Copy Coats: COPI Mimics Clathrin and COPII
2010; Cell Press; Volume: 142; Issue: 1 Linguagem: Inglês
10.1016/j.cell.2010.06.031
ISSN1097-4172
Autores Tópico(s)Cellular transport and secretion
ResumoThe assembly of COPI into a cage-like lattice sculpts membrane vesicles that transport cargo from the Golgi apparatus. Now, Lee and Goldberg, 2010Lee C. Goldberg J. Cell. 2010; (this issue)Google Scholar present X-ray crystal structures of COPI suggesting that these coats combine selected features of two other archetypal coats, clathrin and COPII. The assembly of COPI into a cage-like lattice sculpts membrane vesicles that transport cargo from the Golgi apparatus. Now, Lee and Goldberg, 2010Lee C. Goldberg J. Cell. 2010; (this issue)Google Scholar present X-ray crystal structures of COPI suggesting that these coats combine selected features of two other archetypal coats, clathrin and COPII. Transport vesicles ferry cargo from one cellular compartment to another. Coat proteins drive the formation of these vesicles by polymerizing onto the surface of cellular membranes (Bonifacino and Glick, 2004Bonifacino J.S. Glick B.S. Cell. 2004; 116: 153-166Abstract Full Text Full Text PDF PubMed Scopus (1289) Google Scholar). Because of the central role these coats play in intracellular trafficking in all eukaryotes, their structures and their mechanisms of assembly and disassembly have interested cell biologists for decades. Interestingly, different coats are implicated in budding from different compartments (Figure 1A ). For instance, endocytic vesicles coated with clathrin form at the plasma membrane, whereas vesicles coated with COPII complexes form at exit sites of the endoplasmic reticulum (ER). The third archetypal coat is COPI. ARF family G proteins recruit COPI to the membrane of the Golgi apparatus, which serves as the source of COPI-coated vesicles. Detailed models of clathrin and COPII coats have been built by combining X-ray crystal structures of coat subcomplexes with cryo-electron microscopy (cryo-EM) analyses of coat lattices assembled in vitro (Fath et al., 2007Fath S. Mancias J.D. Bi X. Goldberg J. Cell. 2007; 129: 1325-1336Abstract Full Text Full Text PDF PubMed Scopus (227) Google Scholar, Fotin et al., 2004Fotin A. Cheng Y. Sliz P. Grigorieff N. Harrison S.C. Kirchhausen T. Walz T. Nature. 2004; 432: 573-579Crossref PubMed Scopus (404) Google Scholar, Stagg et al., 2006Stagg S.M. Gürkan C. Fowler D.M. LaPointe P. Foss T.R. Potter C.S. Carragher B. Balch W.E. Nature. 2006; 439: 234-238Crossref PubMed Scopus (242) Google Scholar, Stagg et al., 2008Stagg S.M. LaPointe P. Razvi A. Gürkan C. Potter C.S. Carragher B. Balch W.E. Cell. 2008; 134: 474-484Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar). In this issue of Cell, Lee and Goldberg, 2010Lee C. Goldberg J. Cell. 2010; (this issue)Google Scholar present X-ray crystal structures of COPI components, making it possible to begin comparing the architectures of all three vesicle coats. Vesicle coats have two layers: an inner "adaptor" layer that interacts with cargo and G proteins and an outer layer called a "cage." For clathrin and COPII coats, these layers form sequentially, with adaptor protein complexes binding to the membrane first and then recruiting cage components. In contrast, for COPI coats, both layers are recruited together as a single complex called "coatomer," which contains seven subunits (Hara-Kuge et al., 1994Hara-Kuge S. Kuge O. Orci L. Amherdt M. Ravazzola M. Wieland F.T. Rothman J.E. J. Cell Biol. 1994; 124: 883-892Crossref PubMed Scopus (114) Google Scholar, Waters et al., 1991Waters M.G. Serafini T. Rothman J.E. Nature. 1991; 349: 248-251Crossref PubMed Scopus (379) Google Scholar). Four coatomer subunits, β, δ, γ, and ζ, resemble the subunits of the clathrin adaptor complex. To glean new insights into the architectural principles of the COPI coat, Lee and Goldberg, 2010Lee C. Goldberg J. Cell. 2010; (this issue)Google Scholar turned their attention to the remaining three subunits, α, β′, and ɛ, which together form the outer cage layer. The vesicle cage imparts geometric order to the coat (Figure 1). For example, cryo-electron tomography studies of clathrin-coated vesicles revealed various lattices drawn from the "soccer ball" family (Cheng et al., 2007Cheng Y. Boll W. Kirchhausen T. Harrison S.C. Walz T. J. Mol. Biol. 2007; 365: 892-899Crossref PubMed Scopus (96) Google Scholar). The assembly unit of the clathrin cage is the triskelion, with three elongated clathrin heavy chains joined at a central hub (Figures 1B and 1C). Cryo-EM studies of clathrin cages assembled in vitro revealed, at subnanometer resolution, how leg segments intertwine to form each edge of the lattice (Fotin et al., 2004Fotin A. Cheng Y. Sliz P. Grigorieff N. Harrison S.C. Kirchhausen T. Walz T. Nature. 2004; 432: 573-579Crossref PubMed Scopus (404) Google Scholar). The central hub of the triskelion resides at a vertex where three lattice edges intersect; indeed, the trimeric nature of the assembly unit guarantees that all the vertices in a clathrin cage are formed by the intersection of three edges (Figure 1C). The structure of the COPII cage is remarkably different (Fath et al., 2007Fath S. Mancias J.D. Bi X. Goldberg J. Cell. 2007; 129: 1325-1336Abstract Full Text Full Text PDF PubMed Scopus (227) Google Scholar, Stagg et al., 2006Stagg S.M. Gürkan C. Fowler D.M. LaPointe P. Foss T.R. Potter C.S. Carragher B. Balch W.E. Nature. 