Chapter 6 Rab 7 and Rab 27 a control two motor protein activities competing in melanosomal transport

Melanosomes are lysosome-derived organelles that synthesise, store and transport melanin. In epidermal melanocytes, melanosomes mature and are transferred to surrounding keratinocytes, which is essential for skin and coat colour. Mouse coat colour mutants reveal a critical role for the small GTPase Rab27a, which recruits Myosin Va through its effector protein Melanophilin/Slac2a. Here we have analysed how different Rab GTPases control two motor proteins during the transport of melanosomes. We show that the small GTPase Rab7 mainly associates to early stage melanosomes. Rab7 is able to recruit the microtubule-based dynein motor mediated by its effector RILP and thereby controls microtubule-based transport of melanosomes. Rab27a is detected in two forms on the melanosomes, a rapidly cycling intermediate state and a highly active state on more mature melanosomes. The Rab7-RILPdynein pathway controls early and intermediate stage Rab27a-positive melanosomes. Late stage melanosomes lack Rab7 and remain associated to the actin network through the Rab27aMyoVa link. Thus two Rab proteins regulate two different motor proteins, thereby controlling different phases in melanosome biogenesis: Rab7 controls microtubule-mediated transport of earlier stage melanosomes and Rab27a the subsequent actin-dependent transport of the more mature melanosomes.