

Recent work shows that specific adaptor interactions may sort receptors into distinct cohorts of clathrin-coated vesicles. These different vesicle populations deliver cargo to sorting endosomes of differing motilities and maturation rates, thereby determining the probability of receptor recycling and the duration of endosomal signaling.

Ligands for clathrin-mediated endocytosis are differentially sorted into distinct populations of early endosomes

Cells rely on the correct sorting of endocytic ligands and receptors for proper function. Early endosomes have been considered as the initial sorting station where cargos for degradation separate from those for recycling. Using live-cell imaging to monitor individual endosomes and ligand particles in real time, we have discovered a sorting mechanism that takes place prior to early endosome entry. We show that early endosomes are in fact comprised of two distinct populations: a dynamic population that is highly mobile on microtubules and matures rapidly toward late endosomes and a static population that matures much more slowly. Several cargos destined for degradation are preferentially targeted to the dynamic endosomes, whereas the recycling ligand transferrin is nonselectively delivered to all early endosomes and effectively enriched in the larger, static population. This pre-early endosome sorting process begins at clathrin-coated vesicles, depends on microtubule-dependent motility, and appears to involve endocytic adaptors.

Clathrin light chain b is capable of affecting potently a major protein phosphatase from microtubules (MT-PP1)

Clathrin light chain (CL) b purified from bovine brain postmicrotubule supernatant and identified by mass spectrometry potently inhibited a catalytic activity of a major protein phosphatase (PP) that was copurified with microtubules and recognized by antiPP1 antibodies. CLb similarly affected the catalytic subunit and holoenzyme of the PP, little inhibiting the activity of PP2A. Although the CLb from clathrin-coated vesicles was several hundredfold weaker than our purified CLb, the CLb in the postmicrotubule supernatant, independent of whether it was sedimentable or soluble, was as active as the purified CLb. Thus CLb may be a potent regulator of the PP.

Structural abnormalities in spermatids together with reduced sperm counts and motility underlie the reproductive defect in HIP1−/− mice

Huntingtin interacting protein 1 (HIP1) is an endocytic adaptor protein with clathrin assembly activity that binds to cytoplasmic proteins, such as F-actin, tubulin, and huntingtin (htt). To gain insight into diverse functions of HIP1, we characterized the male reproductive defect of HIP1(-/-) mice from 7 to 30 weeks of age. High levels of HIP1 protein were expressed in the testis of wild-type mice as seen by Western blots and as a reaction over Sertoli cells and elongating spermatids as visualized by immunocytochemistry. Accordingly, major structural abnormalities were evident in HIP1(-/-) mice with vacuolation of seminiferous tubules caused by an apparent loss of postmeiotic spermatids and a significant reduction in mean profile area. Remaining spermatids revealed deformations of their heads, flagella, and/or acrosomes. In some Sertoli cells, ectoplasmic specializations (ES) were absent or altered in appearance accounting for the presence of spherical germ cells in the epididymal lumen. Quantitative analyses of sperm counts from the cauda epididymidis demonstrated a significant decrease in HIP1(-/-) mice compared to wild-type littermates. In addition, computer-assisted sperm analyses indicated that velocities, amplitude of lateral head displacements (ALH), and numbers and percentages of sperm in the motile, rapid, and progressive categories were all significantly reduced in HIP1(-/-) mice, while the numbers and percentages of sperm in the static category were greatly increased. Taken together, these various abnormalities corroborate reduced fertility levels in HIP1(-/-) mice and suggest a role for HIP1 in stabilizing actin and microtubules, which are important cytoskeletal elements enabling normal spermatid and Sertoli cell morphology and function.

Sequence analysis of Arabidopsis thaliana E/ANTH-domain-containing proteins: membrane tethers of the clathrin-dependent vesicle budding machinery

The epsin N-terminal homology (ENTH) domain is a conserved protein module present in cytosolic proteins which are required in clathrin-mediated vesicle budding processes. A highly similar, yet unique module is the AP180 N-terminal homology (ANTH) domain, which is present in a set of proteins that also support clathrin-dependent endocytosis. Both ENTH and ANTH (E/ANTH) domains bind to phospholipids and proteins, in order to support the nucleation of clathrin coats on the plasma membrane or the trans-Golgi-network membrane. Therefore, E/ANTH proteins might be considered as universal tethering components of the clathrin-mediated vesicle budding machinery. Since the E/ANTH protein family appears to be crucial in the first steps of clathrin-coated vesicle budding, we performed data base searches of the Arabidopsis thaliana genome. Sequence analysis revealed three proteins containing the ENTH signature motif and eight proteins containing the ANTH signature motif. Another six proteins were found that do not contain either motif but seem to have the same domain structure and might therefore be seen as VHS-domain-containing plant proteins. Functional analysis of plant E/ANTH proteins are rather scarce, since only one ANTH homolog from A. thaliana, At-AP180, has been characterized so far. At-AP180 displays conserved functions as a clathrin assembly protein and as an alpha-adaptin binding partner, and in addition shows features at the molecular level that seem to be plant-specific.

