Fig. 7: A map of the COPII coat assembly network.

A schematic model for how COPII assembles on membranes. a Overview of COPII organisation on budding ER membrane. Left panel: a fully functional COPII coat promotes budding of cargo-containing membranes. Right panel: a coat whose outer coat is unable to polymerise to form cages is still able to induce budding of cargo-replete, easily deformable membranes by promoting inner coat assembly. b Details of three sets of interactions contribute to coat assembly: 1. Outer-outer coat. Mediated by Sec31 β-propellers forming vertices, by Sec31 β-propellers binding to the α-solenoid domain of a different protomer to create ‘bridging’ rods, and by Sec31 CTD binding to Sec31 α-solenoid domain. It is unclear whether the latter interaction occurs in cis or trans. 2. Outer-inner coat. Mediated by Sec31 disordered region, contributing three interaction sites: triple-proline motifs bind to Sec23 bridging between neighbouring subunits; the active peptide binds across Sec23 and Sar1 to accelerate GAP activity, and positively charged clusters bind to a negatively charged groove on Sec23. 3. Inner–inner coat. Mediated by Sec31 PPP motifs (see above), and by an extended lattice interface which includes Sec23–Sec23 interactions as well as Sec23–Sar1 interactions mediated by the L-loop.