Xu J Q, Harder B A, Uman P, Craig R
Department of Cell Biology, University of Massachusetts Medical School, Worcester, 01655, USA.
J Cell Biol. 1996 Jul;134(1):53-66. doi: 10.1083/jcb.134.1.53.
The in vivo structure of the myosin filaments in vertebrate smooth muscle is unknown. Evidence from purified smooth muscle myosin and from some studies of intact smooth muscle suggests that they may have a nonhelical, side-polar arrangement of crossbridges. However, the bipolar, helical structure characteristic of myosin filaments in striated muscle has not been disproved for smooth muscle. We have used EM to investigate this question in a functionally diverse group of smooth muscles (from the vascular, gastrointestinal, reproductive, and visual systems) from mammalian, amphibian, and avian species. Intact muscle under physiological conditions, rapidly frozen and then freeze substituted, shows many myosin filaments with a square backbone in transverse profile. Transverse sections of fixed, chemically skinned muscles also show square backbones and, in addition, reveal projections (crossbridges) on only two opposite sides of the square. Filaments gently isolated from skinned smooth muscles and observed by negative staining show crossbridges with a 14.5-nm repeat projecting in opposite directions on opposite sides of the filament. Such filaments subjected to low ionic strength conditions show bare filament ends and an antiparallel arrangement of myosin tails along the length of the filament. All of these observations are consistent with a side-polar structure and argue against a bipolar, helical crossbridge arrangement. We conclude that myosin filaments in all smooth muscles, regardless of function, are likely to be side-polar. Such a structure could be an important factor in the ability of smooth muscles to contract by large amounts.
脊椎动物平滑肌中肌球蛋白丝的体内结构尚不清楚。来自纯化的平滑肌肌球蛋白以及一些对完整平滑肌的研究证据表明，它们可能具有非螺旋状、侧极排列的横桥。然而，横纹肌中肌球蛋白丝所特有的双极螺旋结构在平滑肌中并未被推翻。我们利用电子显微镜在来自哺乳动物、两栖动物和鸟类的一组功能多样的平滑肌（来自血管、胃肠道、生殖和视觉系统）中研究了这个问题。在生理条件下完整的肌肉，快速冷冻然后进行冷冻置换，显示出许多肌球蛋白丝在横切面上具有方形主干。固定的、化学去膜肌肉的横切面也显示出方形主干，此外，还揭示了在方形的仅两个相对侧上的突起（横桥）。从去膜平滑肌中轻轻分离并通过负染色观察的丝显示出横桥，其14.5纳米的重复结构在丝的相对侧上向相反方向突出。在低离子强度条件下的此类丝显示出裸露的丝末端以及肌球蛋白尾部沿丝的长度呈反平行排列。所有这些观察结果都与侧极结构一致，并反对双极螺旋横桥排列。我们得出结论，所有平滑肌中的肌球蛋白丝，无论其功能如何，都可能是侧极的。这样的结构可能是平滑肌大量收缩能力的一个重要因素。