Doublecortin interacts with the ubiquitin protease DFFRX, which associates with microtubules in neuronal processes

Doublecortin (DCX) is a microtubule-associated protein involved in neuronal migration, which causes X-linked lissencephaly and subcortical laminar heterotopia (SCLH) when mutated. Here we show that DCX interacts with the ubiquitin-specific protease Drosophila fat facets related on X chromosome (DFFRX). This interaction was confirmed by targeted mutagenesis, colocalization, and immunoprecipitation studies. DFFRX is thought to deubiquitinate specific substrates including beta-catenin, preventing their degradation by the proteasome. Interestingly, unlike beta-catenin, no ubiquitinated forms of DCX could be detected, and indeed we show that DCX interacts with a novel recognition domain in DFFRX, located outside of its catalytic site. We also show that DFFRX associates with microtubules at specific subcellular compartments, including those enriched in DCX. These results thus suggest that in addition to vesicular trafficking, DCX may play a role in the regulation of cell adhesion via its interaction with DFFRX in migrating and differentiating neurons.

Interaction of Sla2p's ANTH domain with PtdIns(4,5)P2 is important for actin-dependent endocytic internalization

A variety of studies have implicated the lipid PtdIns(4,5)P2 in endocytic internalization, but how this lipid mediates its effects is not known. The AP180 N-terminal homology (ANTH) domain is a PtdIns(4,5)P2-binding module found in several proteins that participate in receptor-mediated endocytosis. One such protein is yeast Sla2p, a highly conserved actin-binding protein essential for actin organization and endocytic internalization. To better understand how PtdIns(4,5)P2 binding regulates actin-dependent endocytosis, we investigated the functions of Sla2p's ANTH domain. A liposome-binding assay revealed that Sla2p binds to PtdIns(4,5)P2 specifically through its ANTH domain and identified specific lysine residues required for this interaction. Mutants of Sla2p deficient in PtdIns(4,5)P2 binding showed significant defects in cell growth, actin organization, and endocytic internalization. These defects could be rescued by increasing PtdIns(4,5)P2 levels in vivo. Strikingly, mutant Sla2p defective in PtdIns(4,5)P2 binding localized with the endocytic machinery at the cell cortex, establishing that the ANTH-PtdIns(4,5)P2 interaction is not necessary for this association. In contrast, multicolor real-time fluorescence microscopy and particle-tracking analysis demonstrated that PtdIns(4,5)P2 binding is required during endocytic internalization. These results demonstrate that the interaction of Sla2p's ANTH domain with PtdIns(4,5)P2 plays a key role in regulation of the dynamics of actin-dependent endocytic internalization.

Tandem MS analysis of brain clathrin-coated vesicles reveals their critical involvement in synaptic vesicle recycling

Tandem MS has identified 209 proteins of clathrin-coated vesicles (CCVs) isolated from rat brain. An overwhelming abundance of peptides were assigned to the clathrin coat with a 1:1 stoichiometry observed for clathrin heavy and light chains and a 2:1 stoichiometry of clathrin heavy chain with clathrin adaptor protein heterotetramers. Thirty-two proteins representing many of the known components of synaptic vesicles (SVs) were identified, supporting that a main function for brain CCVs is to recapture SVs after exocytosis. A ratio of vesicle-N-ethylmaleimide-sensitive factor attachment protein receptors to target-N-ethylmaleimide-sensitive factor attachment protein receptors, similar to that previously detected on SVs, supports a single-step model for SV sorting during CCV-mediated recycling of SVs. The uncovering of eight previously undescribed proteins, four of which have to date been linked to clathrin-mediated trafficking, further attests to the value of the current organelle-based proteomics strategy.

