Structural basis for Rab6 GTPase activation by the Ric1‐Rgp1 complex

Abstract

Membrane trafficking is a highly coordinated process in eukaryotes essential for maintaining lipid homeostasis and sorting proteins to the proper location inside cells. At the hub of this dynamic set of pathways, the Golgi complex directs much of this sorting through vesicle mediated transport. Vesicles of varying lipid composition loaded with different protein cargo are in constant flux through the Golgi and precise coordination of these processes requires communication between compartments. Rab GTPases are small signaling proteins that act as molecular switches to regulate vesicular trafficking. Rab6 functions at the Golgi and is important for endosome to Golgi retrograde transport in both yeast and metazoans and for anterograde exit from the TGN in metazoans. The Ric1‐Rgp1 protein complex is a highly conserved guanine nucleotide exchange factor (GEF) required for activation of Rab6. In mammalian cells Ric1‐Rgp1 is known to be regulated by the Rab33 GTPase, but it remains unresolved how the activity of the complex is regulated and how it interacts with Rab6. Furthermore, very little is known about the structure and domain organization of Ric1‐Rgp1. We have used cryo‐EM to determine the high‐resolution structure of the yeast Ric1‐Rgp1‐Rab6(Ypt6) complex, representing the key intermediate of the nucleotide exchange reaction. This structure reveals the overall architecture of the complex and has enabled us to identify the specific interactions that govern activation of Rab6. Ongoing studies aim to test the physiological significance of these interactions and determine the orientation of this complex on the Golgi membrane surface.