Rab27a enables myosin Va-dependent melanosome capture by recruiting the myosin to the organelle

The peripheral accumulation of melanosomes characteristic of wild-type mous e melanocytes is driven by a cooperative process involving long-range, bidi rectional, microtubule-dependent movements coupled to capture and local mov ement in the actin-rich periphery by myosin Va, the product of the dilute l ocus. Genetic evidence suggests that Rab27a, the product of the ashen locus , functions with myosin Va in this process. Here we show that ashen melanoc ytes, like dilute melanocytes, exhibit normal dendritic morphology and mela nosome biogenesis, an abnormal accumulation of end-stage melanosomes in the cell center, and rapid, bidirectional, microtubule-dependent melanosome mo vements between the cell center and the periphery. This phenotype suggests that ashen melanocytes, like dilute melanocytes, are defective in periphera l melanosome capture. Consistent with this, introduction into ashen melanoc ytes of cDNAs encoding wild-type and GTP-bound versions of Rab27a restores the peripheral accumulation of melanosomes in a microtubule-dependent manne r. Conversely, introduction into wild-type melanocytes of the GDP-bound ver sion of Rab27a generates an ashen/dilute phenotype, Rab27a colocalizes with endstage melanosomes in wild-type cells, and is most concentrated in melan osome-rich dendritic tips, where it also colocalizes with myosin Va. Finall y, neither endogenous myosin Va nor an expressed, GFP-tagged, myosin Va tai l domain fusion protein colocalize with melanosomes in ashen melanocytes, i n contrast to that seen previously in wild-type cells. These results argue that Rab27a serves to enable the myosinVa-dependent capture of melanosomes delivered to the periphery by bidirectional, microtubule-dependent transpor t, and that it does so by recruiting the myosin to the melanosome surface. We suggest that Rab27a, in its GTP-bound and melanosome-associated form, pr edominates in the periphery, and that it is this form that recruits the myo sin, enabling capture. These results argue that Rab27a serves as a myosin V a 'receptor', and add to the growing evidence that Rab GTPases regulate ves icle motors as well as SNARE pairing.