Disease mechanism: unravelling Wiskott-Aldrich syndrome

The gene responsible for Wiskott-Aldrich syndrome, a disease affecting platelets and lymphocytes, has been cloned and its protein product (WASp) found to interact with the GTPase Cdc42. WASp seems to provide a link between Cdc42 and the actin cytoskeleton, perhaps explaining the cellular defects underlying the disease.

Direct interaction of the Wiskott-Aldrich syndrome protein with the GTPase Cdc42

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency disorder with the most severe pathology in the T lymphocytes and platelets. The disease arises from mutations in the gene encoding the WAS protein. T lymphocytes of affected males with WAS exhibit a severe disturbance of the actin cytoskeleton, suggesting that the WAS protein could regulate its organization. We show here that WAS protein interacts with a member of the Rho family of GTPases, Cdc42. This interaction, which is guanosine 5'-triphosphate (GTP)-dependent, was detected in cell lysates, in transient transfections and with purified recombinant proteins. A weaker interaction was also detected with Rac1 using WAS protein from cell lysates. It was also found that different mutant WAS proteins from three affected males retained their ability to interact with Cdc42 and that the level of expression of the WAS protein in these mutants was only 2-5% of normal. Taken together these data suggest that the WAS protein might function as a signal transduction adaptor downstream of Cdc42, and in affected males, the cytoskeletal abnormalities may result from a defect in Cdc42 signaling.

Wortmannin alters the transferrin receptor endocytic pathway in vivo and in vitro

Treatment with the phosphatidylinositol 3-kinase inhibitor wortmannin promotes approximately 30% decrease in the steady-state number of cell-surface transferrin receptors. This effect is rapid and dose dependent, with maximal down-regulation elicited with 30 min of treatment and with an IC50 approximately 25 nM wortmannin. Wortmannin-treated cells display an increased endocytic rate constant for transferrin internalization and decreased exocytic rate constants for transferrin recycling. In addition to these effects in vivo, wortmannin is a potent inhibitor (IC50 approximately 15 nM) of a cell-free assay that detects the delivery of endocytosed probes into a common compartment. Inhibition of the in vitro assay involves the inactivation of a membrane-associated factor that can be recruited onto the surface of vesicles from the cytosol. Its effects on the cell-free assay suggest that wortmannin inhibits receptor sorting and/or vesicle budding required for delivery of endocytosed material to "mixing" endosomes. This idea is consistent with morphological changes induced by wortmannin, which include the formation of enlarged transferrin-containing structures and the disruption of the perinuclear endosomal compartment. However, the differential effects of wortmannin, specifically increased transferrin receptor internalization and inhibition of receptor recycling, implicate a role for phosphatidylinositol 3-kinase activity in multiple sorting events in the transferrin receptor's membrane traffic pathway.

A clathrin-binding site in the hinge of the beta 2 chain of mammalian AP-2 complexes

The assembly of cytosolic clathrin into the cytoplasmic face of coated pits and coated vesicles appears to be driven by the clathrin-associated protein (AP) complexes. We have previously shown that one of the large chains of the AP complexes, the beta chain, is sufficient to drive coat assembly in vitro. This chain consists of two domains, the amino-terminal trunk and the carboxyl-terminal ear, linked by a "hinge." We report here that presence of the hinge in recombinant beta trunk or in recombinant beta ear fragments is essential for driving in vitro assembly of clathrin into coats. We have also used a binding assay to map the clathrin-binding site by nested deletion of hinge sequences to a 50-residue region in the center of the hinge. This sequence is conserved in all known beta sequences from multicellular organisms. The interaction of a single beta hinge with a clathrin triskelion is weak, and we propose that recruitment of cytosolic clathrin to a forming coated pit involves simultaneous contacts between the legs of single clathrin trimers and the beta hinges of two or three membrane-bound AP complexes. Uncoating is likely to require interruption of these contacts.

