Sic2為一酵母菌逆境誘發之G1時期細胞週期磷酸酶抑制物

Abstract

在自然界中，細胞必須針對瞬息萬變的外在壓力做出快速並且適當的反應。當細胞遭遇環境壓力，例如滲透壓改變、養份缺乏、氧化壓力或是遺傳物質損害……等等的挑戰，細胞週期會停頓在G1時期，讓細胞適應並且排除壓力。細胞週期中G1到S期的轉換大部份是靠G1週期素和Cdc28週期素依賴性蛋白激酶複合體的活性來調控。G1週期素中的Cln1和Cln2對於細胞週期的啟動提供了特殊的貢獻，而Cln3則是作用在CLN1和CLN2時間點上的表現和其他在G1時期表現的基因。之前的研究顯示出Ypl014w (我們命名為Sic2)是Cdc28/G1週期素複合體的其中一員。然而，這個基因是如何在細胞週期中被調控以及是否影響週期素依賴性蛋白激酶素–G1週期素複合體的活性仍然是未知的。在本篇論文中，我們發現SIC2在細胞遭遇逆境時表現。其在逆境與細胞週期之調控分別由轉錄因子Mns2/4與Mcm1負責。當Sic2大量表現時，細胞週期大規模地停滯在G1時期。利用共同免疫沉澱法證實了Sic2和G1週期素是會互相結合的。此外，Sic2對於滲透壓力造成的暫時性G1週期停頓有重要的貢獻，其功能在此逆境情況下與Sic1有重複性。本篇論文證明Sic2可能是週期素依賴性蛋白激酶的抑制物，且在細胞遭受逆境時調控細胞週期的進行。

In wild life, cells need quick and proper response for adapting to numerous environmental changes. When cells encounter stresses such as osmostress, heat stress, nutrient depletion, oxidative stress, and DNA damage, cells arrest at G1 stage to provide proper adaptation and overcome stress challenge. The G1-S transition in the cell cycle is mainly regulated by the activity of the Cdc28/G1 cyclin-dependent protein kinase complex. Among G1 cyclins, Cln1 and Cln2 appear to make a specific contribution to the initiation of the cell cycle, while Cln3 functions for the timely expression of CLN1, CLN2 and other G1-specific genes. Previous results suggest that Sic2 is a component of the Cdc28/G1 cyclin complex. However, it is still elusive how this gene is cell cycle-regulated and whether it affects the activity of the Cdk/G1 cyclin complex. Here we show that SIC2 is expressed when cells encounter stresses. Its stress- and cell cycle-mediated expression are driven by the transcriptional factors Msn2/4 and Mcm1, respectively. When Sic2 is overexpressed, cell cycle arrest at G1 stage. Co-immunoprecipitation demonstrates the interactions between Sic2 and G1 cyclins. Moreover, Sic2 plays a role in osmostress induced G1 arrest, and it is redundant to Sic1 under such condition. Consequently, Sic2 may act as a CDK inhibitor to modulate cell cycle progression when cells encounter general stresses.