Chloride binding to prestin does not influence very high-frequency complex nonlinear capacitance (cNLC) in the mouse outer hair cell.

Unlabelled: Prestin (SLC26a5) function evolved to enhance auditory sensitivity and frequency selectivity by providing mechanical feedback via outer hair cells (OHC) into the organ of Corti. Its effectiveness is governed by the voltage-dependent kinetics of the protein's charge movements, namely, nonlinear capacitance (NLC). We study the frequency response of NLC in the mouse OHC, a species with ultrasonic hearing. We find that the characteristic frequency cut-off (F ) for the mouse in near 27 kHz. Single point mutations within the chloride binding pocket of prestin (e.g., S396E, S398E) lack the protein's usual anion susceptibility. In agreement, we now show absence of anion binding in these mutants through molecular dynamics (MD) simulations. NLC F in the S396E knock-in mouse is unaltered, indicating that high frequency activity is not governed by chloride, but more likely by viscoelastic loads within the membrane. We also show that the allosteric action of chloride does not underlie piezoelectric-like behavior in prestin, since tension sensitivity of S396E NLC is comparable to that of WT. Because prestin structures of all species studied to-date are essentially indistinguishable, with analogous chloride binding pockets, auditory requirements of individual species for cochlear amplification likely evolved to enhance prestin performance by modifying, not its protein-anion interaction, but instead external mechanical loads on the protein.

Significance: Prestin is believed to provide cochlear amplification in mammals that possess a wide range of frequency sensitivities. Previously, chloride anions have been shown to control prestin kinetics at frequencies below 10 kHz. However, now we find that chloride binding is not influential for prestin kinetics in the very high range of the mouse. We suggest that such high frequency prestin performance is governed by impinging mechanical loads within the membrane, and not interactions with anions.