Role of the regulatory domain of the EGF-receptor cytoplasmic tail in selective binding of the clathrin-associated complex AP-2

BACKGROUND

After stimulation of a cell by the mitogenic epidermal growth factor (EGF), the EGF receptor (EGF-R) is cleared from the cell surface in order to turn off receptor signaling. This internalization is mediated via clathrin-coated pits and coated vesicles, and ultimately the receptors are delivered to the lysosome and destroyed. It is believed that clathrin-associated protein complexes or adaptors (APs) link the entrapment of EGF-R and other nutrient and growth-factor receptors to the formation of the clathrin-coated pit. Two classes of APs are known--AP-2, found at the plasma membrane, and AP-1, found in the trans-Golgi network. Activated EGF-R associates with AP-2s at the plasma membrane, but the mechanism responsible for this association is not known. Here, we investigate, in vivo and in vitro, three aspects of the interaction between APs and EGF-R: firstly, we ask whether EGF-R at the plasma membrane distinguishes between AP-1 and AP-2; secondly, we ask which part of the receptor's cytoplasmic tail is responsible for binding; finally, we ask whether autophosphorylation by EGF-R is essential for the interaction.

RESULTS

We demonstrate that EGF-R displays a selective association for AP-2 over AP-1 in vivo, and that this preferential interaction can also be detected using surface plasmon resonance in vitro. Using a truncated mutant and a kinase-dead mutant of EGF-R, we show that the regulatory domain of the cytoplasmic tail is essential for the recruitment of AP-2 in vivo and that this domain is required for association between purified AP-2 and EGF-R in vitro. Finally, we demonstrate, in vivo and in vitro, that tyrosine auto-phosphorylation by the receptor is not an essential pre-condition for the recruitment of AP-2.

CONCLUSIONS

EGF-R binds selectively to AP-2s, and the regulatory domain of its cytoplasmic tail is required for this interaction. The lack of correlation between receptor autophosphorylation and AP-2 recruitment suggests that activation of the EGF-R kinase stimulates endocytosis by the phosphorylation of a factor distinct from EGF-R itself, as also proposed by others based on experiments measuring receptor traffic and entrapment.

Interaction of tyrosine-based sorting signals with clathrin-associated proteins

Tyrosine-based signals within the cytoplasmic domain of integral membrane proteins mediate clathrin-dependent protein sorting in the endocytic and secretory pathways. A yeast two-hybrid system was used to identify proteins that bind to tyrosine-based signals. The medium chains (mu 1 and mu 2) of two clathrin-associated protein complexes (AP-1 and AP-2, respectively) specifically interacted with tyrosine-based signals of several integral membrane proteins. The interaction was confirmed by in vitro binding assays. Thus, it is likely that the medium chains serve as signal-binding components of the clathrin-dependent sorting machinery.

Saccharomyces cerevisiae Apl2p, a homologue of the mammalian clathrin AP beta subunit, plays a role in clathrin-dependent Golgi functions

Clathrin-coated vesicles mediate selective intracellular protein traffic from the plasma membrane and the trans-Golgi network. At these sites, clathrin-associated protein (AP) complexes have been implicated in both clathrin coat assembly and collection of cargo into nascent vesicles. We have found a gene on yeast chromosome XI that encodes a homologue of the mammalian AP beta subunits. Disruptions of this gene, APl2, and a previously identified beta homologue, APl1, have been engineered in cells expressing wild-type (CHC1) or temperature sensitive (chc1-ts) alleles of the clathrin heavy chain gene. APl1 or APl2 disruptions (apl1 delta or apl2 delta) yield no discernable phenotypes in CHC1 strains, indicating that the Apl proteins are not essential for clathrin function. However, the apl2 delta, but not the apl1 delta, allele enhances the growth and alpha-factor pheromone maturation defects of chc1-ts cells. Disruption of APl2 also partially suppresses the vacuolar sorting defect that occurs in chc1-ts cells immediately after imposition of the non-permissive temperature. These Golgi-specific effects of apl2 delta in chc1-ts cells provide evidence that Apl2p is a component of an AP complex that interacts with clathrin at the Golgi apparatus.

