Abstract
The homotetrameric M2 proton channel of influenza A plays a crucial role in the viral life cycle and is thus an important therapeutic target. It selectively conducts protons against a background of other competing cations whose concentrations are up to a million times greater than the proton concentration. Its selectivity is largely determined by a constricted region of its open pore known as the selectivity filter, which is lined by four absolutely conserved histidines. While the mechanism of proton transport through the channel has been studied, the physical principles underlying the selectivity for protons over other cations in the channel's His4 selectivity filter remain elusive. Furthermore, it is not known if proton selectivity absolutely requires all four histidines with two of the four histidines protonated and if other titratable amino acid residues in lieu of the histidines could bind protons and how they affect proton selectivity. Here, we elucidate how the competition between protons and rival cations such as Na+ depends on the selectivity filter's (1) histidine protonation state, (2) solvent exposure, (3) oligomeric state (the number of protein chains and thus the number of His ligands), and (4) ligand composition by evaluating the free energies for replacing monovalent Na+ with H3O+ in various model selectivity filters. We show that tetrameric His4 filters are more proton-selective than their trimeric His3 counterparts, and a dicationic His4 filter where two of the four histidines are protonated is more proton-selective than tetrameric filters with other charge states/composition (different combinations of His protonation states or different metal-ligating ligands). The [His4]2+ filter achieves proton selectivity by providing suboptimal binding conditions for rival cations such as Na+, which prefers a neutral or negatively charged filter instead of a dicationic one, and three rather than four ligands with oxygen-ligating atoms.
Highlights
The acid-activated M2 proton channel of influenza A virus transports protons across the viral envelope to acidify the virion interior during endocytosis
Can selectivity filter (SF) lined with aa residues other than histidines be selective for protons? To answer this question, we modeled H3O+ and Na+ complexes of homotetrameric SFs lined with backbone groups (−CONHCH3), Ser side chains (−OH), or Asp−/Glu− side chains (COO−)
Previous experimental and theoretical studies have focused on the mechanism of ion transport through the M2A channel rather than the properties of its conserved His[4] SF that controls proton selectivity
Summary
The acid-activated M2 proton channel of influenza A virus (abbreviated as M2A) transports protons across the viral envelope to acidify the virion interior during endocytosis. This step is crucial in the virus life cycle and infection mechanism, as it enables the uncoating of the viral RNA and subsequent viral replication.[1] the M2A channel is an important pharmaceutical target for antiviral drugs, which, by making the pore defunct, prevent the virus from proliferating. To fulfill its role as a proton-carrier, the channel should be very selective for the namesake ion, as the proton concentration in the cellular/extracellular milieu is about a million times less than the concentration of other competing ions such as Na+ or K+.5 the M2A channel has very high affinity for protons, selecting H+ over Na+ or K+ by ∼106.6−12 Unlike its voltagegated counterpart in higher organisms, Hv1, which has perfect proton selectivity conducting no other ions except protons,[5] the M2A channel allows for some small Na+ or K+ currents.[10−12]
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