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 stimuli 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 oligonucleotide primers based upon the deduced amino acid sequence for mammalian myosin I beta, we performed reverse transcription and polymerase chain reactions (PCRs) to produce a candidate cDNA from polyadenylylated mRNA isolated from the frog's brain. The resultant product was used to probe a cDNA library, from which were isolated clones encoding an approximate to 119-kDa isozyme of myosin I beta. PCR amplification disclosed 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 myosin I in hair bundles. This cloned approximate to 119-kDa isozyme of myosin I is accordingly a candidate to be the motor molecule responsible for the adaptation of mechanoelectrical transduction by hair cells.