Catch in certain molluscan muscles is released by an increase in cAMP, and it was suggested that the target of cAMP-dependent protein kinase (PKA) is the high molecular weight protein twitchin [Siegman, M. J., Funabara, J., Kinoshita, S., Watabe, S., Hartshorne, D. J., and Butler, T. M. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 5384-5388]. This study was carried out to investigate the phosphorylation of twitchin by PKA. Twitchin was isolated from Mytilus catch muscles and was phosphorylated by PKA to a stoichiometry of about 3 mol of P/mol of twitchin. There was no evidence of twitchin autophosphorylation. Two phosphorylated peptides were isolated and sequenced, termed D1 and D2. Additional cDNA sequence for twitchin was obtained, and the D2 site was located at the C-terminal side of the putative kinase domain in a linker region between two immunoglobulin C2 repeats. Excess PKA substrates, e.g., D1 and D2, blocked the reduction in force on addition of cAMP, confirming the role for PKA in regulating catch. Papain proteolysis of (32)P-labeled twitchin from permeabilized muscles showed that the D1 site represented about 50% of the (32)P labeling. Proteolysis of in-situ twitchin with thermolysin suggested that the D1 and D2 sites were at the N- and C-terminal ends of the molecule, respectively. Thermolysin proteolysis also indicated that D1 and D2 were major sites of phosphorylation by PKA. The direct phosphorylation of twitchin by PKA is consistent with a regulatory role for twitchin in the catch mechanism and probably involves phosphorylation at the D1 and D2 sites.