Miller L, Reisler E
J Mol Biol. 1985 Mar 20;182(2):271-9. doi: 10.1016/0022-2836(85)90345-6.
The S-1/S-2 swivel in myosin provides a flexible link between the head and tail portions of the molecule. We have investigated the properties of the swivel by employing limited proteolysis methods. Our results indicate that the binding of actin to heavy meromyosin inhibits both the chymotryptic and papain cleavage of the S-1/S-2 swivel, and that this effect is dependent on the presence of intact LC-2 light chains. Actin did not slow digestions carried out using heavy meromyosin previously treated with proteases to nick the LC-2 chains to 17,000 or 14,000 Mr fragments. Although the integrity of the LC-2 light chain appears to be required to transmit the effects of actin binding from the myosin head to the S-1/S-2 swivel, the binding of Ca2+ to the 17,000 Mr LC-2 fragment can still affect the chemical reactivity of SH1 thiol groups. Both chymotryptic and papain digestions of heavy meromyosin containing intact or fragmented LC-2 light chain show substantial temperature sensitivity between 5 degrees C and 35 degrees C. Calculated apparent activation energies for this process indicate that the S-1/S-2 swivel in myosin can undergo temperature-dependent structural changes independently of the state of the LC-2 light chain. Thus, both actin binding and temperature variations can induce structural transitions in the S-1/S-2 swivel.
肌球蛋白中的S-1/S-2旋转区在分子的头部和尾部之间提供了一个灵活的连接。我们通过有限蛋白酶解方法研究了该旋转区的特性。我们的结果表明，肌动蛋白与重酶解肌球蛋白的结合抑制了S-1/S-2旋转区的胰凝乳蛋白酶和木瓜蛋白酶切割，并且这种效应依赖于完整的LC-2轻链的存在。肌动蛋白不会减缓使用先前用蛋白酶处理使LC-2链断裂成17,000或14,000 Mr片段的重酶解肌球蛋白进行的消化。尽管似乎需要LC-2轻链的完整性来将肌动蛋白结合的效应从肌球蛋白头部传递到S-1/S-2旋转区，但Ca2+与17,000 Mr LC-2片段的结合仍可影响SH1巯基的化学反应性。含有完整或片段化LC-2轻链的重酶解肌球蛋白的胰凝乳蛋白酶和木瓜蛋白酶消化在5℃至35℃之间均表现出显著的温度敏感性。该过程的计算表观活化能表明，肌球蛋白中的S-1/S-2旋转区可独立于LC-2轻链的状态发生温度依赖性结构变化。因此，肌动蛋白结合和温度变化均可诱导S-1/S-2旋转区的结构转变。
