Linke W A, Ivemeyer M, Olivieri N, Kolmerer B, Rüegg J C, Labeit S
Institute of Physiology II, University of Heidelberg, Germany.
J Mol Biol. 1996 Aug 9;261(1):62-71. doi: 10.1006/jmbi.1996.0441.
Vertebrate striated muscle behaves elastically when stretched and this property is thought to reside primarily within the giant filamentous protein, titin (connectin). The elastic portion of titin comprises two distinct structural motifs, immunoglobulin (Ig) domains and the PEVK titin, which is a novel motif family rich in proline, glutamate, valine and lysine residues. The respective contributions of the titin Ig and the PEVK sequences to the elastic properties of the molecule have been unknown so far. We have measured both the passive tension in single, isolated myofibrils from cardiac and skeletal muscle and the stretch-induced translational movement of I-band titin antibody epitopes following immunofluorescent labelling of sites adjacent to the PEVK and Ig domain regions. We found that with myofibril stretch, I-band titin does not extend homogeneously. The Ig domain region lengthened predominantly during small stretch, but such lengthening did not result in measurable passive tension and might be explained by straightening, rather than by unfolding, of the Ig repeats. At moderate to extreme stretch, the main extensible region was found to be the PEVK segment whose unravelling was correlated with a steady passive tension increase. In turn, PEVK domain transition from a linearly extended to a folded state appears to be principally responsible for the elasticity of muscle fibers. Thus, the length of the PEVK sequence may determine the tissue-specificity of muscle stiffness, whereas the expression of different Ig domain motif lengths may set the characteristic slack sarcomere length of a muscle type.
脊椎动物的横纹肌在被拉伸时表现出弹性，这种特性被认为主要存在于巨大的丝状蛋白——肌联蛋白（连接蛋白）中。肌联蛋白的弹性部分包含两个不同的结构基序，即免疫球蛋白（Ig）结构域和PEVK肌联蛋白，PEVK肌联蛋白是一个富含脯氨酸、谷氨酸、缬氨酸和赖氨酸残基的新型基序家族。迄今为止，肌联蛋白Ig和PEVK序列对分子弹性特性的各自贡献尚不清楚。我们测量了来自心肌和骨骼肌的单个分离肌原纤维中的被动张力，以及在对与PEVK和Ig结构域区域相邻的位点进行免疫荧光标记后，I带肌联蛋白抗体表位的拉伸诱导平移运动。我们发现，随着肌原纤维的拉伸，I带肌联蛋白并非均匀伸展。在小幅度拉伸时，Ig结构域区域主要伸长，但这种伸长并未导致可测量的被动张力，这可能是由Ig重复序列的伸直而非展开来解释的。在中度到极度拉伸时，主要的可伸展区域是PEVK片段，其解缠与被动张力的稳定增加相关。反过来，PEVK结构域从线性伸展状态转变为折叠状态似乎是肌肉纤维弹性的主要原因。因此，PEVK序列的长度可能决定肌肉硬度的组织特异性，而不同Ig结构域基序长度的表达可能设定肌肉类型的特征性松弛肌节长度。
