Ca2+-induced movement of tropomyosin on native cardiac thin filaments revealed by cryoelectron microscopy
Ca2+-induced movement of tropomyosin on native cardiac thin filaments revealed by cryoelectron microscopy
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DOI：
10.1073/pnas.1700868114
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发表时间：
2017-06-27
影响因子：
11.1
通讯作者：
Galkin, Vitold E.
中科院分区：
文献类型：
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作者：
Risi, Cristina;Eisner, Jamie;Galkin, Vitold E.
肌肉收缩依赖于肌球蛋白马达与F-肌动蛋白的相互作用，而F-肌动蛋白是通过肌钙蛋白复合体对钙离子的反应移位原肌球蛋白来调节的。目前的肌肉调节模型认为，在松弛(低钙)条件下，原肌球蛋白阻断F-肌动蛋白上的肌球蛋白结合位点，而在激活(高钙)条件下，原肌球蛋白易位仅部分暴露F-肌动蛋白上的肌球蛋白结合部位，因此需要僵直的肌球蛋白结合才能完全激活细丝(Tf)。在这里，我们使用单粒子方法在松弛和激活条件下对冷冻水合的天然心脏TF进行螺旋重建，以揭示钙激活时原肌球蛋白在天然心脏TF表面的方位运动。我们证明在松弛或激活条件下，原肌球蛋白并不局限于一种结构状态，而是分布在转铁蛋白表面的三个结构位置之间。我们发现，这些原肌球蛋白中的两个位置抑制了肌动球蛋白的相互作用，而在高钙时显著增强的第三个位置，原肌球蛋白不阻断F-肌动蛋白上的肌球蛋白结合部位。我们的数据为钙离子增强心脏转铁蛋白的激活作用提供了一个结构框架，并为心脏转铁蛋白的调节提供了一个机制模型。
Muscle contraction relies on the interaction of myosin motors with F-actin, which is regulated through a translocation of tropomyosin by the troponin complex in response to Ca2+. The current model of muscle regulation holds that at relaxing (low-Ca2+) conditions tropomyosin blocks myosin binding sites on F-actin, whereas at activating (high-Ca2+) conditions tropomyosin translocation only partially exposes myosin binding sites on F-actin so that binding of rigor myosin is required to fully activate the thin filament (TF). Here we used a single-particle approach to helical reconstruction of frozen hydrated native cardiac TFs under relaxing and activating conditions to reveal the azimuthal movement of the tropomyosin on the surface of the native cardiac TF upon Ca2+ activation. We demonstrate that at either relaxing or activating conditions tropomyosin is not constrained in one structural state, but rather is distributed between three structural positions on the surface of the TF. We show that two of these tropomyosin positions restrain actomyosin interactions, whereas in the third position, which is significantly enhanced at high Ca2+, tropomyosin does not block myosin binding sites on F-actin. Our data provide a structural framework for the enhanced activation of the cardiac TF over the skeletal TF by Ca2+ and lead to a mechanistic model for the regulation of the cardiac TF.