Clathrin Coats— Threads Laid Bare

Abstract

et al. (1998) report in this issue of Cell the crystal structure of the clathrin heavy chain amino-terminal domain, a site of key interactions within clathrin coats. Clathrin Coat Components Flux of membrane and proteins through secretory and endocytic pathways in eukaryotic cells is mediated by Two oligomeric protein complexes, clathrin and adaptor proteins (APs), are major constituents of clathrin coats transport vesicles. Genesis of these vesicles involves assembly of cytoplasmic coat protein complexes onto (reviewed in Schmid, 1997). Clathrin is a three-legged molecule, formed by C-terminal association of three the donor organelle membrane. Three basic classes of vesicle coats have been identified: clathrin, COPI, and elongated heavy chains (HC), each carrying a light chain (LC) (Figure 1B). The distinctive clathrin shape, a triskel-COPII (reviewed in Schmid, 1997, and references therein). Coats are thought to physically deform a patch of the ion, is well-adapted for assembly into polygonal arrays characteristic of clathrin coats (Figure 1C). In low ionic donor organelle membrane into a new vesicle and sequester selected membrane proteins into the emerging strength, mildly acidic, high Ca 2ϩ buffers, purified clathrin triskelia self-assemble into closed polyhedral cages re-vesicle (Figure 1A). Proteins packaged into coated vesicles include cargo in transit to other organelles and sembling the coats on transport vesicles. Removal of LC subunits from triskelia does not prevent cage formation, machinery to guide and dock each vesicle to the target organelle. Thus, coats have a central role in orchestrat-indicating that self-assembly is an inherent property of HC (Ungewickell and Ungewickell, 1991, and references ing traffic between membrane organelles. Once coated vesicles have formed, coat disassembly allows vesicle therein). However, compared to intact clathrin, LC-free triskelia assemble more readily in the absence of Ca 2ϩ . fusion with the target membrane and returns coat complexes to the cytoplasm for additional cycles of vesicle These properties suggest that LC might inhibit spontaneous clathrin assembly, thereby ensuring assembly formation (Figure 1A). Since their discovery in the 1960s, clathrin coats have served as a paradigm for under-only at membrane sites destined for vesiculation (Ungewickell and Ungewickell, 1991). LC subunits can be standing the molecular basis of vesicle formation (reviewed in Schmid, 1997). Clathrin coats act at the plasma phosphorylated and bind Ca 2ϩ , calmodulin, and hsc70, all potential regulatory components (Figure 1B). membrane to form endocytic transport vesicles, at the trans-Golgi network (TGN) to form endosome-targeted AP complexes act in both clathrin assembly and cargo

Coupled Inositide Phosphorylation and Phospholipase D Activation Initiates Clathrin-coat Assembly on Lysosomes

“…This clearly establishes that the recruitment of AP-1 and AP-2 adaptors is regulated differently but, currently, very little is known about the nucleating events that precede AP-2 translocation onto membranes (9,13). Overexpression of several plasma-membrane receptors normally internalized in clathrincoated vesicles does not alter the steady-state distribution of AP-2 (14 -16), arguing that unregulated association between protein-sorting signals and AP-2 adaptors is unlikely to begin the clathrin-coat assembly process.…”

HSF-1 Interacts with Ral-binding Protein 1 in a Stress-responsive, Multiprotein Complex with HSP90 in Vivo

“…RalBP1 has also been shown to interact with the 2 subunit of AP2, which is involved in endocytosis (49,55). Therefore, active RalGTP has been suggested to stabilize association of 2 -RalBP1 with the membrane, leading to inhibition of clathrin-dependent endocytosis of certain receptors such as epidermal growth factor (49).…”

The Synaptojanin-like Protein Inp53/Sjl3 Functions with Clathrin in a Yeast TGN-to-Endosome Pathway Distinct from the GGA Protein-dependent Pathway

“…Severe growth defects have been observed in cells lacking the function of the ␤ subunit of AP-1, Apl2p, and the GGA proteins (Costaguta et al, 2001), suggesting that AP-1 and the GGAs represent distinct clathrin-associated adaptors that function in separate but partially redundant pathways. In addition, clathrin genetically and physically interacts with both AP-1 and the GGA proteins (Pishvaee and Payne, 1998;Yeung et al, 1999;Costaguta et al, 2001). Cells lacking clathrin function are severely compromised for growth and mislocalize TGN resident membrane proteins to the cell surface (Lemmon and Jones, 1987;Payne et al, 1987;Seeger and Payne, 1992), consistent with clathrin being required for both pathways.…”