A Rab1 interactome illuminates a dual role in autophagy and membrane trafficking.

The small GTPase Rab1 is found in all eukaryotes and acts in both ER-to-Golgi transport and autophagy. Several Rab1 effectors and regulators have been identified, but the mechanisms by which Rab1 orchestrates these distinct processes remain incompletely understood. We apply MitoID, a proximity biotinylation approach, to expand the interactome of human Rab1A and Rab1B. We identify new interactors among known membrane traffic and autophagy machinery, as well as previously uncharacterized proteins. One striking set of interactors are the cargo receptors for selective autophagy, indicating a broader role for Rab1 in autophagy than previously supposed. Two cargo receptor interactions are validated in vitro, with the Rab1-binding site in optineurin being required for mitophagy in vivo. We also find an interaction between Rab1 and the dynein adaptor FHIP2A that can only be detected in the presence of membranes. This explains the recruitment of dynein to the ER-Golgi intermediate compartment and demonstrates that conventional methods can miss a subset of effectors of small GTPases.