A protein factor inhibiting the magnesium-activated adenosine triphosphatase of desensitized actomyosin

Abstract

1. The preparation and properties of a myofibrillar protein factor which inhibits the Mg(2+)-activated adenosine triphosphatase of desensitized actomyosin is described. 2. This factor had negligible effect on the Mg(2+)-activated adenosine triphosphatase of natural actomyosin and on the Ca(2+)-activated adenosine triphosphatases of desensitized actomyosin and myosin. 3. The Mg(2+)-activated inosine triphosphatase activity of desensitized actomyosin was not affected by the factor. 4. The inhibitory effect was sensitive to ionic strength. In addition to their ionic effects Mg(2+) and Ca(2+) appeared to have a specific action in reducing the effect of the inhibitor. 5. F-actin reduced the inhibition whereas Bailey-type tropo-myosin had little effect. 6. As far as can be judged from the reported experiments this factor is different from any of the previously described myofibrillar components.

Proteins of Contractile Systems

“…The simultaneous strengthening of troponin-subunit interaction would prevent Tnl dissociation from the thin fila ment. Such a model is in good agreement with the finding of Ca 2 + -insensitive inhibition when isolated Tnl is added to F-actin-TM (148,149). If the ther modynamically favored TM position were the blocking one, further assumptions would have to be introduced.…”

“…The biological function of TnI is the inhibition of actin-myosin interaction in concert with tropomyosin and the remaining troponin subunit (147). Isolated TnI, independent of calcium concentration, inhibits actin-TM cofactor activity when added in a 1:7 molar ratio (148,149). In the absence of tropomyosin, a much higher TnI concentration is needed to achieve inhibition of actin-myosin interaction, which is complete at a molar ratio of 1: 1 (150).…”

The relaxing protein system of striated muscle. Resolution of the troponin complex into inhibitory and calcium ion-sensitizing factors and their relationship to tropomyosin

“…The troponin component of lower electrophoretic mobility appears to be identical with the inhibitory factor reported earlier (Perry et al 1966;Hartshorne et al 1967). Conditions of high ionic strength favoured the spontaneous appearance of the inhibitory factor in preparations of the troponin complex; likewise high ionic strength was earlier reported as essential for its extraction from the myofibril .…”

Some Effects of Pressure Treatment on Actomyosin Systems

“…8) could be due to either an effect on the myosin component or on the association between actin and myosin (for fuller discussion, see for example Perry 1965). The more pronounced effect on the calcium-sensitive Mg2+ ATPase could be due to an effect on the troponin component of the natural actomyosin (Hartshorne et al 1967;Schaub and Perry 1971).…”