Studies on the inhibition of endosome fusion by GTPgammaS-bound ARF

Using a cell free assay, we have previously shown that ARF is not required for endosome fusion but that inhibition of fusion by GTPgammaS is dependent on a cytosolic pool of ARFs. Since ARF is proposed to function in intracellular membrane traffic by promoting vesicle biogenesis, and components of clathrin- and COP-coated vesicles have been localized on endosomal structures, we investigated whether ARF-mediated inhibition of early endosome fusion involves the recruitment or irreversible association of these proteins onto endosomal membranes. We now report that depletion of components of clathrin coated vesicles (clathrin, AP-1 and AP-2) or COPI vesicles (beta COP) does not affect the capacity of GTPgammaS-activated ARF to inhibit endosome fusion. Inhibition of fusion by activated ARF is also independent of endosomal acidification since assays performed in the presence of the vacuolar ATPase inhibitor bafilomycin A1 are equally sensitive to GTPgammaS-bound ARF. Finally, in contrast to reported effects on lysosomes, we demonstrate that ARF-GTPgammaS does not induce endosomal lysis. These combined data argue that sequestration of known coat proteins to membranes by activated ARF is not involved in the inhibition of early endosome fusion and that its capacity to inhibit fusion involves other specific interactions with the endosome surface. These results contrast with the mechanistic action of ARF on intra-Golgi transport and nuclear envelope assembly.

Cytolytic T lymphocyte-associated antigen-4 and the TCR zeta/CD3 complex, but not CD28, interact with clathrin adaptor complexes AP-1 and AP-2

The negative signaling receptor cytolytic T lymphocyte-associated Ag-4 (CTLA-4) resides primarily in intracellular compartments such as the Golgi apparatus of T cells. However, little is known regarding the molecular mechanisms that influence this accumulation. In this study, we demonstrate binding of the clathrin adaptor complex AP-1 with the GVYVKM motif of the cytoplasmic domain of CTLA-4. Binding occurred primarily in the Golgi compartment of T cells, unlike with AP-2 binding that occurs mostly with cell surface CTLA-4. Although evidence was not found to implicate AP-1 binding in the retention of CTLA-4 in the Golgi, AP-1 appears to play a role in shuttling of excess receptor from the Golgi to the lysosomal compartments for degradation. In support of this, increased CTLA-4 synthesis resulted in an increase in CTLA-4/AP-1 binding and a concomitant increase in the appearance of CTLA-4 in the lysosomal compartment. At the same time, the level of intracellular receptor was maintained at a constant level, suggesting that CTLA-4/AP-1 binding represents one mechanism to ensure steady state levels of intracellular CTLA-4 in T cells. Finally, we demonstrate that the TCR zeta/CD3 complex (but not CD28) also binds to AP-1 and AP-2 complexes, thus providing a possible link between these two receptors in the regulation of T cell function.

Cytolytic T Lymphocyte-Associated Antigen-4 and the TCRζ/CD3 Complex, But Not CD28, Interact with Clathrin Adaptor Complexes AP-1 and AP-2

The negative signaling receptor cytolytic T lymphocyte-associated Ag-4 (CTLA-4) resides primarily in intracellular compartments such as the Golgi apparatus of T cells. However, little is known regarding the molecular mechanisms that influence this accumulation. In this study, we demonstrate binding of the clathrin adaptor complex AP-1 with the GVYVKM motif of the cytoplasmic domain of CTLA-4. Binding occurred primarily in the Golgi compartment of T cells, unlike with AP-2 binding that occurs mostly with cell surface CTLA-4. Although evidence was not found to implicate AP-1 binding in the retention of CTLA-4 in the Golgi, AP-1 appears to play a role in shuttling of excess receptor from the Golgi to the lysosomal compartments for degradation. In support of this, increased CTLA-4 synthesis resulted in an increase in CTLA-4/AP-1 binding and a concomitant increase in the appearance of CTLA-4 in the lysosomal compartment. At the same time, the level of intracellular receptor was maintained at a constant level, suggesting that CTLA-4/AP-1 binding represents one mechanism to ensure steady state levels of intracellular CTLA-4 in T cells. Finally, we demonstrate that the TCRζ/CD3 complex (but not CD28) also binds to AP-1 and AP-2 complexes, thus providing a possible link between these two receptors in the regulation of T cell function.

