VanBuren P, Waller G S, Harris D E, Trybus K M, Warshaw D M, Lowey S
Department of Molecular Physiology and Biophysics, University of Vermont, Burlington 05405.
Proc Natl Acad Sci U S A. 1994 Dec 20;91(26):12403-7. doi: 10.1073/pnas.91.26.12403.
Myosin, a molecular motor that is responsible for muscle contraction, is composed of two heavy chains each with two light chains. The crystal structure of subfragment 1 indicates that both the regulatory light chains (RLCs) and the essential light chains (ELCs) stabilize an extended alpha-helical segment of the heavy chain. It has recently been shown in a motility assay that removal of either light chain markedly reduces actin filament sliding velocity without a significant loss in actin-activated ATPase activity. Here we demonstrate by single actin filament force measurements that RLC removal has little effect on isometric force, whereas ELC removal reduces isometric force by over 50%. These data are interpreted with a simple mechanical model where subfragment 1 behaves as a torque motor whose leyer arm length is sensitive to light-chain removal. Although the effect of removing RLCs fits within the confines of this model, altered crossbridge kinetics, as reflected in a reduced unloaded duty cycle, probably contributes to the reduced velocity and force production of ELC-deficient myosins.
肌球蛋白是一种负责肌肉收缩的分子马达，由两条重链组成，每条重链又各有两条轻链。亚片段1的晶体结构表明，调节轻链（RLC）和必需轻链（ELC）都能稳定重链的延伸α螺旋段。最近在一项运动测定中发现，去除任何一条轻链都会显著降低肌动蛋白丝的滑动速度，而肌动蛋白激活的ATP酶活性却没有明显损失。在此，我们通过单根肌动蛋白丝力测量证明，去除RLC对等长力影响不大，而去除ELC则使等长力降低超过50%。这些数据用一个简单的力学模型来解释，其中亚片段1表现为一个扭矩马达，其力臂长度对轻链去除敏感。虽然去除RLC的效果符合该模型的范围，但如卸载工作周期缩短所反映的那样，改变的横桥动力学可能是ELC缺陷型肌球蛋白速度和力产生降低的原因。