2006; 439: 234-238Crossref PubMed Scopus (242) Google Scholar, Stagg et al., 2008Stagg S.M. LaPointe P. Razvi A. Gürkan C. Potter C.S. Carragher B. Balch W.E. Cell. 2008; 134: 474-484Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar). First, the assembly unit is not a triskelion but rather a rod-shaped heterotetramer of two Sec13 and two Sec31 subunits. Each (Sec13/31)2 assembly unit forms a single edge of the COPII cage (Figure 1C). Second, the vertices of the COPII cage are formed by the intersection of four edges, not three. As a result, the possible geometries for COPII and clathrin cages are completely different. The soccer ball lattice, in which all vertices are three-way and each face is either a pentagon or a hexagon, is unavailable to COPII; instead, COPII forms lattices with triangular and square, pentagonal, and/or hexagonal faces. Lee and Goldberg, 2010Lee C. Goldberg J. Cell. 2010; (this issue)Google Scholar now provide insight into the structure of the COPI cage. They began by mapping the interactions among the relevant subunits, α-, β′-, and ɛ-COP. First, they generated soluble αβ′ɛ-COP complexes by coexpressing the full-length versions of the subunits from bovine (Bos taurus) in insect cells. Then, to identify stable core fragments of this complex for crystallography, they subjected αβ′ɛ-COP to limited proteolysis. This approach led to the identification of a subcomplex containing about 40% of the intact αβ′ɛ-COP complex, including the majority of β′-COP and a central region of α-COP. Although the αβ′-COP subcomplex from B. taurus failed to yield useful crystals, the equivalent subcomplex from the budding yeast Saccharomyces cerevisiae generated high-quality crystals that diffracted to 2.5 Å resolution. As anticipated by previous sequence analysis, αβ′-COP is built of structural modules called α-solenoids and β-propellers (Figure 1B). Although the details differ, the structure of the αβ′-COP subcomplex is strongly reminiscent of the Sec13/31 COPII subcomplex (Fath et al., 2007Fath S. Mancias J.D. Bi X. Goldberg J. Cell. 2007; 129: 1325-1336Abstract Full Text Full Text PDF PubMed Scopus (227) Google Scholar); each comprises two β-propeller domains at one end, followed by an extended α-solenoid (Figure 1B). However, unlike Sec13/31, the αβ′-COPI heterodimer is distinctly curved, with an overall shape that resembles a comma. The most striking feature of the X-ray structure presented by Lee and Goldberg is that three αβ′-COP subcomplexes form a triskelion (Figure 1B). If the intermolecular interactions observed in these crystals are indeed physiologically relevant, then it would seem that the vertices of the COPI lattice are three-way intersections, similar to those of the clathrin lattice, rather than four-way intersections as found in the COPII lattice (Figure 1C). In support of this conclusion, a temperature-sensitive mutation in α-COP (sec27-95) (Prinz et al., 2000Prinz W.A. Grzyb L. Veenhuis M. Kahana J.A. Silver P.A. Rapoport T.A. J. Cell Biol. 2000; 150: 461-474Crossref PubMed Scopus (223) Google Scholar) maps to the three-way interface observed in the crystal structure. It is important to note that Lee and Goldberg, 2010Lee C. Goldberg J. Cell. 2010; (this issue)Google Scholar are not proposing that COPI triskelia are stable outside the context of the assembled cage. Indeed, they find no sign of triskelia in solution, even at high concentrations of αβ′-COP (i.e., 30 mg/ml). These results are consistent with weak interactions at the vertices of the lattice, a design feature that may facilitate efficient remodeling and disassembly of both COPI and COPII cages. Nonetheless, a major goal of future studies must be to discover conditions that promote the assembly of COPI cages in vitro; then the nature of the lattice can be examined directly. Another challenge for the future will be to complete the model of the COPI cage by filling in the missing regions between the vertices. Clearly, the remaining 60% of αβ′ɛ-COP must be involved. The authors hypothesize that N-terminal regions of the α subunit, which were not included in the αβ′-COP subcomplex, mediate formation of symmetrical (αβ′ɛ)2 dimers and that these dimers represent the assembly unit of the COPI cage. This (αβ′ɛ)2 dimer would be roughly analogous to the symmetrical (Sec13/31)2 dimer that serves as the assembly unit for the COPII cage, but, instead of being straight, it would be S shaped. Taken together, these results suggest that the COPI cage combines features of both the COPII and clathrin cages. As with COPII, the vertices of the COPI lattice appear to involve interactions among N-terminal β-propeller domains (Figure 1B). On the other hand, by forming three-way vertices rather than four-way vertices, COPI polymerization is expected to generate the soccer-ball family of lattices heretofore associated exclusively with clathrin; it will be fascinating to see whether this prediction is borne out by future studies. In the meantime, the results of Lee and Goldberg provide an exciting first glimpse of the COPI cage and set the stage for mechanistic studies of coat assembly and disassembly. Structure of Coatomer Cage Proteins and the Relationship among COPI, COPII, and Clathrin Vesicle CoatsLee et al.CellJune 24, 2010In BriefCOPI-coated vesicles form at the Golgi apparatus from two cytosolic components, ARF G protein and coatomer, a heptameric complex that can polymerize into a cage to deform the membrane into a bud. Although coatomer shares a common evolutionary origin with COPII and clathrin vesicle coat proteins, the architectural relationship among the three cages is unclear. Strikingly, the αβ′-COP core of coatomer crystallizes as a triskelion in which three copies of a β′-COP β-propeller domain converge through their axial ends. Full-Text PDF Open Archive
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