Functional organization of clathrin in coats: combining electron cryomicroscopy and X-ray crystallography

The sorting of specific proteins into clathrin-coated pits and the mechanics of membrane invagination are determined by assembly of the clathrin lattice. Recent structures of a six-fold barrel clathrin coat at 21 A resolution by electron cryomicroscopy and of the clathrin terminal domain and linker at 2.6 A by X-ray crystallography together show how domains of clathrin interact and orient within the coat and reveal the strongly puckered shape and conformational variability of individual triskelions. The beta propeller of the terminal domain faces the membrane so that recognition segments from adaptor proteins can extend along its lateral grooves. Clathrin legs adapt to different coat environments in the barrel by flexing along a segment at the knee that is free of contacts with other molecules.

The interaction between N-WASP and the Arp2/3 complex links Cdc42-dependent signals to actin assembly

Although small GTP-binding proteins of the Rho family have been implicated in signaling to the actin cytoskeleton, the exact nature of the linkage has remained obscure. We describe a novel mechanism that links one Rho family member, Cdc42, to actin polymerization. N-WASP, a ubiquitously expressed Cdc42-interacting protein, is required for Cdc42-stimulated actin polymerization in Xenopus egg extracts. The C terminus of N-WASP binds to the Arp2/3 complex and dramatically stimulates its ability to nucleate actin polymerization. Although full-length N-WASP is less effective, its activity can be greatly enhanced by Cdc42 and phosphatidylinositol (4,5) bisphosphate. Therefore, N-WASP and the Arp2/3 complex comprise a core mechanism that directly connects signal transduction pathways to the stimulation of actin polymerization.

Atomic structure of clathrin: a beta propeller terminal domain joins an alpha zigzag linker

Clathrin triskelions form the lattice that organizes recruitment of proteins to coated pits and helps drive vesiculation of the lipid bilayer. We report the crystal structure at 2.6 A resolution of a 55 kDa N-terminal fragment from the 190 kDa clathrin heavy chain. The structure comprises the globular "terminal domain" and the linker that joins it to the end of a triskelion leg. The terminal domain is a seven-blade beta propeller, a structure well adapted to interaction with multiple partners, such as the AP-1 and AP-2 sorting adaptor complexes and the nonvisual arrestins. The linker is an alpha-helical zigzag emanating from the propeller domain. We propose that this simple motif may extend into the rest of the clathrin leg.

Vesicle formation: dynamic dynamin lives up to its name

The GTP-binding protein dynamin was initially thought to be required for just the final stages of clathrin-dependent vesicle formation, but recent results indicate that it can actually catalyse many of the essential steps in the vesiculation pathway.

A dileucine motif in HIV-1 Nef is essential for sorting into clathrin-coated pits and for downregulation of CD4

Nef, a approximately 200 residue multifunctional regulatory protein of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV), interacts with components of host cell signal transduction and clathrin-dependent protein sorting pathways. The downregulation of surface CD4 molecules and major histocompatibility complex (MHC) class I antigens by Nef is believed to be important in AIDS pathogenesis [1-7]. Nef contains a globular core domain and two disordered segments--a myristylated arm at the amino terminus and a carboxy-terminal loop projecting from the globular core [8,9]. Here, we aimed to determine the sorting signals in HIV-1 Nef that were responsible for its involvement in the clathrin-mediated pathway. We found that a sequence in the carboxy-terminal disordered loop of Nef is essential for downregulation of CD4. This sequence resembles the dileucine motif, one of two well-characterized sorting signals that target membrane proteins to clathrin-coated vesicles. The dileucine-motif-containing segment of Nef bound directly and specifically to the beta-adaptin subunit of the clathrin adaptor complexes AP-1 and AP-2, which are responsible for recruiting sorted proteins into coated pits. Unlike wild-type Nef, a mutant form of Nef that lacked the dileucine motif did not localize to clathrin-coated pits and did not downregulate CD4 expression, although it could downregulate MHC class I surface expression. Thus, the dileucine motif in HIV-1 is required for CD4 downregulation and for interaction with clathrin adaptor complexes.

