Department of Biological Science and Institute of Molecular Biophysics, Florida State University, 91 Chieftan Way, Tallahassee, FL 32306, USA; Department of Physiology and Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118-2526, USA.
J Mol Biol. 2013 Nov 15;425(22):4544-55. doi: 10.1016/j.jmb.2013.08.020. Epub 2013 Sep 8.
Tropomyosin (Tm) is a key factor in the molecular mechanisms that regulate the binding of myosin motors to actin filaments (F-Actins) in most eukaryotic cells. This regulation is achieved by the azimuthal repositioning of Tm along the actin (Ac):Tm:troponin (Tn) thin filament to block or expose myosin binding sites on Ac. In striated muscle, including involuntary cardiac muscle, Tm regulates muscle contraction by coupling Ca(2+) binding to Tn with myosin binding to the thin filament. In smooth muscle, the switch is the posttranslational modification of the myosin. Depending on the activation state of Tn and the binding state of myosin, Tm can occupy the blocked, closed, or open position on Ac. Using native cryogenic 3DEM (three-dimensional electron microscopy), we have directly resolved and visualized cardiac and gizzard muscle Tm on filamentous Ac in the position that corresponds to the closed state. From the 8-Å-resolution structure of the reconstituted Ac:Tm filament formed with gizzard-derived Tm, we discuss two possible mechanisms for the transition from closed to open state and describe the role Tm plays in blocking myosin tight binding in the closed-state position.
原肌球蛋白（Tm）是调节大多数真核细胞中肌球蛋白马达与肌动蛋白丝（F-肌动蛋白）结合的分子机制中的关键因素。这种调节是通过 Tm 在肌动蛋白（Ac）上的方位重新定位来实现的，从而阻止或暴露 Ac 上的肌球蛋白结合位点。在横纹肌中，包括不随意心肌，Tm 通过将 Tn 与 Ca(2+) 的结合与肌球蛋白与细丝的结合相偶联来调节肌肉收缩。在平滑肌中，开关是肌球蛋白的翻译后修饰。根据 Tn 的激活状态和肌球蛋白的结合状态，Tm 可以占据 Ac 上的封闭、关闭或开放位置。使用原生低温 3DEM（三维电子显微镜），我们直接解析并可视化了对应于关闭状态的丝状 Ac 上的心脏和肌胃肌 Tm。从与肌胃衍生的 Tm 形成的重建 Ac:Tm 细丝的 8-Å 分辨率结构，我们讨论了从关闭状态到开放状态的两种可能机制，并描述了 Tm 在阻止肌球蛋白在封闭状态下紧密结合中所起的作用。
