The Structure of the Sec13/31 COPII Cage Bound to Sec23

Abstract

Structural studies have revealed some of the organizing principles and mechanisms involved in the assembly of the COPII coat including the location of the Sec23/24 adapter layer. Previous studies, however, were unable to unambiguously determine the positions of Sec23 and Sec24 in the coat. Here we have determined a cryogenic electron microscopic structure of Sec13/31 together with Sec23. Electron tomography revealed that the binding of Sec23 induces Sec13/31 to form a variety of different geometries including a cuboctahedron, as was previously characterized for Sec13/31 alone. Single particle reconstruction of the Sec13/31-23 cuboctahedra revealed that the binding of Sec23 induces a conformational change in Sec13/31 resulting in a more extended conformation. Docking Sec23 crystal structures into the EM map suggested that Sec24 projects its cargo binding surface out into the large open faces of the coat. These results have implications for the mechanisms by which COPII transports large cargos, cargos with large intracellular domains, and for tethering complexes that must project out of the coat in order to interact with their binding partners. Furthermore, Sec23 binds Sec13/31 at two unique sites in the coat, which suggests that each site may have unique roles in the mechanisms of COPII vesiculation.

Cargo adaptors: structures illuminate mechanisms regulating vesicle biogenesis

“…This same study also revealed that the Sec23/24 cargo adaptor assembled into a regular lattice on the membrane surface [48] . Taken together, the findings from this and earlier work [43, 44, 49, 46, 47] indicate that the COPII coat can adopt a variety of geometries to accommodate a range of vesicle sizes, with the Sec23/24 adaptor itself playing an unexpected structural role in determining vesicle shape [48] .…”

Organization of the ER–Golgi interface for membrane traffic control

“…Several lines of evidence indicate that the COPII cage is flexible 35,36 , and this would allow the formation of larger COPII-coated intermediates and thereby the transport of large cargo. A combination of structural approaches has revealed that SEC13–SEC31 subunits can self-assemble into polyhedral cages of distinct orders through moderate variation in geometries of subunit interactions 37–39 (FIG. 2).…”

“…c | The structures for the SEC13–SEC31 assembly unit 38 and the SEC13–SEC31 cage 37 in the presence of SEC23 (REF. 147) and SEC23–SEC24 (REF. 35) have been reported.…”

GTPase Sar1 regulates the trafficking and secretion of the virulence factor gp63 in Leishmania

“…Subsequently, it recruits Sec23/24 heterodimer via specific interactions with Sec23 to form 'prebudding' complex (23,24) in which cargo is captured by Sec24 (25,26). Eventually, Sec13/31 heterotetramer is recruited that drives the membrane deformation and stimulates the GAP activity of Sec23 towards Sar1 resulting in the release of cargo containing COPII coated vesicles from the ER membrane (27)(28)(29). However, COPII complex and its role in the secretory pathway in Leishmania is not known.…”