Myosin isozymes are essential for hair cells, the sensory cells of the inner 
ear. Because a myosin-I subfamily member may mediate adaptation of 
mechanoelectrical transduction, we examined expression of all eight myosin-I 
isozymes in rodent auditory and vestibular epithelia. Using RT-PCR, we found 
prominent expression of three isozymes, Myo1b (also known as myosin-Ia or myr 
1). Myo1c (myosin-Ib or myr 2). and Myo1e (myr 3). By contrast, Myo1a 
(brush-border myosin-I), Myo1d (myosin lg or myr 4). Myo1f, Myo1g, and Myo1h 
were less readily amplified. Because sequence analysis demonstrated that the 
RT-PCR products encoded the appropriate isozymes, this represents the first 
demonstration of expression of all eight mouse myosin-I genes. Using 
immunocytochemistry with isozyme-selective antibodies, we found that Myo1b was 
located at apical surfaces of supporting cells that surround hair cells in 
auditory epithelia of postnatal rats. In vestibular epithelia, Myo1b was present 
in a ring within the apical pole of the hair cell. In both cases, expression was 
prominent only immediately after birth. Myo1e was found in hair cells of the 
auditory and vestibular epithelia; this isozyme was enriched in the cuticular 
plate, the actin meshwork that anchors the stereocilia. Myo1c was found in 
hair-cell stereocilia, concentrated towards their tips; we confirmed this 
localization by using adenovirus vectors to direct expression of a GFP-Myo1c 
tail fusion protein; this fusion protein localized to plasma membranes, often 
concentrating at stereociliary tips. Myo1c therefore remains the myosin isozyme 
best localized to carry out transducer adaptation.