Paratropomyosin: a new myofibrillar protein that modifies the actin-myosin interaction in postrigor skeletal muscle. II. Distinct function from tropomyosin.

The functional differences between paratropomyosin and tropomyosin were studied. The weight ratio of maximally bound paratropomyosin to F-actin was 1 : 7.6, whereas that of tropomyosin was 1 : 3.9. The actin-myosin interaction was markedly depressed by paratropomyosin under conditions where it was promoted by tropomyosin. Paratropomyosin had sensitized the actin-myosin-troponin system to Ca2+, but was much less effective than tropomyosin. Paratropomyosin showed an additive effect on the actomyosin systems containing tropomyosin, indicating that the binding site of paratropomyosin on F-actin is distinct from that of tropomyosin. Probably, due to greater affinity for myosin binding site on F-actin, paratropomyosin competes for the binding site and thus modifies the actin-myosin interaction. The role of paratropomyosin in tenderization of meat during postmortem ageing is also discussed.