Paramyosin and the catch mechanism.

We propose here the formation by molluscan and notochord muscles in the catch state of three-dimensional, entangled network structures composed of bent and sometimes entwined paramyosin thick filaments including myosin intermediate filaments and disordered actin thin filaments; in the relaxed state the three forms lie in parallel. The intact forms of bivalve (Andonta pacifica, Heude) muscle paramyosin are those of 120 and 95 kDa (beta-paramyosin). The 102 kDa form (alpha-paramyosin) is the proteinase cleavage product of 120 kDa paramyosin. Paramyosin could be phosphorylated in vitro by cyclic AMP-dependent protein kinase. The amino acid phosphorylated was at the serine residue. Paramyosin from muscles treated with acetylcholine (catch state) was phosphorylated to a greater extent than that of untreated muscles (normal state) and even more so in the case of serotonin-treated muscles (relaxed state). Actin markedly inhibited the phosphorylation of paramyosin in vitro.