BioCAT, Department of Biology, Illinois Institute of Technology, Chicago, IL.
Department of Biochemistry, Bristol Myers Squibb, Brisbane, CA.
J Gen Physiol. 2022 Dec 5;154(12). doi: 10.1085/jgp.202213213. Epub 2022 Nov 1.
Classically, striated muscle contraction is initiated by calcium (Ca2+)-dependent structural changes in regulatory proteins on actin-containing thin filaments, which allow the binding of myosin motors to generate force. Additionally, dynamic switching between resting off and active on myosin states has been shown to regulate muscle contractility, a recently validated mechanism by novel myosin-targeted therapeutics. The molecular nature of this switching, however, is not understood. Here, using a combination of small-angle x-ray fiber diffraction and biochemical assays with reconstituted systems, we show that cardiac thick filaments are directly Ca2+-regulated. We find that Ca2+ induces a structural transition of myosin heads from ordered off states close to the thick filament to disordered on states closer to the thin filaments. Biochemical assays show a Ca2+-induced transition from an inactive super-relaxed (SRX) state(s) to an active disordered-relaxed (DRX) state(s) in synthetic thick filaments. We show that these transitions are an intrinsic property of cardiac myosin only when assembled into thick filaments and provide a fresh perspective on nature's two orthogonal mechanisms to regulate muscle contraction through the thin and the thick filaments.
经典理论认为，肌球蛋白丝上含有的肌动蛋白调节蛋白的钙离子（Ca2+）依赖性结构变化引发横纹肌收缩，使肌球蛋白马达与肌动蛋白结合以产生力。此外，肌球蛋白从静息的关闭状态到活跃的开启状态的动态切换已被证明可以调节肌肉收缩能力，这是一种最近通过新型肌球蛋白靶向治疗方法验证的机制。然而，这种切换的分子本质尚不清楚。在这里，我们使用小角度 X 射线纤维衍射和重组系统的生化分析相结合的方法，表明心肌粗丝是直接受 Ca2+调控的。我们发现 Ca2+诱导肌球蛋白头部从靠近粗丝的有序关闭状态向更靠近细丝的无序开启状态的结构转变。生化分析表明，在合成的粗丝中，Ca2+诱导从无活性的超松弛（SRX）状态到活性的无序松弛（DRX）状态的转变。我们表明，只有当心肌球蛋白组装成粗丝时，这些转变才是其固有特性，并为通过细肌丝和粗肌丝调节肌肉收缩的两种自然正交机制提供了新的视角。
