Department of Biochemistry, Stanford University, Stanford, CA, USA.
Nat Struct Mol Biol. 2019 Jun;26(6):450-459. doi: 10.1038/s41594-019-0230-1. Epub 2019 May 27.
Stalled translation produces incomplete, ribosome-tethered polypeptides that the ribosome-associated quality control (RQC) pathway targets for degradation via the E3 ubiquitin ligase Ltn1. During this process, the protein Rqc2 and the large ribosomal subunit elongate stalled polypeptides with carboxy-terminal alanine and threonine residues (CAT tails). Failure to degrade CAT-tailed proteins disrupts global protein homeostasis, as CAT-tailed proteins can aggregate and sequester chaperones. Why cells employ such a potentially toxic process during RQC is unclear. Here, we developed quantitative techniques to assess how CAT tails affect stalled polypeptide degradation in Saccharomyces cerevisiae. We found that CAT tails enhance the efficiency of Ltn1 in targeting structured polypeptides, which are otherwise poor Ltn1 substrates. If Ltn1 fails to ubiquitylate those stalled polypeptides or becomes limiting, CAT tails act as degrons, marking proteins for proteasomal degradation off the ribosome. Thus, CAT tails functionalize the carboxy termini of stalled polypeptides to drive their degradation on and off the ribosome.
翻译暂停产生不完整的、与核糖体相连的多肽，核糖体相关的质量控制（RQC）途径通过 E3 泛素连接酶 Ltn1 将其靶向降解。在此过程中，蛋白质 Rqc2 和大亚基核糖体使带有羧基末端丙氨酸和苏氨酸残基（CAT 尾巴）的停滞后的多肽延伸。未能降解 CAT 尾巴蛋白会破坏全局蛋白质平衡，因为 CAT 尾巴蛋白可以聚集并隔离伴侣蛋白。为什么细胞在 RQC 期间采用这种潜在有毒的过程尚不清楚。在这里，我们开发了定量技术来评估 CAT 尾巴如何影响酿酒酵母中停滞后的多肽降解。我们发现 CAT 尾巴增强了 Ltn1 靶向结构多肽的效率，否则这些多肽是 Ltn1 的不良底物。如果 Ltn1 未能泛素化这些停滞后的多肽或变得有限，CAT 尾巴作为 degrons 起作用，将蛋白质标记为核糖体上的蛋白酶体降解。因此，CAT 尾巴使停滞后的多肽的羧基末端功能化，以驱动它们在核糖体上和核糖体上的降解。
