Interrogating the structure and function of the human voltage-gated proton channel (hHv1) with a fluorescent noncanonical amino acid.

The human voltage-gated proton channel (hHv1) is a dimer of voltage-sensor domains (VSDs) containing highly selective proton permeation pathways in each monomer. In addition to voltage, hHv1 is regulated by other stimuli, including pH gradients, mechanical forces, and ligands such as Zn2+. Aside from the VSDs, this membrane protein contains an N-terminal domain and a C-terminal coiled-coil domain (CC) formed between the monomers. To address the need for direct measurements of conformational rearrangements in hHv1, we developed a Forster resonance energy transfer (FRET) approach to measuring the conformational rearrangements in full-length hHv1 purified from E. coli. We used genetic code expansion (GCE) to generate a library of 14 full-length hHv1 constructs, each incorporating the fluorescent noncanonical amino acid acridon-2-ylalanine (Acd) at a different site throughout the various structural domains. Following the expression and purification of these hHv1-Acd proteins, we found that 12 sites yielded stable and functional proton-permeable channels. The fluorescence properties of Acd at each site showed small site-specific differences. Furthermore, we measured site-specific FRET efficiencies from tryptophan (Trp) and tyrosine (Tyr) to Acd in the hHv1-Acd proteins and found results consistent with correct folding in detergent micelles. Finally, the addition of Zn2+ produced reversible changes in FRET, with affected residues clustered on the intracellular side of the channel.