Department of Physiology and Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA; Computational Biochemistry Group, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, Im Neuenheimer Feld 368, Heidelberg D69120, Germany.
Department of Physiology and Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA.
Biochem Biophys Res Commun. 2014 Mar 28;446(1):304-8. doi: 10.1016/j.bbrc.2014.02.097. Epub 2014 Mar 4.
To be effective as a gatekeeper regulating the access of binding proteins to the actin filament, adjacent tropomyosin molecules associate head-to-tail to form a continuous super-helical cable running along the filament surface. Chimeric head-to-tail structures have been solved by NMR and X-ray crystallography for N- and C-terminal segments of smooth and striated muscle tropomyosin spliced onto non-native coiled-coil forming peptides. The resulting 4-helix complexes have a tight coiled-coil N-terminus inserted into a separated pair of C-terminal helices, with some helical unfolding of the terminal chains in the striated muscle peptides. These overlap complexes are distinctly curved, much more so than elsewhere along the superhelical tropomyosin cable. To verify whether the non-native protein adducts (needed to stabilize the coiled-coil chimeras) perturb the overlap, we carried out Molecular Dynamics simulations of head-to-tail structures having only native tropomyosin sequences. We observe that the splayed chains all refold and become helical. Significantly, the curvature of both the smooth and the striated muscle overlap domain is reduced and becomes comparable to that of the rest of the tropomyosin cable. Moreover, the measured flexibility across the junction is small. This and the reduced curvature ensure that the super-helical cable matches the contours of F-actin without manifesting localized kinking and excessive flexibility, thus enabling the high degree of cooperativity in the regulation of myosin accessibility to actin filaments.
作为一种调节结合蛋白与肌动蛋白丝结合的守门人，相邻的原肌球蛋白分子以头对头的方式结合形成一个连续的超螺旋电缆，沿着纤维表面运行。通过 NMR 和 X 射线晶体学已经解决了来自平滑肌和横纹肌原肌球蛋白的 N 和 C 末端片段与非天然卷曲螺旋形成肽拼接的头对头结构。产生的 4 螺旋复合物具有紧密卷曲螺旋的 N 末端插入分离的 C 末端螺旋对，在横纹肌肽中末端链有一些螺旋展开。这些重叠复合物明显弯曲，比超螺旋原肌球蛋白电缆的其他部分弯曲得多。为了验证非天然蛋白加合物（稳定卷曲螺旋嵌合体所需）是否会干扰重叠，我们对只有天然原肌球蛋白序列的头对头结构进行了分子动力学模拟。我们观察到，张开的链都重新折叠成螺旋状。重要的是，平滑和横纹肌重叠域的曲率都减小了，并变得与原肌球蛋白电缆的其余部分相当。此外，连接处的测量灵活性很小。这和曲率的降低确保了超螺旋电缆与 F-肌动蛋白的轮廓相匹配，而不会表现出局部扭曲和过度的灵活性，从而使肌球蛋白对肌动蛋白丝的可及性的高度协调性成为可能。
