Functional variation of troponin, a muscle regulatory protein, during evolution of chordate animals

在脊椎动物横纹肌中，肌钙蛋白以Ca^<++>依赖性方式调节肌肉收缩，并在收缩期间作为肌动蛋白-肌球蛋白相互作用的抑制剂。然而，在海鞘（Halocynthia roretzi）平滑肌中，肌钙蛋白在钙离子存在下作为激活剂调节肌动蛋白-肌球蛋白收缩，而不是像脊椎动物那样在钙离子不存在下作为抑制剂。在这项研究中，肌钙蛋白在原索动物的横纹肌和平滑肌功能的变化进行了研究。我们使用的动物主要是文昌鱼，Branchiostoma belcheri，它在整个身体中有丰富的横纹肌，海鞘Ciona ciona在成体和幼虫中有平滑肌和横纹肌，但在成体和幼虫中使用共同的TnI，肌钙蛋白的抑制亚基。为了进行功能分析，我们使用了在E. coli表达系统。肌钙蛋白组分用于肌动蛋白-肌球蛋白ATP酶活性测定，以确定肌钙蛋白在肌肉收缩期间肌动蛋白-肌球蛋白相互作用中的作用。我们的结果总结如下。1)文昌鱼横纹肌肌钙蛋白TnT、TnI和TnC复合体的功能与脊椎动物横纹肌肌钙蛋白相似，而与海鞘、海鞘、文昌鱼平滑肌肌钙蛋白不同。roretzi。2)海鞘肌钙蛋白复合体在成体平滑肌和幼体横纹肌中使用一个共同的肌钙蛋白I，其功能类似于海鞘平滑肌中的肌钙蛋白。roretzi和不同于脊椎动物横纹肌肌钙蛋白。这些结果表明肌钙蛋白在原索动物中的作用存在进化变异。

In vertebrate striated muscles, troponin regulates muscle contraction in a Ca^<++>-dependent manner and acts as an inhibitor of actin-myosin interaction during contraction. In ascidian (Halocynthia roretzi) smooth muscle, however, troponin regulates actin-myosin contraction as an activator in the presence of calcium ions and not an inhibitor in the absence of calcium ions as in vertebrates. In this study, functional variation of troponin in striated and smooth muscles of protochordates was examined. The animals we used mainly are Amphioxus, Branchiostoma belcheri, that has striated muscle abundantly in the entire body and ascidian Ciona intestinalis that has smooth muscle in adult and striated muscle in larva but uses a common TnI, an inhibitory subunit of troponin, in both adult and larval ages. For the functional analysis, we used recombinant troponin subunits, TnT, TnI, and TnC that were generated in an E. coli expression system. The troponin components were used in an actin-myosin ATPase activity assay to determine the role of troponin in the actin-myosin interaction during muscle contraction. Our results are summarized as follows. 1) The troponin complex of TnT, TnI, and TnC in amphioxus striated muscle functions similar to the troponin in vertebrate striated muscle and different from the troponin in smooth muscle of ascidian, H. roretzi. 2) The troponin complex in ascidian (Ciona intestinalis) which uses a common TnI in both adult smooth and larval striated muscles functions similar to the troponin in the smooth muscle of ascidian, H. roretzi and different from vertebrate striated muscle troponin. These results show evolutionary variations in the role of troponin in protochordates.