Model at 140 mM and 1 mM chloride, respectively. The stimulus protocol and analysis were the same as the biophysical measures. Note the similarity to the biophysical data. (E) Cm versus frequency as measured by the fast two-state Boltzmann model (PD173074 site forward/backward rate constants of 1.5e5 s?). Note the absence of NLC decline across frequency. (F) Cm versus frequency as measured by the slower two-state Boltzmann model (forward/backward rate constants of 0.5e4 s?). Note the gradual roll-off due to reduced single-exponential transitions. (G) Cm versus frequency as measured by the electrical cell model with Rs ?10 MU, Rm ?300 MU, Cm ?17 pF, all nominal. Note the flat Cm response across frequency and voltage. (H) Cm versus frequency as measured by the same electrical model, with an additional 5 pF Cm switched in using a magnetically activated reed relay with minimal additional stray capacitance. To see this figure in color, go online.2556 Biophysical Journal 110, 2551?561, June 7,Chloride Controls Prestin Kineticshighlighting, in the case of the OHC, the need to consider interrogation frequency effects when assessing prestin’s voltage-sensor Qmax, namely, total sensor charge in a given cell. DISCUSSION Characterizing sensor-charge movement in voltage-sensitive proteins provides a host of important information on protein function, including operating voltage range and maximum charge moved (Qmax). The latter metric aids in quantifying protein content within the membrane, and our data indicate that prestin may be present at densities higher than the long-held estimates (26,36). For over a decade, chloride has been believed to be a key player in prestin function (13?6), influencing the quantity of measured sensor charge. However, our new data point to a role of chloride in controlling prestin kinetics and not in limiting the quantity of charge movement. Indeed, we previously showed that the maximum OHC eM magnitude, which is expected to correspond to the charge moved, since eM is voltagedriven, is little affected by chloride (18). Does chloride underlie prestin’s voltage-driven charge movement? Zheng et al. (7) identified the OHC molecular motor as the fifth member of the mammalian SLC26 family of anion exchangers, of which 10 members have been identified (5,37). These anion exchangers facilitate the transmembrane movements of monovalent and divalent anions; however, prestin’s transport capabilities are controversial, with some studies showing transport capabilities and others not (38?3). It is interesting to note that the influence of Necrosulfonamide site anions on NLC had been observed before the identification of prestin. For example, lipophilic anions, but not cations, were shown to influence OHC eM and NLC (44), and it has been known since the mid 1990s that the anion salicylate blocks NLC and eM, working on the intracellular aspect of the OHC (45,46). Notwithstanding the controversy of anion transport, the existence of voltage-dependent displacement currents, or NLC, has been taken to indicate an evolutionary change that enables eM, since SLC26a5’s closest mammalian homolog, SLC26a6, lacks this capability, as assessed by standard high-frequency admittance techniques (13). Whether other SLC26 family members actually possess NLC is a subject for future investigation, since our data indicate that we must now consider the occurrence of charge movements that are slower than typically expected. Should other family members possess slow voltage-sensor charge movements,.Model at 140 mM and 1 mM chloride, respectively. The stimulus protocol and analysis were the same as the biophysical measures. Note the similarity to the biophysical data. (E) Cm versus frequency as measured by the fast two-state Boltzmann model (forward/backward rate constants of 1.5e5 s?). Note the absence of NLC decline across frequency. (F) Cm versus frequency as measured by the slower two-state Boltzmann model (forward/backward rate constants of 0.5e4 s?). Note the gradual roll-off due to reduced single-exponential transitions. (G) Cm versus frequency as measured by the electrical cell model with Rs ?10 MU, Rm ?300 MU, Cm ?17 pF, all nominal. Note the flat Cm response across frequency and voltage. (H) Cm versus frequency as measured by the same electrical model, with an additional 5 pF Cm switched in using a magnetically activated reed relay with minimal additional stray capacitance. To see this figure in color, go online.2556 Biophysical Journal 110, 2551?561, June 7,Chloride Controls Prestin Kineticshighlighting, in the case of the OHC, the need to consider interrogation frequency effects when assessing prestin’s voltage-sensor Qmax, namely, total sensor charge in a given cell. DISCUSSION Characterizing sensor-charge movement in voltage-sensitive proteins provides a host of important information on protein function, including operating voltage range and maximum charge moved (Qmax). The latter metric aids in quantifying protein content within the membrane, and our data indicate that prestin may be present at densities higher than the long-held estimates (26,36). For over a decade, chloride has been believed to be a key player in prestin function (13?6), influencing the quantity of measured sensor charge. However, our new data point to a role of chloride in controlling prestin kinetics and not in limiting the quantity of charge movement. Indeed, we previously showed that the maximum OHC eM magnitude, which is expected to correspond to the charge moved, since eM is voltagedriven, is little affected by chloride (18). Does chloride underlie prestin’s voltage-driven charge movement? Zheng et al. (7) identified the OHC molecular motor as the fifth member of the mammalian SLC26 family of anion exchangers, of which 10 members have been identified (5,37). These anion exchangers facilitate the transmembrane movements of monovalent and divalent anions; however, prestin’s transport capabilities are controversial, with some studies showing transport capabilities and others not (38?3). It is interesting to note that the influence of anions on NLC had been observed before the identification of prestin. For example, lipophilic anions, but not cations, were shown to influence OHC eM and NLC (44), and it has been known since the mid 1990s that the anion salicylate blocks NLC and eM, working on the intracellular aspect of the OHC (45,46). Notwithstanding the controversy of anion transport, the existence of voltage-dependent displacement currents, or NLC, has been taken to indicate an evolutionary change that enables eM, since SLC26a5’s closest mammalian homolog, SLC26a6, lacks this capability, as assessed by standard high-frequency admittance techniques (13). Whether other SLC26 family members actually possess NLC is a subject for future investigation, since our data indicate that we must now consider the occurrence of charge movements that are slower than typically expected. Should other family members possess slow voltage-sensor charge movements,.