Geeves M A, Lehrer S S
Department of Biochemistry, University of Bristol, England.
Biophys J. 1994 Jul;67(1):273-82. doi: 10.1016/S0006-3495(94)80478-3.
Actin thin filaments containing bound tropomyosin (Tm) or tropomyosin troponin (Tm.Tn) exist in two states ("off" and "on") with different affinities for myosin heads (S1), which results in the cooperative binding of S1. The rate of S1 binding to, and dissociating from, actin, Tm.actin, and Tm.Tn.actin, monitored by light scattering (LS), was compared with the rate of change in state, monitored by the excimer fluorescence (Fl) of a pyrene label attached to Tm. The ATP-induced S1 dissociation showed similar exponential decreases in LS for actin.S1, Tm.actin.S1, and Tm.Tn.actin.S1 +/- Ca2+. The Fl change, however, showed a delay that was greater for Tm.Tn.actin than Tm.actin, independent of Ca2+. The S1 binding kinetics gave observed rate constants for the S1-induced change in state that were 5-6 times the observed rate constants of S1 binding to Tm.actin, which were increased to 10-12 for Tm.Tn.actin, independent of Ca2+. The rate of the Fl signals showed that the on/off states were in rapid equilibrium. These data indicate that the apparent cooperative unit for Tm.actin is 5-6 actin subunits rather than the minimum structural unit size of 7, and is increased to 10-12 subunits for Tm.Tn.actin, independent of the presence of Ca2+. Thus, Tm appears semi-flexible, and Tn increases communication between neighboring structural units. A general model for the dynamic transitions involved in muscle regulation is presented.
含有结合原肌球蛋白（Tm）或原肌球蛋白 - 肌钙蛋白（Tm.Tn）的肌动蛋白细肌丝以两种状态（“关闭”和“开启”）存在，对肌球蛋白头部（S1）具有不同的亲和力，这导致了S1的协同结合。通过光散射（LS）监测S1与肌动蛋白、Tm.肌动蛋白和Tm.Tn.肌动蛋白结合和解离的速率，并与通过连接到Tm上的芘标记的准分子荧光（Fl）监测的状态变化速率进行比较。ATP诱导的S1解离在肌动蛋白 - S1、Tm.肌动蛋白 - S1和Tm.Tn.肌动蛋白 - S1±Ca2+的LS中显示出类似的指数下降。然而，Fl变化显示出延迟，对于Tm.Tn.肌动蛋白比Tm.肌动蛋白更大，与Ca2+无关。S1结合动力学给出了S1诱导状态变化的观测速率常数，其是S1与Tm.肌动蛋白结合观测速率常数的5 - 6倍，对于Tm.Tn.肌动蛋白增加到10 - 12倍，与Ca2+无关。Fl信号的速率表明开启/关闭状态处于快速平衡。这些数据表明，Tm.肌动蛋白的表观协同单位是5 - 6个肌动蛋白亚基，而不是最小结构单位大小7，对于Tm.Tn.肌动蛋白增加到10 - 12个亚基，与Ca2+的存在无关。因此，Tm似乎是半柔性的，而Tn增加了相邻结构单位之间的通讯。本文提出了一个涉及肌肉调节的动态转变的通用模型。
