Bershitsky Sergey Y, Koubassova Natalia A, Ferenczi Michael A, Kopylova Galina V, Narayanan Theyencheri, Tsaturyan Andrey K
Institute of Immunology and Physiology, Russian Academy of Sciences, Laboratory of Biological Motility, Yekaterinburg, Russia.
Institute of Mechanics, M.V. Lomonosov Moscow University, Moscow, Russia.
Biophys J. 2017 Apr 11;112(7):1455-1461. doi: 10.1016/j.bpj.2017.02.017.
Muscle contraction is powered by actin-myosin interaction controlled by Ca via the regulatory proteins troponin (Tn) and tropomyosin (Tpm), which are associated with actin filaments. Tpm forms coiled-coil dimers, which assemble into a helical strand that runs along the whole ∼1 μm length of a thin filament. In the absence of Ca, Tn that is tightly bound to Tpm binds actin and holds the Tpm strand in the blocked, or B, state, where Tpm shields actin from the binding of myosin heads. Ca binding to Tn releases the Tpm from actin so that it moves azimuthally around the filament axis to a closed, or C, state, where actin is partially available for weak binding of myosin heads. Upon transition of the weak actin-myosin bond into a strong, stereo-specific complex, the myosin heads push Tpm strand to the open, or O, state allowing myosin binding sites on several neighboring actin monomers to become open for myosin binding. We used low-angle x-ray diffraction at the European Synchrotron Radiation Facility to check whether the O- to C-state transition in fully activated fibers of fast skeletal muscle of the rabbit occurs during transition from isometric contraction to shortening under low load. No decrease in the intensity of the second actin layer line at reciprocal radii in the range of 0.15-0.275 nm was observed during shortening suggesting that an azimuthal Tpm movement from the O- to C-state does not occur, although during shortening muscle stiffness is reduced compared to the isometric state, and the intensities of other actin layer lines demonstrate a ∼2-fold decrease in the fraction of myosin heads strongly bound to actin. The data show that a small fraction of actin-bound myosin heads is sufficient for supporting the O-state and, therefore the C-state is not occupied in fully activated skeletal muscle that produces mechanical work at low load.
肌肉收缩由肌动蛋白-肌球蛋白相互作用提供动力，这种相互作用由钙离子通过调节蛋白肌钙蛋白（Tn）和原肌球蛋白（Tpm）进行控制，它们与肌动蛋白丝相关联。Tpm形成卷曲螺旋二聚体，这些二聚体组装成一条螺旋链，沿着细肌丝约1μm的全长延伸。在没有钙离子的情况下，紧密结合Tpm的Tn与肌动蛋白结合，并将Tpm链保持在阻断状态，即B状态，此时Tpm屏蔽肌动蛋白，使其无法与肌球蛋白头部结合。钙离子与Tn结合后，Tpm从肌动蛋白上释放，从而绕着细丝轴在方位上移动到闭合状态，即C状态，此时肌动蛋白部分可用于与肌球蛋白头部进行弱结合。当弱的肌动蛋白-肌球蛋白键转变为强的、立体特异性复合物时，肌球蛋白头部将Tpm链推到开放状态，即O状态，使几个相邻肌动蛋白单体上的肌球蛋白结合位点开放，可供肌球蛋白结合。我们利用欧洲同步辐射装置的低角度X射线衍射来检查兔子快速骨骼肌完全激活纤维中从O状态到C状态的转变是否发生在从等长收缩到低负荷缩短的过程中。在缩短过程中，未观察到在0.15 - 0.275nm倒易半径范围内第二肌动蛋白层线强度的降低，这表明Tpm没有发生从O状态到C状态的方位移动，尽管在缩短过程中肌肉硬度相较于等长状态有所降低，并且其他肌动蛋白层线的强度表明与肌动蛋白紧密结合的肌球蛋白头部比例下降了约2倍。数据表明，一小部分与肌动蛋白结合的肌球蛋白头部就足以维持O状态，因此在低负荷下产生机械功的完全激活骨骼肌中C状态并未占据主导。