Molecular characterization of an apical early endosomal glycoprotein from developing rat intestinal epithelial cells

The apical endosomal compartment is thought to be involved in the sorting and selective transport of receptors and ligands across polarized epithelia. To learn about the protein components of this compartment, we have isolated and sequenced a cDNA that encodes a glycoprotein that is located in the apical endosomal tubules of developing rat intestinal epithelial cells. The deduced amino acid sequence predicts a protein of 1216 amino acids with a molecular mass of 133,769 Da. The deduced amino acid sequence together with amino-terminal amino acid sequencing indicate that there is a cleaved 21-amino acid signal sequence at the NH2-terminal portion of the molecule. There is a single hydrophobic region near the carboxyl terminus that has the characteristics of a membrane-spanning domain and a 36-amino acid cytoplasmic tail. We have found that the major form of this protein in intestinal epithelial cells has a molecular mass of 55-60 kDa, which is significantly smaller than the size predicted from the cDNA sequence, suggesting that the protein is synthesized as a large precursor and processed to the smaller form. The smaller form remains associated with the membrane, however, possibly through noncovalent association with the transmembrane portion of the molecule or with another membrane protein. The extracytoplasmic domain is cysteine-rich, with three cysteine-rich repeats that are similar to cysteine repeats present in several receptor proteins. However, there is no other significant similarity to other proteins in the GenBank. The cytoplasmic tail contains a possible internalization motif and several consensus motifs for serine/threonine kinases. Northern blot analysis suggests a single abundant message, and Southern blot analysis is consistent with a single gene and the absence of pseudogenes for this unique endosomal protein.

Stoichiometric interaction of the epidermal growth factor receptor with the clathrin-associated protein complex AP-2

Plasma membrane clathrin-associated protein complexes (AP-2) have been shown to co-immunoprecipitate with the epidermal growth factor (EGF) receptor (Sorkin A., and Carpenter, G. (1993) Science 261, 612-615). Hence, we analyzed the stoichiometry of the EGF receptor interaction with AP-2 using a new antibody that efficiently immunoprecipitates native AP-2. EGF receptor AP-2 complexes were isolated from 35S-labeled cells treated with EGF by EGF receptor affinity chromatography followed by precipitation with the antibody to AP-2. Quantitation of the relative molar concentrations of the proteins found in the complex revealed that 1 mol of AP-2 was associated with approximately 1.1 mol of EGF receptor. No other proteins were present in significant molar concentrations relative to AP-2, indicating that other proteins are not stoichiometrically involved in the interaction of EGF receptors and AP-2 in vivo. Co-immunoprecipitation experiments in cells expressing a mutant EGF receptor demonstrated that the cytoplasmic carboxyl-terminal 214 residues of the EGF receptor are essential for interaction with AP-2.

A late Golgi sorting function for Saccharomyces cerevisiae Apm1p, but not for Apm2p, a second yeast clathrin AP medium chain-related protein

Mammalian clathrin-associated protein (AP) complexes, AP-1 (trans-Golgi network) and AP-2 (plasma membrane), are composed of two large subunits of 91-107 kDa, one medium chain (mu) of 47-50 kDa and one small chain (sigma) of 17-19 kDa. Two yeast genes, APM1 and APM2, have been identified that encode proteins related to AP mu chains. APM1, whose sequence was reported previously, codes for a protein of 54 kDa that has greatest similarity to the mammalian 47-kDa mu 1 chain of AP-1. APM2 encodes an AP medium chain-related protein of 605 amino acids (predicted molecular weight of 70 kDa) that is only 30-33% identical to the other family members. In yeast containing a normal clathrin heavy chain gene (CHC1), disruptions of the APM genes, singly or in combination, had no detectable phenotypic consequences. However, deletion of APM1 greatly enhanced the temperature-sensitive growth phenotype and the alpha-factor processing defect displayed by cells carrying a temperature-sensitive allele of the clathrin heavy chain gene. In contrast, deletion of APM2 caused no synthetic phenotypes with clathrin mutants. Biochemical analysis indicated that Apm1p and Apm2p are components of distinct high molecular weight complexes. Apm1p, Apm2p, and clathrin cofractionated in a discrete vesicle population, and the association of Apm1p with the vesicles was disrupted in CHC1 deletion strains. These results suggest that Apm1p is a component of an AP-1-like complex that participates with clathrin in sorting at the trans-Golgi in yeast. We propose that Apm2p represents a new class of AP-medium chain-related proteins that may be involved in a nonclathrin-mediated vesicular transport process in eukaryotic cells.

