Labeit S, Kolmerer B
European Molecular Biology Laboratory, Heidelberg, Germany.
Science. 1995 Oct 13;270(5234):293-6. doi: 10.1126/science.270.5234.293.
In addition to thick and thin filaments, vertebrate striated muscle contains a third filament system formed by the giant protein titin. Single titin molecules extend from Z discs to M lines and are longer than 1 micrometer. The titin filament contributes to muscle assembly and resting tension, but more details are not known because of the large size of the protein. The complete complementary DNA sequence of human cardiac titin was determined. The 82-kilobase complementary DNA predicts a 3-megadalton protein composed of 244 copies of immunoglobulin and fibronectin type III (FN3) domains. The architecture of sequences in the A band region of titin suggests why thick filament structure is conserved among vertebrates. In the I band region, comparison of titin sequences from muscles of different passive tension identifies two elements that correlate with tissue stiffness. This suggests that titin may act as two springs in series. The differential expression of the springs provides a molecular explanation for the diversity of sarcomere length and resting tension in vertebrate striated muscles.
除了粗肌丝和细肌丝外，脊椎动物的横纹肌还包含由巨大的肌联蛋白形成的第三种丝系统。单个肌联蛋白分子从Z盘延伸到M线，长度超过1微米。肌联蛋白丝有助于肌肉组装和静息张力，但由于该蛋白质体积巨大，更多细节尚不清楚。已确定人类心脏肌联蛋白的完整互补DNA序列。这个82千碱基的互补DNA预测了一种3兆道尔顿的蛋白质，它由244个免疫球蛋白和III型纤连蛋白（FN3）结构域组成。肌联蛋白A带区域的序列结构揭示了为何粗肌丝结构在脊椎动物中得以保守。在I带区域，对不同静息张力肌肉的肌联蛋白序列进行比较，确定了两个与组织硬度相关的元件。这表明肌联蛋白可能像两个串联的弹簧一样起作用。弹簧的差异表达为脊椎动物横纹肌肌节长度和静息张力的多样性提供了分子解释。
