Karamysheva Zemfira, Diaz-Martinez Laura A, Crow Sara E, Li Bing, Yu Hongtao
Department of Pharmacology, Howard Hughes Medical Institute, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, 75390, USA.
J Biol Chem. 2009 Jan 16;284(3):1772-80. doi: 10.1074/jbc.M807083200. Epub 2008 Nov 17.
Shugoshin 1 (Sgo1) protects centromeric sister-chromatid cohesion in early mitosis and, thus, prevents premature sister-chromatid separation. The protein level of Sgo1 is regulated during the cell cycle; it peaks in mitosis and is down-regulated in G1/S. Here we show that Sgo1 is degraded during the exit from mitosis, and its degradation depends on the anaphase-promoting complex/cyclosome (APC/C). Overexpression of Cdh1 reduces the protein levels of ectopically expressed Sgo1 in human cells. Sgo1 is ubiquitinated by APC/C bound to Cdh1 (APC/C(Cdh1)) in vitro. We have further identified two functional degradation motifs in Sgo1; that is, a KEN (Lys-Glu-Asn) box and a destruction box (D box). Although removal of either motif is not sufficient to stabilize Sgo1, Sgo1 with both KEN box and D box deleted is stable in cells. Surprisingly, mitosis progresses normally in the presence of non-degradable Sgo1, indicating that degradation of Sgo1 is not required for sister-chromatid separation or mitotic exit. Finally, we show that the spindle checkpoint kinase Bub1 contributes to the maintenance of Sgo1 steady-state protein levels in an APC/C-independent mechanism.
守护蛋白1（Sgo1）在有丝分裂早期保护着丝粒姐妹染色单体黏连，从而防止姐妹染色单体过早分离。Sgo1的蛋白水平在细胞周期中受到调控；它在有丝分裂期达到峰值，在G1/S期下调。在此我们表明，Sgo1在有丝分裂退出过程中会被降解，其降解依赖后期促进复合物/细胞周期体（APC/C）。Cdh1的过表达会降低人细胞中异位表达的Sgo1的蛋白水平。在体外，Sgo1被与Cdh1结合的APC/C（APC/C(Cdh1)）泛素化。我们进一步在Sgo1中鉴定出两个功能性降解基序；即一个KEN（赖氨酸 - 谷氨酸 - 天冬酰胺）框和一个破坏框（D框）。虽然去除任何一个基序都不足以稳定Sgo1，但同时缺失KEN框和D框的Sgo1在细胞中是稳定的。令人惊讶的是，在存在不可降解的Sgo1的情况下有丝分裂仍能正常进行，这表明Sgo1的降解对于姐妹染色单体分离或有丝分裂退出并非必需。最后，我们表明纺锤体检查点激酶Bub1以一种不依赖APC/C的机制有助于维持Sgo1的稳态蛋白水平。
