Structural revelations of the human proton channel

Abstract

The voltage-gated proton channel, H V 1, is notoriously unique among ion channels (1), and plays key roles in the health and disease of diverse tissues and species (2). Li et al. (3) combine biochemical, computational, and electron paramagnetic resonance (EPR) spectroscopic approaches to shed light on structural aspects of the human proton channel, hH V 1. Their results advance the field in several key areas, culminating in a bold new model for gating.The voltage-sensing domain (VSD) is the part of voltage-gated ion channels that senses the electrical potential across the cell membrane where the channel resides. Most such channels open upon membrane depolarization, which is accomplished mainly by cationic amino acids located in the fourth transmembrane helical segment (S4) moving outward when the inside of the cell is made more positive. This movement is transduced from the VSD (S1-S4) to the pore region (S5-S6), opening a conduction pathway. In addition to biologically important voltagegated K + , Na + , Ca 2+ , and H + channels, other classes of membrane proteins with VSDs exist that are not channels at all. One example is a voltage-sensing phosphatase (VSP), an enzyme whose activity is regulated by membrane potential. In a landmark study in 2014 (4), the Li-Perozo group reported crystal structures for CiVSP in both "down" and "up" conformations, the first VSD-containing molecule to have structures determined in both states. Voltage-gated ion channels are closed at negative voltages, and open upon depolarization as the S4 helix moves "up" through the membrane electrical field. Because the VSP is not a channel, "closed" and "open" become "down" and "up." The gene for hH V 1 was identified only in 2006 (5). To the astonishment of everyone, the gene product bore a striking resemblance to the VSD of other voltage-gated ion channels, so much so that the simultaneously identified mouse (mH V 1) and Ciona intestinalis (CiH V 1) gene products were dubbed VSOP or "voltage sensor-only protein" (6). H V 1 has only four membrane-spanning helices (S1-S4); these sense voltage but also contain the proton conduction pathway (7).Crystal structures are great, up to a point. They provide tremendously detailed information about molecules, but they have limitations that are sometimes overlooked. They tell us about structure, but only the structure of whatever exists in the crystal. Proteins have many conformations, and one must determine or guess which one was captured during crystallization, and hope it is a native conformation and not a broken one. Forming crystals of membrane proteins is challenging, and often the protein is modified to facilitate crystallization. Ligands or chaperone-like proteins are included, parts of the molecule are truncated, chimerae are produced: whatever it takes to get a good crystal. H V 1 has not been successfully crystallized in its entirety. First came crystal structures of the C terminus alone that lacked the entire transmembrane region (8, 9). Then in 2014, the first exciting glimpse of H V 1 ap...

Insights into the structure and function of HV1 from a meta-analysis of mutation studies

“…The channels are viewed from the side with the extracellular end at the top. An EPR study of hH V 1 generally agreed with the structure of the mH V 1 chimera, except in the EPR study the S2 helix (with F150 and E153) was one turn of the helix lower and S3 (with D174 and D185) was one turn higher relative to S1 and S4 ( DeCoursey, 2015a ; Li et al, 2015 ). E153 is the first amino acid replaced by the spliced-in CiVSP segment, and is actually D in the crystal.…”

“…As the positively charged Arg residues in S4 move outwards during a depolarization that opens the channel, they move past Phe 150 , which serves as a delimiter of internal and external accessibility. Bezanilla and colleagues include Phe 150 along with two other hydrophobic residues, Val 109 and Val 178 , in a hydrophobic gasket (or “plug”) that functions similarly (labeled HG in Table 1 ; Lacroix et al, 2014 ; Li et al, 2014 , 2015 ; DeCoursey, 2015a ). The global purpose of having a narrow isthmus of protein between the two aqueous vestibules is to focus the electric field ( Yang et al, 1996 , 1997 ; Starace and Bezanilla, 2001 , 2004 ).…”

Identification of a vacuolar proton channel that triggers the bioluminescent flash in dinoflagellates

“…The Asp in the middle of the S1 segment is crucial to establishing proton selectivity [ 9 , 12 , 14 , 16 ], the Arg in S4 are thought to confer voltage sensing as in other VSDs, and the Trp affects multiple properties of H V 1 [ 37 ]. LpH V 1 has residues (Phe 171 , Leu 42 and Ile 199 ) that we propose form the hydrophobic gasket that has been identified in other voltage gated ion channels [ 52 – 55 ]. These three hydrophobic residues are aligned horizontally near the middle of the membrane, where they separate internally and externally accessible aqueous vestibules.…”

Voltage and pH sensing by the voltage-gated proton channel, HV1

“…All VSDs have four transmembrane helices, with a series of cationic Arg or Lys in S4 that are thought to sense voltage, and several conserved acidic amino acids in S1–S3 that are thought to interact with the cationic residues to stabilize closed, open or intermediate states. The basic groups in S4 are thought to move outwards during channel opening, passing through a ‘hydrophobic gasket’ [ 91 , 100 , 195 , 196 ,] or ‘charge transfer centre’ [ 143 ] that includes an extremely highly conserved Phe (Phe 150 in hH V 1), which is the delimiter between inner and outer vestibules that access internal and external aqueous solutions, respectively. Furthermore, Cys scanning indicates that the general movement of S4 relative to the other domains (S1–S3) in proton channels [ 85 ] is qualitatively similar to the movement that occurs in other voltage-gated ion channels [ 63 , 133 – 135 ].…”