Rab3a and SNARE proteins: potential regulators of melanosome movement

Melanosomes are specialized organelles that undergo a dynamic process of transport along the melanocyte dendrite to the dendrite tip and transfer to keratinocytes. We hypothesized that soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE), which are involved in membrane fusion, and rab3a, a GTP-binding protein involved in exocytosis in neuronal cells and in SNARE complex assembly, may play a part in melanosome transport and transfer. By reverse transcription-polymerase chain reaction we identified transcripts for rab3a, vesicle-associated membrane protein-2, synaptosome-associated proteins of 23 kDa and 25 kDa, and syntaxin-4 in murine melanocytic cells. We also showed that purified melanosome preparations contain rab3a and SNARE, including vesicle-associated membrane protein-2, syntaxin-4, synaptosome-associated proteins 23 kDa and 25 kDa, and the SNARE accessory protein, alpha-soluble N-ethylmaleimide-sensitive factor attachment protein. Ultraviolet radiation is a potent stimulus for melanosome transport and transfer. We show that ultraviolet radiation rapidly suppresses melanosome-associated rab3a expression and that this occurs at the protein and mRNA level. Finally, we show that vesicle-associated membrane protein-2 and synaptosome-associated protein 23 kDa coimmunoprecipitate from purified melanocytic cell membranes, suggesting that they form complexes. The presence of rab3a and SNARE on melanosomes, and of SNARE complexes in melanocytic cell membranes suggests that these proteins play a part in targeting melanosomes to the plasma membrane, to melanosome transfer to keratinocytes, or both.