Structural Study of Whirlin, A Crucial PDZ Containing Protein Involved in the Mechanotransduction of Auditory Hair Cells

Abstract

Mammals perceive sound thanks to mechanosensory hair cells located in the inner ear. To detect sound waves, the eardrum produces vibrations that are transmitted to the cochlea where they induce motion of hair-cell cilia. These cilia are tightly bound together in bundle by a network of cadherins and scaffolding proteins. Stretching of this network is directly responsible for the opening of an ion channel that triggers a neuronal message, translating the original sound wave into electric signals transmissible to the brain. Nearly all proteins involved in this cilia-associated network contain short C-terminal motifs of interaction with PDZ domains. Only two proteins of this network encompass PDZ domains: Harmonin and Whirlin. Both of them are multi-domain scaffolding proteins containing three PDZ domains among others protein-protein interaction domains. With tens of potential partners in hair cells, these two proteins have a central role in connecting the extracellular protein links, the cytoskeleton and the ion channel. However their interactome have been only partially described and very little information are available on their molecular organization. We focus our work on several aspect of Whirlin. The N-terminal half of the protein encompasses two PDZ domains and two HHD domains (Harmonin Homology Domain). By homology to related systems, we suspect that HHD and PDZ domain can interact. Using sequence alignment, we identified the second domain HHD downstream to Whirlin second PDZ domain, creating a symmetric organization: HHD1-PDZ1-PDZ2-HHD2. We are investigating the interplay potentially occurring between those four domains and are in the process of determining inter- and intramolecular interactions. We also document the network of interaction of Whirlin in the inner ear, and more generally of all PDZ ligand motifs present in the ear by using a new high-throughput method.