Blankenfeldt Wulf, Thomä Nicolas H, Wray John S, Gautel Mathias, Schlichting Ilme
Max Planck Institute of Molecular Physiology, Department of Physical Biochemistry, 44227 Dortmund, Germany.
Proc Natl Acad Sci U S A. 2006 Nov 21;103(47):17713-7. doi: 10.1073/pnas.0606741103. Epub 2006 Nov 9.
Myosin II is the major component of the muscle thick filament. It consists of two N-terminal S1 subfragments ("heads") connected to a long dimeric coiled-coil rod. The rod is in itself twofold symmetric, but in the filament, the two heads point away from the filament surface and are therefore not equivalent. This breaking of symmetry requires the initial section of the rod, subfragment 2 (S2), to be relatively flexible. S2 is an important functional element, involved in various mechanisms by which the activity of smooth and striated muscle is regulated. We have determined crystal structures of the 126 N-terminal residues of S2 from human cardiac beta-myosin II (S2-Delta), of both WT and the disease-associated E924K mutant. S2-Delta is a straight parallel dimeric coiled coil, but the N terminus of one chain is disordered in WT-S2-Delta due to crystal contacts, indicative of unstable local structure. Bulky noncanonical side chains pack into a/d positions of S2-Delta's N terminus, leading to defined local asymmetry and axial stagger, which could induce nonequivalence of the S1 subfragments. Additionally, S2 possesses a conserved charge distribution with three prominent rings of negative potential within S2-Delta, the first of which may provide a binding interface for the "blocked head" of smooth muscle myosin in the OFF state. The observation that many disease-associated mutations affect the second negatively charged ring further suggests that charge interactions play an important role in regulation of cardiac muscle activity through myosin-binding protein C.
肌球蛋白II是肌肉粗肌丝的主要成分。它由两个N端S1亚片段（“头部”）连接到一个长的二聚体卷曲螺旋杆组成。杆本身具有二重对称性，但在肌丝中，两个头部指向远离肌丝表面的方向，因此并不等同。这种对称性的破坏要求杆的起始部分，即亚片段2（S2）相对灵活。S2是一个重要的功能元件，参与平滑肌和横纹肌活动调节的各种机制。我们已经确定了来自人心脏β-肌球蛋白II的S2的126个N端残基（S2-Δ）的晶体结构，包括野生型和与疾病相关的E924K突变体。S2-Δ是一个直的平行二聚体卷曲螺旋，但由于晶体接触，在野生型S2-Δ中一条链的N端无序，表明局部结构不稳定。大的非经典侧链堆积到S2-Δ的N端的a/d位置，导致确定的局部不对称和轴向错位，这可能导致S1亚片段的不等同。此外，S2具有保守的电荷分布，在S2-Δ内有三个突出的负电位环，其中第一个可能为处于关闭状态的平滑肌肌球蛋白的“受阻头部”提供结合界面。许多与疾病相关的突变影响第二个带负电荷的环这一观察结果进一步表明，电荷相互作用在通过肌球蛋白结合蛋白C调节心肌活动中起重要作用。
