Intracellular traffic is a fundamental biological process in which 10 to 20% of the proteins present in a eukaryotic cell have been estimated to, directly or indirectly, participate. Among them, small GTPases of the Rab family (over sixty proteins in humans) play a central regulatory role. Localized on the external side of all cellular compartments, they cycle between an inactive (GDP-bound) and active (GTP-bound) form. The GDP/GTP cycle is tightly coupled to a membrane association/dissociation process. Rab proteins are involved in many aspects of the life of transport intermediates (vesicles or tubules) that shuttle between compartments, such as budding from donor membranes, movement along components of the cytoskeleton, and docking/fusion events. Another important function of Rab GTPases is to regulate the dynamic formation of membrane domains on organelles. To perform their multiple tasks, Rab GTPases interact with a wide variety of effectors. Rab effectors include scaffolding proteins, adaptor proteins, lipid kinases, lipid phosphatases and tethering factors. This proposal will focus on one important class of Rab effectors, i.e. molecular motors. Three motor protein superfamilies are present in mammalian cells (kinesin, dynein and myosin) and members of all of them have been involved in a wide variety of transport events. Our group was the first to identify a direct interaction between a Rab GTPase (Rab6) and a kinesin-like protein (Rabkinesin-6, also named MKlP2 or KIF20A). Since then, several motors have been shown to be specific effectors of many Rab GTPases. It is generally thought that Rab GTPases are involved in the recruitment of molecular motors to specific membranes and/or in the regulation of motor activity. However, direct evidence for such a role for Rab GTPases is still lacking. The general objective of the present proposal is to use a combination of cell biology, biochemical and biophysical approaches to assemble a comprehensive model for the functional basis of the interactions between three Rab GTPase (Rab6, Rab8 and Rab11) with their molecular motor partners throughout their functional cycle during intracellular traffic. It involves two groups of the UMR 144 (Institut Curie, Paris) who have strong expertise in their respective fields, Rab GTPases (Bruno Goud) and 3D structure of molecular motors (Anne Houdusse). We propose to perform extensive structural-functional characterization of a range of complexes between several Rab GTPases and their interacting motor proteins to reveal the molecular basis of their selectivity.