Zot Henry G, Chase P Bryant, Hasbun Javier E, Pinto Jose R
Department of Biology, University of West Georgia, Carrollton, Georgia, USA; Department of Biomedical Sciences, Florida State University, Tallahassee, Florida, USA.
Department of Biological Science, Florida State University, Tallahassee, Florida, USA.
J Biol Chem. 2020 Nov 20;295(47):15913-15922. doi: 10.1074/jbc.RA120.014438. Epub 2020 Sep 8.
Vertebrate striated muscle thin filaments are thought to be thermodynamically activated in response to an increase in Ca concentration. We tested this hypothesis by measuring time intervals for gliding runs and pauses of individual skeletal muscle thin filaments in cycling myosin motility assays. A classic thermodynamic mechanism predicts that if chemical potential is constant, transitions between runs and pauses of gliding thin filaments will occur at constant rate as given by a Poisson distribution. In this scenario, rate is given by the odds of a pause, and hence, run times between pauses fit an exponential distribution that slopes negatively for all observable run times. However, we determined that relative density of observed run times fits an exponential only at low Ca levels that activate filament gliding. Further titration with Ca, or adding excess regulatory proteins tropomyosin and troponin, shifted the relative density of short run times to fit the positive slope of a gamma distribution, which derives from waiting times between Poisson events. Events that arise during a run and prevent the chance of ending a run for a random interval of time account for the observed run time distributions, suggesting that the events originate with cycling myosin. We propose that regulatory proteins of the thin filament require the mechanical force of cycling myosin to achieve the transition state for activation. During activation, combinations of cycling myosin that contribute insufficient activation energy delay deactivation.
脊椎动物的横纹肌细肌丝被认为会随着钙离子浓度的增加而发生热力学激活。我们通过在循环肌球蛋白运动性测定中测量单个骨骼肌细肌丝滑行和停顿的时间间隔来检验这一假设。一种经典的热力学机制预测，如果化学势恒定，滑行细肌丝的滑行和停顿之间的转变将以泊松分布给出的恒定速率发生。在这种情况下，速率由停顿的几率给出，因此，停顿之间的滑行时间符合指数分布，对于所有可观察到的滑行时间，该分布呈负斜率。然而，我们确定，仅在激活细肌丝滑行的低钙水平下，观察到的滑行时间的相对密度才符合指数分布。进一步用钙滴定，或添加过量的调节蛋白原肌球蛋白和肌钙蛋白，会使短滑行时间的相对密度发生变化，以符合伽马分布的正斜率，伽马分布源自泊松事件之间的等待时间。在一次滑行过程中出现的、在随机时间间隔内阻止滑行结束的事件解释了观察到的滑行时间分布，这表明这些事件源于循环的肌球蛋白。我们提出，细肌丝的调节蛋白需要循环肌球蛋白的机械力来达到激活的过渡状态。在激活过程中，提供的激活能量不足的循环肌球蛋白组合会延迟失活。
