Rab1A brings a touch of colour

Mammalian skin pigmentation is determined by melanocytes, a type of skin cell that synthesises melanin pigments and stores them in melanosomes. These specialized organelles are formed and mature around the nucleus of the melanocyte, and are then transported to the cell periphery by coordinated bidirectional microtubule-dependent and unidirectional actin-dependent movements. The mechanisms of retrograde microtubule-dependent and actin-dependent melanosome transport have been determined, but very little is known about anterograde melanosome transport. Here (p. 5177), by performing a genome-wide screen of members of the mouse Rab family, Mitsunori Fukuda and colleagues identify the small GTPase Rab1A as a potential component of the anterograde microtubule-dependent melanosome transport machinery in mouse melanocytes. The authors report that Rab1A, which was originally described as a Golgi-resident Rab, localizes to mature melanosomes in cultured melanocytes, and show that functional ablation of Rab1A induces perinuclear melanosome aggregation. In addition, they use time-lapse microscopy to show that anterograde, but not retrograde, melanosome transport is suppressed in Rab1A-deficient melanocytes. Together, these results provide the first clues to the machinery involved in anterograde melanosome transport.