Tryptophan 207 is crucial to the unique properties of the human voltage-gated proton channel, hH V 1

What is it about?

Part of the “signature sequence” that defines voltage gated proton channels (HV1) is a tryptophan residue adjacent to the second Arg in the S4 transmembrane helix: RxWRxxR, that is perfectly conserved in all high-confidence HV1 genes. Replacing Trp207 in human HV1 with Ala, Ser, or Phe facilitated gating, accelerating channel opening by 100-fold, and closing by 30-fold. Mutant channels opened at more negative voltages than WT channels, indicating that in WT channels Trp favors a closed state. The Ea for channel opening decreased to 22 kcal/mol from 30-38 kcal/mol for WT, confirming that Trp207 establishes the major energy barrier between closed and open hHV1. Cation-pi interaction between Trp207 and Arg211 latches the channel closed. Trp207 mutants lost proton selectivity at pHo > 8.0. Finally, gating that depends on the transmembrane pH gradient (pH dependent gating), a universal feature of HV1 that is essential to its biological functions, was compromised. In the WT hHV1 channel pH dependent gating is shown to saturate above pHi or pHo 8, consistent with a single pH sensor with alternating access to internal and external solutions. However, saturation occurred independently of pH, indicating the existence of distinct internal and external pH sensors. In Trp207 mutants pH dependent gating saturated at lower pHo but not at lower pHi. That Trp207 mutation selectively alters pHo sensing further supports the existence of distinct internal and external pH sensors. Analogous mutations in HV1 from the unicellular species Karlodinium veneficum and Emiliania huxleyi produced generally similar consequences. Saturation of pH dependent gating occurred at the same pHo and pHi in HV1 of all three species, suggesting that the same or similar group(s) are involved in pH sensing. Therefore, Trp enables four characteristic properties: slow channel opening, highly temperature dependent gating kinetics, proton selectivity, and pH dependent gating.

Featured Image