Intersectin, a novel adaptor protein with two Eps15 homology and five Src homology 3 domains

The p21 Rho-activating toxin cytotoxic necrotizing factor 1 is endocytosed by a clathrin-independent mechanism and enters the cytosol by an acidic-dependent membrane translocation step

Cytotoxic necrotizing factor 1 (CNF1), a protein produced by pathogenic strains of Escherichia coli, activates the p21 Rho-GTP-binding protein, inducing a profound reorganization of the actin cytoskeleton. CNF1 binds to its cell surface receptor on HEp-2 cells with high affinity (K(d) = 20 pM). In HEp-2 cells the action of CNF1 is not blocked in the presence of filipin, a drug described to reduce cholera toxin internalization by the caveolae-like mechanism. Moreover, HEp-2 cells, which express a dominant negative form of proteins that impair the formation of clathrin coated-vesicles and internalization of transferrin (Eps15, dynamin or intersectin-Src homology 3), are still sensitive to CNF1. In this respect, the endocytosis of CNF1 is similar to the plant toxin ricin. However, unlike ricin toxin, CNF1 does not cross the Golgi apparatus and requires an acidic cell compartment to transfer its enzymatic activity into the cytosol in a manner similar to that required by diphtheria toxin. As shown for diphtheria toxin, the pH-dependent membrane translocation step of CNF1 could be mimicked at the level of the plasma membrane by a brief exposure to a pH of </=5.2. CNF1 is the first bacterial toxin described that uses both a clathrin-independent endocytic mechanism and an acidic-dependent membrane translocation step in its delivery of the catalytic domain to the cell cytosol.

Solution structure of Eps15's third EH domain reveals coincident Phe-Trp and Asn-Pro-Phe binding sites

Eps15 homology (EH) domains interact with proteins involved in endocytosis and signal transduction. EH domains bind to Asn-Pro-Phe (NPF) consensus motifs of target proteins. A few EH domains, such as the third EH domain (EH(3)) of human Eps15, prefer to bind Phe-Trp (FW) sequences. The structure of EH(3) has been solved by nuclear magnetic resonance (NMR) spectroscopy and is the first of an FW- and NPF-binding EH domain. Both FW and NPF sequences bind in the same hydrophobic pocket as shown by heteronuclear chemical shift mapping. EH(3) contains the dual EF-hand fold characteristic of the EH domain family, but it binds calcium with high affinity in the first EF-hand rather than the usual coordination in the second EF-hand. Point mutations were designed based on differences in the EH(3) and the second EH domain (EH(2)) of human Eps15 that alter the affinity of the domains for FW or NPF motif peptides. Peptides that mimic binding sites in the potential EH(3) targets Rab, synaptojanin, and the cation-dependent mannose 6-phosphate receptor were used to explore wild-type and mutant affinities. Characterization of the structure and binding properties of an FW- and NPF-binding EH domain and comparison to an NPF-specific EH domain provide important insights into the mechanisms of EH domain ligand recognition.

Application of Alu-splice PCR on chromosome 21: DSCR1 and Intersectin

Down syndrome (DS) is a major cause of mental retardation and congenital heart defects, with an overall incidence of one in 700 live births. DS is caused by increases in the amounts of a number of normal gene products, the exact number and identity of which are presently unknown. Elucidating the molecular basis of DS relies on the identification of the gene products whose augmentation by 50% or more causes symptoms of the disease. With the aim of contributing to the transcriptional map of human chromosome 21 and to identify new genes with potential involvement in DS, we developed a technique to isolate expressed sequences called Alu-splice PCR, which is very simple to perform and is independent of gene expression patterns. Putative exons are PCR amplified in genomic DNA by virtue of their proximity to Alu repeats using primers designed from splice-site consensus sequences in combination with specific Alu repeat primers. The Alu repeats, which are repetitive DNA elements found exclusively and at high frequency in the genomes of primates, impart the human specificity to the method. The splice-site consensus sequences were used to direct primers to exon boundaries. Using the Alu-splice technique, we have identified at least three new genes. We trapped an exon of DSCR1 (Down Syndrome Candidate Region 1) and two different exons of a gene called human Intersectin (ITSN). Presently, we are working with another novel trapped exon to identify the corresponding gene. The major advantage of Alu-splice PCR is that the technique can be readily established in any laboratory which has the basic facilities for molecular biology because no specialised materials or expertise is required.