A rab protein is required for the assembly of SNARE complexes in the docking of transport vesicles

Rab proteins are generally required for transport vesicle docking. We have exploited yeast secretion mutants to demonstrate that a rab protein is required for v-SNAREs and t-SNAREs to assemble. The absence of the rab protein in the docking complex suggests that, in a broad sense, rab proteins participate in a reaction catalyzing SNARE complex assembly. In so doing, rab proteins could help impart an additional layer of specificity to vesicle docking. This mechanism likely involves the Sec1 homolog Sly1, which we identified in isolated docking complexes. We also report the identification of a novel v-SNARE (Ykt6p) component of the yeast ER-Golgi docking complex that has a CAAX box and is predicted to be lipid anchored. The surprising finding that docking complexes can contain many distinct species of SNAREs (Sed5p, Bos1p, Sec22p, Ykt6p, and likely Bet1p, p28, and p14) suggests that multimeric interactions are features of the fusion machinery, and may also improve the fidelity of vesicle targeting.

The Saccharomyces cerevisiae APS1 gene encodes a homolog of the small subunit of the mammalian clathrin AP-1 complex: evidence for functional interaction with clathrin at the Golgi complex

Clathrin-associated protein (AP) complexes have been implicated in the assembly of clathrin coats and the selectivity of clathrin-mediated protein transport processes. We have identified a yeast gene, APS1, encoding a homolog of the small (referred to herein as sigma) subunits of the mammalian AP-1 complex. Sequence comparisons have shown that Aps1p is more similar to the sigma subunit of the Golgi-localized mammalian AP-1 complex than Aps2p, which is more related to the plasma membrane AP-2 sigma subunit. Like their mammalian counterparts, Aps1p and Aps2p are components of distinct, large (> 200 kDa) complexes and a significant portion of the Aps proteins co-fractionate with clathrin-coated vesicles during gel filtration chromatography. Unexpectedly, even though the evolutionary conservation of AP small subunits is substantial (50% identity between mammalian and yeast proteins), disruptions of APS1 (aps1 delta) and APS2 (aps2 delta), individually or in combination, elicit no detectable mutant phenotypes. These data indicate that the Aps proteins are not absolutely required for clathrin-mediated selective protein transport in cells expressing wild type clathrin. However, aps1 delta accentuated the slow growth and alpha-factor pheromone maturation defect of cells carrying a temperature-sensitive allele of clathrin heavy chain (Chc) (chc1-ts). In contrast, aps1 delta did not influence the effects of chc1-ts on vacuolar protein sorting or receptor-mediated endocytosis. The aps2 delta mutation resulted in a slight effect on chc1-ts cell growth but had no additional effects. The growth defect of cells completely lacking Chc was compounded by aps1 delta but not aps2 delta. These results comprise evidence that Aps1p is involved in a subset of clathrin functions at the Golgi apparatus. The effect of aps1 delta on cells devoid of clathrin function suggests that Aps1p also participates in clathrin-independent processes.

Arrangement of domains, and amino acid residues required for binding of vascular cell adhesion molecule-1 to its counter-receptor VLA-4 (alpha 4 beta 1)

Interaction of the vascular cell adhesion molecule (VCAM-1) with its counter-receptor very late antigen-4 (VLA-4) (integrin alpha 4 beta 1) is important for a number of developmental pathways and inflammatory functions. We are investigating the molecular mechanism of this binding, in the interest of developing new anti-inflammatory drugs that block it. In a previous report, we showed that the predominant form of VCAM-1 on stimulated endothelial cells, seven-domain VCAM (VCAM-7D), is a functionally bivalent molecule. One binding site requires the first and the other requires the homologous immunoglobulin-like domain. Rotary shadowing and electron microscopy of recombinant soluble VCAM-7D molecules suggests that the seven Ig-like domains are extended in a slightly bent linear array, rather than compactly folded together. We have systematically mutagenized the first domain of VCAM-6D (a monovalent, alternately spliced version mission domain 4) by replacing 3-4 amino acids of the VCAM sequence with corresponding portions of the related ICAM-1 molecule. Specific amino acids, important for binding VLA-4 include aspartate 40 (D40), which corresponds to the acidic ICAM-1 residue glutamate 34 (E34) previously reported to be essential for binding of ICAM-1 to its integrin counter-receptor LFA-1. A small region of VCAM including D40, QIDS, can be replaced by the similar ICAM-1 sequence, GIET, without affecting function or epitopes, indicating that this region is part of a general integrin-binding structure rather than a determinant of binding specificity for a particular integrin. The VCAM-1 sequence G65NEH also appears to be involved in binding VLA-4.

