Myosin V associates with melanosomes in mouse melanocytes: evidence that myosin V is an organelle motor.

Mice with mutations at the dilute locus exhibit a 'washed out' or 'diluted' coat color. The pigments that are responsible for the coloration of mammalian hair are produced by melanocytes within a specialized organelle, the melanosome. Each melanocyte is responsible for delivering melanosomes via its extensive dendritic arbor to numerous keratinocytes, which go on to form the pigmented hair shaft. In this study, we show by light immunofluorescence microscopy and immunoelectron microscopy that the myosin V isoform encoded by the dilute locus associates with melanosomes. This association, which was seen in all mouse melanocyte cell lines examined and with two independent myosin V antibodies, was evident not only within completely melanized cells, but also within cells undergoing the process of melanosome biogenesis, where coordinate changes in the distributions of a melanosome marker and myosin V were seen. To determine where myosin V, a known actin-based motor, might play a role in melanosome transport, we also examined the cellular distribution of F-actin. The only region where myosin V and F-actin were both concentrated was in dendrites and dendritic tips, which represent the sole destination for melanosomes and where they accumulate in cultured melanocytes. These results support the idea that myosin V serves as the motor for the outward movement of melanosomes within dendritic extensions, and, together with the available information regarding the phenotype of mutant melanocytes in vitro, argue that coat color dilution is caused by the absense of this myosin V-dependent melanosome transport system.