The Ca2+sensitivity of the actin-activated ATPase of scallop heavy meromyosin

AbstractThe actin-activated scallop heavy meromyosin (HMM) ATPase was monitored turbidometrically during a limited number of turnovers. At 4 μM actin, the turnover rate in the presence of Ca2+ (1.2 s−1 per head) was 650-fold higher than in its absence (1.8 × 10−3 s−1), a Ca2+ sensitivity which approaches that expected in vivo. The extent of Ca2+ activation was much larger than the observed by steady-state measurements, where the rate, in the absence of Ca2+, is dominated by a small proportion of unregulated molecules. Acto-HMM formation, and its dissociation by ATP, were Ca2+ insensitive