Melanosomes are specialized organelles that produce the photoprotective pigment melanin in skin. Synthesized in melanocytes, they are captured at dendritic tips by the motor protein myosin Va and the GTPase Rab27a. These organelles are subsequently transferred to surrounding keratinocytes - their ultimate destination - but the mechanism involved has proved elusive. Glynis Scott and co-workers now reveal how transfer might occur: through filopodia(see ). Employing time-lapse digital imaging and electron microscopy, they observe that melanocytes extend filopodia that contact neighbouring keratinocytes and that these filopodia act as conduits for melanosome transfer. The authors also demonstrate that expression of Cdc42 (a GTPase known to induce filopodia)produces a dendritic phenotype in melanocytes and more melanosome-containing filopodia. In addition, they show that melanosome-enriched fractions from human melanocytes contain the Cdc42 effectors PAK1 and N-WASP. Given that Cdc42 is activated following UV-induced release of interleukin 1 and tumour necrosis factor α, it could be part of a mechanism that upregulates melanosome transfer in response to UV irradiation.
