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Rab 38 and Rab 32 control post-Golgi traffi cking of melanogenic enzymes

Melanocytes are specialized pigment-producing cells residing in the skin and eyes of mammals. Melanin is synthesized and stored in melanosomes, membrane-bound subcellular organelles. These share some characteristics with lysosomes, but also possess several melanosome-specifi c components, including matrix proteins and melanin-synthesizing enzymes. Melanosome biogenesis can be divided into four morphologically distinct stages. Stage I premelanosomes are nonpigmented vacuoles derived from endosomes, which then acquire internal striations (stage II). Melanin formation results in pigment being deposited onto these striations, giving rise to stage III, and eventually fully melanized electron-dense stage IV melanosomes. The highly dendritic architecture of melanocytes in the skin allows transfer of mature pigment granules to a large number of keratinocytes, resulting in normal pigmentation (Marks and Seabra, 2001; Raposo and Marks, 2002; Hearing, 2005). The study of mouse mutants displaying alterations in coat color has identifi ed a large number of gene products that affect pigmentation (Bennett and Lamoreux, 2003). Many of these are involved in melanosome function. They include the enzymes required for melanin synthesis, primarily tyrosinase, as well as tyrosinase-related protein (Tyrp/TRP) 1 and Dct (TRP2; Hearing, 2005), and molecules with structural roles in melanosome formation, such as Pmel17 (Theos et al., 2005b). Others are involved in the regulation of intracellular protein traffi cking and organelle biogenesis. Most mouse models for Hermansky-Pudlak syndrome (HPS), a heterogeneous group of disorders associated with albinism in humans, fall into this category. They include subunits of the biogenesis of lysosome-related organelles complexes (BLOCs), of adaptor protein 3 (AP-3), or of the homotypic vacuolar protein sorting complex (Li et al., 2004; Pietro and Dell’Angelica, 2005; Wei, 2006). Others again are required for correct intracellular distribution of melanosomes, for example, Rab27a, melanophilin, and myosinVa (Seabra and Coudrier, 2004). Analysis of the coat color mutant “chocolate” (cht) identifi ed the small GTPase Rab38 as another gene involved in the regulation of pigmentation (Loftus et al., 2002). The Rab family of proteins consists of >60 members in mammalian cells (Pereira-Leal and Seabra, 2001). Some are ubiquitously expressed, whereas others are highly tissue specifi c, refl ecting more specialized roles. Each Rab protein shows a characteristic subcellular distribution and may thus represent an important determinant of organelle identity (Pfeffer, 2001; Munro, 2002; Seabra and Wasmeier, 2004). Rabs serve as key regulators of vesicular traffi cking between subcellular compartments. They have been Rab38 and Rab32 control post-Golgi traffi cking of melanogenic enzymes