Identification of an organelle receptor for myosin-Va.
Little is known about how molecular motors bind to their vesicular cargo.  Here we show that myosin-Va, an actin-based vesicle motor, binds to one of its cargoes, the melanosome, by interacting with a receptor-protein complex containing Rab27a and melanophilin, a postulated Rab27a effector.  Rab27a binds to the melanosome first and then recruits melanophilin, which in turn recruits myosin-Va. Melanophilin creates this link by binding to Rab27a in a GTP-dependent fashion through its amino terminus, and to myosin-Va through its carboxy terminus.  Moreover, this latter interaction, similar to the ability of myosin-Va to colocalize with melanosomes and influence their distribution in vivo, is absolutely dependent on the presence of exon-F, an alternatively spliced exon in the myosin-Va tail.  These results provide the first molecular description of an organelle receptor for an actin-based motor, illustrate how alternate exon usage can be used to specify cargo, and further expand the functional repertoire of Rab GTPases and their effectors.