Myosin Mg-ATPase of molluscan muscles is slightly activated by F-actin under catch state in vitro

Molluscan muscle twitchin, a titin/connectin-related giant protein, regulates interactions between actin and myosin filaments at low Ca²⁺ concentrations. When it is dephosphorylated, actin filaments tightly bind to myosin filaments, resulting in the catch state known as the state of high passive tension with very low energy consumption. Yet when twitchin is phosphorylated actin filaments detach from the myosin filaments, resulting in relaxation of the catch. Here, steady-state Mg-ATPase activities of purified myosin were measured under various conditions: without twitchin, with dephosphorylated twitchin, or with phosphorylated twitchin; with or without phalloidin-stabilized F-actin; and at various Ca²⁺ concentrations. At low Ca²⁺ concentration, Mg-ATPase was activated by F-actin only in the presence of dephosphorylated twitchin (catch state). The activation was about two orders lower than that fully activated by Ca²⁺ and F-actin. In the absence of F-actin, twitchin and its phosphorylation state did not affect Mg-ATPase activities in any of the conditions we tested. Based on these results, we propose a molecular mechanism for the catch, where twitchin alone does not interact with the myosin catalytic motor domain but its complex with F-actin does, forming the bridge between actin and myosin filaments and the myosin slowly hydrolyzes Mg-ATP in the catch state.