The Human CDK8 Subcomplex Is a Histone Kinase That Requires Med12 for Activity and Can Function Independently of Mediator

The four proteins CDK8, cyclin C, Med12, and Med13 can associate with Mediator and are presumed to form a stable “CDK8 subcomplex” in cells. We describe here the isolation and enzymatic activity of the 600-kDa CDK8 subcomplex purified directly from human cells and also via recombinant expression in insect cells. Biochemical analysis of the recombinant CDK8 subcomplex identifies predicted (TFIIH and RNA polymerase II C-terminal domain [Pol II CTD]) and novel (histone H3, Med13, and CDK8 itself) substrates for the CDK8 kinase. Notably, these novel substrates appear to be metazoan-specific. Such diverse targets imply strict regulation of CDK8 kinase activity. Along these lines, we observe that Mediator itself enables CDK8 kinase activity on chromatin, and we identify Med12—but not Med13—to be essential for activating the CDK8 kinase. Moreover, mass spectrometry analysis of the endogenous CDK8 subcomplex reveals several associated factors, including GCN1L1 and the TRiC chaperonin, that may help control its biological function. In support of this, electron microscopy analysis suggests TRiC sequesters the CDK8 subcomplex and kinase assays reveal the endogenous CDK8 subcomplex—unlike the recombinant submodule—is unable to phosphorylate the Pol II CTD.     

整合因子复合物——基因转录调控的多能选手

整合因子复合物(integrator complex,INT)的发现极大地拓展了对小核RNA转录成熟和基因转录调控的认知,也重新掀起了相关领域的研究热潮.INT是1个至少由14个亚基组成、分子量超过1.4 MD的蛋白质复合物.它一方面通过内切酶活性切割转录本,执行功能;另一方面与PP2A磷酸酶结合,调节RNA聚合酶Ⅱ上...