Twitchin, a thick-filament protein from molluscan catch muscle, interacts with F-actin in a phosphorylation-dependent way.

Twitchin belongs to the titin-like giant proteins family, it is co-localized with thick filaments in molluscan catch muscles and regulates the catch state depending on its level of phosphorylation. The mechanism by which twitchin controls the catch state remains to be established. We report for the first time the ability of twitchin to interact with F-actin. The interaction is observed at low and physiological ionic strengths, irrespective of the presence or absence of Ca(2+). It was demonstrated by viscosity and turbidity measurements, low- and high-speed co-sedimentation, and with the light-scattering particle size analysis revealing the specific twitchin-actin particles. The twitchin-actin interaction is regulated by twitchin phosphorylation: in vitro phosphorylated twitchin does not interact with F-actin. We speculate that the catch muscle twitchin might provide a mechanical link between thin and thick filaments, which contributes to catch force maintenance.