Interaction of paratropomyosin with beta-connectin and its 400-kilodalton fragment from chicken skeletal muscle as influenced by the calcium ion concentration.

The binding of paratropomyosin to beta-connectin, which has been suggested to interact at the A-I junction of a sarcomere, was confirmed by measuring the changes in turbidity of a mixture with changing NaCl concentration, pH and free calcium ions, and by morphological observation and a coprecipitation assay of the aggregates formed in the mixture. Paratropomyosin also bound to the 400-kDa fragment which is the N-terminal portion of beta-connectin and contains the A-I junction region. Moreover, the interaction of paratropomyosin with the 400-kDa fragment was enhanced by a calcium ion concentration from 10(-7) M to 10(-5) M and markedly suppressed above 10(-4) M calcium ions. We conclude that paratropomyosin probably binds to the 400-kDa fragment of beta-connectin in the A-I junction region in living and pre-rigor skeletal muscle. In postmortem skeletal muscle paratropomyosin may be released from the 400-kDa portion of the connectin filament by increased calcium ion concentration and translocated on to thin filaments to induce meat tenderization.