Bloom Joanna, Amador Virginia, Bartolini Francesca, DeMartino George, Pagano Michele
New York University Cancer Institute and New York University School of Medicine, New York, NY 10016, USA.
Cell. 2003 Oct 3;115(1):71-82. doi: 10.1016/s0092-8674(03)00755-4.
We examined the mechanism responsible for the degradation of p21, a negative regulator of the cell division cycle. We found that p21 proteolysis requires functional ubiquitin and Nedd8 systems. Ubiquitinylated forms of p21 and p21(K0), a p21 mutant missing all lysines, are detected in vivo and in vitro, showing that the presence of lysines is dispensable for p21 ubiquitinylation. Instead, the free amino group of the N-terminal methionine of p21 is a site for ubiquitinylation in vivo. Although wild-type p21 is more abundantly ubiquitinylated than p21(K0) mutant due to the presence of internal lysine residues, their rates of proteolysis are indistinguishable. These results demonstrate that proteasomal degradation of p21 is regulated by the ubiquitin pathway and suggest that the site of the ubiquitin chain is critical in making p21 a competent substrate for the proteasome.
我们研究了细胞分裂周期负调控因子p21降解的机制。我们发现p21的蛋白水解需要功能性泛素和Nedd8系统。在体内和体外均检测到泛素化形式的p21和p21(K0)（一种缺失所有赖氨酸的p21突变体），这表明赖氨酸的存在对于p21的泛素化并非必需。相反，p21 N端甲硫氨酸的游离氨基是体内泛素化的位点。尽管由于内部赖氨酸残基的存在，野生型p21比p21(K0)突变体更大量地被泛素化，但其蛋白水解速率并无差异。这些结果表明，p21的蛋白酶体降解受泛素途径调控，并提示泛素链的位点对于使p21成为蛋白酶体的合适底物至关重要。
