Cooke P H, Fay F S, Craig R
Department of Cell Biology, University of Massachusetts Medical School, Worcester 01655.
J Muscle Res Cell Motil. 1989 Jun;10(3):206-20. doi: 10.1007/BF01739811.
The structure of myosin filaments isolated from skinned toad stomach smooth muscle cells has been examined by electron microscopy as a step toward identifying the in vivo structure. When negatively stained following exposure to relaxing conditions, the filaments exhibited a continuous 14-nm axial repeat of crossbridge projections with no central bare zone. The filaments thus differed from the bipolar filaments found in striated muscle and displayed instead features resembling side-polar and mixed-polarity filament models. By rotation of isolated filaments around their longitudinal axes it was found that cross bridges occurred only along two sides of the filament, an arrangement consistent with the side-polar but not the mixed-polarity model. The polarity is thus similar to that proposed for ribbons (Small & Squire, J. molec. Biol. 67, (1972) 17-149) and for synthetic smooth muscle myosin filaments (Craig and Megerman, J. Cell Biol. 75, (1977) 990-996); their appearance in cross-section, however, shows that these structures are filaments (i.e. with two axes of similar dimensions) and not broad ribbons. As the filaments were derived directly from skinned cells which contracted and relaxed in response to physiological levels of MgATP and Ca2+ at rates comparable to those of native, isolated cells, this unusual arrangement of cross bridges appears to be an effective, functional form of myosin in the contractile apparatus. Side-polar filaments therefore merit consideration as plausible candidates for the native organization of myosin in vertebrate smooth muscle cells.
从剥制的蟾蜍胃平滑肌细胞中分离出的肌球蛋白丝的结构已通过电子显微镜进行了检查，以此作为确定其体内结构的第一步。在松弛条件下进行负染色后，这些丝呈现出14纳米的连续轴向重复横纹肌投影，没有中央裸区。因此，这些丝与横纹肌中发现的双极丝不同，而是呈现出类似于侧极和混合极性丝模型的特征。通过使分离的丝绕其纵轴旋转，发现横纹肌仅沿丝的两侧出现，这种排列与侧极模型一致，但与混合极性模型不一致。因此，其极性与为带（Small & Squire，《分子生物学杂志》67卷，（1972年）17 - 149页）和合成平滑肌肌球蛋白丝（Craig和Megerman，《细胞生物学杂志》75卷，（1977年）990 - 996页）所提出的极性相似；然而，它们在横截面上的外观表明这些结构是丝（即具有两个相似尺寸的轴）而不是宽带。由于这些丝直接来源于剥制的细胞，这些细胞在生理水平的MgATP和Ca2 +作用下收缩和松弛的速率与天然分离细胞相当，这种横纹肌的不寻常排列似乎是收缩装置中肌球蛋白的一种有效功能形式。因此，侧极丝值得被视为脊椎动物平滑肌细胞中肌球蛋白天然组织的合理候选者。
