Galan J M, Cantegrit B, Garnier C, Namy O, Haguenauer-Tsapis R
Institut J. Monod, Université Paris VII-CNRS, France.
FASEB J. 1998 Mar;12(3):315-23. doi: 10.1096/fasebj.12.3.315.
The yeast plasma membrane, uracil permease, undergoes ubiquitin-dependent endocytosis and subsequent degradation in the vacuole via a process that does not involve the proteasome. Cell-surface ubiquitination of this protein is mediated by the ubiquitin-protein ligase Npi1p/Rsp5p and involves Lys63-linked ubiquitin chains. This report describes the intracellular fate of a mutant form of uracil permease carrying a three amino acid insertion in a cytoplasmic loop. Most of this protein is not deployed beyond the ER, and is degraded by the 26S proteasome. Mutant permease degradation is almost unaffected in cells with impaired Npi1p/Rsp5p, but is dependent on the Ubc6p and Ubc7p ubiquitin-conjugating enzymes, suggesting that proteolysis of the protein requires its prior ubiquitination. Overproduction of a derivative of ubiquitin with a modified Lys48 strongly impairs mutant permease degradation. This suggests that, like other proteasome substrates, mutant permease might be polyubiquitinated with Lys48-linked ubiquitin chains. These findings provide an example of a yeast plasma membrane protein that is routed to the 'ER degradation' pathway, and highlight the versatility of the ubiquitin system.
酵母质膜上的尿嘧啶通透酶会经历泛素依赖性内吞作用，并随后通过一个不涉及蛋白酶体的过程在液泡中降解。该蛋白的细胞表面泛素化由泛素 - 蛋白连接酶Npi1p/Rsp5p介导，并涉及赖氨酸63连接的泛素链。本报告描述了一种尿嘧啶通透酶突变体的细胞内命运，该突变体在细胞质环中有三个氨基酸插入。这种蛋白的大部分没有转运到内质网之外，而是被26S蛋白酶体降解。在Npi1p/Rsp5p功能受损的细胞中，突变体通透酶的降解几乎不受影响，但依赖于Ubc6p和Ubc7p泛素结合酶，这表明该蛋白的蛋白水解需要其先前的泛素化。过量表达赖氨酸48修饰的泛素衍生物会严重损害突变体通透酶的降解。这表明，与其他蛋白酶体底物一样，突变体通透酶可能被赖氨酸48连接的泛素链多泛素化。这些发现提供了一个酵母质膜蛋白进入“内质网降解”途径的例子，并突出了泛素系统的多功能性。
