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A Voltage Sensor-Domain Protein Is a Voltage-Gated Proton Channel

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Science  28 Apr 2006:
Vol. 312, Issue 5773, pp. 589-592
DOI: 10.1126/science.1122352

Abstract

Voltage-gated proton channels have been widely observed but have not been identified at a molecular level. Here we report that a four-transmembrane protein similar to the voltage-sensor domain of voltage-gated ion channels is a voltage-gated proton channel. Cells overexpressing this protein showed depolarization-induced outward currents accompanied by tail currents. Current reversal occured at equilibrium potentials for protons. The currents exhibited pH-dependent gating and zinc ion sensitivity, two features which are characteristic of voltage-gated proton channels. Responses of voltage dependence to sequence changes suggest that mouse voltage-sensor domain–only protein is itself a channel, rather than a regulator of another channel protein.

Voltage-gated ion channels are composed of six transmembrane segments (S1 to S6). S5 and S6 form the hydrophilic pore, while S1 to S4 constitute the voltage-sensor domain (VSD) (1, 2). S4 has positively charged amino acids that are periodically aligned at every third position, and these are known to be essential for sensing the change in membrane potential (1). Amino acid substitution in S4 can confer ion-conducting activity to the VSD (36). We recently identified a voltage-sensor–containing phosphatase protein (VSP) that contains a VSD-like domain and a phosphatase domain (7). The VSD-like domain was shown to regulate enzymatic activity of the phosphatase domain (7). Thus, VSD domains may have functions beyond voltage sensing and may be distributed more widely than only in channel proteins.

We used the amino acid sequence of the VSD of Ciona intestinalis VSP (Ci-VSP) (7) as a query for searching with Washington University Basic Local Alignment Search Tool (WUBLAST) software (8) to identify a mouse cDNA, registered as RIKEN cDNA 0610039P13 in the GenBank database. The encoded protein consists of four transmembrane segments with overall homology to the VSD, but it lacks any structure corresponding to the pore domain of voltage-gated channels (fig. S1). We thus named it mVSOP (mouse voltage-sensor domain–only protein). Ortholog genes were found in the genomes of ascidians, zebrafish, Xenopus, and mammals. The putative S4 segment of the deduced protein shows alignment of the positively charged residues similar to that conserved in conventional voltage-gated channels (fig. S1). The putative S2 and S3 segments contain negatively charged residues (fig. S1). These charge distributions are conserved among all ortholog proteins.

Based on the similarity of mVSOP to the VSD of voltage-gated channels, we tested whether it exhibits gating currents. We transfected tsA201 cells, which are derivatives of HEK293 cells, with mVSOP cDNA. We used whole-cell patch clamping and did not observe any trace of gating currents. Instead, robust voltage-dependent outward currents were elicited (Fig. 1), despite the apparent lack of the pore domain. These currents were activated slowly during the depolarizing pulse, and activation became faster as membrane potentials became more positive (Fig. 1A). The average maximum amplitude was 60 ± 38 pA/pF (n = 16 cells) for a 100-mV pulse applied for 500 ms. Inward tail currents were observed during repolarization, indicating that mVSOP is not acting as a pump. Untransfected tsA201 cells showed almost no outward or inward current under the same conditions. mVSOP-induced currents were detected even when both intracellular and extracellular alkali ions and divalent cations were replaced by N-methyl-d-glutamate (NMDG), or when chloride ions were replaced by methanesulfonate. This result suggested that protons are the permeant ions. To verify this, we measured the reversal potential (Vrev) by using extracellular and intracellular solutions with different pHs. Tail currents during repolarization to various potentials were elicited after 500 ms of depolarizing prepulses (fig. S2). The Vrevs shifted in the negative direction as pHin was decreased or pHout was increased (Fig. 2A), and they agreed well with values predicted by the Nernst equation for proton permeability (Fig. 2B). Reversal potentials did not shift from Nernst values even when all NMDG was replaced by the mixture of K+ and Na+ (Fig. 2B), indicating that the channel is selective for protons. To test whether mVSOP-induced currents regulate intracellular pH, changes in intracellular pH during depolarization were measured after acid loading using the pH-sensitive fluorescent dye 2′,7′-bis-(2-carboxyethyl)-5-(6)-carboxyfluorescein–acetoxymethyl ester (BCECF-AM) in HEK293 cells. For acid loading, cells were pretreated with ammonium chloride, then washed with ammonium chloride–free solution (9). Some cells expressing mVSOP showed recovery of pHin even without stimulating membrane depolarization by high K+ concentrations. Increasing extracellular potassium concentration after intracellular acidification led to rapid proton efflux in most mVSOP-expressing cells (Fig. 2, C and D). These results suggest that the channels expressed in mVSOP-transfected cells are proton-selective, voltage-dependent channels.