A putative vacuolar cargo receptor partially colocalizes with AtPEP12p on a prevacuolar compartment in Arabidopsis roots

Targeting of protein cargo to the vacuole/lysosome is a multistep process that appears to have conserved features between mammalian, yeast, and plant cells. In each case, some soluble vacuolar/lysosomal proteins are believed to be bound by transmembrane cargo receptors in the trans-Golgi network (TGN) that redirect these proteins into clathrin-coated vesicles. These vesicles then appear to be transported to the prevacuole/endosome by a trafficking machinery that requires components identified in other vesicle-targeting steps such as N-ethylmaleimide-sensitive factor (NSF), soluble NSF attachment protein (SNAP), SNAP receptors (SNAREs), rab-type GTPases, and Sec1p homologs. Two likely members of this trafficking machinery have been characterized from Arabidopsis thaliana: AtPEP12p, a t-SNARE that resides on a what we now call a prevacuolar compartment, and AtELP, a protein that shares many common features with mammalian and yeast transmembrane cargo receptors. Here, we have further investigated the intracellular distribution of AtELP. We have found that AtELP is located at the trans-Golgi of Arabidopsis root cells, and that its C terminus can preferentially interact in vitro with the mammalian TGN-specific AP-1 clathrin-adapter complex, suggesting a likely role in clathrin-coated, vesicle-directed trafficking at the TGN. Further, consistent with a role in trafficking of vacuolar cargo, we have found that AtELP partially colocalizes with AtPEP12p on a prevacuolar compartment.

The clathrin endocytic pathway in viral infection

How important is the clathrin-dependent endocytic pathway for entry of viruses into host cells? While it is widely accepted that Semliki Forest virus (SFV), an enveloped virus, requires this pathway there are conflicting data concerning the closely related Sindbis virus, as well as varying results with picornaviruses such as human rhinovirus 14 (HRV 14) and poliovirus. We have examined the entry mode of SFV, Sindbis virus, HRV 14 and poliovirus using a method that identifies single infected cells. This assay takes advantage of the observation that the clathrin-dependent endocytic pathway is specifically and potently arrested by overexpression of dynamin mutants that prevent clathrin-coated pit budding. Using HeLa cells and conditions of low multiplicity of infection to favor use of the most avid pathway of cell entry, it was found that SFV, Sindbis virus and HRV 14 require an active clathrin-dependent endocytic pathway for successful infection. In marked contrast, infection of HeLa cells by poliovirus did not appear to require the clathrin pathway.

The neural cell adhesion molecule L1 interacts with the AP-2 adaptor and is endocytosed via the clathrin-mediated pathway

Cell-cell interactions mediated via cell adhesion molecules (CAMs) are dynamically regulated during nervous system development. One mechanism to control the amount of cell surface CAMs is to regulate their recycling from the plasma membrane. The L1 subfamily of CAMs has a highly conserved cytoplasmic domain that contains a tyrosine, followed by the alternatively spliced RSLE (Arg-Ser-Leu-Glu) sequence. The resulting sequence of YRSL conforms to a tyrosine-based sorting signal that mediates clathrin-dependent endocytosis of signal-bearing proteins. The present study shows that L1 associates in rat brain with AP-2, a clathrin adaptor that captures plasma membrane proteins with tyrosine-based signals for endocytosis by coated pits. In vitro assays demonstrate that this interaction occurs via the YRSL sequence of L1 and the mu 2 chain of AP-2. In L1-transfected 3T3 cells, L1 endocytosis is blocked by dominant-negative dynamin that specifically disrupts clathrin-mediated internalization. Furthermore, endocytosed L1 colocalizes with the transferrin receptor (TfR), a marker for clathrin-mediated internalization. Mutant forms of L1 that lack the YRSL do not colocalize with TfR, indicating that the YRSL mediates endocytosis of L1. In neurons, L1 is endocytosed preferentially at the rear of axonal growth cones, colocalizing with Eps15, another marker for the clathrin endocytic pathway. These results establish a mechanism by which L1 can be internalized from the cell surface and suggest that an active region of L1 endocytosis at the rear of growth cones is important in L1-dependent axon growth.