The beta 1 and beta 2 subunits of the AP complexes are the clathrin coat assembly components

The beta 1 and beta 2 subunits are the closely-related large chains of the trans-Golgi network AP-1 and the plasma membrane AP-2 clathrin-associated protein complexes, respectively. Recombinant beta 1 and beta 2 subunits have been generated in Escherichia coli. It was found that, in the absence of all the other AP subunits, beta 1 and beta 2 interact with clathrin and drive the efficient assembly of clathrin coats. In addition, beta 2 subunits and AP complexes compete for the same clathrin binding site. The appearance of the clathrin/beta coats is the same as the barrel-shaped structures formed with native AP complexes. It is proposed that the principal function of the beta subunits is to initiate coat formation, while the remaining subunits of the AP complexes have other roles in coated pit and coated vesicle function.

Immunoelectron microscopic evidence for the extended conformation of light chains in clathrin trimers

Clathrin is a major component of the basket-like network of hexagons and pentagons that forms the "coat" on the cytoplasmic face of the plasma membrane and the trans Golgi network during the invagination of coated pits. Soluble clathrin is a three-legged structure (triskelion) comprising three identical heavy chains and three different light chains located toward the center of the triskelion on the proximal segment of the leg. All mammalian light chains contain a central domain of 10 heptad repeats, which is necessary for the interaction with heavy chain. Because the repeats are characteristic of alpha helical coiled coils, we proposed that the central domain had an extended conformation (Kirchhausen, T., Scarmato, P., and Harrison, S. C. et al. (1987) Science 236, 320-324). However, an alternative model has recently been proposed (Nathke, I. S., Heuser, J., Lupas, A., Stock, J., Turck, C. W., and Brodsky, F. M. (1992) Cell 68, 899-910). Here, we use single-molecule electron microscopy of clathrin decorated with monoclonal antibodies directed against different epitopes on light chains to show that the light chain central domain has an extended conformation and reaches along most of the proximal segment of the heavy chain leg.

Location of the domains of ICAM-1 by immunolabeling and single-molecule electron microscopy

Intercellular adhesion molecule 1 (ICAM-1), a member of the immunoglobulin gene superfamily, is a cell surface glycoprotein with an extracellular domain comprising five immunoglobulin-like domains. Soluble ICAM-1, a recombinant protein truncated at the transmembrane domain, has a rod-like shape, 19 nm long overall, with a characteristic bend 7.6 nm from one end of the molecule. Because the link between domain D2 and domain D3 is proline rich, it has been proposed that the short arm contains domains D1 and D2 and the long arm contains domains D3-D5. We used single-molecule electron microscopy of soluble ICAM-1 decorated with monoclonal antibodies specific for domains D1 and D4 to show that the bend instead lies between domains D3 and D4. Therefore, the short arm lies closer to the plasma membrane, whereas the long arm, containing all the known ligand binding sites on ICAM-1, is positioned toward the target cell surface.

The medium chains of the mammalian clathrin-associated proteins have a homolog in yeast

We have cloned and sequenced mouse brain AP47, the medium chain of the trans-Golgi network clathrin-associated protein complex AP-1. The predicted protein sequence of AP47 is closely related to rat and calf brain AP50, the corresponding medium chain of the plasma-membrane clathrin-associated protein complex AP-2. We have also identified in the yeast genome an open reading frame encoding a protein of previously unknown function. Referred to here as YAP54, its predicted protein sequence displays a striking homology to AP47. We therefore propose that Yap54 is the medium chain subunit of a putative AP-1 complex in yeast. From the analyses of the optimized sequence alignments of AP47, AP50 and Yap54p, we suggest a model for the domain organization of the medium chains.

AP17 and AP19, the mammalian small chains of the clathrin-associated protein complexes show homology to Yap17p, their putative homolog in yeast

AP17 and AP19 are the smallest polypeptide chain components of AP-2 and AP-1, the clathrin-associated protein complexes found in coated structures of the plasma membrane and Golgi apparatus of mammalian cells. cDNA clones representing the entire coding sequence of AP17 and AP19 were isolated from rat and mouse brain cDNA libraries, respectively. Determination of their nucleotide sequence predicts proteins of 142 and 158 amino acids with Mr 17,018 and 18,733. A sequence comparison of rat brain AP17 with mouse brain AP19 demonstrates that the small chains are highly related. A computer search for other related proteins has uncovered in yeast a previously unknown gene whose DNA sequence encodes a protein homologous to the small chain of AP complexes. The yeast sequence predicts Yap17p, a protein with 147 amino acids and a Mr of 17,373 that is slightly more related to the mammalian AP17 chain than to its AP19 counterpart.