The endocytic protein intersectin is a major binding partner for the Ras exchange factor mSos1 in rat brain

We recently identified intersectin, a protein containing two EH and five SH3 domains, as a component of the endocytic machinery. The N-terminal SH3 domain (SH3A), unlike other SH3 domains from intersectin or various endocytic proteins, specifically inhibits intermediate events leading to the formation of clathrin-coated pits. We have now identified a brain-enriched, 170 kDa protein (p170) that interacts specifically with SH3A. Screening of combinatorial peptides reveals the optimal ligand for SH3A as Pp(V/I)PPR, and the 170 kDa mammalian son-of-sevenless (mSos1) protein, a guanine-nucleotide exchange factor for Ras, con- tains two copies of the matching sequence, PPVPPR. Immunodepletion studies confirm that p170 is mSos1. Intersectin and mSos1 are co-enriched in nerve terminals and are co-immunoprecipitated from brain extracts. SH3A competes with the SH3 domains of Grb2 in binding to mSos1, and the intersectin-mSos1 complex can be separated from Grb2 by sucrose gradient centrifugation. Overexpression of the SH3 domains of intersectin blocks epidermal growth factor-mediated Ras activation. These results suggest that intersectin functions in cell signaling in addition to its role in endocytosis and may link these cellular processes.

Epsin binds to clathrin by associating directly with the clathrin-terminal domain. Evidence for cooperative binding through two discrete sites

Epsin is a recently identified protein that appears to play an important role in clathrin-mediated endocytosis. The central region of epsin 1, the so-called DPW domain, binds to the heterotetrameric AP-2 adaptor complex by associating directly with the globular appendage of the alpha subunit. We have found that this central portion of epsin 1 also associates with clathrin. The interaction with clathrin is direct and not mediated by epsin-bound AP-2. Alanine scanning mutagenesis shows that clathrin binding depends on the sequence (257)LMDLADV located within the epsin 1 DPW domain. This sequence, related to the known clathrin-binding sequences in the adaptor beta subunits, amphiphysin, and beta-arrestin, facilitates the association of epsin 1 with the terminal domain of the clathrin heavy chain. Unexpectedly, inhibiting the binding of AP-2 to the GST-epsin DPW fusion protein by progressively deleting DPW triplets but leaving the LMDLADV sequence intact, diminishes the association of clathrin in parallel with AP-2. Because the beta subunit of the AP-2 complex also contains a clathrin-binding site, optimal association with soluble clathrin appears to depend on the presence of at least two distinct clathrin-binding sites, and we show that a second clathrin-binding sequence (480)LVDLD, located within the carboxyl-terminal segment of epsin 1, also interacts with clathrin directly. The LMDLADV and LVDLD sequences act cooperatively in clathrin recruitment assays, suggesting that they bind to different sites on the clathrin-terminal domain. The evolutionary conservation of similar clathrin-binding sequences in several metazoan epsin-like molecules suggests that the ability to establish multiple protein-protein contacts within a developing clathrin-coated bud is an important aspect of epsin function.

Dynamin and its role in membrane fission

Dynamin, a 100-kDa GTPase, is an essential component of vesicle formation in receptor-mediated endocytosis, synaptic vesicle recycling, caveolae internalization, and possibly vesicle trafficking in and out of the Golgi. In addition to the GTPase domain, dynamin also contains a pleckstrin homology domain (PH) implicated in membrane binding, a GTPase effector domain (GED) shown to be essential for self-assembly and stimulated GTPase activity, and a C-terminal proline-rich domain (PRD), which contains several SH3-binding sites. Dynamin partners bind to the PRD and may either stimulate dynamin's GTPase activity or target dynamin to the plasma membrane. Purified dynamin readily self-assembles into rings or spirals. This striking structural property supports the hypothesis that dynamin wraps around the necks of budding vesicles where it plays a key role in membrane fission. The focus of this review is on the relationship between the GTPase and self-assembly properties of dynamin and its cellular function.