Wiskott-Aldrich syndrome: a gene, a multifunctional protein and the beginnings of an explanation

Patients with Wiskott-Aldrich syndrome show various defects in the normal function of platelets and lymphocytes. The recent identification of the gene responsible for this syndrome has led to a surge of studies aimed at solving the puzzle posed by the varied phenotype observed in this disease. It is now known that WASP, the protein product of this gene, can interact with a large number of other proteins known to be involved in the regulation of signal transduction and cytoskeletal organization. Thus, WASP appears to integrate these two basic and fundamental cellular mechanisms. Several groups are now focusing on understanding the function of WASP in detail, and translating this new knowledge into improved therapies.

Dileucine-based sorting signals bind to the beta chain of AP-1 at a site distinct and regulated differently from the tyrosine-based motif-binding site

In previous work, we showed that peptides from endocytosed proteins containing the tyrosine YXXphi sorting motif are recognized by the mu 2 subunit of AP-2, the plasma membrane clathrin adaptor protein complex. This interaction is activated by phosphoinositide lipids that are phosphorylated at the D-3 position of the inositol ring, and is also enhanced by the formation of clathrin-AP-2 coats. Here, we describe the detection of a specific interaction between peptides containing a second sorting motif, the dileucine motif, and AP-1, the clathrin adaptor complex responsible for sorting proteins at the trans-Golgi network (TGN). Surprisingly, the site of dileucine binding is the beta1 subunit, not mu 1. A YXXphi-containing peptide from a protein trafficked within the TGN does bind to mu 1, however. Phosphatidylinositol 3,4-diphosphate and 3,4, 5-triphosphate did not activate the interaction between dileucine-containing peptides and AP-1 but instead inhibited it, and clathrin-AP-1 coat formation did not alter the interaction. Thus, there are at least two physically separate binding sites for sorting signals on APs, which are also regulated independently.

Assembly of clathrin coats disrupts the association between Eps15 and AP-2 adaptors

Eps15 is a phosphorylation substrate of the epidermal growth factor receptor kinase. In vivo, it is largely found in complex with AP-2, the plasma membrane clathrin adaptor protein complex. Although AP-2 is uniformly distributed across the surface of clathrin-coated pits and vesicles, Eps15 is preferentially found in the rims of endocytic clathrin-coated pits (1). This observation suggests that Eps15 may disengage from AP-2 during coat formation. Here we use two new anti-Eps15 antibodies to show that, contrary to our own earlier suggestion, coated vesicles isolated from brain do not contain detectable amounts of Eps15. Furthermore, when AP-2 complexes that are saturated with Eps15 are used for in vitro assembly of clathrin-AP-2 coats, normal structures are formed that contain the expected amounts of clathrin and AP-2, but the amount of Eps15 present is dramatically lower than that of AP-2. We propose that during coated pit formation, addition of clathrin to the growing edge at the rim of the pit releases Eps15 from AP-2.

Interaction of the bovine papillomavirus E6 protein with the clathrin adaptor complex AP-1

The E6 gene of the bovine papillomavirus type 1 (BPV-1) is expressed in fibropapillomas caused by BPV-1 and in tissue culture cells transformed by BPV-1. It encodes one of the two major oncoproteins of BPV-1. In this study, we demonstrate an interaction between the BPV-1 E6 protein and AP-1, the TGN (trans-Golgi network)-specific clathrin adaptor complex. AP-1 is a four-subunit protein complex required for clathrin-mediated cellular transport from the TGN. The AP-1/E6 interaction was observed in vitro and in cells. The E6 binding site on AP-1 was mapped to the N-terminal trunk domain of the gamma subunit. BPV-1 E6 preferentially associated with membrane-bound AP-1 in cells but not with free cytosolic AP-1. BPV-1 E6 was further shown to be recruited to isolated Golgi membranes and to copurify with clathrin-coated vesicles. The recruitment of BPV-1 E6 to Golgi membranes was AP-1 independent, but the E6 interaction with AP-1 was required for its association with clathrin-coated vesicles. Furthermore, AP-1 proteins could compete with BPV-1 E6 for binding to Golgi membranes, suggesting that the recruitment of BPV-1 E6 and AP-1 to Golgi membranes involves a common factor. Taken together, our results suggest that cytosolic BPV-1 E6 is first recruited to the TGN, where it is then recognized by membrane-bound AP-1 and subsequently recruited into TGN-derived clathrin-coated vesicles. We propose that BPV-1 E6, through its interaction with AP-1, can affect cellular processes involving clathrin-mediated trafficking pathway.