Fig. 1.

Depolarization-activated whole-cell current induced by mVSOP. (A) A family of current traces recorded from an mVSOP-transfected tsA201 cell under voltage clamp in the whole-cell patch configuration. Pulses stepped by 3 s were applied in 10-mV increments ranging from –30 mV to 100 mV from a holding potential of –60 mV. NMDG solutions were used as patch and bath solutions. pHin/pHout = 7.0/7.0. (B) The current-voltage relationship of the current traces shown in (A).

Fig. 2.

Evidence that VSOP-induced currents are proton selective. (A) Plots of tail current amplitude against membrane potential under various pH conditions. Reversal potentials were determined from the intercept of the current-voltage relationship of tail currents. (B) Comparison of the reversal potentials obtained from tail currents with proton equilibrium potentials (EH) predicted from Nernst equation. The solid line indicates linear fitting of reversal potentials against ΔpH (56.0 mV/ΔpH). The dashed line shows EH calculated by the Nernst equation (59.3 mV/ΔpH). Junction potentials ranging up to 4 mV were corrected in the plot. In the normal solution, a mixture of sodium, potassium, and calcium was substituted for NMDG. (C) Ratiometric fluorescence measurements with pH-sensitive dye of pHin in mVSOP-transfected cells [beads (+), red] and nontransfected cells [beads (–), blue]. (D) Differences of pHin before and after depolarization were quantified. pHin immediately after intracellular acidification by NH4Cl [time 0 in (C)] and that at 10 min after the start of perfusion [arrow in (C)] of high-potassium solution were measured. Transfection (–) denotes results from cells without transfection (n = 14). Beads (+) denotes results from transfected cells (n = 25). Beads (–) denotes cells that did not express CD8 in the same dish for beads (+) cells (n = 18).

Voltage-gated proton channels (Hv channels) have previously been described in mammalian eosinophils (10, 11), macrophages (12, 13), microglia (14), and snail neurons (1517). A characteristic feature of the native Hv channel is its unique pH dependence of gating (18), in which the voltage activation relationship depends strongly on both the intracellular and extracellular pH. Current-voltage (I-V) relations measured from the same cell at three distinct extracellular pHs showed that when the extracellular pH was decreased from 7.0 to 6.1, the I-V curve shifted in the positive direction by about 50 mV (Fig. 3A and fig. S2). When intracellular pH was altered, the I-V relationship shifted in the opposite direction by a similar value (fig. S3). Outward currents were activated at a similar threshold of membrane potential (Vthreshold), when measured at two different conditions of extracellular pH = 6.1 and 6.5, with a small pH gradient across the cell membrane (fig. S3). These findings are consistent with the observation that the voltage activation curve is predicted from the gradient between the extracellular and the intracellular pH in native Hv channels (11). Vthreshold was always more positive than Vrev within the range of pH gradients we examined (–1.5 < ΔpH < 2.5; ΔpH is the difference of pH across the cell membrane, pHout – pHin). Therefore, mVSOP opens at a range of membrane potentials over the equilibrium potential for protons, thus enabling outwardly rectifying property for proton flow. Activation kinetics were also pH-dependent; the time for half activation at varied membrane potentials became slower as the extracellular pH decreased (Fig. 3B). This pH-dependent gating of VSOP-induced currents agrees well with a reported behavior of native Hv currents (12, 18).

Fig. 3.

pH-dependent gating and inhibition by divalent metal cations of mVSOP-induced currents. (A) The current-voltage relationships evoked by a series of voltage steps in 10-mV increments (–80 to 100 mV) under pHin = 6.5 and pHout = 7.0, 6.5, or 6.1. The pulse duration was 500 ms. Currents were measured from the same sets of cells. Current amplitudes at the end of the depolarizing pulse obtained under each condition of pHout were normalized by those at 20 mV recorded under pHout = 7.0 for individual cells. (B) Voltage dependence and pH dependence of the time required for half-maximal activation. Maximal current was measured as the amplitude at the end of depolarizing pulse. The pulse duration was 500 ms. Representative current traces for (A) and (B) are shown in fig. S2. Averaged values ± SE are shown (n = 9) in (A) and (B). (C) Dose-response curves of inhibition by zinc and cadmium ions. Small circles are plots from individual cells, and squares denote average values. The holding potential was –80 mV. (D) Tissue distributions of VSOP mRNA examined by real-time RT-PCR. The expression level of VSOP mRNA was normalized by expression in the spleen. L8 ribosomal protein was used as internal control.

