Funatsu T, Kono E, Higuchi H, Kimura S, Ishiwata S, Yoshioka T, Maruyama K, Tsukita S
Department of Information Physiology, National Institute for Physiological Sciences, Aichi, Japan.
J Cell Biol. 1993 Feb;120(3):711-24. doi: 10.1083/jcb.120.3.711.
To clarify the full picture of the connectin (titin) filament network in situ, we selectively removed actin and myosin filaments from cardiac muscle fibers by gelsolin and potassium acetate treatment, respectively, and observed the residual elastic filament network by deep-etch replica electron microscopy. In the A bands, elastic filaments of uniform diameter (6-7 nm) projecting from the M line ran parallel, and extended into the I bands. At the junction line in the I bands, which may correspond to the N2 line in skeletal muscle, individual elastic filaments branched into two or more thinner strands, which repeatedly joined and branched to reach the Z line. Considering that cardiac muscle lacks nebulin, it is very likely that these elastic filaments were composed predominantly of connectin molecules; indeed, anti-connectin monoclonal antibody specifically stained these elastic filaments. Further, striations of approximately 4 nm, characteristic of isolated connectin molecules, were also observed in the elastic filaments. Taking recent analyses of the structure of isolated connectin molecules into consideration, we concluded that individual connectin molecules stretched between the M and Z lines and that each elastic filament consisted of laterally-associated connectin molecules. Close comparison of these images with the replica images of intact and S1-decorated sarcomeres led us to conclude that, in intact sarcomeres, the elastic filaments were laterally associated with myosin and actin filaments in the A and I bands, respectively. Interestingly, it was shown that the elastic property of connectin filaments was not restricted by their lateral association with actin filaments in intact sarcomeres. Finally, we have proposed a new structural model of the cardiac muscle sarcomere that includes connectin filaments.
为了阐明原位连接蛋白（肌联蛋白）丝网络的全貌，我们分别用凝溶胶蛋白和醋酸钾处理从心肌纤维中选择性去除肌动蛋白丝和肌球蛋白丝，并通过深蚀刻复型电子显微镜观察残留的弹性丝网络。在A带中，从M线伸出的直径均匀（6 - 7纳米）的弹性丝平行排列，并延伸到I带。在I带中的交界线处，可能对应于骨骼肌中的N2线，单个弹性丝分支成两条或更多更细的链，这些链反复连接和分支以到达Z线。考虑到心肌缺乏伴肌动蛋白，很可能这些弹性丝主要由连接蛋白分子组成；事实上，抗连接蛋白单克隆抗体特异性地标记了这些弹性丝。此外，在弹性丝中还观察到了约4纳米的条纹，这是分离的连接蛋白分子的特征。结合最近对分离的连接蛋白分子结构的分析，我们得出结论，单个连接蛋白分子在M线和Z线之间伸展，并且每条弹性丝由横向关联的连接蛋白分子组成。将这些图像与完整和S1装饰的肌节的复型图像进行仔细比较，我们得出结论，在完整的肌节中，弹性丝分别在A带和I带与肌球蛋白丝和肌动蛋白丝横向关联。有趣的是，结果表明在完整的肌节中，连接蛋白丝的弹性特性不受其与肌动蛋白丝横向关联的限制。最后，我们提出了一个包含连接蛋白丝的心肌肌节新结构模型。