Pan1p, End3p, and S1a1p, three yeast proteins required for normal cortical actin cytoskeleton organization, associate with each other and play essential roles in cell wall morphogenesis

The EH domain proteins Pan1p and End3p of budding yeast have been known to form a complex in vivo and play important roles in organization of the actin cytoskeleton and endocytosis. In this report, we describe new findings concerning the function of the Pan1p-End3p complex. First, we found that the Pan1p-End3p complex associates with Sla1p, another protein known to be required for the assembly of cortical actin structures. Sla1p interacts with the first long repeat region of Pan1p and the N-terminal EH domain of End3p, thus leaving the Pan1p-End3p interaction, which requires the second long repeat of Pan1p and the C-terminal repeat region of End3p, undisturbed. Second, Pan1p, End3p, and Sla1p are also required for normal cell wall morphogenesis. Each of the Pan1-4, sla1Delta, and end3Delta mutants displays the abnormal cell wall morphology previously reported for the act1-1 mutant. These cell wall defects are also exhibited by wild-type cells overproducing the C-terminal region of Sla1p that is responsible for interactions with Pan1p and End3p. These results indicate that the functions of Pan1p, End3p, and Sla1p in cell wall morphogenesis may depend on the formation of a heterotrimeric complex. Interestingly, the cell wall abnormalities exhibited by these cells are independent of the actin cytoskeleton organization on the cell cortex, as they manifest despite the presence of apparently normal cortical actin cytoskeleton. Examination of several act1 mutants also supports this conclusion. These observations suggest that the Pan1p-End3p-Sla1p complex is required not only for normal actin cytoskeleton organization but also for normal cell wall morphogenesis in yeast.

Piccolo, a presynaptic zinc finger protein structurally related to bassoon

Piccolo is a novel component of the presynaptic cytoskeletal matrix (PCM) assembled at the active zone of neurotransmitter release. Analysis of its primary structure reveals that Piccolo is a multidomain zinc finger protein structurally related to Bassoon, another PCM protein. Both proteins were found to be shared components of glutamatergic and GABAergic CNS synapses but not of the cholinergic neuromuscular junction. The Piccolo zinc fingers were found to interact with the dual prenylated rab3A and VAMP2/Synaptobrevin II receptor PRA1. We show that PRA1 is a synaptic vesicle-associated protein that is colocalized with Piccolo in nerve terminals of hippocampal primary neurons. These data suggest that Piccolo plays a role in the trafficking of synaptic vesicles (SVs) at the active zone.

The eps15 homology (EH) domain-based interaction between eps15 and hrb connects the molecular machinery of endocytosis to that of nucleocytosolic transport

The Eps15 homology (EH) module is a protein-protein interaction domain that establishes a network of connections involved in various aspects of endocytosis and sorting. The finding that EH-containing proteins bind to Hrb (a cellular cofactor of the Rev protein) and to the related protein Hrbl raised the possibility that the EH network might also influence the so-called Rev export pathway, which mediates nucleocytoplasmic transfer of proteins and RNAs. In this study, we demonstrate that Eps15 and Eps15R, two EH-containing proteins, synergize with Hrb and Hrbl to enhance the function of Rev in the export pathway. In addition, the EH-mediated association between Eps15 and Hrb is required for the synergistic effect. The interaction between Eps15 and Hrb occurs in the cytoplasm, thus pointing to an unexpected site of action of Hrb, and to a possible role of the Eps15-Hrb complex in regulating the stability of Rev.

The endocytic machinery in nerve terminals surrounds sites of exocytosis

In most models of endocytosis, the endocytic machinery is recruited from the cytoplasm by cytoplasmic tails of the plasma membrane proteins that are to be internalized. This does not appear to be true at synapses where the endocytic machinery required for synaptic vesicle recycling is localized to membrane-associated 'hot spots' [1] [2]. In Drosophila neuromuscular junctions, the multi-domain protein Dap160 is also localized to hot spots [3] and has some characteristics expected of an anchoring protein. Anchoring the endocytic machinery to the plasma membrane might help contribute to the remarkable speed of synaptic vesicle recycling [4]. Here, we report that the endocytic machinery surrounds sites that are believed to be sites of exocytosis. We propose that the radial distribution of the synaptic vesicle recycling machinery already present on the plasma membrane in unstimulated nerve terminals is a fundamental unit of pre-synaptic organization and allows the nerve terminal to extract maximum recycling efficiency out of conventional endocytic machinery.

