The phosphorylation state of the myosin thick filament-associated mini-titin, twitchin, regulates catch force maintenance in molluscan smooth muscle. The full-length cDNA for twitchin from the anterior byssus retractor muscle of the mussel Mytilus was obtained using PCR and 5'rapid amplification of cDNA ends, and its derived amino acid sequence showed a large molecule ( approximately 530 kDa) with a motif arrangement as follows: (Ig)11(IgFn2)2Ig(Fn)3Ig(Fn)2Ig(Fn)3(Ig)2(Fn)2(Ig)2 FnKinase(Ig)4. Other regions of note include a 79-residue sequence between Ig domains 6 and 7 (from the N terminus) in which more than 60% of the residues are Pro, Glu, Val, or Lys and between the 7th and 8th Ig domains, a DFRXXL motif similar to that thought to be necessary for high affinity binding of myosin light chain kinase to F-actin. Two major phosphorylation sites, i.e. D1 and D2, were located in linker regions between Ig domains 7 and 8 and Ig domains 21 and 22, respectively. Correlation of the phosphorylation state of twitchin, using antibodies specific to D1 and D2, with mechanical properties suggested that phosphorylation of both D1 and D2 is required for relaxation from the catch state.