Parallel dimers and anti-parallel tetramers formed by epidermal growth factor receptor pathway substrate clone 15

The recently discovered localization of epidermal growth factor receptor pathway substrate clone 15 (Eps15) to plasma membrane clathrin-coated pits and its constitutive association with the endocytic clathrin adaptor protein complex, AP-2, strongly suggest that Eps15 has an important role in the pathway of clathrin-dependent endocytic traffic. We report here that Eps15 forms dimers and tetramers of distinct shape. The Eps15 dimer is an elongated molecule, 32 nm in length. There is a globular "head" at one end of the molecule and an extended "stalk" of 25 nm which is kinked at about 17 nm away from the head. In the Eps15 dimer, two subunits are arranged parallel to each other, so that the head corresponds to two side by side copies of the N-terminal region I, which contains the three Eps15 homology domains. The proximal part of the stalk is the coiled-coil central region II containing 20 heptad repeats. The kink is at the boundary between region II and the C-terminal region III, which contains the AP-2 binding site, 15 aspartic-proline-phenylalanine repeats, and proline-rich Src homology domain ligand sites. The Eps15 tetramer has a "dumbbell" shape, approximately 31 nm in length; it is formed by the anti-parallel association of two Eps15 dimers. Formation of these Eps15 tetramers appears to require contacts between regions I of one dimer and regions III of a second apposing dimer. The extended shapes of the Eps15 dimers and tetramers suggest how Eps15 oligomers are located in the clathrin coat. We discuss the implications for accessibility to partners and for proposed functions of Eps15.

Linking cargo to vesicle formation: receptor tail interactions with coat proteins

How soluble cargo molecules concentrate into budding vesicles is the subject of intensive current research. Clathrin-based vesiculation from the plasma membrane and the trans-Golgi network constitutes the best described system that supports this sorting process. Soluble ligands bind to specific transmembrane receptors which have been shown to interact directly with clathrin adaptor complexes, components of clathrin coats. At the same time, these clathrin adaptors facilitate clathrin coat assembly and probably regulate the recruitment of the rest of the coat components. Recent studies have looked at both the interaction of receptor tails with adaptors and the assembly of the clathrin coat. Progress has also been made in elucidating how soluble cargo molecules may be concentrated for exit from the endoplasmic reticulum.

A role for the hinge/ear domain of the beta chains in the incorporation of AP complexes into clathrin-coated pits and coated vesicles

The clathrin-associated adaptor protein (AP) complexes drive the polymerization of clathrin in coated pits to form coated vesicles. It has previously been shown that the carboxyl-terminal hinge/ear domain of the beta 2 chain contains a binding site for clathrin and that removal of this domain from APs or from isolated beta 2 chains abrogates their ability to form clathrin coats in vitro. We show here that the hinge/ear domain is necessary for efficient incorporation of AP complexes into coated pits and coated vesicles in cells, a result that is consistent with the view that the beta chains indeed provide an important interaction between the AP complexes and clathrin.

Regulatory interactions in the recognition of endocytic sorting signals by AP-2 complexes

Many plasma membrane proteins destined for endocytosis are concentrated into clathrin-coated pits through the recognition of a tyrosine-based motif in their cytosolic domains by an adaptor (AP-2) complex. The mu2 subunit of isolated AP-2 complexes binds specifically, but rather weakly, to proteins bearing the tyrosine-based signal. We now demonstrate, using peptides with a photoreactive probe, that this binding is strengthened significantly when the AP-2 complex is present in clathrin coats, indicating that there is cooperativity between receptor-AP-2 interactions and coat formation. Phosphoinositides with a phosphate at the D-3 position of the inositol ring, but not other isomers, also increase the affinity of the AP-2 complex for the tyrosine-based motif. AP-2 is the first protein known (in any context) to interact with phosphatidylinositol 3-phosphate. Our findings indicate that receptor recruitment can be coupled to clathrin coat assembly and suggest a mechanism for regulation of membrane traffic by lipid products of phosphoinositide 3-kinases.

