The Voltage-Gated Proton Channel: A Riddle, Wrapped in a Mystery, inside an Enigma

This review will highlight some of the distinctive features of the voltage gated proton channel (HVCN1 or HV1). The title borrows from Winston Churchill’s observation on the unpredictability of Russian behavior and intends to evoke the elusiveness of the proton channel. HV1 prehistory begins in 1806 with a classical paper “Sur la decomposition de l’eau et des corps qu’elle tient en dissolution a l’aide de l’electricite galvanique,” by C.J.T. de Grotthuss (a.k.a. Freiherr Christian J. Theodor von Grotthuss)1, the namesake of the “Grotthuss mechanism.” Reading a translation of this paper2, one realizes that the mechanism proposed differs substantially from the current view3, not least because Grotthuss described the water molecule as OH. Details aside, the contribution of Grotthuss was acknowledged in 1905 in “Notiz uber Ionengeschwindigkeiten”4, in which Danneel provided a lucid explanation of the anomalously rapid conductance of H+ (which has a five-fold higher mobility than other ions), and to a lesser extent, OH−. Danneel recognized that the proton, alone among cations, could bind reversibly to water, and further that in protonated water, H3O+, the three protons are equivalent and interchangeable. One proton binds to water and another proton leaves on the far side, saving distance to be traveled. It was perhaps inevitable that an ion with such a unique conduction mechanism in water would follow different rules in its interaction with proteins. Indeed, both groups who identified HVCN1 channel genes in 20065,6 noted the presence of a voltage-sensing domain resembling that found in other voltage gated ion channels, but the striking absence of any traditional pore domain (Figure 1). In light of the fact that ion channels exist in order to create regulated passive flow of specific ions across cell membranes, the lack of a permeation pathway seems a deplorable omission, almost a fatal flaw. Nevertheless, the gene product alone, when incorporated into liposomes, functions as a proton conductor (Figure 2), confirming that no accessory protein, such as a separate pore domain, is required7. Figure 1 Topology of three molecules that contain voltage-sensing domains (VSD): a generic K+ channel, the proton channel, and a voltage-sensitive phosphatase. The upper row shows the orientation of a monomer in the membrane, with the transmembrane helical segments ... Figure 2 Fluorescence-based H+ flux assay for vesicles containing hHV1 (dark blue), the VSD of KvAP channels (green), KvAP (dark green), the paddle chimera (cyan), and empty vesicles (red). Valinomycin and protonophore CCCP were added at the indicated time points. ... Traditional ion channels16 are thought of as containing a pore through which a column of water mole-cules17 interspersed with ions march in single file18. The single-file region usually comprises a small part of the distance across the membrane, with relatively wide aqueous vestibules at either end19. Protons can hop through the single-file region of a water-filled pore without displacing the water molecules20, whereas ions must wait for the waters in front to pass through17,21. Protons do not need to travel with an entourage of waters of hydration, and can in fact move quite well as a bare proton, hopping from one ligand to another through hydrogen bonds22–25. As will be discussed later, this ability underlies the mechanism by which HV1 channels are able to select for protons. Consequently, it is appropriate that these channels be termed “proton channels” as opposed to “hydrogen ion channels,” because the permeating species is the proton, not the hydronium ion, H3O+(26–29) or the other common forms of hydrated hydrogen ion, H5O2+, recently named the “Zundel cation”30 but described by Huggins in 193631, and H9O4+, the “Eigen cation”32,33. For the same reason, the official (HUGO Gene Nomenclature Committee) gene product name HVCN1 (Hydrogen voltage-gated channel 1) is a misnomer. Even in water, bodily diffusion of H3O+ is negligible34,35. If we know one thing about proton movement through channels, it is that H3O+ is not the permeating species22,28,36,37.