ATPASE ACTIVITY OF MYOSIN IN HAIR BUNDLES OF THE BULLFROGS SACCULUS

Mechanoelectrical transduction by a hair cell displays adaptation, whi ch is thought to occur as myosin-based molecular motors within the mec hanically sensitive hair bundle adjust the tension transmitted to tran sduction channels. To assess the enzymatic capabilities of the myosin isozymes in hair bundles, we examined the actin-dependent ATPase activ ity 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 myo sin isozymes from that of other hair-bundle enzymes, we inhibited the interaction of hair-bundle myosin with actin and determined the reduct ion in ATPase activity. N-ethylmaleimide (NEM) decreased neither physi ologically 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 l argely inhibited by calmidazolium, but was unaffected by the addition of exogenous calmodulin. These results, which indicate that hair bundl es 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.