Department of Biomedical Engineering, Tohoku University, Japan.
Biochem Biophys Res Commun. 2010 May 7;395(3):301-6. doi: 10.1016/j.bbrc.2010.03.150. Epub 2010 Mar 28.
Stress fibers (SFs) composed of nonmuscle actin and myosin II play critical roles in various cellular functions such as structural remodeling in response to changes in cell stress or strain. Previous studies report that SFs rapidly disassemble upon loss of tension caused by reduced myosin activity or sudden cell shortening, but the mechanism is unclear. Here, we showed that Rho-kinase inhibition with Y-27632 led to detachment of intact actin filaments from the SFs rather than depolymerization. Loss of tension may allow SFs to shorten via MgATP-driven cross-bridge cycling, thus we investigated the effects of MgATP concentration on SF shortening and stability. We performed the experiments using extracted SFs to allow control over MgATP concentration. SF contraction and disassembly rates each increased with increasing MgATP concentration. SFs transitioned from conventional SF shortening to rapid disassembly as MgATP concentration increases from 2 to 5mM, which is within the physiological range of intracellular MgATP concentrations. Thus, we submit that SFs in intact cells are inherently on the verge of disassembly, which is likely due to the small number of actomyosin cross-bridges in SFs compared to those found in relatively stable myofibrils. Given that recent studies have revealed that loss of resistive force against myosin II could lower the fraction of the MgATPase cycle time that the myosin head is attached to actin (i.e., the duty ratio), binding of cytoplasmic levels of MgATP to myosin II may be sufficient to cause the disassembly of unloaded SFs. The present study thus describes a putative mechanism for rapid SF disassembly caused by decreased myosin activity or sudden cell shortening.
应力纤维（SFs）由非肌肉肌动蛋白和肌球蛋白 II 组成，在各种细胞功能中发挥着关键作用，例如响应细胞应激或应变的结构重塑。先前的研究报告称，SFs 在肌球蛋白活性降低或细胞突然缩短导致张力丧失时会迅速解体，但机制尚不清楚。在这里，我们表明 Rho 激酶抑制 Y-27632 导致完整的肌动蛋白丝从 SFs 上脱离，而不是解聚。张力的丧失可能允许 SFs 通过 MgATP 驱动的横桥循环缩短，因此我们研究了 MgATP 浓度对 SF 缩短和稳定性的影响。我们使用提取的 SF 进行实验，以控制 MgATP 浓度。SF 收缩和解体速率都随 MgATP 浓度的增加而增加。随着 MgATP 浓度从 2mM 增加到 5mM，SF 从传统的 SF 缩短转变为快速解体，这在细胞内 MgATP 浓度的生理范围内。因此，我们提出，完整细胞中的 SF 本质上处于解体的边缘，这可能是由于 SF 中的肌球蛋白肌动蛋白交联数量较少，而相对稳定的肌原纤维中则存在大量肌球蛋白肌动蛋白交联。鉴于最近的研究表明，抵抗肌球蛋白 II 的阻力丧失可能会降低肌球蛋白头部与肌动蛋白结合的 MgATP 酶循环时间的分数（即占空比），细胞质水平的 MgATP 与肌球蛋白 II 的结合可能足以导致未加载的 SF 解体。因此，本研究描述了一种由于肌球蛋白活性降低或细胞突然缩短导致 SF 快速解体的假设机制。
