A pH-dependent conformational ensemble mediates proton transport through the influenza A/M2 protein.
The influenza A/M2 protein exhibits inwardly rectifying, pH-activated proton transport that saturates at low pH. A comparison of high-resolution structures of the transmembrane domain at high and low pH suggests that pH-dependent conformational changes may facilitate proton conduction by alternately changing the accessibility of the N-terminal and C-terminal regions of the channel as a proton transits through the transmembrane domain. Here, we show that M2 functionally reconstituted in liposomes populates at least three different conformational states over a physiologically relevant pH range, with transition midpoints that are consistent with previously reported His37 pK(a) values. We then develop and test two similar, quantitative mechanistic models of proton transport, where protonation shifts the equilibrium between structural states having different proton affinities and solvent accessibilities. The models account well for a collection of experimental data sets over a wide range of pH values and voltages and require only a small number of adjustable parameters to accurately describe the data. While the kinetic models do not require any specific conformation for the protein, they nevertheless are consistent with a large body of structural information based on high-resolution nuclear magnetic resonance and crystallographic structures, optical spectroscopy, and molecular dynamics calculations.
Department of Biochemistry and Biophysics, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6059, United States.
Alexei L
AL
Polishchuk
James D
JD
Lear
Chunlong
C
Ma
Robert A
RA
Lamb
Lawrence H
LH
Pinto
William F
WF
Degrado
D000818:Animals; D006863:Hydrogen-Ion Concentration; D009980:Influenza A virus / Q000378:metabolism; D007473:Ion Channels / Q000502:physiology; D007700:Kinetics; D008081:Liposomes; D009865:Oocytes / Q000378:metabolism; D011487:Protein Conformation / Q000187:drug effects*; D011522:Protons; D014763:Viral Matrix Proteins / Q000737:chemistry*; D014981:Xenopus; D000086082:Viroporin Proteins
D007473:Ion Channels; D008081:Liposomes; D011522:Protons; D014763:Viral Matrix Proteins; C488937:M2 protein, Influenza A virus; D000086082:Viroporin Proteins