Eps15 is a component of clathrin-coated pits and vesicles and is located at the rim of coated pits

Eps15, a phosphorylation substrate of the epidermal growth factor (EGF) receptor kinase, has been shown to bind to the alpha-subunit of the clathrin-associated protein complex AP-2. Here we report that in cells, virtually all Eps15 interacts with the cytosol and membrane-bound forms of AP-2. This association is not affected by the treatment of cells with EGF. Immunofluorescence microscopy reveals nearly absolute co-localization of Eps15 with AP-2 and clathrin, and analysis by immunoelectron microscopy shows that the localization of membrane-associated Eps15 is restricted to the profiles corresponding to endocytic coated pits and vesicles. Unexpectedly, Eps15 was found at the edge of forming coated pits and at the rim of budding coated vesicles. This asymmetric distribution is in sharp contrast to the localization of AP-2 that shows an even distribution along the same types of clathrin-coated structures. These findings suggest several possible regulatory roles of Eps15 during the formation of coated pits.

Sequence requirements for the recognition of tyrosine-based endocytic signals by clathrin AP-2 complexes

We recently determined that fusion proteins containing tyrosine-based endocytic signals bind to the mu 2 subunit of AP-2, the complex that drives clathrin coat formation and mediates endocytosis from the plasma membrane. Here we analyze the selectivity of peptide recognition by mu 2 and by AP-2 using combinatorial selection methods and surface plasmon resonance. Both mu 2 and AP-2 are shown to interact with various sequences of the form tyrosine-polar-polar-hydrophobic (Yppø) found on receptors that follow the clathrin pathway. The optimal sequence for interaction with mu 2 and with AP-2 has tyrosine as an anchor and prefers arginine at position Y + 2 and leucine at position Y + 3. In contrast, no preferred sequence is detected surrounding the Yppø signal, indicating that recognition of the Yppø endocytic signal does not require a prefolded structure. We conclude that sorting into the endocytic pathway is governed by a surprisingly simple interaction between the mu 2 chain and a tyrosine-containing tetrapeptide sequence.

Identification of Itk/Tsk Src homology 3 domain ligands

The tyrosine kinase Itk/Tsk is a T cell specific analog of Btk, the tyrosine kinase defective in the human immunodeficiency X-linked agammaglobulinemia and in xid mice. T lymphocytes from Itk-deficient mice are refractory to mitogenic stimuli delivered through the T cell receptor (TCR). To gain insights into the biochemical role of Itk, the binding properties of the Itk SH3 domain were examined. An optimal Itk SH3 binding motif was derived by screening biased phage display libraries; peptides based on this motif bound with high affinity and selectivity to the Itk SH3 domain. Initial studies with T cell lysates indicated that the Itk SH3 domain bound Cbl, Fyn, and other tyrosine phosphoproteins from TCR-stimulated Jurkat cells. Under conditions of increased detergent stringency Sam 68, Wiskott-Aldrich Syndrome protein, and hnRNP-K, but not Cbl and Fyn, were bound to the Itk SH3 domain. By examining the ability of different SH3 domains to interact with deletion variants of Sam 68 and WASP, we demonstrated that the Itk-SH3 domain and the SH3 domains of Src family kinases bind to overlapping but distinct sets of proline-rich regions in Sam 68 and WASP.

Mechanisms of signal-mediated protein sorting in the endocytic and secretory pathways

The ability to sort proteins to different intracellular compartments is an essential property of all cells. Many diseases are caused by a failure of certain proteins to be sorted properly in the endocytic and secretory pathways. In addition, various intracellular pathogens use their hosts' protein-sorting machinery at different stages of their life cycles. These facts underscore the importance of understanding the mechanisms of protein sorting at a molecular level. In this article, we review recent advances in the identification of signals that direct proteins to their correct intracellular locations and of the recognition molecules that bind to the signals. The implications of these findings for the trafficking of various proteins are discussed.