1. The Scripps Research, Institute Department of Cell Biology,
2. Institute for Childhood and Neglected Disease, La Jolla, California 92037, USA
3. Laboratory of Cellular Biology, National Institute on Deafness and other Communication Disorders, National Institutes of Health, Bethesda, Maryland 20892, USA
4. These authors contributed equally to this work.

Correspondence to: Ulrich Müller^1,2,4Bechara Kachar^3,4 Correspondence and requests for materials should be addressed to U.M. (Email: umueller@scripps.edu) or B.K. (Email: kacharb@nidcd.nih.gov).

Abstract

Hair cells of the inner ear are mechanosensors that transduce mechanical forces arising from sound waves and head movement into electrochemical signals to provide our sense of hearing and balance. Each hair cell contains at the apical surface a bundle of stereocilia. Mechanoelectrical transduction takes place close to the tips of stereocilia in proximity to extracellular tip-link filaments that connect the stereocilia and are thought to gate the mechanoelectrical transduction channel^{1, 2, 3}. Recent reports on the composition^{4, 5, 6, 7, 8}, properties and function^{9, 10, 11} of tip links are conflicting²⁹. Here we demonstrate that two cadherins that are linked to inherited forms of deafness in humans^{12, 13, 14, 15} interact to form tip links. Immunohistochemical studies using rodent hair cells show that cadherin 23 (CDH23) and protocadherin 15 (PCDH15) localize to the upper and lower part of tip links, respectively. The amino termini of the two cadherins co-localize on tip-link filaments. Biochemical experiments show that CDH23 homodimers interact in trans with PCDH15 homodimers to form a filament with structural similarity to tip links. Ions that affect tip-link integrity and a mutation in PCDH15 that causes a recessive form of deafness¹⁶ disrupt interactions between CDH23 and PCDH15. Our studies define the molecular composition of tip links and provide a conceptual base for exploring the mechanisms of sensory impairment associated with mutations in CDH23 and PCDH15.

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