Farmer K, Catala F, Wright W E
Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas 75235.
J Biol Chem. 1992 Mar 15;267(8):5631-6.
The native molecular weight of the basic helix-loop-helix (bHLH) proteins myogenin, MyoD, and E12 was calculated from their mobilities on sucrose gradients and molecular sieve chromatography. The muscle bHLH proteins associate to form a variety of higher order complexes, most of which are larger than dimers. Homodimers bind to DNA sequences such as the MEF-1 site in the creatine kinase enhancer whereas homotetramers and larger forms do not recognize this DNA sequence. The ubiquitous bHLH protein E12 forms monomers or homodimers with little evidence for higher order complexes. Mixtures of myogenin and E12 show some heterodimeric structures, but most of the myogenin remains in large complexes. This result using purified proteins is also obtained in nuclear extracts from differentiated myotubes, in which most of the myogenin is present in large complexes that do not bind to the creatine kinase enhancer. A fusion protein containing only the myogenin HLH region forms large homomeric complexes. A model is presented in which each helix associates with a different subunit to form chains or ring structures to explain these observations. The partition of myogenin in nuclear extracts into dimers that recognize known DNA sequences and higher order complexes that do not raises important new issues concerning the regulation of skeletal muscle bHLH protein activity during myogenesis.
通过碱性螺旋-环-螺旋（bHLH）蛋白肌细胞生成素、肌分化因子（MyoD）和E12在蔗糖梯度和分子筛色谱上的迁移率计算出它们的天然分子量。肌肉bHLH蛋白相互结合形成多种高阶复合物，其中大多数大于二聚体。同二聚体可结合DNA序列，如肌酸激酶增强子中的MEF-1位点，而异四聚体及更大的形式则不能识别该DNA序列。普遍存在的bHLH蛋白E12形成单体或同二聚体，几乎没有高阶复合物形成的证据。肌细胞生成素和E12的混合物显示出一些异二聚体结构，但大多数肌细胞生成素仍存在于大复合物中。使用纯化蛋白得到的这一结果，在分化的肌管核提取物中也能得到，其中大多数肌细胞生成素存在于不与肌酸激酶增强子结合的大复合物中。仅包含肌细胞生成素HLH区域的融合蛋白形成大的同聚复合物。本文提出了一个模型，其中每个螺旋与不同的亚基结合形成链状或环状结构来解释这些观察结果。肌细胞生成素在核提取物中分配为识别已知DNA序列的二聚体和不识别已知DNA序列的高阶复合物，这引发了关于成肌过程中骨骼肌bHLH蛋白活性调控的重要新问题。
