Biophysics Program, Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742;
Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742.
Proc Natl Acad Sci U S A. 2017 May 30;114(22):E4389-E4398. doi: 10.1073/pnas.1615708114. Epub 2017 May 16.
Myosin VI (MVI) is the only known member of the myosin superfamily that, upon dimerization, walks processively toward the pointed end of the actin filament. The leading head of the dimer directs the trailing head forward with a power stroke, a conformational change of the motor domain exaggerated by the lever arm. Using a unique coarse-grained model for the power stroke of a single MVI, we provide the molecular basis for its motility. We show that the power stroke occurs in two major steps. First, the motor domain attains the poststroke conformation without directing the lever arm forward; and second, the lever arm reaches the poststroke orientation by undergoing a rotational diffusion. From the analysis of the trajectories, we discover that the potential that directs the rotating lever arm toward the poststroke conformation is almost flat, implying that the lever arm rotation is mostly uncoupled from the motor domain. Because a backward load comparable to the largest interhead tension in a MVI dimer prevents the rotation of the lever arm, our model suggests that the leading-head lever arm of a MVI dimer is uncoupled, in accord with the inference drawn from polarized total internal reflection fluorescence (polTIRF) experiments. Without any adjustable parameter, our simulations lead to quantitative agreement with polTIRF experiments, which validates the structural insights. Finally, in addition to making testable predictions, we also discuss the implications of our model in explaining the broad step-size distribution of the MVI stepping pattern.
肌球蛋白 VI（MVI）是肌球蛋白超家族中唯一已知的成员，当二聚化时，它会朝着肌动蛋白丝的尖端进行程序性运动。二聚体的前导头通过力冲程，即通过杠杆臂放大的马达结构域的构象变化，向前引导尾随头。我们使用一种独特的粗粒化模型来模拟单个 MVI 的力冲程，为其运动提供了分子基础。我们表明，力冲程分为两个主要步骤。首先，马达结构域达到后冲程构象，而不向前引导杠杆臂；其次，杠杆臂通过旋转扩散达到后冲程取向。通过对轨迹的分析，我们发现引导旋转杠杆臂进入后冲程构象的势能几乎是平坦的，这意味着杠杆臂的旋转与马达结构域的耦合度较低。由于向后的负载与 MVI 二聚体中最大的头间张力相当，阻止了杠杆臂的旋转，因此我们的模型表明，MVI 二聚体的前导头杠杆臂是解耦的，这与偏振全内反射荧光（polTIRF）实验的推断一致。我们的模拟无需任何可调参数，就能与 polTIRF 实验定量吻合，验证了该结构的合理性。最后，除了做出可测试的预测外，我们还讨论了我们的模型在解释 MVI 步进模式广泛的步长分布方面的意义。
