Conformational changes underlying electromechanical transduction in prestin resemble a transport transition in pendrin

Prestin (SLC26A5), a membrane protein in cochlear outer hair cells, drives electromechanical transduction essential for mammalian hearing. Unlike other SLC26 anion transporters, prestin functions as a voltage-dependent molecular motor, transitioning between contracted and expanded conformations. How this transition relates to the transporter cycle of SLC26 family members remains unclear. Here, multi-microsecond molecular dynamics (MD) simulations starting from a contracted conformation reveal a rapid, spontaneous transition to an expanded state that resembles the inward-facing conformation of the anion exchanger pendrin (SLC26A4 from mouse). An accompanying transmembrane area expansion is localized to the inner membrane leaflet. In line with this observation, reduced unitary sensor charge movement accompanies neutralization of charged residues localized near the inner leaflet. Simulations uncover a previously uncharacterized contracted conformation that resembles outward-facing pendrin; the MD also predicts an extracellular anion-binding site in prestin. A 3.27-Å resolution cryo-electron microscopy structure of prestin in the presence of thiocyanate anions confirms this site, which appears to be common in the SLC26 family, notably also in pendrin. Based on the simulations, the extracellular site in pendrin is involved in facilitating anion access to the canonical intracellular site in a conformation-dependent manner while in prestin, the intracellular site is not accessible from the extracellular side. Furthermore, like prestin, pendrin exhibits a non-linear capacitance, an indication of voltage-dependent conformational switching. Together, these findings suggest that prestin and pendrin share core structural and functional properties, notably parallels between expansion-contraction states and anion binding sites, though transition speeds and conformation-dependent ion-accessibility may differ.