The epsins define a family of proteins that interact with components of the clathrin coat and contain a new protein module

Epsin (epsin 1) is an interacting partner for the EH domain-containing region of Eps15 and has been implicated in conjunction with Eps15 in clathrin-mediated endocytosis. We report here the characterization of a similar protein (epsin 2), which we have cloned from human and rat brain libraries. Epsin 1 and 2 are most similar in their NH(2)-terminal region, which represents a module (epsin NH(2) terminal homology domain, ENTH domain) found in a variety of other proteins of the data base. The multiple DPW motifs, typical of the central region of epsin 1, are only partially conserved in epsin 2. Both proteins, however, interact through this central region with the clathrin adaptor AP-2. In addition, we show here that both epsin 1 and 2 interact with clathrin. The three NPF motifs of the COOH-terminal region of epsin 1 are conserved in the corresponding region of epsin 2, consistent with the binding of both proteins to Eps15. Epsin 2, like epsin 1, is enriched in brain, is present in a brain-derived clathrin-coated vesicle fraction, is concentrated in the peri-Golgi region and at the cell periphery of transfected cells, and partially colocalizes with clathrin. High overexpression of green fluorescent protein-epsin 2 mislocalizes components of the clathrin coat and inhibits clathrin-mediated endocytosis. The epsins define a new protein family implicated in membrane dynamics at the cell surface.

The EH network

The EH domain is an evolutionary conserved protein-protein interaction domain present in a growing number of proteins from yeast to mammals. Even though the domain was discovered just 5 years ago, a great deal has been learned regarding its three-dimensional structure and binding specificities. Moreover, a number of cellular ligands of the domain have been identified and demonstrated to define a complex network of protein-protein interactions in the eukaryotic cell. Interestingly, many of the EH-containing and EH-binding proteins display characteristics of endocytic "accessory" proteins, suggesting that the principal function of the EH network is to regulate various steps in endocytosis. In addition, recent evidence suggests that the EH network might work as an "integrator" of signals controlling cellular pathways as diverse as endocytosis, nucleocytosolic export, and ultimately cell proliferation.

Molecular cloning and functional expression of two splice forms of human N-acetylglucosamine-1-phosphodiester alpha-N-acetylglucosaminidase

We have isolated and sequenced human cDNA and mouse genomic DNA clones encoding N-acetylglucosamine-1-phosphodiester alpha-N-acetylglucosaminidase (phosphodiester alpha-GlcNAcase) which catalyzes the second step in the synthesis of the mannose 6-phosphate recognition signal on lysosomal enzymes. The gene is organized into 10 exons. The protein sequence encoded by the clones shows 80% identity between human and mouse phosphodiester alpha-GlcNAcase and no homology to other known proteins. It predicts a type I membrane-spanning glycoprotein of 514 amino acids containing a 24-amino acid signal sequence, a luminal domain of 422 residues with six potential N-linked glycosylation sites, a single 27-residue transmembrane region, and a 41-residue cytoplasmic tail that contains both a tyrosine-based and an NPF internalization motif. Human brain expressed sequence tags lack a 102-base pair region present in human liver cDNA that corresponds to exon 8 in the genomic DNA and probably arises via alternative splicing. COS cells transfected with the human cDNA expressed 50-100-fold increases in phosphodiester alpha-GlcNAcase activity proving that the cDNA encodes the subunits of the tetrameric enzyme. Transfection with cDNA lacking the 102-base pair region also gave active enzyme. The complete genomic sequence of human phosphodiester alpha-GlcNAcase was recently deposited in the data base. It showed that our cDNA clone was missing only the 5'-untranslated region and initiator methionine and revealed that the human genomic DNA has the same exon organization as the mouse gene.

