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Proton (H + ) channels are special: They select protons against other ions that are up to a millionfold more abundant. Only a few pro- ton channels have been identified so far. Here, we identify a fam- ily of voltage -gated ?pacemaker ? channels, HCNL1, that are exquisitely selective for protons. HCNL1 activates during hyperpo- larization and conducts protons into the cytosol. Surprisingly, pro- tons permeate through the channel ?s voltage -sensing domain, whereas the pore domain is nonfunctional. Key to proton perme- ation is a methionine residue that interrupts the series of regularly spaced arginine residues in the S4 voltage sensor. HCNL1 forms a tetramer and thus contains four proton pores. Unlike classic HCN channels, HCNL1 is not gated by cyclic nucleotides. The channel is present in zebrafish sperm and carries a proton inward current that acidifies the cytosol. Our results suggest that protons rather than cyclic nucleotides serve as cellular messengers in zebrafish sperm. Through small modifications in two key functional do- mains, HCNL1 evolutionarily adapted to a low-Na + freshwater en- vironment to conserve sperm ?s ability to depolarize.