Horowits R, Maruyama K, Podolsky R J
National Institute of Arthritis and Musculoskeletal and Skin Diseases, Bethesda, Maryland 20892.
J Cell Biol. 1989 Nov;109(5):2169-76. doi: 10.1083/jcb.109.5.2169.
Connectin (also called titin) is a huge, striated muscle protein that binds to thick filaments and links them to the Z-disc. Using an mAb that binds to connectin in the I-band region of the molecule, we studied the behavior of connectin in both relaxed and activated skinned rabbit psoas fibers by immunoelectron microscopy. In relaxed fibers, antibody binding is visualized as two extra striations per sarcomere arranged symmetrically about the M-line. These striations move away from both the nearest Z-disc and the thick filaments when the sarcomere is stretched, confirming the elastic behavior of connectin within the I-band of relaxed sarcomeres as previously observed by several investigators. When the fiber is activated, thick filaments in sarcomeres shorter than 2.8 microns tend to move from the center to the side of the sarcomere. This translocation of thick filaments within the sarcomere is accompanied by movement of the antibody label in the same direction. In that half-sarcomere in which the thick filaments move away from the Z-disc, the spacings between the Z-disc and the antibody and between the antibody and the thick filaments both increase. Conversely, on the side of the sarcomere in which the thick filaments move nearer to the Z-line, these spacings decrease. Regardless of whether I-band spacing is varied by stretch of a relaxed sarcomere or by active sliding of thick filaments within a sarcomere of constant length, the spacings between the Z-line and the antibody and between the antibody and the thick filaments increase with I-band length identically. These results indicate that the connectin filaments remain bound to the thick filaments in active fibers, and that the elastic properties of connectin are unaltered by calcium ions and cross-bridge activity.
连接蛋白（也称为肌联蛋白）是一种巨大的横纹肌蛋白，它与粗肌丝结合并将它们连接到Z盘。我们使用一种能与分子I带区域的连接蛋白结合的单克隆抗体，通过免疫电子显微镜研究了松弛和激活的去皮兔腰大肌纤维中连接蛋白的行为。在松弛的纤维中，抗体结合表现为每个肌节有两条额外的横纹，围绕M线对称排列。当肌节被拉伸时，这些横纹会远离最近的Z盘和粗肌丝，这证实了连接蛋白在松弛肌节I带内的弹性行为，正如之前几位研究者所观察到的那样。当纤维被激活时，长度小于2.8微米的肌节中的粗肌丝往往会从肌节中心向肌节一侧移动。粗肌丝在肌节内的这种移位伴随着抗体标记物向相同方向移动。在粗肌丝远离Z盘的那半个肌节中，Z盘与抗体之间以及抗体与粗肌丝之间的间距都会增加。相反，在肌节中粗肌丝向Z线靠近的一侧，这些间距会减小。无论I带间距是通过松弛肌节的拉伸而变化，还是通过恒定长度肌节内粗肌丝的主动滑动而变化，Z线与抗体之间以及抗体与粗肌丝之间的间距都会随着I带长度的增加而以相同的方式增加。这些结果表明，连接蛋白丝在活性纤维中仍与粗肌丝结合，并且连接蛋白的弹性特性不受钙离子和横桥活性的影响。
