MYOSIN I-BETA IS LOCATED AT TIP LINK ANCHORS IN VESTIBULAR HAIR BUNDLES

Recent studies have suggested that myosin I beta mediates the adaptati on of mechanoelectrical transduction in vestibular hair cells. An impo rtant prediction of this hypothesis is that myosin I beta should be fo und in the side insertional plaque, an osmiophilic hair bundle structu re that anchors tip links and is thought to house the adaptation motor . To determine whether myosin I beta was situated properly to perform adaptation, we used immunofluorescence and immunoelectron microscopy w ith the monoclonal antibody mT2 to examine the distribution of myosin I beta in hair bundles of the bullfrog utricle. Although utricular hai r cells differ in their rates and extent of adaptation [Baird RA (1994 ) Comparative transduction mechanisms of hair cells in the bullfrog ut riculus. II. Sensitivity and response dynamics to hair bundle displace ment. J Neurophysiol 71:685-705.], myosin I beta was present in all ha ir bundles, regardless of adaptation kinetics. Confirming that, nevert heless, it was positioned properly to mediate adaptation, myosin I bet a was found at significantly higher levels in the side insertional pla que. Myosin I beta was also present at elevated levels at the second t ip link anchor of a hair bundle, the tip insertional plaque, found at the tip of a stereocilium. These data support myosin I beta as the ada ptation motor and are consistent with the suggestion that the motor se rves to restore tension applied to transduction channels to an optimal level, albeit with different kinetics in different cell types.