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Twitchin, also called mini-titin, is structurally related to the giant elastic protein connectin/titin, and has been found in not only striated but also smooth muscles of bivalves. Many bivalve smooth muscles such as byssus retractor muscles and the opaque part of adductor muscles are known as catch muscles that can maintain high passive tension with little expenditure of energy after they have actively contracted. Twitchin is phosphorylated when this high-tension state (catch state) ceases. Our recent studies revealed that the catch tension is due to interactions between thick and thin filaments in the presence of MgATP at low free Ca2+ concentrations, which can be visualized in vitro under a light microscope (Yamada et al., 2001 Proc Natl Acad Sci USA 98: 6635-6640). We also found that twitchin is essential for the interactions of the catch state in mussel (Mytilus galloprovincialis) catch muscles. In the presence of twitchin, actin filaments bound to purified myosin filaments when twitchin was dephosphorylated by Ser/Thr protein phosphatase 2B, while they did not when it was phosphorylated by cAMP-dependent protein kinase. In the current study we demonstrate the same essential components of the catch state for another bivalve that exhibits catch, i.e., Japanese oyster (Crassostrea gigas).

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