Melanosomes are specialized intracellular organelles that produce and store melanin pigments in melanocytes, which are present in several mammalian tissues and organs, including the skin, hair, and eyes. Melanosomes form and mature stepwise (stages I–IV) in melanocytes and then are transported toward the plasma membrane along the cytoskeleton. They are subsequently transferred to neighboring keratinocytes by a largely unknown mechanism, and incorporated melanosomes are transported to the perinuclear region of the keratinocytes where they form melanin caps. Melanocytes also extend several dendrites that facilitate the efficient transfer of the melanosomes to the keratinocytes. Since the melanosome biogenesis, transport, and transfer steps require multiple membrane trafficking processes, Rab GTPases that are conserved key regulators of membrane traffic in all eukaryotes are crucial for skin and hair pigmentation. Dysfunctions of two Rab isoforms, Rab27A and Rab38, are known to cause a hypopigmentation phenotype in human type 2 Griscelli syndrome patients and in chocolate mice (related to Hermansky–Pudlak syndrome), respectively. In this review article, I review the literature on the functions of each Rab isoform and its upstream and downstream regulators in mammalian melanocytes and keratinocytes.

中文翻译：

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Rab GTPases：黑素体生物发生、运输和转移的关键参与者

黑色素体是专门的细胞内细胞器，可在黑色素细胞中产生和储存黑色素，黑色素细胞存在于多种哺乳动物组织和器官中，包括皮肤、头发和眼睛。黑素体在黑素细胞中逐步形成和成熟（I-IV 期），然后沿着细胞骨架向质膜转运。它们随后通过一种很大程度上未知的机制转移到邻近的角质形成细胞，并且掺入的黑素体被运输到角质形成细胞的核周区域，在那里它们形成黑色素帽。黑素细胞还扩展了几个树突，促进黑素体向角质形成细胞的有效转移。由于黑素体的生物发生、运输和转移步骤需要多个膜运输过程，Rab GTP 酶是所有真核生物中膜运输的保守关键调节剂，对于皮肤和毛发色素沉着至关重要。已知两种 Rab 亚型 Rab27A 和 Rab38 的功能障碍会导致人类 2 型 Griscelli 综合征患者和巧克力小鼠（与 Hermansky-Pudlak 综合征有关），分别。在这篇评论文章中，我回顾了有关哺乳动物黑素细胞和角质形成细胞中每种 Rab 亚型及其上游和下游调节因子的功能的文献。