Davis J S
Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218.
Biochemistry. 1993 Apr 20;32(15):4035-42. doi: 10.1021/bi00066a026.
Subunit exchange between groups of myosin filaments at equilibrium in a volume similar to a sarcomere is simulated using Monte Carlo (probabilistic) methods. Five published kinetic parameters (three rate constants and two cooperativity parameters) which govern the assembly of thick filaments from purified myosin at pH 8.0 are used for the computations. Filament length distributions equivalent to those measured experimentally in the electron microscope result. Distinctive patterns of exchange emerge because cooperativity in myosin assembly is not confined to nucleation but functions throughout growth. Fluctuations in filament size, first apparent in the millisecond time domain, mediate exchange which first occurs at the tips of the filaments and then gradually progresses inwards toward the central bare-zone. Exchange rates decreased by an approximate factor of 10 per decade of time: full exchange takes years, 50% takes 28 h, and 10% takes a brief 100 ms. These data represent the fastest possible rates of exchange because synthetic myosin filaments lack the overall stabilizing influence of the copolymerizing proteins of native filaments. Exchange at equilibrium is therefore too slow to explain, for example, the much faster rates recorded in vivo for the complete replacement of one myosin isoform by another. Facilitated exchange where partial or complete filament dissociation is followed by the introduction of new subunits during reassembly offers a means of accelerating exchange. In this context, it is shown that the requisite disassembly and reassembly of myosin thick filaments can be completed in a minimum of a few seconds.
使用蒙特卡罗（概率）方法模拟了肌球蛋白丝组之间在类似于肌节的体积中达到平衡时的亚基交换。计算中使用了五个已发表的动力学参数（三个速率常数和两个协同性参数），这些参数在pH 8.0时控制着由纯化的肌球蛋白组装粗丝。得到了与在电子显微镜下实验测量结果相当的丝长度分布。由于肌球蛋白组装中的协同性不仅限于成核，而是在整个生长过程中起作用，因此出现了独特的交换模式。丝尺寸的波动首先在毫秒时域中显现，介导了交换，交换首先发生在丝的末端，然后逐渐向内朝着中央裸区推进。交换速率每十年下降约10倍：完全交换需要数年时间，50%的交换需要28小时，10%的交换只需短暂的100毫秒。这些数据代表了最快可能的交换速率，因为合成肌球蛋白丝缺乏天然丝中共聚蛋白的整体稳定作用。因此，平衡时的交换太慢，无法解释例如体内一种肌球蛋白同工型被另一种完全替代时记录的快得多的速率。在重新组装过程中，部分或完全丝解离后引入新亚基的促进交换提供了一种加速交换的方法。在这种情况下，结果表明肌球蛋白粗丝所需的拆卸和重新组装可以在最少几秒钟内完成。