Epsin binds to the EH domain of POB1 and regulates receptor-mediated endocytosis

POB1 has been identified as a RalBP1-binding protein and has the Eps15 homology (EH) domain. The EH domain-containing proteins have been suggested to be involved in clathrin-dependent endocytosis. To clarify the function of POB1, we purified a protein which binds to the EH domain of POB1 from bovine brain cytosol and identified it as Epsin, which is known to bind to the EH domain of Eps15. Epsin has three Asn-Pro-Phe (NPF) motifs in the C-terminal region, which are known to form the core sequence for the binding to the EH domain. The EH domain of POB1 interacted directly with the region containing the NPF motifs of Epsin. Expression of Epsin in CHO-IR cells inhibited internalization of insulin although it affected neither insulin-binding nor autophosphorylation activities of the insulin receptor. Taken together with the observations that Epsin is involved in internalization of the receptors for epidermal growth factor and transferrin, these results suggest that Epsin is a binding partner of POB1 and their binding regulates receptor-mediated endocytosis.

Clathrin assembly lymphoid myeloid leukemia (CALM) protein: localization in endocytic-coated pits, interactions with clathrin, and the impact of overexpression on clathrin-mediated traffic

The clathrin assembly lymphoid myeloid leukemia (CALM) gene encodes a putative homologue of the clathrin assembly synaptic protein AP180. Hence the biochemical properties, the subcellular localization, and the role in endocytosis of a CALM protein were studied. In vitro binding and coimmunoprecipitation demonstrated that the clathrin heavy chain is the major binding partner of CALM. The bulk of cellular CALM was associated with the membrane fractions of the cell and localized to clathrin-coated areas of the plasma membrane. In the membrane fraction, CALM was present at near stoichiometric amounts relative to clathrin. To perform structure-function analysis of CALM, we engineered chimeric fusion proteins of CALM and its fragments with the green fluorescent protein (GFP). GFP-CALM was targeted to the plasma membrane-coated pits and also found colocalized with clathrin in the Golgi area. High levels of expression of GFP-CALM or its fragments with clathrin-binding activity inhibited the endocytosis of transferrin and epidermal growth factor receptors and altered the steady-state distribution of the mannose-6-phosphate receptor in the cell. In addition, GFP-CALM overexpression caused the loss of clathrin accumulation in the trans-Golgi network area, whereas the localization of the clathrin adaptor protein complex 1 in the trans-Golgi network remained unaffected. The ability of the GFP-tagged fragments of CALM to affect clathrin-mediated processes correlated with the targeting of the fragments to clathrin-coated areas and their clathrin-binding capacities. Clathrin-CALM interaction seems to be regulated by multiple contact interfaces. The C-terminal part of CALM binds clathrin heavy chain, although the full-length protein exhibited maximal ability for interaction. Altogether, the data suggest that CALM is an important component of coated pit internalization machinery, possibly involved in the regulation of clathrin recruitment to the membrane and/or the formation of the coated pit.

Epidermal growth factor pathway substrate 15, Eps15

Eps15 was originally identified as a substrate for the kinase activity of the epidermal growth factor receptor (EGFR). Eps15 has a tripartite structure comprising a NH2-terminal portion, which contains three EH domains, a central putative coiled-coil region, and a COOH-terminal domain containing multiple copies of the amino acid triplet Aspartate-Proline-Phenylalanine. A pool of Eps15 is localized at clathrin coated pits where it interacts with the clathrin assembly complex AP-2 and a novel AP-2 binding protein, Epsin. Perturbation of Eps15 and Epsin function inhibits receptor-mediated endocytosis of EGF and transferrin, demonstrating that both proteins are components of the endocytic machinery. Since the family of EH-containing proteins is implicated in various aspects of intracellular sorting, biomolecular strategies aimed at interfering with these processes can now be envisioned. These strategies have potentially far reaching implications extending to the control of cell proliferation. In this regard, it is of note that Eps15 has the potential of transforming NIH-3T3 cells and that the eps15 gene is rearranged with the HRX/ALL/MLL gene in acute myelogeneous leukemias, thus implicating this protein in the subversion of cell proliferation in neoplasia.

