Hill L E, Mehegan J P, Butters C A, Tobacman L S
Department of Internal Medicine, College of Medicine, University of Iowa, Iowa City 52242.
J Biol Chem. 1992 Aug 15;267(23):16106-13.
The binding of tropomyosin to actin and troponin-tropomyosin to actin was analyzed according to a linear lattice model which quantifies two parameters: Ko, the affinity of the ligand for an isolated site on the actin filament, and gamma, the fold increase in affinity when binding is contiguous to an occupied site (cooperativity). Tropomyosin-actin binding is very cooperative (gamma = 90-137). Troponin strengthens tropomyosin-actin binding greatly but, surprisingly, does so solely by an 80-130-fold increase in Ko, while cooperativity actually decreases. Additionally, troponin complexes containing TnT subunits with deletions of either amino acids 1-69 (troponin70-259) or 1-158 (troponin159-259) were examined. Deletion of amino acids 1-69 had only small effects on Ko and y, despite this peptide's location spanning the joint between adjacent tropomyosins. Ca2+ reduced Ko by half for both troponin and troponin70-159 and had no detectable effect on cooperativity. Troponin159-259 had much weaker effects on tropomyosin-actin binding than did troponin70-259 and had no effect at all in the presence of Ca2+. This suggests the importance of Ca(2+)-insensitive interactions between tropomyosin and troponin T residues 70-159. Cooperativity was slightly lower for troponin159-259 than tropomyosin alone, suggesting that the globular head region of troponin affects tropomyosin-tropomyosin interactions along the thin filament.
根据线性晶格模型分析了原肌球蛋白与肌动蛋白以及肌钙蛋白 - 原肌球蛋白与肌动蛋白的结合情况，该模型量化了两个参数：Ko，即配体对肌动蛋白丝上孤立位点的亲和力；γ，即当结合与一个被占据的位点相邻时亲和力增加的倍数（协同性）。原肌球蛋白 - 肌动蛋白结合具有很强的协同性（γ = 90 - 137）。肌钙蛋白极大地增强了原肌球蛋白 - 肌动蛋白的结合，但令人惊讶的是，它只是通过将Ko提高80 - 130倍来实现的，而协同性实际上却降低了。此外，还研究了含有TnT亚基缺失氨基酸1 - 69（肌钙蛋白70 - 259）或1 - 158（肌钙蛋白159 - 259）的肌钙蛋白复合物。尽管该肽段的位置跨越相邻原肌球蛋白之间的连接处，但缺失氨基酸1 - 69对Ko和γ的影响很小。Ca2 +使肌钙蛋白和肌钙蛋白70 - 159的Ko降低了一半，并且对协同性没有可检测到的影响。肌钙蛋白159 - 259对原肌球蛋白 - 肌动蛋白结合的影响比肌钙蛋白70 - 259弱得多，并且在有Ca2 +存在时完全没有影响。这表明原肌球蛋白与肌钙蛋白T残基70 - 159之间的Ca(2 +)不敏感相互作用很重要。肌钙蛋白159 - 259的协同性略低于单独的原肌球蛋白，这表明肌钙蛋白的球状头部区域会影响细肌丝上原肌球蛋白 - 原肌球蛋白的相互作用。
