Aspartate(112) is the Selectivity Filter of the Human Voltage Gated Proton Channel

The ion selectivity of pumps and channels is central to their ability to perform a multitude of functions. Here we investigate the mechanism of the extraordinary selectivity of the human voltage gated proton channel(1), hH(V)1. This selectivity is essential to its ability to regulate reactive oxygen species production by leukocytes(2–4), histamine secretion by basophils(5), sperm capacitation(6), and airway pH(7). The most selective ion channel known, H(V)1 shows no detectable permeability to other ions(1). Opposing classes of selectivity mechanisms postulate that (a) a titratable amino acid residue in the permeation pathway imparts proton selectivity(1, 8–11), or (b) water molecules “frozen” in a narrow pore conduct protons while excluding other ions(12). Here we identify Aspartate(112) as a crucial component of the selectivity filter of hH(V)1. When a neutral amino acid replaced Asp(112), the mutant channel lost proton specificity and became anion selective or did not conduct. Only the glutamate mutant remained proton specific. Mutation of the nearby Asp(185) did not impair proton selectivity, suggesting that Asp(112) plays a unique role. Although histidine shuttles protons in other proteins, when histidine or lysine replaced Asp(112), the mutant channel was still anion permeable. Evidently, the proton specificity of hH(V)1 requires an acidic group at the selectivity filter.