The three members of the pocket proteins family share the ability to repress E2F activity through recruitment of a histone deacetylase

The transcription factor E2F plays a major role in cell cycle control in mammalian cells. E2F binding sites, which are present in the promoters of a variety of genes required for S phase, shift from a negative to a positive role in transcription at the commitment point, a crucial point in G(1) that precedes the G(1)/S transition. Before the commitment point, E2F activity is repressed by members of the pocket proteins family. This repression is believed to be crucial for the proper control of cell growth. We have previously shown that Rb, the founding member of the pocket proteins family, represses E2F1 activity by recruiting the histone deacetylase HDAC1. Here, we show that the two other members of the pocket proteins family, p107 and p130, also are able to interact physically with HDAC1 in live cells. HDAC1 interacts with p107 and Rb through an “LXCXE”-like motif, similar to that used by viral transforming proteins to bind and inactivate pocket proteins. Indeed, we find that the viral transforming protein E1A competes with HDAC1 for p107 interaction. We also demonstrate that p107 is able to interact simultaneously with HDAC1 and E2F4, suggesting a model in which p107 recruits HDAC1 to repress E2F sites. Indeed, we demonstrate that histone deacetylase activity is involved in the p107- or p130-induced repression of E2F4. Taken together, our data suggest that all members of the E2F family are regulated in early G(1) by similar complexes, containing a pocket protein and the histone deacetylase HDAC1.

转录因子E2F（transcription factor E2F）在哺乳动物细胞的细胞周期调控中发挥核心作用。位于多种S期必需基因启动子区域的E2F结合位点，在G1期紧邻G1/S转换的关键节点处，其转录调控功能会从负调控转为正调控。在该节点出现之前，E2F的活性会被口袋蛋白家族（pocket proteins family）的成员所阻遏。这种阻遏作用被认为对于细胞生长的精准调控至关重要。我们此前的研究证实，口袋蛋白家族的原型成员Rb，可通过招募组蛋白去乙酰化酶HDAC1（histone deacetylase HDAC1）来抑制E2F1的活性。本研究发现，口袋蛋白家族的另外两个成员p107与p130，同样可在活细胞中与HDAC1发生物理性相互作用。HDAC1通过一段类似“LXCXE”的基序与p107和Rb结合，这一结合模式与病毒转化蛋白结合并灭活口袋蛋白所使用的基序高度相似。实验表明，病毒转化蛋白E1A会与HDAC1竞争结合p107。我们还证实，p107可同时与HDAC1和E2F4发生相互作用，这提示了p107招募HDAC1以阻遏E2F结合位点的作用模型。进一步实验证明，组蛋白去乙酰化酶活性确实参与了p107或p130介导的E2F4转录阻遏过程。综上，本研究数据表明，E2F家族的所有成员在G1早期均受到由口袋蛋白与组蛋白去乙酰化酶HDAC1组成的类似复合物的调控。