Delineating the shape of COPII coated membrane bud

Curvature-generating proteins that direct membrane trafficking assemble on the surface of lipid bilayers to bud transport intermediates, which move protein and lipid cargoes from one cellular compartment to another. Our recent study on the COPII protein Sar1 showed that the inserted volume of the protein into the membrane determines the degree of membrane bending. However, it is unclear what controls the overall shape of the membrane bud once curvature induction has begun. In vitro experiments showed that excessive concentrations of Sar1 promoted the formation of membrane tubules from synthetic vesicles, while COPII-coated transport intermediates in cells are generally more spherical or lobed in shape. To understand the origin of these morphological dissimilarities, we employ atomistic, coarse-grained (CG), and continuum mesoscopic simulations of membranes in the presence of multiple curvature-generating proteins. We first demonstrate the membrane bending ability of amphipathic peptides derived from the amino terminus of Sar1, as a function of inter-peptide angle and concentration using an atomistic bicelle simulation protocol. Then, we employ CG (MARTINI) simulations to reveal that Sec23 and Sec24 control the relative spacing between Sar1 protomers and form the inner-coat unit through an attachment with Sar1. Finally, using Dynamical Triangulated Surface (DTS) simulations based on the Helfrich Hamiltonian we demonstrate that the uniform distribution of spacer molecules among curvature-generating proteins is crucial to the spherical budding of the membrane. Overall, we show that Sec23 and Sec24 act as a spacer to preserve a dispersed arrangement of Sar1 protomers and to help determine the overall shape of the membrane bud.