Trombitás K, Pollack G H
University of Washington, Seattle 98195.
J Muscle Res Cell Motil. 1993 Aug;14(4):416-22. doi: 10.1007/BF00121293.
The characteristics of the titin filament in the vicinity of the Z-line were investigated using immunoelectron microscopy. We used monoclonal titin antibodies T-11 and T-12 on single fibres of frog skeletal muscle, and on Z-line-extracted fibres. It is well established that the I-band region of titin is elastic. We find, however, that the elastic properties are not uniform. The T-12 epitope, which binds near the Z-line at the N1-line level, hardly changes position relative to the Z-line as the sarcomere is stretched. This demonstrates the functional inextensibility of the N1-Z-line region. After extreme stretch (above 6-microns sarcomere length), this zone finally does elongate; thus, the titin molecule in this region is intrinsically elastic. The functional inextensibility seen at shorter sarcomere lengths may, therefore, be a result of binding of titin to the actin filament in the zone near the Z-line. When the Z-line was extracted, the T-12 epitope remained in the same position as in the unextracted fibres; it did not retract from the Z-line. Failure to retract implies that functional anchoring of titin is not exclusive to the Z-line, but includes some site closer to the A-band. Combined with the results of the above-mentioned stretch experiment, this result implies a likely binding of titin to the thin filament either focally at the N1 line or all along the entire N1-Z region. Thus, this region of titin is functionally stiff, but intrinsically elastic.
利用免疫电子显微镜研究了Z线附近肌联蛋白丝的特性。我们将单克隆肌联蛋白抗体T-11和T-12应用于青蛙骨骼肌的单根肌纤维以及去除Z线的肌纤维上。众所周知，肌联蛋白的I带区域具有弹性。然而，我们发现其弹性特性并不均匀。在N1线水平靠近Z线处结合的T-12表位，随着肌节的拉伸，相对于Z线几乎不改变位置。这证明了N1-Z线区域在功能上不可伸展。在极度拉伸（肌节长度超过6微米）后，该区域最终确实会伸长；因此，该区域的肌联蛋白分子具有内在弹性。因此，在较短肌节长度下观察到的功能上的不可伸展性，可能是肌联蛋白与Z线附近区域的肌动蛋白丝结合的结果。当去除Z线时，T-12表位与未去除Z线的肌纤维处于相同位置；它没有从Z线缩回。未缩回意味着肌联蛋白的功能锚定并非仅局限于Z线，而是包括一些更靠近A带的位点。结合上述拉伸实验的结果，这一结果表明肌联蛋白可能在N1线处局部或在整个N1-Z区域与细肌丝结合。因此，肌联蛋白的这一区域在功能上是刚性的，但具有内在弹性。
