Genetic Study of Pigment Granule Transport in the Mouse

淡色(d)、灰白（ash）和铅色（ln）毛色突变为哺乳动物囊泡运输的研究提供了一个独特的模型系统。所有三个突变位点编码的基因都是黑色素小体（黑色素细胞的特化、含有色素的细胞器）向邻近的角质形成细胞并最终进入被毛的极化运输所必需的。此外，这三种突变都被第四种突变稀释抑制因子（dsu）所抑制。遗传学研究表明，d、ln和ash在相同或重叠的途径中起作用，生化研究表明d编码一种基于肌动蛋白的黑素小体运输马达MyoVA。我们还表明，灰分编码Rab27a，这是一种小的gtp结合蛋白，先前与囊泡运输有关。在与John Hammer实验室的合作中，我们证明了Rab27a定位于黑素小体，并提供证据表明Rab27a在黑素小体上充当MyoVA受体。随后，许多研究者证实了我们的预测，并表明Rab27a附着在黑素体上是MyoVA进入转运复合体的必要条件。Myo5a和Rab27a也在人类疾病中发生突变。格里塞利综合征是一种罕见的常染色体隐性遗传病，患者表现为皮肤和头发色素沉着。一些患者还表现出神经功能障碍，这与MYO5A基因突变有关；其他人除了色素沉着改变外还表现出免疫紊乱，这种表型与RAB27A突变相关。最近，我们根据它在小鼠1号染色体上的位置克隆了ln突变。有趣的是，ln编码了一种我们命名为嗜黑素（Mlph）的Rab效应物。正在进行的研究表明，Mlph是黑素体转运复合体的关键组成部分。因此，正如遗传学研究预测的那样，这三种蛋白质似乎都是哺乳动物细胞中囊泡运输所必需的综合运动复合体的一部分。研究也在进行中，以确定dsu的产物，并了解蛋白质如何参与定向囊泡运输。

The dilute (d), ashen (ash), and leaden (ln) coat color mutations provide a unique model system for the study of vesicle transport in mammals. All three mutant loci encode genes that are required for the polarized transport of melanosomes, the specialized, pigment-containing organelles of melanocytes, to the neighboring keratinocytes and eventually into coat hairs. Furthermore, all three mutations are suppressed by a fourth mutation, dilute suppressor (dsu). Genetic studies suggest that d, ln, and ash function in the same or overlapping pathways and are supported by biochemical studies showing that d encodes an actin-based melanosome transport motor, MyoVA. We have also shown that ash encodes Rab27a, a small GTP-binding protein previously implicated in vesicle transport. In collaboration with John Hammer's laboratory, we demonstrated that Rab27a localizes to melanosomes and provided evidence suggesting that Rab27a serves as the MyoVA receptor on melanosomes. Subsequently, a number of investigators have confirmed our predictions and showed that Rab27a attachment to the melanosome is required for the recruitment of MyoVA into the transport complex. Myo5a and Rab27a are also mutated in human disease. Patients with a rare autosomal recessive disorder, Griscelli Syndrome, display hypopigmentation of the skin and hair. Some patients also display neurological impairment, which correlates with mutations in MYO5A; others show immune disorders in addition to the pigmentation alterations and this phenotype correlates with mutations in RAB27A. Very recently, we cloned the ln mutation based on its position on mouse chromosome 1. Interestingly, ln encodes a Rab effector that we have named melanophilin (Mlph). Ongoing work suggests that Mlph is a critical component of the melanosomal transport complex. Thus, as predicted by genetic studies, all three proteins appear to function as part of an integrated motor complex that is essential for vesicle transport in mammalian cells. Studies are also in progress to identify the product of dsu and to understand how the protein participates in directional vesicular transport.