Paratropomyosin, a new myofibrillar protein, weakens rigor linkages formed between actin and myosin.

We have used an enzymatic technique to determine the weakening effect of paratropomyosin, a new myofibrillar protein, on rigor linkages formed between actin and myosin, and to clarify the distinct function of paratropomyosin, as to that of tropomyosin. Paratropomyosin inhibited the Mg-ATPase activity and enhanced the K-ATPase activity of reconstituted actomyosin stoichiometrically, and its maximal binding to actin was estimated to occur at a molar ratio of 1: 12.5. Paratropomyosin also inhibited the myofibrillar Mg-ATPase activity by 49% and enhanced the myofibrillar K-ATPase activity to 126%, while tropomyosin had no effect on these ATPases. These results indicate that paratropomyosin is able to bind to thin filaments of myofibrils, because the binding site for paratropomyosin on F-actin is different from that for tropomyosin, and that, due to its greater affinity for the myosin binding site on actin, paratropomyosin competes for the binding site and helps weaken rigor linkages.