Funabara Daisuke, Kanoh Satoshi, Siegman Marion J, Butler Thomas M, Hartshorne David J, Watabe Shugo
Faculty of Bioresources, Mie University, Kurimamachiya 1577, 514-8507, Tsu, Mie, Japan.
J Muscle Res Cell Motil. 2005;26(6-8):455-60. doi: 10.1007/s10974-005-9029-2.
Molluscan catch muscle can maintain tension for a long time with little energy consumption. This unique phenomenon is regulated by phosphorylation and dephosphorylation of twitchin, a member of the titin/connectin family. The catch state is induced by a decrease of intracellular Ca2+ after the active contraction and is terminated by the phosphorylation of twitchin by the cAMP-dependent protein kinase (PKA). Twitchin, from the well-known catch muscle, the anterior byssus retractor muscle (ABRM) of the mollusc Mytilus, incorporates three phosphates into two major sites D1 and D2, and some minor sites. Dephosphorylation is required for re-entering the catch state. Myosin, actin and twitchin are essential players in the mechanism responsible for catch during which force is maintained while myosin cross-bridge cycling is very slow. Dephosphorylation of twitchin allows it to bind to F-actin, whereas phosphorylation decreases the affinity of the two proteins. Twitchin has been also been shown to be a thick filament-binding protein. These findings raise the possibility that twitchin regulates the myosin cross-bridge cycle and force output by interacting with both actin and myosin resulting in a structure that connects thick and thin filaments in a phosphorylation-dependent manner.
软体动物的捕捉肌能够长时间维持张力，且能量消耗极少。这种独特现象受肌联蛋白/伴肌动蛋白家族成员肌动蛋白结合蛋白的磷酸化和去磷酸化调节。捕捉状态在主动收缩后由细胞内Ca2+浓度降低诱导产生，并通过环磷酸腺苷依赖性蛋白激酶（PKA）对肌动蛋白结合蛋白的磷酸化作用而终止。来自著名的捕捉肌——贻贝的前足丝牵缩肌（ABRM）的肌动蛋白结合蛋白，会在两个主要位点D1和D2以及一些次要位点掺入三个磷酸基团。重新进入捕捉状态需要去磷酸化。肌球蛋白、肌动蛋白和肌动蛋白结合蛋白是捕捉机制中的关键参与者，在此过程中，当肌球蛋白横桥循环非常缓慢时，张力得以维持。肌动蛋白结合蛋白的去磷酸化使其能够与F-肌动蛋白结合，而磷酸化则降低了这两种蛋白质之间的亲和力。肌动蛋白结合蛋白也已被证明是一种粗肌丝结合蛋白。这些发现增加了一种可能性，即肌动蛋白结合蛋白通过与肌动蛋白和肌球蛋白相互作用来调节肌球蛋白横桥循环和力输出，从而形成一种以磷酸化依赖方式连接粗肌丝和细肌丝的结构。
