Onions J, Hermann S, Grundström T
Division of Tumour Biology, Department of Cell and Molecular Biology, Umeå University, S-901 87 Umeå, Sweden.
Biochemistry. 2000 Apr 18;39(15):4366-74. doi: 10.1021/bi992533u.
Calmodulin is the predominant intracellular receptor for Ca(2+) signals, mediating the regulation of numerous cellular processes. Previous studies have shown that calcium-loaded calmodulin can bind to and inhibit the activity of certain basic helix-loop-helix (bHLH) transcription factors. The basic sequence within the bHLH domain is the primary target for calmodulin binding, and sequences modulating the calmodulin interaction reside directly N-terminal to the basic sequence. Here we show that the interaction of calmodulin with bHLH proteins is of a novel type, displaying characteristics very different from those of previously characterized calmodulin-target complexes. We show that calmodulin interacts much stronger with a dimeric basic sequence than with the monomeric form. The calmodulin-bHLH protein complex contains equimolar amounts of calmodulin and bHLH chains. The interaction is unusual in being to a large extent polar in nature, and it is highly resistant to tested calmodulin inhibitors. Both the N-terminal and C-terminal domains of calmodulin can independently bind to and inhibit the DNA binding of bHLH proteins. The C-terminal domain preferentially binds to the basic sequence, whereas the N-terminal domain is essential for the effect of the modulatory sequence. We propose a model for the calmodulin-bHLH complex where two calmodulin molecules interact with one bHLH dimer, with one domain of calmodulin preferentially binding to the basic sequence of bHLH proteins and the other domain interacting with the modulatory sequence.
钙调蛋白是细胞内Ca(2+)信号的主要受体，介导众多细胞过程的调节。先前的研究表明，钙负载的钙调蛋白可结合并抑制某些碱性螺旋-环-螺旋（bHLH）转录因子的活性。bHLH结构域内的碱性序列是钙调蛋白结合的主要靶点，调节钙调蛋白相互作用的序列直接位于碱性序列的N端。在此我们表明，钙调蛋白与bHLH蛋白的相互作用是一种新型的相互作用，其表现出的特征与先前表征的钙调蛋白-靶标复合物的特征非常不同。我们表明，钙调蛋白与二聚体碱性序列的相互作用比与单体形式的相互作用要强得多。钙调蛋白-bHLH蛋白复合物包含等摩尔量的钙调蛋白和bHLH链。这种相互作用的不寻常之处在于其在很大程度上具有极性，并且对所测试的钙调蛋白抑制剂具有高度抗性。钙调蛋白的N端和C端结构域均可独立结合并抑制bHLH蛋白的DNA结合。C端结构域优先结合碱性序列，而N端结构域对于调节序列的作用至关重要。我们提出了一种钙调蛋白-bHLH复合物的模型，其中两个钙调蛋白分子与一个bHLH二聚体相互作用，钙调蛋白的一个结构域优先结合bHLH蛋白的碱性序列，另一个结构域与调节序列相互作用。
