Department of Biochemistry, University of Washington, Seattle, Washington 98195-7350, USA.
J Biol Chem. 2013 Jun 21;288(25):18162-71. doi: 10.1074/jbc.M113.457069. Epub 2013 Apr 23.
VPS9 domains can act as guanosine nucleotide exchange factors (GEFs) against small G proteins of the Rab5 family. Saccharomyces cerevisiae vps9Δ mutants have trafficking defects considerably less severe than multiple deletions of the three cognate Rab5 paralogs (Vps21, Ypt52, and Ypt53). Here, we show that Muk1, which also contains a VPS9 domain, acts as a second GEF against Vps21, Ypt52, and Ypt53. Muk1 is partially redundant with Vps9 in vivo, with vps9Δ muk1Δ double mutant cells displaying hypersensitivity to temperature and ionic stress, as well as profound impairments in endocytic and Golgi endosome trafficking, including defects in sorting through the multivesicular body. Cells lacking both Vps9 and Muk1 closely phenocopy double and triple knock-out strains lacking Rab5 paralogs. Microscopy and overexpression experiments demonstrate that Vps9 and Muk1 have distinct localization determinants. These experiments establish Muk1 as the second Rab5 GEF in budding yeast.
VPS9 结构域可以作为鸟嘌呤核苷酸交换因子（GEFs），针对 Rab5 家族的小 G 蛋白发挥作用。酿酒酵母 vps9Δ 突变体的运输缺陷比三个同源 Rab5 旁系同源物（Vps21、Ypt52 和 Ypt53）的多次缺失要轻得多。在这里，我们表明 Muk1 也含有一个 VPS9 结构域，可作为第二个 GEF 针对 Vps21、Ypt52 和 Ypt53 发挥作用。Muk1 在体内与 Vps9 部分冗余，vps9Δ muk1Δ 双突变体细胞对温度和离子应激表现出超敏反应，以及在内吞作用和高尔基体内体运输方面的严重损伤，包括通过多泡体的分选缺陷。缺乏 Vps9 和 Muk1 的细胞与缺乏 Rab5 旁系同源物的双和三敲除菌株非常相似。显微镜和过表达实验表明，Vps9 和 Muk1 具有不同的定位决定因素。这些实验确立了 Muk1 为芽殖酵母中的第二个 Rab5 GEF。
