Microtubule motor transport in the delivery of melanosomes to the actin-rich apical domain of the retinal pigment epithelium.

Melanosomes are motile, light-absorbing organelles that are present in pigment cells of the skin and eye. It has been proposed that melanosome localization, in both skin melanocytes and the retinal pigment epithelium (RPE), involves melanosome capture from microtubule motors by an unconventional myosin, which dynamically tethers the melanosomes to actin filaments. Recent studies with melanocytes have questioned this cooperative capture model. Here, we test the model in RPE cells by imaging melanosomes associated with labeled actin filaments and microtubules, and by investigating the roles of different motor proteins. We found that a deficiency in cytoplasmic dynein phenocopies the lack of myosin-7a, in that melanosomes undergo fewer of the slow myosin-7a-dependent movements and are absent from the RPE apical domain. These results indicate that microtubule-based motility is required for the delivery of melanosomes to the actin-rich apical domain and support a capture mechanism that involves both microtubule and actin motors.