在G1,细胞周期蛋白依赖性激酶(CDK)的活性被保持在由CDK抑制剂(抑制因子)的p27和p21的检查,从而防止过早进入S期(参见Guardavaccaro和Pagano的,2006)。这两个抑制因子是通过泛素途径在G1晚期降解(Pagano等,1995; Bloom等人,2003),涉及泛素连接酶SCF(Skp2的)(Tsvetkov等,1999;卡拉诺等,1999。; Sutterluty等人,1999,伯恩斯坦等人,2003)和细胞周期调节蛋白CKS1(Ganoth等人,2001; Spruck等人2001;伯恩斯坦等人,2003)。通过SCF(Skp2的)和其随后的遍在蛋白化p27蛋白的识别依赖于细胞周期蛋白E / A:苏氨酸在p27蛋白的187 Cdk2-介导的磷酸(Montagnoli等人,1999)。有证据表明,细胞周期蛋白A / B:对Cdk1复合物也结合和上Th187磷酸化的p27(Nakayama等人,2004)。通过26S蛋白酶体多泛素化的p27的降解促进驱动细胞进入S期的CDK的活性。(Montagnoli等人,1999; Tsvetkov等人,1999,卡拉诺等人,1999)。SCF(Skp2蛋白)的p21介导的降解机制类似于p27蛋白在其CKS1的和CDK2的存在/细胞周期蛋白E / A(伯恩斯坦等人,2003要求方面; Wang等人,2005。)。此外,作为用于观察p27蛋白,在特定位点的p21磷酸化(Ser130)刺激其泛素化。与此相反,以p27蛋白,然而,p21的泛素化可以采取在没有磷酸化的地方,虽然具有较少的效率(伯恩斯坦等人,2003)。SCF(Skp2的)P27 / P21的介导降解通过M-相从G1后期继续。 During G0 and from early G1 to G1/S, Skp2 is degraded by the anaphase promoting complex/Cyclosome and its activator Cdh1 [APC/C(Cdh1)] (Bashir et al, 2004; Wei et al, 2004). The tight regulation of APC/C(Cdh1) activity ensures the timely elimination Skp2 and, thus, plays a critical role in controlling the G1/S transition. APC/C(Cdh1) becomes active in late M-phase by the association of unphosphorylated Cdh1 with the APC/C. APC/C(Cdh1) remains active until the G1/S phase at which time it interacts with the inhibitory protein, Emi1 (Hsu et al., 2002). Inhibition of APC/C(Cdh1) activity results in an accumulation of cyclins, which leads to the phosphorylation and consequently to a further inactivation of Cdh1 at G1/S (Lukas et al., 1999). Finally, to make the inactivation of APC/C(Cdh1) permanent, Cdh1 and its E2, namely Ubc10, are eliminated in an auto-ubiquitination event (Listovsky et al., 2004; Rape and Kirschner, 2004). At G1/S, Skp2 reaccumulates as Cdh1 is inactivated, thus allowing the ubiquitination of p21 and p27 and resulting in a further increase in CDK activity.