Structural Characterization of Whirlin Reveals an Unexpected and Dynamic Supramodule Conformation of Its PDZ Tandem

Hearing relies on the transduction of sound-evoked vibrations into electric signals, occurring in the stereocilia bundle of hair cells. The bundle is organized in a staircase pattern formed by rows of packed stereocilia. This architecture is pivotal to transduction and involves a network of scaffolding proteins with hitherto uncharacterized features. Key interactions in this network are mediated by PDZ domains. Here, we describe the architecture of the first two PDZ domains of whirlin, a protein involved in these assemblies and associated with congenital deaf-blindness. C-terminal hairpin extensions of the PDZ domains mediate the transient supramodular assembly, which improves the binding capacity of the first domain. We determined a detailed structural model of the closed conformation of the PDZ tandem and characterized its equilibrium with an ensemble of open conformations. The structural and dynamic behavior of this PDZ tandem provides key insights into the regulatory mechanisms involved in the hearing machinery.