Structure and function of connectin, muscle elastic protein

连接素是一种很长的丝状蛋白质，存在于脊椎动物的横纹肌原纤维中，将肌球蛋白细丝与肌节中相邻的Z线连接起来。在本研究中，圆二色谱表明连接素由70%的β折叠结构和30%的无规卷曲组成。人造连接素纤维显示出与丝素纤维中所见的β片层结构相对应的X射线衍射。随着张力的产生，纤维可以可逆地拉伸到初始长度的30%。张力产生的温度依赖性表明，这种弹性不是简单的熵弹性。连接蛋白细丝的弹性结构还需要进一步的研究，观察到在β-肌动蛋白存在下，超声分散的肌动蛋白短纤维通过加入连接蛋白而形成网络。通过共印迹实验研究了连接素与肌动蛋白网络的结合。原肌球蛋白不干扰结合。然而，Sl，肌球蛋白头抑制它。因此，当将S1添加到连接蛋白-肌动蛋白网络中时，修饰了S1的肌动蛋白细丝逐渐从聚集体中释放出来。连接素明显不干扰肌动蛋白与肌球蛋白的相互作用，而肌球蛋白相互作用是滑动的驱动力。从目前的研究来看，连接素细丝松散地结合在I带区域的肌动蛋白细丝上，并在肌动蛋白细丝发生滑动时起到指导作用。连接素丝与肌球蛋白丝结合，但认为肌球蛋白和肌动蛋白的相互作用完全不受连接素的阻碍。

Connectin is a very long filamentous protein present in vertebrate striated myofibrils linking myosin filaments to neighboring Z lines in a sarcomere. This is the third filament in addition to myosin and actin filaments in muscle.In the present study, circular dichroism spectra suggested that connectin consists of 70 % beta sheet structure and 30 % random coil. Artifially made connectin fibers showed X ray diffraction corrresponding to beta sheet structure as seen in silk fiboin. The fibers could be reversibly stretched up to 30 % of the initial length with tention generation. Temperature dependence of tention generation suggested that the elasticity was not a simple entropy elasticity. Further work is required for the detailed elastic structure of the connectin filaments.It was observed that short actin filaments dispersed by sonication in the presence of beta-actinin formed a network by the addition of connectin. Binding of connectin to actin meshwork was studied by cosedimentation experiments. Tropomyosin did not interfere with the binding. However, Sl, myosin heads inhibited it. Thus when Sl was added to connectin-actin network, actin filaments decorated with Sl gradually released from the aggregate. Connectin evidently does not interfere with the actin myosin interaction, driving force for sliding.From the present study, it is proposed that connectin filaments loosely bound to actin filaments in the I band region and acts as a guideline when sliding of the actin filaments occurs. Connectin filaments bind to the myosin filaments but it is regarded that myosin and actin interactions are not hindered by connectin at all.