Folding of Prestin's Anion-Binding Site and the Mechanism of Outer Hair Cell Electromotility.

Prestin (SLC26A5) responds to transmembrane voltage fluctuations by changing its cross-sectional area, a process underlying the electromotility of outer hair cells and cochlear amplification. Prestin's voltage-dependent conformational rearrangements partially depend on the binding of Cl - to an electrostatic gap between the TM3 and TM10 helices. Using hydrogen-deuterium exchange mass spectrometry, we find that prestin displays an unstable anion-binding site, where folding of the amino termini of TM3 and TM10 is coupled to Cl - binding. This event shortens the TM3-TM10 electrostatic gap, thereby reducing prestin's cross-sectional area. These folding events upon anion-binding are absent in SLC26A9, an anion transporter closely related to prestin. We observe helix fraying of prestin's anion-binding site but cooperative unfolding of multiple peripheral helices, with implications to prestin's fast electromechanical rearrangements. These results highlight a novel role of the folding equilibrium of the anion-binding site in defining prestin's unique voltage-sensing mechanism and electromotility.