OPPOSITE EFFECTS OF MYOSIN SUBFRAGMENT-1 ON BINDING OF CARDIAC TROPONIN AND TROPOMYOSIN TO THE THIN FILAMENT

To better understand the regulation of striated muscle contraction, th e effects of myosin subfragment 1 (S-1) on the actin binding of cardia c troponin and tropomyosin were investigated, Troponin's affinity for actin-tropomyosin was 4-fold stronger in the absence than in the prese nce of myosin S-1. CaCl2 had no effect on troponin binding to the thin filament in the presence of myosin S-1, The binding curve was weakly cooperative, implying interactions between adjacent troponin molecules , Myosin S-1 increased (40-200-fold) the affinity of tropomyosin for t he thin filament, an effect opposite to the effect of myosin on tropon in, This effect was highly cooperative and occurred in the presence of ADP or in the absence of nucleotide; Myosin altered the effect of ion ic conditions on tropomyosin-actin binding, consistent with tropomyosi n binding to a different site on F-actin in the presence of myosin. Th e results indicate that troponin-tropomyosin and strongly binding myos in crossbridges do not compete for an F-actin binding site, Although r epositioning of troponin-tropomyosin on the actin filament may be ster ically required for tight myosin-actin binding, a myosin-induced confo rmational change in actin provides a better explanation for the comple x effects of myosin on thin filament assembly.