Hv currents are known to be inhibited by submillimolar concentrations of Zn2+, Cd2+, and other divalent cations (11, 19, 20). Likewise, mVSOP-derived currents were reversibly decreased by submillimolar concentrations of Zn2+ and Cd2+ (Fig. 3C and fig. S2). Consistent with results for native Hv currents (19, 20), Zn2+ had a stronger effect (dissociation constant Kd = 0.48 μM) than Cd2+.

Previous electrophysiological studies have revealed Hv currents in mammalian blood cells, alveolar epithelium cells of the lung, and microglia of the brain (11, 12, 14). Consistent with this, quantitative reverse transcriptase polymerase chain reaction (RT-PCR) (Fig. 3D) showed marked gene expression of mVSOP in the spleen, whole blood, bone marrow, and macrophages.

Does mVSOP directly encode Hv channels, or just activate endogenous Hv channels? To address this, we tested whether modification of molecular structure leads to changes in the properties of the Hv current. Changes in voltage dependence were examined for mutations at each of the three positively charged amino acids of the S4-like segment. The conductance-voltage (G-V) curve was obtained with the same pH (7.0) in both the extracellular and intracellular solutions. In the R204Q mutant, where Arg204 is replaced by Gln, detailed analysis of current kinetics was not possible due to a low level of protein expression to the cell surface. The G-V curve for R207Q was indistinguishable from that of the wild-type channel (fig. S4), whereas R201Q showed much faster kinetics of activation (Fig. 4A). The G-V curve of the R201Q mutant was shifted in a negative direction by about 50 mV (Fig. 4B). The steepness of activation (z value) became slightly sharper (1.9 ± 0.25 for R201Q, n = 9 cells, versus 1.4 ± 0.15 for wild type, n = 5). In this mutant, Vtheshold was more negative than Vrev, suggesting that inward current flows at a membrane potential more negative than Vrev. In fact, an inward proton current was clearly seen at acidic extracellular pH (Fig. 4, C and D). R201 may therefore help the channel to maintain a Vthreshold that is more positive than the equilibrium potential for protons, which enables the outwardly-rectifying property of the channel.

Fig. 4.

Mutation in the S4-like segment alters the voltage dependence of channel gating. (A) Current traces recorded from cells expressing wild-type mVSOP (wt) (left) or R201Q (right). Traces with depolarizing steps (–20 to 100 mV) are superimposed. pHin/pHout = 7.0/7.0. The pulse protocol is shown in fig. S2. (B) Voltage dependency of conductance is plotted for wt (n = 5; circles) and R201Q (n = 9; squares) and fitted by the Boltzmann equation Embedded Image where k is the Boltzmann constant, e is the elementary electric charge, T is temperature, and z is the valence. V½ values are 63.7 ± 7.6 mV and 14.8 ± 8.0 mV, and z values are 1.4 ± 0.15 and 1.9 ± 0.25 for wt and R201Q, respectively. Error bars indicate SD. (C) Current traces under pHin/pHout = 7.0/6.1 for wt (left) and R201Q (right). Traces with depolarizing steps (20 to 90 mV) are superimposed. In R201Q, tail currents are scaled out in this magnification. (D) The current-voltage relationships for wt (n = 8) and R201Q mutant (n = 12) in pHin/pHout = 7.0/6.1. Averaged current densities at the end of depolarization pulses are plotted. Inward current is evident for R201Q. In [(A) to (D)], the holding potential was –60 mV. Error bars indicate SD.

The ortholog of mVSOP was also isolated from an ascidian, C. intestinalis (called Ci-VSOP). Ci-VSOP showed marked homology to mVSOP in the putative S1 to S4 segments, although the N-terminal and C-terminal cytoplasmic regions of the two VSOP proteins were highly divergent (fig. S1). This ascidian ortholog protein also exhibited outward-rectifying proton currents activated by membrane depolarization when expressed in tsA201 cells. Ci-VSOP showed 27 times lower sensitivity to Zn2+ (Fig. 3C) and significantly faster activation kinetics than mVSOP (fig. S5). The shifted voltage dependency of activation in the mutant R201Q of mVSOP, together with the differences in kinetics and pharmacology between ascidian and mammalian orthologs, suggest that VSOP proteins are the principal subunit of the Hv channel, rather than a regulator or accessory subunit of an endogenous proton channel.

We identified a protein consisting primarily of a VSD as an Hv channel, providing the first example of a protein in which the VSD functions beyond voltage sensing. Proton efflux through Hv channels, accompanied by membrane depolarization, facilitates acute production of reactive oxygen species in phagocytosis (11, 14, 21). Further studies of VSOP will not only provide new clues to general principles of proton permeation and gating of membrane proteins (10, 11, 14), but may also open avenues for advances in understanding biological events related to respiratory burst and phagocytosis.

Supporting Online Material

www.sciencemag.org/cgi/content/full/1122352/DC1

Materials and Methods

Figs. S1 to S5

References

References and Notes

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