The N terminus of amphiphysin II mediates dimerization and plasma membrane targeting

Amphiphysin I and II are nerve terminal-enriched proteins containing SH3 domains that interact with dynamin and synaptojanin. The amphiphysins may function in synaptic vesicle endocytosis by targeting synaptojanin and dynamin to emerging endocytic buds through SH3 domain-independent interactions with clathrin and AP2. We have recently identified and cloned several amphiphysin II splice variants that differentially incorporate clathrin-binding domains. To determine whether these domains function in membrane targeting, we used immunofluorescence to examine the potential localization of amphiphysin II variants to clathrin-coated pits on plasma membranes purified from transfected COS-7 cells. Full-length amphiphysin II targets to the plasma membrane where it partially co-localizes with clathrin. However, splice variants and deletion constructs lacking clathrin-binding domains still target to the plasma membrane, and removal of clathrin from the membrane does not affect amphiphysin II distribution. Surprisingly, plasma membrane targeting was dependent on the presence of a 31-amino acid alternatively spliced sequence at the N terminus of amphiphysin II, a result confirmed using subcellular fractionation. In binding assays, the 31-amino acid sequence was also found to facilitate amphiphysin dimerization mediated through the N terminus. Taken together, these data support a role for the N terminus of amphiphysin II in membrane targeting during endocytosis.

EHSH1/intersectin, a protein that contains EH and SH3 domains and binds to dynamin and SNAP-25. A protein connection between exocytosis and endocytosis?

In yeast two-hybrid screens for proteins that bind to SNAP-25 and may be involved in exocytosis, we isolated a protein called EHSH1 (for EH domain/SH3 domain-containing protein). Cloning of full-length cDNAs revealed that EHSH1 is composed of an N-terminal region with two EH domains, a central region that is enriched in lysine, leucine, glutamate, arginine, and glutamine (KLERQ domain), and a C-terminal region comprised of five SH3 domains. The third SH3 domain is alternatively spliced. Data bank searches demonstrated that EHSH1 is very similar to Xenopus and human intersectins and to human SH3P17. In addition, we identified expressed sequence tags that encode a second isoform of EHSH1, called EHSH2. EHSH1 is abundantly expressed in brain and at lower levels in all other tissues tested. In binding studies, we found that the central KLERQ domain of EHSH1 binds to recombinant or native brain SNAP-25 and SNAP-23. The C-terminal SH3 domains, by contrast, quantitatively interact with dynamin, a protein involved in endocytosis. Dynamin strongly binds to the alternatively spliced central SH3 domain (SH3C) and the two C-terminal SH3 domains (SH3D and SH3E) but not to the N-terminal SH3 domains (SH3A and SH3B). Immunoprecipitations confirmed that both dynamin and SNAP-25 are complexed to EHSH1 in brain. Our data suggest that EHSH1/intersectin may be a novel adaptor protein that couples endocytic membrane traffic to exocytosis. The ability of multiple SH3 domains in EHSH1 to bind to dynamin suggests that EHSH1 can cluster several dynamin molecules in a manner that is regulated by alternative splicing.

SH3-domain-containing proteins function at distinct steps in clathrin-coated vesicle formation

Several SH3-domain-containing proteins have been implicated in endocytosis by virtue of their interactions with dynamin; however, their functions remain undefined. Here we report the efficient reconstitution of ATP-, GTP-, cytosol- and dynamin-dependent formation of clathrin-coated vesicles in permeabilized 3T3-L1 cells. The SH3 domains of intersectin, endophilin I, syndapin I and amphiphysin II inhibit coated-vesicle formation in vitro through interactions with membrane-associated proteins. Most of the SH3 domains tested selectively inhibit late events involving membrane fission, but the SH3A domain of intersectin uniquely inhibits intermediate events leading to the formation of constricted coated pits. These results suggest that interactions between SH3 domains and their partners function sequentially in endocytic coated-vesicle formation.

