THE CORRELATION BETWEEN CA2-DEPENDENT MO VEMENT OF CROSS-BRIDGES IN MYOSIN-FILAMENTS AND CA2+-SENSITIVITY OF ACTIN-ACTIVATED ATPASE OF SKELETAL-MUSCLE MYOSIN()

The dependence of actin-activated ATPase activity and myosin filament structure have been studied on Ca2+-concentration in the range between pCa 7,5 and pCa 4,6. Rabbit skeletal muscle myosin with dephosphoryla ted regulatory light chains column-purified with DEAE-Sephadex A-50 fo r the removal of minor proteins and actin without regulatory proteins have been used. Considerable increase in actomyosin ATPase activity (b y 70%) is revealed with increasing Ca2+-level from pCa 7,5 up to pCa 4 ,6. Electron microscopic observations on the structures of reconstitut ed myosin filaments have revealed Ca2+-dependent movement of myosin cr oss-bridges (head+subfragment-2) from and to the backbone of myosin fi laments. The correlation between the manifestation of Ca2+-sensitivity of ATPase properties of myosins and Ca2+-dependent mobility of cross- bridges has been established. In particular, the increase in the mobil ity of cross-bridges and their moving away from the surface of myosin filaments at pCa 4,6 correlates with the increase in actin-activated A TPase of the same myosin preparations. It is supposed that the interre lation between the above properties observed in in vitro system can be of importance for force generation and its regulation in muscle.