Twitchin as a regulator of catch contraction in molluscan smooth muscle

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.

软体动物的闭锁肌能够以低能耗长时间维持张力。这种独特现象是由肌联蛋白/伴肌动蛋白家族成员——粗丝调节蛋白的磷酸化和去磷酸化所调控的。闭锁状态是在主动收缩后细胞内钙离子浓度降低所诱导的，并由环腺苷酸依赖性蛋白激酶（PKA）对粗丝调节蛋白进行磷酸化而终止。来自软体动物贻贝中著名的闭锁肌——前足丝收缩肌（ABRM）的粗丝调节蛋白在两个主要位点D1和D2以及一些次要位点结合三个磷酸基团。重新进入闭锁状态需要去磷酸化。肌球蛋白、肌动蛋白和粗丝调节蛋白是负责闭锁机制中的关键参与者，在闭锁期间，当肌球蛋白横桥循环非常缓慢时，力得以维持。粗丝调节蛋白的去磷酸化使其能够与F - 肌动蛋白结合，而磷酸化则降低这两种蛋白质的亲和力。粗丝调节蛋白也已被证明是一种粗丝结合蛋白。这些发现增加了一种可能性，即粗丝调节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的输出，从而形成一种以磷酸化依赖的方式连接粗细丝的结构。