Exploring the Structural Elements Responsible for Cis-Homodimerization of Inner Ear Cadherin-23

Abstract

Cadherin-23 (CDH23) is an enormous protein essential for hearing, balance, and proper eyesight. There are over 100 mutations in CDH23 that affect these processes with varying severity, some leading to deafness, balance disorders, and progressive blindness (Usher Syndrome). In the inner ear, CDH23 makes up the upper half of a proteinaceous filament known as the tip link, which is essential for hearing. Upon stimulation by sound or head movements, the tip link is stretched and conveys force to open the ion channels in the inner ear, thereby leading to the conversion of vibrational stimulus into electrical signals interpreted by the brain as sound. CDH23 is a non-classical cadherin with 27 extracellular cadherin (EC) repeats and a membrane adjacent domain (MAD28). The EC repeats are connected by a linker region containing highly conserved residues that bind calcium ions essential for tip-link function. Electron microscopy images suggest that CDH23 exists as a cis-homodimer within the tip link, however, the structural elements mediating this dimerization are not well determined. To better understand inner-ear mechanotransduction at the molecular level, we have solved high- resolution X-ray crystal structures of 18 CDH23 EC repeats along with 13 of the 26 EC linker regions (Jaiganesh et al., 2018). Here, we present several biochemical experiments that suggest potential sites of parallel dimerization on the extracellular domain of CDH23. Additionally, we present structures of various fragments of CDH23 allowing for closer analysis of deafness causing mutations. These results provide information on the cis-homodimerization of CDH23 and provide deeper insights about how mutations can result in inherited deafness.