The architecture of the membrane associated retromer-sorting nexin complex revealed by cryo-electron tomography
- CIMR, Cambridge.
- MRC-LMB, Cambridge.
- UNSW, Sydney.
- UQ, Brisbane.
Compartmentalisation is a defining feature of all eukaryotic cells, and we have evolved highly sophisticated protein machineries to control the flow of transmembrane molecules and membrane lipids between different organelles. The retromer protein complex is required for generating cargo-selective tubulovesicular carriers from endosomal membranes (1,2). Retromer-mediated trafficking is an essential process in all eukaryotes, controlling the cellular localisation and homeostasis of hundreds of transmembrane proteins, and its disruption is associated with major neurodegenerative disorders (3). However, how retromer is assembled and how it is recruited to form membrane tubules remains unknown. Here we describe the structure of the Chaetomium thermophilum trimeric retromer complex (Vps26-Vps29-Vps35) assembled on membrane tubules with the sorting nexin protein Vps5, using X-ray crystallography, molecular modelling and cryo-electron tomography with sub-tomogram averaging at sub-nanometre resolution. The structure reveals two interwoven layers where Vps5 forms a membrane-associated lattice, while the outer lattice consists of arches of retromer that extend away from the membrane surface. Vps35 forms the legs of these arches, Vps29 sits at the apex where it is free to interact with regulatory factors, and the feet of the arches connect to each other through Vps26. Vps26 also forms the primary interface with Vps5 via the same site previously shown to mediate Snx3 and cargo interactions (4), suggesting the existence of distinct retromer-sorting nexin complexes. The architecture of the tubulovesicular retromer coat revealed here provides key insights into the conserved mechanisms of retromer assembly and retromer-mediated endosomal trafficking.