Gruen M, Prinz H, Gautel M
Max-Planck-Institute für molekulare Physiologie, Abt. Physikalische Biochemie, Dortmund, Germany.
FEBS Lett. 1999 Jun 25;453(3):254-9. doi: 10.1016/s0014-5793(99)00727-9.
Myosin binding protein C is a protein of the myosin filaments of striated muscle which is expressed in isoforms specific for cardiac and skeletal muscle. The cardiac isoform is phosphorylated rapidly upon adrenergic stimulation of myocardium by cAMP-dependent protein kinase, and together with the phosphorylation of troponin-I and phospholamban contributes to the positive inotropy that results from adrenergic stimulation of the heart. Cardiac myosin binding protein C is phosphorylated by cAMP-dependent protein kinase on three sites in a myosin binding protein C specific N-terminal domain which binds to myosin-S2. This interaction with myosin close to the motor domain is likely to mediate the regulatory function of the protein. Cardiac myosin binding protein C is a common target gene of familial hypertrophic cardiomyopathy and most mutations encode N-terminal subfragments of myosin binding protein C. The understanding of the signalling interactions of the N-terminal region is therefore important for understanding the pathophysiology of myosin binding protein C associated cardiomyopathy. We demonstrate here by cosedimentation assays and isothermal titration calorimetry that the myosin-S2 binding properties of the myosin binding protein C motif are abolished by cAMP-dependent protein kinase-mediated tris-phosphorylation, decreasing the S2 affinity from a Kd of approximately 5 microM to undetectable levels. We show that the slow and fast skeletal muscle isoforms are no cAMP-dependent protein kinase substrates and that the S2 interaction of these myosin binding protein C isoforms is therefore constitutively on. The regulation of cardiac contractility by myosin binding protein C therefore appears to be a 'brake-off' mechanism that will free a specific subset of myosin heads from sterical constraints imposed by the binding to the myosin binding protein C motif.
肌球蛋白结合蛋白C是横纹肌肌球蛋白丝的一种蛋白质，以心肌和骨骼肌特异的同工型形式表达。心脏同工型在心肌受到肾上腺素能刺激时，会被cAMP依赖性蛋白激酶迅速磷酸化，并且与肌钙蛋白I和受磷蛋白的磷酸化一起，促成肾上腺素能刺激心脏所产生的正性肌力作用。心脏肌球蛋白结合蛋白C在肌球蛋白结合蛋白C特异的N端结构域中的三个位点被cAMP依赖性蛋白激酶磷酸化，该结构域与肌球蛋白-S2结合。这种与靠近运动结构域的肌球蛋白的相互作用可能介导了该蛋白的调节功能。心脏肌球蛋白结合蛋白C是家族性肥厚型心肌病的常见靶基因，大多数突变编码肌球蛋白结合蛋白C的N端亚片段。因此，了解N端区域的信号相互作用对于理解与肌球蛋白结合蛋白C相关的心肌病的病理生理学很重要。我们在此通过共沉降分析和等温滴定量热法证明，cAMP依赖性蛋白激酶介导的三磷酸化消除了肌球蛋白结合蛋白C基序的肌球蛋白-S2结合特性，将S2亲和力从约5 microM的解离常数降低到无法检测的水平。我们表明，慢肌和快肌骨骼肌同工型不是cAMP依赖性蛋白激酶的底物，因此这些肌球蛋白结合蛋白C同工型与S2的相互作用是持续存在的。因此，肌球蛋白结合蛋白C对心脏收缩力的调节似乎是一种“松开刹车”机制，它将使特定子集的肌球蛋白头部从与肌球蛋白结合蛋白C基序结合所施加的空间限制中释放出来。