Recruitment of an alternatively spliced form of synaptojanin 2 to mitochondria by the interaction with the PDZ domain of a mitochondrial outer membrane protein

Synaptojanin 1 is an inositol 5'-phosphatase highly enriched in nerve terminals with a putative role in recycling of synaptic vesicles. We have previously described synaptojanin 2, which is more broadly expressed as multiple alternatively spliced forms. Here we have identified and characterized a novel mitochondrial outer membrane protein, OMP25, with a single PDZ domain that specifically binds to a unique motif in the C-terminus of synaptojanin 2A. This motif is encoded by the exon sequence specific to synaptojanin 2A. OMP25 mRNA is widely expressed in rat tissues. OMP25 is localized to the mitochondrial outer membrane via the C-terminal transmembrane region, with the PDZ domain facing the cytoplasm. Overexpression of OMP25 results in perinuclear clustering of mitochondria in transfected cells. This effect is mimicked by enforced expression of synaptojanin 2A on the mitochondrial outer membrane, but not by the synaptojanin 2A mutants lacking the inositol 5'-phosphatase domain. Our findings provide evidence that OMP25 mediates recruitment of synaptojanin 2A to mitochondria and that modulation of inositol phospholipids by synaptojanin 2A may play a role in maintenance of the intracellular distribution of mitochondria.

Splice variants of intersectin are components of the endocytic machinery in neurons and nonneuronal cells

We recently identified and cloned intersectin, a protein containing two Eps15 homology (EH) domains and five Src homology 3 (SH3) domains. Using a newly developed intersectin antibody, we demonstrate that endogenous COS-7 cell intersectin localizes to clathrin-coated pits, and transfection studies suggest that the EH domains may direct this localization. Through alternative splicing in a stop codon, a long form of intersectin is generated with a C-terminal extension containing Dbl homology (DH), pleckstrin homology (PH), and C2 domains. Western blots reveal that the long form of intersectin is expressed specifically in neurons, whereas the short isoform is expressed at lower levels in glia and other nonneuronal cells. Immunofluorescence analysis of cultured hippocampal neurons reveals that intersectin is found at the plasma membrane where it is co-localized with clathrin. Ibp2, a protein identified based on its interactions with the EH domains of intersectin, binds to clathrin through the N terminus of the heavy chain, suggesting a mechanism for the localization of intersectin at clathrin-coated pits. Ibp2 also binds to the clathrin adaptor AP2, and antibodies against intersectin co-immunoprecipitate clathrin, AP2, and dynamin from brain extracts. These data suggest that the long and short forms of intersectin are components of the endocytic machinery in neurons and nonneuronal cells.

The EH and SH3 domain Ese proteins regulate endocytosis by linking to dynamin and Eps15

Clathrin-mediated endocytosis is a multistep process which requires interaction between a number of conserved proteins. We have cloned two mammalian genes which code for a number of endocytic adaptor proteins. Two of these proteins, termed Ese1 and Ese2, contain two N-terminal EH domains, a central coiled-coil domain and five C-terminal SH3 domains. Ese1 is constitutively associated with Eps15 proteins to form a complex with at least 14 protein-protein interaction surfaces. Yeast two-hybrid assays have revealed that Ese1 EH and SH3 domains bind epsin family proteins and dynamin, respectively. Overexpression of Ese1 is sufficient to block clathrin-mediated endocytosis in cultured cells, presumably through disruption of higher order protein complexes, which are assembled on the endogenous Ese1-Eps15 scaffold. The Ese1-Eps15 scaffold therefore links dynamin, epsin and other endocytic pathway components.

Identification of a novel domain shared by putative components of the endocytic and cytoskeletal machinery

We have identified a approximately 140 amino acid domain that is shared by a variety of proteins in budding and fission yeast, nematode, rat, mouse, frog, oat, and man. Typically, this domain is located within 20 residues of the N-terminus of the various proteins. The percent identity among the domains in the 12 proteins ranges from 42 to 93%, with 16 absolutely conserved residues: N-x(11-13)-V-x2-A-T-x(34-36)-R-x(7-8)-W-R-x3-K-x12-G-x-E-x15 -L-x11-12-D-x-G-R-x11-D-x7-R. Even though these proteins share little beyond their segment of homology, data are emerging that several of the proteins are involved in endocytosis and or regulation of cytoskeletal organization. We have named this protein segment the ENTH domain, for Epsin N-terminal Homology domain, and hypothesize that it is a candidate for binding specific ligands and/or enzymatic activity in the cell.

Endocytosis: EH domains lend a hand

A number of proteins that have been implicated in endocytosis feature a conserved protein-interaction module known as an EH domain. The three-dimensional structure of an EH domain has recently been solved, and is likely to presage significant advances in understanding molecular mechanisms of endocytosis.