A novel rare splice variant of the human voltage-dependent proton channel hH_V1

Diversity among human voltage-gated proton channels (hH_V1) arises exclusively from alternative splicing or post-translational modification. In addition to the canonical 273-amino acid form (hH_V1-l), two shorter isoforms, HVCN1_S and H_V1Sper, have been described, both characterized by truncated N-terminal domains. To date, isoforms longer than hH_V1-l were unidentified in humans. Here, we report a novel 'very-long' hH_V1 splice variant (hH_V1-vl) containing a 47 amino acid N-terminal insertion derived from alternative splicing of exon 12 of the HVCN1 gene. The additional amino acid sequence is predicted to be largely unstructured and contains several potential phosphorylation sites. Molecular dynamics simulations further suggest that the conserved poly-glutamate motif downstream of Thr29 can interact with cytosolic arginine residues at the base of the S4 segment. Nested PCR unambiguously identified hH_V1-vl in healthy tissues (pons and choroid plexus epithelial cells) as well as in three different types of lymphoma. However, quantitative PCR showed that hH_V1-vl is expressed at low levels in the analyzed tissues, 0.3-2% of the total HVCN1 transcripts. Heterologous expression of hH_V1-vl followed by patch-clamp recordings demonstrated that this isoform retains the key properties of proton channels, including high proton selectivity and voltage- and pH-dependent conductance. Compared with the canonical 273-amino acid hH_V1-l isoform, hH_V1-vl exhibits similar gating parameters, including proton selectivity, ΔpH-dependent gating, conductance density, g_H-V relationship, and closing kinetics, but shows markedly slower activation kinetics.