Rab GTPases and myosin motors in organelle motility.

The actin cytoskeleton is essential to ensure the proper location of, and communication between, intracellular organelles. Some actin-based myosin motors have been implicated in this process, particularly members of the class V myosins. We discuss here the emerging role of the Ras-like GTPases of the Rab family as regulators of myosin function in organelle transport. Evidence from yeast secretory vesicles and mitochondria, and mammalian melanosomes and endosomes suggests that Rab GTPases are crucial components of the myosin organelle receptor machinery. Better understood is the case of the melanosome where Rab27a recruits a specific effector called melanophilin, which in turn binds myosin Va. The presence of a linker protein between a Rab and a myosin may represent a general mechanism. We argue that Rabs are ideally suited to perform this role as they are exquisite organelle markers. Furthermore, the molecular switch property of Rabs may enable them to regulate the timing of the myosin association with the target organelle.