Myosin XVa localizes to the tips of inner ear sensory cell stereocilia and is essential for staircase formation of the hair bundle.

Mutations of the gene encoding unconventional myosin XVa are associated with sensorineural deafness in humans (DFNB3) and shaker (Myo15sh2) mice. In deaf Myo15sh2/sh2 mice, stereocilia are short, nearly equal in length, and lack myosin XVa immunoreactivity. We previously reported that myosin XVa mRNA and protein are expressed in cochlear hair cells. We now show that in the mouse, rat, and guinea pig, endogenous myosin XVa localizes to the tips of the stereocilia of the cochlear and vestibular hair cells. Myosin XVa localization overlaps with the barbed ends of actin filaments and extends to the apical plasma membrane of the stereocilia. Gene gun-mediated transfection of mouse inner ear sensory epithelia explants shows selective accumulation of myosin XVa-GFP at the tips of stereocilia, confirming the localization of native myosin XVa. Expression in COS7 cells also reveals targeting of myosin XVa-GFP to the dynamic actin region at the tips of filopodia. In a wild-type mouse, during auditory and vestibular hair cell development, myosin XVa appears at the tips of stereocilia at the time when the hair bundle begins to develop its characteristic staircase pattern. We propose that myosin XVa is essential for the graded elongation of stereocilia during their functional maturation.