Rab27a regulates the peripheral distribution of melanosomes in melanocytes

Rab GTPases are regulators of intracellular membrane traffic. We report a p ossible function of Rab27a, a protein implicated in several diseases, inclu ding Griscelli syndrome, choroideremia, and the Hermansky-Pudlak syndrome m ouse model, gunmetal. We studied endogenous Rab27a and overexpressed enhanc ed GFP-Rab27a fusion protein in several cultured melanocyte and melanoma-de rived cell lines. In pigmented cells, we observed that Rab27a decorates mel anosomes, whereas in nonpigmented cells Rab27a colocalizes with melanosome- resident proteins. When dominant interfering Rab27a mutants were expressed in pigmented cells, we observed a redistribution of pigment granules with p erinuclear clustering. This phenotype is similar to that observed by others in melanocytes derived from the ashen and dilute mutant mice, which bear m utations in the Rab27a and MyoVa loci, respectively. We also found that myo sinVa coimmunoprecipitates with Rab27a in extracts from melanocytes and tha t both Rab27a and myosinVa colocalize on the cytoplasmic face of peripheral melanosomes in wildtype melanocytes. However, the amount of myosinVa in me lanosomes from Rab27a-deficient ashen melanocytes is greatly reduced. These results, together with recent data implicating myosinVa in the peripheral capture of melanosomes, suggest that Rab27a is necessary for the recruitmen t of myosinVa, so allowing the peripheral retention of melanosomes in melan ocytes.