Department of Biochemistry and Molecular Genetics and Center for Cell Signaling, University of Virginia, 800577 HSC, Charlottesville, VA 22908, USA.
J Mol Biol. 2010 May 21;398(5):657-71. doi: 10.1016/j.jmb.2010.03.048. Epub 2010 Mar 31.
Pc2 (Cbx4) is a member of the chromobox family of polycomb proteins, and is a SUMO E3 ligase for the transcriptional corepressor CtBP1. Here, we show that both CtBP1 and Pc2 are phosphorylated by the kinase Akt1, which is activated by growth factor signaling via the PI3-kinase pathway. In the presence of Pc2, phosphorylation of CtBP1 is increased, and this requires interaction of both CtBP1 and Akt1 with Pc2. Pc2 promotes CtBP1 phosphorylation by recruiting Akt1 and, in part, by preventing de-phosphorylation of activated Akt1. Alteration of the Akt-phosphorylated residue in CtBP1 to a phosphomimetic results in decreased CtBP1 dimerization, but does not prevent interaction with other transcriptional regulators. The phosphomimetic mutant of CtBP1 is expressed at a lower level than the wild type protein, resulting in decreased transcriptional repression. We show that this CtBP1 mutant is targeted for poly-ubiquitylation and is less stable than the wild type protein. Co-expression of Pc2 and Akt1 results in both phosphorylation and ubiquitylation of CtBP1, thereby targeting CtBP1 for degradation. This work suggests that Pc2 might coordinate multiple enzymatic activities to regulate CtBP1 function.
Pc2（Cbx4）是多梳蛋白家族的成员，也是转录共抑制因子 CtBP1 的 SUMO E3 连接酶。在这里，我们表明 Akt1 激酶可以磷酸化 CtBP1 和 Pc2，Akt1 是通过 PI3-激酶途径被生长因子信号激活的。在 Pc2 存在的情况下，CtBP1 的磷酸化增加，这需要 CtBP1 和 Akt1 与 Pc2 的相互作用。Pc2 通过招募 Akt1 并部分防止激活的 Akt1 去磷酸化来促进 CtBP1 的磷酸化。将 CtBP1 中的 Akt 磷酸化残基改变为磷酸模拟物会导致 CtBP1 二聚体减少，但不会阻止与其他转录调节剂的相互作用。CtBP1 的磷酸模拟突变体的表达水平低于野生型蛋白，导致转录抑制减少。我们表明，这种 CtBP1 突变体被多泛素化靶向，并且比野生型蛋白更不稳定。Pc2 和 Akt1 的共表达导致 CtBP1 的磷酸化和泛素化，从而将 CtBP1 靶向降解。这项工作表明，Pc2 可能协调多种酶活性来调节 CtBP1 功能。
