Towards an atomistic model of the vertebrate inner-ear transduction apparatus.

Abstract

At the foundation of vertebrate hearing and balance is the process of mechanotransduction, in which forces from sound and head movements are transduced into electrochemical signals in the inner ear. Mechanotransduction takes place in inner-ear hair cells and involves tip-link filaments that pull on ion channels to trigger sensory perception. Each tip link is made of cadherin-23 (CDH23) and protocadherin-15 (PCDH15) proteins, while the pore forming subunits of the transduction ion channel are formed by transmembrane channel-like proteins (TMCs) 1 and 2, likely coupled to accessory units formed by calcium- and integrin-binding proteins (CIBs) 2 and 3, the transmembrane inner-ear expressed protein TMIE, and the tetraspan membrane protein of hair-cell stereocilia TMHS (also known as LHFPL5). Here we present structural, biochemical, and computational studies aimed at elucidating the molecular mechanisms underlying function of the hair-cell transduction apparatus components. Data from X-ray crystallography, small-angle X-ray scattering, analytical ultracentrifugation experiments and low-resolution cryo-EM along with AlphaFold2 predictions allowed us to build atomistic models of the entire tip link. Similarly, we used AlphaFold2 models and nuclear magnetic resonance data to build TMC protein models in complex with CIBs suggesting a ‘clamp-like’ architecture involving TMC cytosolic domains. Molecular dynamics simulations of these models and of TMHS coupled to PCDH15 provide additional insights into cation conduction and possible mechanisms underlying the role of membrane tension in TMC gating. These data and models, obtained from a combination of experimental and computational approaches, are providing a rigorous molecular view of mechanotransduction in normal and impaired hearing and balance.