Small GTPase Rab3A is associated with melanosomes in melanoma cells

Rab3A is a small guanosine triphosphate (GTP)-binding protein that has been recently implicated in intracellular vesicle transport and the secretion o f neurotransmitters in neuronal cells, We demonstrate here that Rab3A is as sociated with melanosomes in pigment cells. Rab3A as well as Rabphilin3A, a putative target protein of Rab3A, mere detected in the melanosome fraction , purified from B16 murine melanoma cells by sucrose density gradient ultra centrifugation, Tn contrast, Rab GDP dissociation inhibitor (GDI), a GDP/GT P exchange protein for Rab3A, was found in the cytosol fraction, Further st udies using confocal laser scanning microscopy and immunoelectron microscop y revealed that immunoreactive Rab3A. is localized in conjunction with the melanosomal membrane, These results suggest the possibility of involvement of Rab3A-Rabphilin3A complex, regulated by Rab GDI, in the intracellular tr ansport of melanosomes in pigment cells.