Specific Interaction between SNAREs and Epsin N-terminal Homology (ENTH) Domains of Epsin-related Proteins in trans-Golgi Network to Endosome Transport

SNARE proteins on transport vesicles and target membranes have important roles in vesicle targeting and fusion. Therefore, localization and activity of SNAREs have to be tightly controlled. Regulatory proteins bind to N-terminal domains of some SNAREs. vti1b is a mammalian SNARE that functions in late endosomal fusion. To investigate the role of the N terminus of vti1b we performed a yeast two-hybrid screen. The N terminus of vti1b interacted specifically with the epsin N-terminal homology (ENTH) domain of enthoprotin/CLINT/epsinR. The interaction was confirmed using in vitro binding assays. This complex formation between a SNARE and an ENTH domain was conserved between mammals and yeast. Yeast Vti1p interacted with the ENTH domain of Ent3p. ENTH proteins are involved in the formation of clathrin-coated vesicles. Both epsinR and Ent3p bind adaptor proteins at the trans-Golgi network. Vti1p is required for multiple transport steps in the endosomal system. Genetic interactions between VTI1 and ENT3 were investigated. Synthetic defects suggested that Vti1p and Ent3p cooperate in transport from the trans-Golgi network to the prevacuolar endosome. Our experiments identified the first cytoplasmic protein binding to specific ENTH domains. These results point toward a novel function of the ENTH domain and a connection between proteins that function either in vesicle formation or in vesicle fusion.

HIP1 and HIP1r stabilize receptor tyrosine kinases and bind 3-phosphoinositides via epsin N-terminal homology domains

Huntingtin-interacting protein 1-related (HIP1r) is the only known mammalian relative of huntingtin-interacting protein 1 (HIP1), a protein that transforms fibroblasts via undefined mechanisms. Here we demonstrate that both HIP1r and HIP1 bind inositol lipids via their epsin N-terminal homology (ENTH) domains. In contrast to other ENTH domain-containing proteins, lipid binding is preferential to the 3-phosphate-containing inositol lipids, phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,5-bisphosphate. Furthermore, the HIP1r ENTH domain, like that of HIP1, is necessary for lipid binding, and expression of an ENTH domain-deletion mutant, HIP1r/deltaE, induces apoptosis. Consistent with the ability of HIP1r and HIP1 to affect cell survival, full-length HIP1 and HIP1r stabilize pools of growth factor receptors by prolonging their half-life following ligand-induced endocytosis. Although HIP1r and HIP1 display only a partially overlapping pattern of protein interactions, these data suggest that both proteins share a functional homology by binding 3-phosphorylated inositol lipids and stabilizing receptor tyrosine kinases in a fashion that may contribute to their ability to alter cell growth and survival.

ENTH/ANTH proteins and clathrin-mediated membrane budding

The epsin N-terminal homology (ENTH) domain is an evolutionarily conserved protein module found primarily in proteins that participate in clathrin-mediated endocytosis. Structural analyses and ligand-binding studies have shown that a set of proteins previously designated as harboring an ENTH domain in fact contain a highly similar, yet unique module referred to as an AP180 N-terminal homology (ANTH) domain. ENTH and ANTH (E/ANTH) domains bind both inositol phospholipids and proteins and contribute to the nucleation and formation of clathrin coats on membranes. ENTH domains also function in the development of membrane curvature through lipid remodeling during the formation of clathrin-coated vesicles. E/ANTH-bearing proteins have recently been shown to function with adaptor protein-1 and GGA adaptors at the trans-Golgi network, which suggests that E/ANTH domains are universal components of the machinery for clathrin-mediated membrane budding.

Characterization of a γ-adaptin ear-binding motif in enthoprotin

Enthoprotin, a newly identified component of clathrin-coated vesicles, interacts with the trans-Golgi network (TGN) clathrin adapters AP-1 and GGA2. Here we perform a multi-faceted analysis of the site in enthoprotin that is responsible for the binding to the gamma-adaptin ear (gamma-ear) domain of AP-1. Alanine scan mutagenesis and nuclear magnetic resonance (NMR) studies reveal the full extent of the site as well as critical residues for this interaction. NMR studies of the gamma-ear in complex with a synthetic peptide from enthoprotin provide structural details of the binding site for TGN accessory proteins within the gamma-ear.

Disruption of the endocytic protein HIP1 results in neurological deficits and decreased AMPA receptor trafficking

Huntingtin interacting protein 1 (HIP1) is a recently identified component of clathrin-coated vesicles that plays a role in clathrin-mediated endocytosis. To explore the normal function of HIP1 in vivo, we created mice with targeted mutation in the HIP1 gene (HIP1(-/-)). HIP1(-/-) mice develop a neurological phenotype by 3 months of age manifest with a failure to thrive, tremor and a gait ataxia secondary to a rigid thoracolumbar kyphosis accompanied by decreased assembly of endocytic protein complexes on liposomal membranes. In primary hippocampal neurons, HIP1 colocalizes with GluR1-containing AMPA receptors and becomes concentrated in cell bodies following AMPA stimulation. Moreover, a profound dose-dependent defect in clathrin-mediated internalization of GluR1-containing AMPA receptors was observed in neurons from HIP1(-/-) mice. Together, these data provide strong evidence that HIP1 regulates AMPA receptor trafficking in the central nervous system through its function in clathrin-mediated endocytosis.