Keratinocytes uptake melanosomes from melanocytes and retain them in the perinuclear region, where they form melanin caps. Although these processes are crucial to protecting nuclear DNA against ultraviolet injury, the molecular basis of melanosome uptake and decomposition in keratinocytes is poorly understood. One of the major reasons for its being poorly understood is the lack of a specific marker protein that can be used to visualize or monitor melanosomes (or melanosome-containing compartments) that have been incorporated into keratinocytes. In this study, we performed a comprehensive localization screening for mammalian Rab family small GTPases (Rab1-45) and succeeded in identifying 11 Rabs that were enriched around melanosomes that had been incorporated into keratinocytes. We also established a new assay by using a recently developed melanosome probe (called M-INK) as a means of quantitatively assessing the degradation of proteins on incorporated melanosomes in control and each of a series of Rab-knockdown keratinocytes. The results showed that knockdown or CRISPR/Cas9-mediated knockout of Rab7B (also identified as Rab42) in keratinocytes caused strong inhibition of protein degradation on melanosomes. Our findings indicated that Rab7B/42 is recruited to melanosome-containing compartments and that it promotes protein degradation on melanosomes in keratinocytes.Key words: degradation, keratinocytes, melanocytes, melanosome, Rab small GTPase.

中文翻译：

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Rab7B / 42在功能上参与了角质形成细胞中黑素体上的蛋白质降解。

角质形成细胞从黑素细胞摄取黑素体，并将它们保留在核周区域，在那里形成黑素帽。尽管这些过程对于保护核DNA免受紫外线伤害至关重要，但对角质形成细胞中黑素体摄取和分解的分子基础知之甚少。对其了解不足的主要原因之一是缺乏可用于可视化或监视已掺入角质形成细胞的黑素体（或含黑素体的区室）的特异性标记蛋白。在这项研究中，我们对哺乳动物的Rab家族小GTPases（Rab1-45）进行了全面的定位筛选，并成功地鉴定了11个Rab，这些Rab富含在已掺入角质形成细胞的黑素体周围。我们还通过使用最近开发的黑素体探针（称为M-INK）建立了一种新的测定方法，以此作为定量评估对照和一系列Rab-knocked角质形成细胞中掺入的黑素体上蛋白质降解的手段。结果表明，敲减或CRISPR / Cas9介导的角质形成细胞中Rab7B（也称为Rab42）的敲除对黑素体的蛋白质降解产生了强烈的抑制作用。我们的发现表明，Rab7B / 42被募集到含有黑素体的区室中，并促进角质形成细胞中黑素体上的蛋白质降解。