Institute of Biochemistry and Molecular Biology, ZBMZ, University of Freiburg, Freiburg, Germany.
Mol Cell Biol. 2012 Dec;32(23):4769-79. doi: 10.1128/MCB.00809-12. Epub 2012 Sep 24.
When a polyadenylated nonstop transcript is fully translated, a complex consisting of the ribosome, the nonstop mRNA, and the C-terminally polylysine-tagged protein is generated. In Saccharomyces cerevisiae, a 3-step quality control system prevents formation of such dead-end complexes. Nonstop mRNA is rapidly degraded, translation of nonstop mRNA is repressed, and finally, nonstop proteins are cotranslationally degraded. Nonstop mRNA degradation depends on Ski7 and the exosome; nonstop protein degradation depends on the ribosome-bound E3 ligase Ltn1 and the proteasome. However, components which mediate translational repression of nonstop mRNA have previously not been identified. Here we show that the ribosome-bound chaperone system consisting of the ribosome-associated complex (RAC) and the Hsp70 homolog Ssb is required to stabilize translationally repressed ribosome-polylysine protein complexes, without affecting the folding or the degradation of polylysine proteins. As a consequence, in the absence of RAC/Ssb, polylysine proteins escaped translational repression and subsequently folded into their native conformation. This active role of RAC/Ssb in the quality control of polylysine proteins significantly contributed to the low level of expression of nonstop transcripts in vivo.
当多聚腺苷酸化的无终止转录物被完全翻译时，会产生一个由核糖体、无终止 mRNA 和 C 端聚赖氨酸标记的蛋白质组成的复合物。在酿酒酵母中，一个三步质量控制系统可以防止这种无终止复合物的形成。无终止 mRNA 迅速降解，无终止 mRNA 的翻译受到抑制，最后，无终止蛋白质被共翻译降解。无终止 mRNA 的降解依赖于 Ski7 和核酶体；无终止蛋白质的降解依赖于核糖体结合的 E3 连接酶 Ltn1 和蛋白酶体。然而，以前没有鉴定出介导无终止 mRNA 翻译抑制的成分。在这里，我们表明，由核糖体相关复合物（RAC）和 Hsp70 同源物 Ssb 组成的核糖体结合伴侣系统对于稳定翻译抑制的核糖体-聚赖氨酸蛋白复合物是必需的，而不会影响聚赖氨酸蛋白的折叠或降解。因此，在没有 RAC/Ssb 的情况下，聚赖氨酸蛋白逃脱了翻译抑制，随后折叠成其天然构象。RAC/Ssb 在聚赖氨酸蛋白质量控制中的这种积极作用显著降低了体内无终止转录物的表达水平。
