Institute for Biophysical Dynamics, University of Chicago, Chicago, Illinois, USA.
Biophys J. 2011 Jun 8;100(11):2698-705. doi: 10.1016/j.bpj.2011.04.031.
Contractile actomyosin bundles are critical for numerous aspects of muscle and nonmuscle cell physiology. Due to the varying composition and structure of actomyosin bundles in vivo, the minimal requirements for their contraction remain unclear. Here, we demonstrate that actin filaments and filaments of smooth muscle myosin motors can self-assemble into bundles with contractile elements that efficiently transmit actomyosin forces to cellular length scales. The contractile and force-generating potential of these minimal actomyosin bundles is sharply sensitive to the myosin density. Above a critical myosin density, these bundles are contractile and generate large tensile forces. Below this threshold, insufficient cross-linking of F-actin by myosin thick filaments prevents efficient force transmission and can result in rapid bundle disintegration. For contractile bundles, the rate of contraction decreases as forces build and stalls under loads of ∼0.5 nN. The dependence of contraction speed and stall force on bundle length is consistent with bundle contraction occurring by several contractile elements connected in series. Thus, contraction in reconstituted actomyosin bundles captures essential biophysical characteristics of myofibrils while lacking numerous molecular constituents and structural signatures of sarcomeres. These results provide insight into nonsarcomeric mechanisms of actomyosin contraction found in smooth muscle and nonmuscle cells.
收缩性肌球蛋白肌动球蛋白束对于肌肉和非肌肉细胞生理学的许多方面都至关重要。由于肌球蛋白束在体内的组成和结构不同，其收缩的最小要求仍不清楚。在这里，我们证明了肌动蛋白丝和平滑肌肌球蛋白马达丝可以自组装成具有收缩元件的束，这些收缩元件可以有效地将肌球蛋白力传递到细胞长度尺度。这些最小肌球蛋白束的收缩和产生力的潜力对肌球蛋白密度非常敏感。在肌球蛋白密度超过临界值时，这些束是可收缩的，并产生大的拉伸力。低于此阈值时，肌球蛋白粗丝对 F-肌动蛋白的交联不足会阻止有效力的传递，并可能导致束迅速解体。对于收缩束，当力建立时收缩速度会降低，并在约 0.5 nN 的负载下停顿。收缩速度和停顿力对束长度的依赖性与通过串联连接的几个收缩元件的束收缩一致。因此，重组肌球蛋白肌动球蛋白束中的收缩捕获了平滑肌和非肌肉细胞中肌球蛋白收缩的基本物理特性，而没有肌节的许多分子成分和结构特征。这些结果为平滑肌和非肌肉细胞中发现的非肌节肌球蛋白收缩机制提供了深入的了解。
