ABSTRACT
The dependence of actin-activated ATPase activity and myosin filament structure have been studied on Ca(2+)-concentration in the range between pCa 7.5 and pCa 4.6. Rabbit skeletal muscle myosin with dephosphorylated regulatory light chains column-purified with DEAE-Sephadex A-50 for the removal of minor proteins and actin without regulatory proteins have been used. Considerable increase in actomyosin ATPase activity (by 70%) is revealed with increasing Ca(2+)-level from pCa 7.5 up to pCa 4.6. Electron microscopic observations on the structures of reconstituted myosin filaments have revealed Ca(2+)-dependent movement of myosin cross-bridges (head + subfragment-2) from and to the backbone of myosin filaments. The correlation between the manifestation of Ca(2+)-sensitivity of ATPase properties of myosins and Ca(2+)-dependent mobility of cross-bridges has been established. In particular, the increase in the mobility of cross-bridges and their moving away from the surface of myosin filaments at pCa 4.6 correlates with the increase in actin-activated ATPase of the same myosin preparations. It is supposed that the interrelation between the above properties observed in in vitro system can be of importance for force generation and its regulation in muscle.
