STRUCTURAL CHANGES IN ACTIN-CONTAINING AND MYOSIN-CONTAINING FILAMENTS DURING CONTRACTION
STRUCTURAL CHANGES IN ACTIN-CONTAINING AND MYOSIN-CONTAINING FILAMENTS DURING CONTRACTION
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DOI：
10.1101/sqb.1973.037.01.046
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发表时间：
1973-01-01
期刊：
影响因子：
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通讯作者：
HUXLEY, HE
中科院分区：
文献类型：
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作者：
HUXLEY, HE
早期对横纹肌低角度x射线衍射图的研究表明，在收缩过程中，横纹肌衍射图的一些特征几乎保持不变，而另一些特征则以特有的方式发生变化。因此，细丝中的亚基重复序列和肌动蛋白螺旋的螺距明显保持不变，肌球蛋白丝中的亚基重复序列（143a交叉桥间距）也几乎保持不变（Elliott等人，1966,1967；Huxley等人，1965；Huxley和Brown， 1967），尽管更广泛的测量显示收缩期间间距略有增加，约为1%（Huxley和Brown, 1967; Haselgrove, 1967, 1970）。另一方面，厚纤维上交叉桥螺旋排列的x射线图的强度有很大的下降，特别是在图案的非子午线部分，这被解释为表明任何给定纤维上的交叉桥从活松弛肌肉的相对有序的排列特征移动到收缩肌肉的更随机的排列。这些结果表明，交叉桥一定有一定的轴向运动，结合更广泛的径向和/或方位紊乱，这是由交叉桥在活动期间的异步运动引起的，因为它们各自经历了肌动蛋白连接和ATP分解的收缩周期（Huxley和Brown， 1967）。赤道反射的观测（由肌动蛋白和肌球蛋白丝在重叠区域的并排排列产生）显示，当肌肉变得活跃时，间距只有非常小的变化（Elliott等人，1965年，1967年），但赤道反射相对强度的变化与严格度（Huxley， 1968年）或活动(Haselgrove， 1970年；Haselgrove和Huxley（在prep.）指出，当细丝之间的相互作用发生时，质量会发生非常可观的横向再分布。在活的松弛肌肉中，最初与肌凝蛋白丝相关的物质与肌动蛋白丝密切相关；严格来说，涉及的物质数量大约相当于所有肌球蛋白$1亚基的质量，而在活动肌肉中，涉及的物质数量约为其一半。有人提出（Huxley, 1968），这可能代表交叉桥的活性端向侧面倾斜以附着在肌动蛋白上，并指出这种方案提供了一种很好的方法，解决了迄今为止似乎存在的困难，这些困难涉及肌动蛋白和肌凝蛋白丝之间在可变侧距上的直接物理相互作用。还注意到，从力学角度考虑，产生主动力的区域可能是81头亚基与其附着的肌动蛋白单体之间的接触区域（Huxley, 1968, 1969）。因此，一些x射线观测（还有一些我在这里没有篇幅提及，但由Haselgrove[1970]和Haselgrove和Huxley在准备中描述的观测）有力地支持了滑动细丝模型，在该模型中，细丝的长度实际上是不变的，力是通过移动的交叉桥产生的。结果还提示了力产生机构本身的一些新特征。然而，在一些地方，对x射线模式的解释远远不够完整，或者需要额外的数据；本文将讨论其中的两个问题。第一部分是关于努力达到一个更现实的理解的性质和…
Earlier studies on the low-angle X-ray diffraction diagrams given by striated muscles showed that some features of the diagrams remained virtually unchanged during contraction, whereas others changed in a characteristic manner. Thus the subunit repeat and the pitch of the actin helices in the thin filaments remained apparently constant, and the subunit repeat in the myosin filaments (the 143 A cross-bridge spacing) remained almost constant too (Elliott et al., 1965, 1967; Huxley et al., 1965; Huxley and Brown, 1967), although more extensive measurements revealed a small increase in spacing during contraction, by about 1%(Huxley and Brown, 1967; Haselgrove, 1967, 1970). On the other hand, there was a very substantial decrease in the intensity of the X-ray diagram given by the helical arrangement of cross-bridges on the thick filaments, especially in the off-meridional parts of the pattern, which was interpreted as showing that the cross-bridges on any given filament moved from a relatively well-ordered arrangement characteristic of live relaxed muscle to a more random arrangement in contracting muscle. These results showed that there must be some axial movement of the cross-bridges, combined with a more extensive radial and/or azimuthal disordering, and it was suggested that this was brought about by the asynchonized movement of the cross-bridges during activity as they each went through their individual contractile cycles of actin attachment and ATP breakdown (Huxley and Brown, 1967). Observations on the equatorial reflections (which arise from the side-by-side arrangement of the actin and myosin filaments in the region of overlap) showed only very small changes in spacing when the muscle became active (Elliott et al., 1965, 1967), but observations on the changes in the relative intensities of the equatorial reflections associated with rigor (Huxley, 1968) or with activity (Haselgrove, 1970; Haselgrove and Huxley, in prep.) indicated that a very substantial sideways redistribution of mass occurred when interaction between the filaments took place. Material originally associated with the myosin filaments in live relaxed muscles became closely associated with the actin filaments; in rigor the amount of material involved corresponded approximately to the mass of all the myosin $1 subunits, whereas in active muscle about half that amount was involved. It was suggested (Huxley, 1968) that this might represent the active end of the cross-bridge leaning out sideways to attach to actin, and it was pointed out that such a scheme provided a good way out of the difficulties which hitherto had seemed to exist with models involving direct physical interaction between actin and myosin filaments across a variable side-spacing. It was also noted that mechanical considerations made it probable that the region of active force generation was the contact area between the 81 head subunit and the actin monomer to which it attached (Huxley, 1968, 1969). Thus several of the X-ray observations (and also others that I do not have space to mention here but are described by Haselgrove [1970] and Haselgrove and Huxley, in prep.) gave strong support to the sliding-filament model, in which the filaments are of virtually invariant length, and force is developed by moving cross-bridges. The results also suggested some new features of the force-generating mechanism itself. However there were a number of points at which the interpretation of the X-ray patterns was far from complete or where additional data were needed; the present paper will deal with two of these. The first part is concerned with efforts to reach a more realistic understanding of the properties and …