contributed equally to this work Deaf-blindness in three distinct genetic forms of Usher type I syndrome (USH1) is caused by defects in myosin VIIa, harmonin and cadherin 23. Despite being critical for hearing, the functions of these proteins in the inner ear remain elusive. Here we show that harmonin, a PDZ domain-containing protein, and cadherin 23 are both present in the growing stereocilia and that they bind to each other. Moreover, we demonstrate that harmonin b is an F-actin-bundling protein, which is thus likely to anchor cadherin 23 to the stereocilia micro®laments, thereby identifying a novel anchorage mode of the cadherins to the actin cytoskeleton. Moreover, harmonin b interacts directly with myosin VIIa, and is absent from the disorganized hair bundles of myosin VIIa mutant mice, suggesting that myosin VIIa conveys harmonin b along the actin core of the developing stereocilia. We propose that the shaping of the hair bundle relies on a functional unit composed of myosin VIIa, harmonin b and cadherin 23 that is essential to ensure the cohesion of the stereocilia.

contributed equally to this work Defects in myosin VIIA are responsible for deafness in the human and mouse. The role of this unconventional myosin in the sensory hair cells of the inner ear is not yet understood. Here we show that the C-terminal FERM domain of myosin VIIA binds to a novel transmembrane protein, vezatin, which we identi®ed by a yeast two-hybrid screen. Vezatin is a ubiquitous protein of adherens cell±cell junctions, where it interacts with both myosin VIIA and the cadherin±catenins complex. Its recruitment to adherens junctions implicates the C-terminal region of a-catenin. Taken together, these data suggest that myosin VIIA, anchored by vezatin to the cadherin±catenins complex, creates a tension force between adherens junctions and the actin cytoskeleton that is expected to strengthen cell±cell adhesion. In the inner ear sensory hair cells vezatin is, in addition, concentrated at another membrane±membrane interaction site, namely at the ®brillar links interconnecting the bases of adjacent stereocilia. In myosin VIIA-defective mutants, inactivity of the vezatin±myosin VIIA complex at both sites could account for splaying out of the hair cell stereocilia.