Does Aspartate112 Mutation Convert the Human Voltage Gated Proton Channel into a Hydroxide Channel?

Abstract

Voltage gated proton channels (HV1) are the most selective channels known, with no detectable permeability to any ion besides H+. We recently identified the selectivity filter of the human voltage gated proton channel (hHV1). Mutation of an aspartate residue, Asp112, in the middle of the S1 transmembrane domain resulted in loss of proton specificity. Surprisingly, mutant channels were anion selective. Cation substitution did not affect Vrev at all. Replacing CH3SO3- by Cl- shifted Vrev negatively, showing that that Cl- is more permeable than CH3SO3-. Dilution of the bath solution by 90% with isotonic sucrose does not change pH (or pOH), but shifted the measured reversal potential positively, in both Cl- and CH3SO3- bath solutions, indicating anion selectivity. However, although 10-fold reduction in ionic strength produced large Vrev (reversal potential) shifts of up to +40 mV, it did not shift Vrev by +57 mV, as expected for pure anion selectivity. The sub-Nernstian shift means that the mutant channels must be permeable to either H+ or OH-, the only ions whose concentration does not change. Because EH and EOH are identical, it is difficult to distinguish these species. We will present and discuss several types of evidence, most of which suggests that OH- and not H+ is permeant in mutant hHV1 channels.