Malek N P, Sundberg H, McGrew S, Nakayama K, Kyriakides T R, Roberts J M
Howard Hughes Medical Institute, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104, USA.
Nature. 2001 Sep 20;413(6853):323-7. doi: 10.1038/35095083.
The protein p27Kip1 is an inhibitor of cell division. An increase in p27 causes proliferating cells to exit from the cell cycle, and a decrease in p27 is necessary for quiescent cells to resume division. Abnormally low amounts of p27 are associated with pathological states of excessive cell proliferation, especially cancers. In normal and tumour cells, p27 is regulated primarily at the level of translation and protein turnover. Phosphorylation of p27 on threonine 187 (T187) by cyclin-dependent kinase 2 (Cdk2) is thought to initiate the major pathway for p27 proteolysis. To critically test the importance of this pathway in vivo, we replaced the murine p27 gene with one that encoded alanine instead of threonine at position 187 (p27T187A). Here we show that cells expressing p27T187A were unable to downregulate p27 during the S and G2 phases of the cell cycle, but that this had a surprisingly modest effect on cell proliferation both in vitro and in vivo. Our efforts to explain this unexpected result led to the discovery of a second proteolytic pathway for controlling p27, one that is activated by mitogens and degrades p27 exclusively during G1.
蛋白质p27Kip1是一种细胞分裂抑制剂。p27水平升高会使增殖细胞退出细胞周期，而静止细胞重新开始分裂则需要p27水平降低。p27含量异常低与细胞过度增殖的病理状态有关，尤其是癌症。在正常细胞和肿瘤细胞中，p27主要在翻译和蛋白质周转水平受到调控。细胞周期蛋白依赖性激酶2（Cdk2）使p27的苏氨酸187（T187）位点磷酸化，这被认为是启动p27蛋白水解的主要途径。为了严格测试该途径在体内的重要性，我们用一个在187位编码丙氨酸而非苏氨酸的基因取代了小鼠的p27基因（p27T187A）。我们在此表明，表达p27T187A的细胞在细胞周期的S期和G2期无法下调p27，但这在体外和体内对细胞增殖的影响都出人意料地小。我们为解释这一意外结果所做的努力，促成了另一条控制p27的蛋白水解途径的发现，该途径由有丝分裂原激活，且仅在G1期降解p27。
