Structure of the Sec13/31 COPII coat cage

Abstract

Endomembranes of eukaryotic cells are dynamic structures that are in continuous communication through the activity of specialized cellular machineries, such as the coat protein complex II (COPII), which mediates cargo export from the endoplasmic reticulum (ER). COPII consists of the Sar1 GTPase, Sec23 and Sec24 (Sec23/24), where Sec23 is a Sar1-specific GTPase-activating protein and Sec24 functions in cargo selection, and Sec13 and Sec31 (Sec13/31), which has a structural role. Whereas recent results have shown that Sec23/24 and Sec13/31 can self-assemble to form COPII cage-like particles, we now show that Sec13/31 can self-assemble to form minimal cages in the absence of Sec23/24. We present a three-dimensional reconstruction of these Sec13/31 cages at 30 A resolution using cryo-electron microscopy and single particle analysis. These results reveal a novel cuboctahedron geometry with the potential to form a flexible lattice and to generate a diverse range of containers. Our data are consistent with a model for COPII coat complex assembly in which Sec23/24 has a non-structural role as a multivalent ligand localizing the self-assembly of Sec13/31 to form a cage lattice driving ER cargo export.

ArfGAP1 promotes COPI vesicle formation by facilitating coatomer polymerization

“…49,50 COPII has also been found to assemble independent of membrane binding. [51][52][53] The function of ArfGAP1 to facilitate assembly of COPI independent of membranes is analogous to that of clathrin adaptors AP-1 and AP-2 and the catalytic function is analogous to that of Sec23 in COPII, which is thought to be analogous to clathrin adaptors. [54][55][56][57] Thus, our results are consistent with previous work identifying ArfGAP1 as a coat component.…”

Two Mammalian Sec16 Homologues Have Nonredundant Functions in Endoplasmic Reticulum (ER) Export and Transitional ER Organization

“…Sar1-GTP recruits the Sec23/Sec24 coat protein complex. The Sec13/Sec31 coat protein complex then binds and apparently polymerizes to create the coat lattice (Stagg et al, 2006). Scission of the vesicle is thought to be driven by membrane insertion of an amphipathic Nterminal helix of Sar1 (Bielli et al, 2005;Lee et al, 2005).…”

Erv26p Directs Pro-Alkaline Phosphatase into Endoplasmic Reticulum–derived Coat Protein Complex II Transport Vesicles

“…Current models envisage that the cargo recognition and binding by Sec24p is stabilized through assembly of prebudding complexes consisting of Sec23/24 and Sar1p-GTP bound to secretory cargo on the ER membrane surface. These prebudding complexes are then incorporated into an outer layer Sec13/31 scaffold structure that deforms the membrane and produces COPII-coated vesicles (Lee et al, 2004;Stagg et al, 2006).…”

“…Current models envisage that the cargo recognition and binding by Sec24p is stabilized through assembly of prebudding complexes consisting of Sec23/24 and Sar1p-GTP bound to secretory cargo on the ER membrane surface. These prebudding complexes are then incorporated into an outer layer Sec13/31 scaffold structure that deforms the membrane and produces COPII-coated vesicles (Lee et al, 2004;Stagg et al, 2006).In addition to the COPII coat proteins, cargo-specific accessory factors are required to accommodate the variety of secretory cargo exported from the ER in COPII vesicles. For example, efficient export of certain soluble secretory proteins from the ER depends on transmembrane receptor-like proteins.…”