Retromer oligomerization drives SNX-BAR coat assembly and membrane constriction

The retromer coat mediates protein exit from endosomes and impacts many signaling pathways, lysosomal biogenesis, and diseases such as Parkinsons, Alzheimers and COVID-19. Retromer complexes (CSC in yeast) form coats by interconnecting sorting nexins (SNX). The dynamics of this process is poorly explored. Here, we analyze the oligomerization of CSC/SNX-BAR retromer coats on oriented synthetic lipid tubules. SNX-BARs and CSC assemble a static tubular coat that does not exchange subunits. Coat formation proceeds bidirectionally, adding new subunits at both ends of the coat. High concentrations of SNX-BARs alone suffice to constrict membrane tubes to an invariant radius of 19 nm. At lower concentrations, CSC-complexes must drive constriction, which requires their oligomerization. CSCs populate the SNX-BAR layer at densities that increase with the starting radius of the membrane tube. We hence propose that retromer-mediated crosslinking of SNX-BARs at variable densities tunes the coat according to the energy required to deform the membrane. This model is supported by the effects of mutations interfering with retromer oligomerization, which impair retromer function in yeast and human cells.