The Exomer cargo adaptor features a flexible hinge domain

Exomer is a cargo adaptor that mediates the sorting of specific plasma membrane proteins into vesicles at the trans-Golgi network. Cargo adaptors must bind to multiple partners, including their cargo, regulatory proteins, and the membrane surface. During biogenesis of a vesicle, the membrane makes a transition from a relatively flat surface to one of high curvature, requiring cargo adaptors to somehow maintain protein-protein and protein-membrane interactions on a changing membrane environment. Here, we present the crystal structure of a tetrameric Chs5/Bch1 exomer complex and use small-angle X-ray scattering to demonstrate its flexibility in solution. The structural data suggest that the complex flexes primarily around the dimeric N-terminal domain of the Chs5 subunits, which adopts a noncanonical β sandwich fold. We propose that this flexible hinge domain enables exomer to maintain interactions in the context of a dynamic membrane environment.