Clathrin domains involved in recognition by assembly protein AP-2

The domains on clathrin responsible for interaction with the plasma membrane-associated assembly protein AP-2 have been studied using a novel cage binding assay. AP-2 bound to pure clathrin cages but not to coat structures already containing AP that had been prepared by coassembly. Binding to preassembled cages also occurred in the presence of elevated Tris-HCl concentrations (greater than or equal to 200 mM) which block AP-2 interactions with free clathrin. AP-2 interactions with assembled cages could also be distinguished from AP-2 binding to clathrin trimers by sodium tripolyphosphate (NaPPPi), which binds to the alpha subunit of AP-2 (Beck, K., and Keen, J. H. (1991) J. Biol. Chem. 266, 4442-4447). At concentrations of 1-5 mM, NaPPPi blocked clathrin-triskelion binding; in contrast, interactions with cages persisted in the presence of 25 mM NaPPPi. To begin to identify the region(s) of the clathrin molecule important in recognition by AP-2, clathrin cages were proteolyzed to remove heavy chain terminal domains and portions of the distal leg as well as all of the light chains. AP-2 bound to these "clipped cages"; however, unlike the interaction with native cages, binding of AP-2 to clipped cages was sensitive to the lower concentrations of both Tris-HCl and NaPPPi which disrupt interactions of AP-2 with clathrin trimers. Reconstitution of the clipped cages with clathrin light chains did not restore resistance of AP-2 binding to Tris-HCl. We conclude that one binding site for AP-2 resides on the hub and/or proximal part of the clathrin triskelion whereas a second site is likely to involve the terminal domain and/or distal leg; the second site is manifested only in the assembled lattice structure. We suggest that these two distinct binding interactions may be mediated by the two unique large subunits within the AP-2 complex, acting sequentially during assembly.

Stabilization of clathrin coats by the core of the clathrin-associated protein complex AP-2

AP-2 is the class of clathrin-associated protein complex found in coated vesicles derived from the plasma membrane of eukaryotic cells. We demonstrate here, using a chemical method, that an AP-2 complex is an asymmetric structure consisting of one large alpha chain, one large beta chain, one medium AP50 chain, and one small AP17 chain. The complex has been shown to contain a core and two appendages. The AP core includes the small AP17 and the medium AP50 chains together with the amino-terminal domains of the large alpha and beta chains. One appendage corresponds to the carboxy-terminal domain of the beta chain. We find that as in the case of the beta chains, the carboxy-terminal portion of the alpha chains is an independently folded domain corresponding to the second appendage. We use limited tryptic proteolysis of clathrin/AP-2 coats to show the release of the appendages from the interior of the coats and the retention of the AP core by the remaining clathrin lattice. In addition, we find that the AP core stabilizes the coat and prevents its depolymerization. These results are consistent with the proposal that the AP core contains the binding site(s) for clathrin, while the alpha- and beta-chain appendages interact with membrane components of coated pits and coated vesicles.

Identification of a putative yeast homolog of the mammalian beta chains of the clathrin-associated protein complexes

The clathrin-associated protein complexes are heterotetrameric structures believed to interact with clathrin and with membrane components of mammalian coated pits and coated vesicles. I have identified a yeast homolog of the mammalian beta-type large chains, suggesting the existence in yeast cells of clathrin-associated protein complexes. A sequence comparison between the putative yeast beta-type chain and its mammalian counterparts shows that their amino-terminal domains are related over their entire length and that their carboxyl-terminal domains diverge completely. This observation is consistent with our earlier proposal (T. Kurchhausen et al., Proc. Natl. Acad. Sci. USA 86:2612-2616, 1989) for the bifunctional-domain organization of the large chains, in which the invariant amino-terminal region interacts with conserved proteins of the coat while the variable carboxyl-terminal domain interacts with different membrane components of coated pits and coated vesicles.