Elucidating Proton Sensitivity and Activation in the Gloeobacter Violaceus Ligand-Gated Ion Channel

Abstract

The pentameric ligand-gated ion channel (pLGIC) family plays an important role in the chemical-to-electrical transduction of neuronal signaling. Although human pLGICs are endogenously activated by acetylcholine, serotonin, zinc, glycine, or gamma-aminobutyric acid, some of them notably glycine and gamma-aminobutyric acid receptors are modulated by protons. The Gloeobacter violaceus ligand-gated ion channel (GLIC) is a prokaryotic proton-gated ion channel of the family and was the first pLGIC to be structurally resolved in both an open and closed conformation offering a structural perspective to the mechanism of activation for the pLGIC family. Determining the proton modulatory site(s) of receptors, including acid-sensing and ATP-sensitive K+ channels, may be a complicated endeavor. Although residues have been identified in these receptors as key proton-sensitive sites the underlying molecular mechanism of modulation is not always clear. GLIC has been shown to be a robust model of the pLGIC family, and understanding how a single or several proton(s) can stabilize the open conformation would shed light on the intricate mechanism involved in channel opening of human pLGICs. This comprehension might also pave the way to reveal proton modulatory mechanisms of other receptors. Here we represent a systematic functional evaluation of titratable residues in GLIC to pinpoint the key proton sensitive sites that play a role in the extracellular initiation of the activation mechanism. We have found two key inter-subunit interacting regions, distinct from the ligand-binding pocket, which play key roles in the opening of the channel.