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 coprecipitati on assay of the aggregates formed in the mixture. Paratropomyosin also boun d 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 paratro pomyosin with the 400-kDa fragment was enhanced by a calcium ion concentrat ion from 10(-7) M to 10(-5) M and markedly suppressed above 10(-4) M calciu m ions. We conclude that paratropomyosin probably binds to the 400-kDa frag ment of beta-connectin in the A-I junction region in living and pre-rigor s keletal muscle. In postmortem skeletal muscle paratropomyosin may be releas ed from the 400-kDa portion of the connectin filament by increased calcium ion concentration and translocated on to thin filaments to induce meat tend erization.