MOLECULAR-CLONING OF A MYOSIN I-BETA ISOZYME THAT MAY MEDIATE ADAPTATION BY HAIR-CELLS OF THE BULLFROGS INTERNAL EAR

The internal ear's sensory receptor, or hair cell, responds when stimu li deflect its mechanoreceptive hair bundle. As a hair cell adapts to sustained stimulation, mechanical adjustments within the bundle reset its position of sensitivity. Because several lines of experimentation suggest that a form of myosin I mediates adaptation, we endeavored to clone cDNAs encoding this motor molecule. By using degenerate oligonuc leotide primers based upon the deduced amino acid sequence for mammali an myosin I beta, we performed reverse transcription and polymerase ch ain reactions (PCRs) to produce a candidate cDNA from polyadenylylated mRNA isolated from the frog's brain. The resultant product was used t o probe a cDNA library, from which were isolated clones encoding an ap proximate to 119-kDa isozyme of myosin I beta. PCR amplification discl osed the presence of mRNA encoding the same isozyme in tissue from the bullfrog's sacculus, an organ of the internal ear. When expressed as a bacterial fusion protein, a domain from the tail region of this form of myosin I was recognized by monoclonal antibodies that react with m yosin I in hair bundles. This cloned approximate to 119-kDa isozyme of myosin I is accordingly a candidate to be the motor molecule responsi ble for the adaptation of mechanoelectrical transduction by hair cells .