Reconditi Massimo, Linari Marco, Lucii Leonardo, Stewart Alex, Sun Yin-Biao, Boesecke Peter, Narayanan Theyencheri, Fischetti Robert F, Irving Tom, Piazzesi Gabriella, Irving Malcom, Lombardi Vincenzo
Laboratorio di Fisiologia, DBAG, Università di Firenze, I-50134 Firenze, and OGG, Istituto Nazionale di Fisica della Materia, Italy.
Nature. 2004 Apr 1;428(6982):578-81. doi: 10.1038/nature02380.
Muscle contraction is driven by the motor protein myosin II, which binds transiently to an actin filament, generates a unitary filament displacement or 'working stroke', then detaches and repeats the cycle. The stroke size has been measured previously using isolated myosin II molecules at low load, with rather variable results, but not at the higher loads that the motor works against during muscle contraction. Here we used a novel X-ray-interference technique to measure the working stroke of myosin II at constant load in an intact muscle cell, preserving the native structure and function of the motor. We show that the stroke is smaller and slower at higher load. The stroke size at low load is likely to be set by a structural limit; at higher loads, the motor detaches from actin before reaching this limit. The load dependence of the myosin II stroke is the primary molecular determinant of the mechanical performance and efficiency of skeletal muscle.
肌肉收缩由运动蛋白肌球蛋白II驱动，它与肌动蛋白丝短暂结合，产生一个单位的丝位移或“工作冲程”，然后分离并重复这个循环。冲程大小此前已在低负荷下使用分离的肌球蛋白II分子进行测量，结果差异较大，但未在肌肉收缩过程中运动蛋白所对抗的较高负荷下进行测量。在这里，我们使用一种新颖的X射线干涉技术，在完整的肌肉细胞中以恒定负荷测量肌球蛋白II的工作冲程，保留了运动蛋白的天然结构和功能。我们发现，在较高负荷下冲程更小且更慢。低负荷下的冲程大小可能由结构限制设定；在较高负荷下，运动蛋白在达到这个极限之前就会从肌动蛋白上分离。肌球蛋白II冲程的负荷依赖性是骨骼肌机械性能和效率的主要分子决定因素。
