Tongaonkar P, Chen L, Lambertson D, Ko B, Madura K
Department of Biochemistry, Robert Wood Johnson Medical School-UMDNJ, Piscataway, NJ 08854, USA.
Mol Cell Biol. 2000 Jul;20(13):4691-8. doi: 10.1128/MCB.20.13.4691-4698.2000.
The targeting of proteolytic substrates is accomplished by a family of ubiquitin-conjugating (E2) enzymes and a diverse set of substrate recognition (E3) factors. The ligation of a multiubiquitin chain to a substrate can promote its degradation by the proteasome. However, the mechanism that facilitates the translocation of a substrate to the proteasome in vivo is poorly understood. We have discovered that E2 proteins, including Ubc1, Ubc2, Ubc4, and Ubc5, can interact with the 26S proteasome. Significantly, the interaction between Ubc4 and the proteasome is strongly induced by heat stress, consistent with the requirement for this E2 for efficient stress tolerance. A catalytically inactive derivative of Ubc4 (Ubc4(C86A)), which causes toxicity in yeast cells, can also bind the proteasome. Purified proteasomes can ligate ubiquitin to a test substrate without the addition of exogenous E2 protein, suggesting that the ubiquitylation of some proteolytic substrates might be directly coupled to degradation by the proteasome.
蛋白水解底物的靶向作用是由一类泛素结合（E2）酶和多种底物识别（E3）因子完成的。多泛素链与底物的连接可促进其被蛋白酶体降解。然而，在体内促进底物向蛋白酶体转运的机制却知之甚少。我们发现，包括Ubc1、Ubc2、Ubc4和Ubc5在内的E2蛋白可与26S蛋白酶体相互作用。值得注意的是，Ubc4与蛋白酶体之间的相互作用在热应激下会强烈诱导，这与该E2对有效应激耐受性的需求一致。Ubc4的一种催化无活性衍生物（Ubc4(C86A)）在酵母细胞中具有毒性，它也能结合蛋白酶体。纯化的蛋白酶体在不添加外源E2蛋白的情况下就能将泛素连接到测试底物上，这表明一些蛋白水解底物的泛素化可能直接与蛋白酶体的降解相关联。
