Ju Donghong, Xie Youming
Barbara Ann Karmanos Cancer Institute, Department of Pathology, Wayne State, University School of Medicine, Detroit, Michigan 48201, USA.
J Biol Chem. 2004 Jun 4;279(23):23851-4. doi: 10.1074/jbc.C400111200. Epub 2004 Apr 16.
Substrates of the ubiquitin system are degraded by the 26 S proteasome, a complex protease consisting of at least 32 different subunits. Recent studies showed that RPN4 (also named SON1 and UFD5) is a transcriptional activator required for normal expression of the Saccharomyces cerevisiae proteasome genes. Interestingly, RPN4 is extremely short-lived and degraded by the 26 S proteasome, establishing a feedback circuit that controls the homeostatic abundance of the 26 S proteasome. The mechanism underlying the degradation of RPN4, however, remains unclear. Here we demonstrate that the proteasomal degradation of RPN4 is mediated by two independent degradation signals (degron). One degron leads to ubiquitylation on internal lysine(s), whereas the other is independent of ubiquitylation. Stabilization of RPN4 requires inhibition of internal ubiquitylation and inactivation of the ubiquitin-independent degron. RPN4 represents the first proteasomal substrate in S. cerevisiae that can be degraded through ubiquitylation or without prior ubiquitylation. This finding makes it possible to use both yeast genetics and biochemical analysis to investigate the mechanism of ubiquitin-independent proteolysis.
泛素系统的底物由26S蛋白酶体降解，26S蛋白酶体是一种由至少32个不同亚基组成的复合蛋白酶。最近的研究表明，RPN4（也称为SON1和UFD5）是酿酒酵母蛋白酶体基因正常表达所需的转录激活因子。有趣的是，RPN4寿命极短，会被26S蛋白酶体降解，从而建立了一个控制26S蛋白酶体稳态丰度的反馈回路。然而，RPN4降解的潜在机制仍不清楚。在这里，我们证明RPN4的蛋白酶体降解是由两个独立的降解信号（降解子）介导的。一个降解子导致内部赖氨酸的泛素化，而另一个则不依赖于泛素化。RPN4的稳定需要抑制内部泛素化并使不依赖泛素的降解子失活。RPN4是酿酒酵母中第一个可以通过泛素化或无需预先泛素化而降解的蛋白酶体底物。这一发现使得利用酵母遗传学和生化分析来研究不依赖泛素的蛋白水解机制成为可能。
