Retromer oligomerization drives SNX‐BAR coat assembly and membrane constriction

Abstract

Proteins exit from endosomes through tubular carriers coated by retromer, a complex that impacts cellular signaling, lysosomal biogenesis and numerous diseases. The coat must overcome membrane tension to form tubules. We explored the dynamics and driving force of this process by reconstituting coat formation with yeast retromer and the BAR‐domain sorting nexins Vps5 and Vps17 on oriented synthetic lipid tubules. This coat oligomerizes bidirectionally, forming a static tubular structure that does not exchange subunits. High concentrations of sorting nexins alone constrict membrane tubes to an invariant radius of 19 nm. At lower concentrations, oligomers of retromer must bind and interconnect the sorting nexins to drive constriction. Constricting less curved membranes into tubes, which requires more energy, coincides with an increased surface density of retromer on the sorting nexin layer. Retromer‐mediated crosslinking of sorting nexins at variable densities may thus tune the energy that the coat can generate to deform the membrane. In line with this, genetic ablation of retromer oligomerization impairs endosomal protein exit in yeast and human cells. image Endosomes export proteins through tubular membrane carriers. This study explores the formation of one of the coats forming these carriers, composed of retromer and SNX‐BAR proteins. Supported membrane tubes allow for the study of coat formation and membrane constriction in real time The coat grows bidirectionally and forms a static tubular structure that does not exchange subunits Retromer oligomerization provides driving force to constrict the membrane Retromer binds the lipid‐associated SNX‐BAR protein layer at variable retromer/SNX‐BAR ratios, depending on the radius of the membrane tubule to be coated.