Cross Frederick R, Schroeder Lea, Bean James M
The Rockefeller University, New York, New York 10021, USA.
Genetics. 2007 Jul;176(3):1541-55. doi: 10.1534/genetics.107.073494. Epub 2007 May 4.
In budding yeast, B-type cyclin (Clb)-dependent kinase activity is essential for S phase and mitosis. In newborn G(1) cells, Clb kinase accumulation is blocked, in part because of the Sic1 stoichiometric inhibitor. Previous results strongly suggested that G(1) cyclin-dependent Sic1 phosphorylation, and its consequent degradation, is essential for S phase. However, cells containing a precise endogenous gene replacement of SIC1 with SIC1-0P (all nine phosphorylation sites mutated) were fully viable. Unphosphorylatable Sic1 was abundant and nuclear throughout the cell cycle and effectively inhibited Clb kinase in vitro. SIC1-0P cells had a lengthened G(1) and increased G(1) cyclin transcriptional activation and variable delays in the budded part of the cell cycle. SIC1-0P was lethal when combined with deletion of CLB2, CLB3, or CLB5, the major B-type cyclins. Sic1 phosphorylation provides a sharp link between G(1) cyclin activation and Clb kinase activation, but failure of Sic1 phosphorylation and proteolysis imposes a variable cell cycle delay and extreme sensitivity to B-type cyclin dosage, rather than a lethal cell cycle block.
在出芽酵母中，B型细胞周期蛋白（Clb）依赖性激酶活性对于S期和有丝分裂至关重要。在新生的G1期细胞中，Clb激酶的积累受到阻碍，部分原因是由于化学计量的Sic1抑制剂。先前的结果强烈表明，G1期细胞周期蛋白依赖性的Sic1磷酸化及其随后的降解对于S期至关重要。然而，含有用SIC1-0P（所有九个磷酸化位点均发生突变）精确替换SIC1的内源性基因的细胞完全可以存活。不可磷酸化的Sic1在整个细胞周期中都很丰富且位于细胞核内，并在体外有效抑制Clb激酶。SIC1-0P细胞的G1期延长，G1期细胞周期蛋白转录激活增加，并且在细胞周期的出芽部分存在可变延迟。当与主要的B型细胞周期蛋白CLB2、CLB3或CLB5的缺失相结合时，SIC1-0P是致死性的。Sic1磷酸化在G1期细胞周期蛋白激活和Clb激酶激活之间提供了一个紧密的联系，但Sic1磷酸化和蛋白水解的失败会导致可变的细胞周期延迟以及对B型细胞周期蛋白剂量的极端敏感性，而不是致死性的细胞周期阻滞。
