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

Melanosomes are motile, light-absorbing organelles, 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 have tested the model in RPE cells by imaging melanosomes associated with labeled actin filaments and microtubules, and by investigating the roles of different motor proteins. In particular, we found that a deficiency of 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 the requirement of microtubule-based motility for the delivery of melanosomes to the actin-rich, apical domain, and support a capture mechanism that involves both microtubule and actin motors.