ATPase activity of myosin in hair bundles of the bullfrog's sacculus

Mechanoelectrical transduction by a hair cell displays adaptation, which is thought to occur as myosin-based molecular motors within the mechanically sensitive hair bundle adjust the tension transmitted to transduction channels. To assess the enzymatic capabilities of the myosin isozymes in hair bundles, we examined the actin-dependent ATPase activity of bundles isolated from the bullfrog's sacculus. Separation of P-32-labeled inorganic phosphate from unreacted [gamma(32)P]ATP by thin-layer chromatography enabled us to measure the liberation of as little as 0.1 fmol phosphate. To distinguish the Mg2+-ATPase activity of myosin isozymes from that of other hair-bundle enzymes, we inhibited the interaction of hair-bundle myosin with actin and determined the reduction in ATPase activity. N-ethylmaleimide (NEM) decreased neither physiologically measured adaptation nor the nucleotide-hydrolytic activity of a 120-kDa protein thought to be myosin I beta. The NEM-insensitive, actin-activated ATPase activity of myosin increased from 1.0 fmolxs(-1) in 1 mM EGTA to 2.3 fmolxs(-1) in 10 mu M Ca2+. This activity was largely inhibited by calmidazolium, but was unaffected by the addition of exogenous calmodulin. These results, which indicate that hair bundles contain enzymatically active, Ca2+-sensitive myosin molecules, are consistent with the role of Ca2+ in adaptation and with the hypothesis that myosin forms the hair cell's adaptation motor.