CIB2 as a Ca ²⁺ Sensor for Auditory Mechano-Electrical Transduction and Linked to Genetic-Heterogeneity of TMC1 in Hearing Loss

Non-syndromic sensorineural hearing loss is characterized by genetic heterogeneity, leading to potential clinical misdiagnosis. TMC1 , a unique causative gene associated with deafness, exhibits variants with autosomal dominant and recessive inheritance patterns. TMC1 codes for the transmembrane channel-like 1 (TMC1), a key component of the mechano-electrical transduction (MET) machinery for hearing. However, the molecular mechanism of Ca ²⁺ regulation in MET, which is essential for sound perception, remains unclear. CIB2, another MET component associated with deafness, can bind with calcium and may play a role in MET gating. Our study reveals that the TMC1-CIB2 complex undergoes a Ca ²⁺ -induced conformational change, highlighting the crucial role of Ca ²⁺ in MET. We identified a vertebrate-specific binding site on TMC1, named CR3, that interacts with apo CIB2, with the binding interface linked to hearing loss. Using an in-vivo animal model, we demonstrated that disruption of the calcium-sensing site of CIB2 perturbs the MET channel conductivity, confirming its role as a Ca ²⁺ sensor for MET. Additionally, after systematically analyzing the hearing-loss variants, we observed that dominant mutations of TMC1 are often located near the ion pore or at the binding interfaces with the Ca ²⁺ sensor CIB2. This provides a detailed mechanism of the genetic heterogeneity of TMC1 in hearing loss at an atomic level.