Zhou J, Olson E N
Department of Biochemistry and Molecular Biology, University of Texas M.D. Anderson Cancer Center, Houston 77030.
Mol Cell Biol. 1994 Sep;14(9):6232-43. doi: 10.1128/mcb.14.9.6232-6243.1994.
The muscle-specific basic helix-loop-helix (bHLH) protein myogenin activates muscle transcription by binding to target sequences in muscle-specific promoters and enhancers as a heterodimer with ubiquitous bHLH proteins, such as the E2A gene products E12 and E47. We show that dimerization with E2A products potentiates phosphorylation of myogenin at sites within its amino- and carboxyl-terminal transcription activation domains. Phosphorylation of myogenin at these sites was mediated by the bHLH region of E2A products and was dependent on dimerization but not on DNA binding. Mutations of the dimerization-dependent phosphorylation sites resulted in enhanced transcriptional activity of myogenin, suggesting that their phosphorylation diminishes myogenin's transcriptional activity. The ability of E2A products to potentiate myogenin phosphorylation suggests that dimerization induces a conformational change in myogenin that unmasks otherwise cryptic phosphorylation sites or that E2A proteins recruit a kinase for which myogenin is a substrate. That phosphorylation of these dimerization-dependent sites diminished myogenin's transcriptional activity suggests that these sites are targets for a kinase that interferes with muscle-specific gene expression.
肌肉特异性碱性螺旋-环-螺旋（bHLH）蛋白肌细胞生成素，作为异二聚体与普遍存在的bHLH蛋白（如E2A基因产物E12和E47）结合，通过与肌肉特异性启动子和增强子中的靶序列结合来激活肌肉转录。我们发现，与E2A产物二聚化可增强肌细胞生成素在其氨基末端和羧基末端转录激活域内位点的磷酸化。肌细胞生成素在这些位点的磷酸化由E2A产物的bHLH区域介导，并且依赖于二聚化而非DNA结合。依赖二聚化的磷酸化位点的突变导致肌细胞生成素的转录活性增强，这表明它们的磷酸化会降低肌细胞生成素 的转录活性。E2A产物增强肌细胞生成素磷酸化的能力表明，二聚化诱导了肌细胞生成素的构象变化，从而暴露了原本隐蔽的磷酸化位点，或者E2A蛋白招募了一种以肌细胞生成素为底物的激酶。这些依赖二聚化的位点的磷酸化降低了肌细胞生成素的转录活性，这表明这些位点是一种干扰肌肉特异性基因表达的激酶的作用靶点。
