Proton Conduction Pathways in Proton Channel Proteins

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PROJECT ABSTRACT: Proton channel proteins potentiate the flow of protons across cell membranes, and have evolved fine control over proton selectivity and conductivity to efficiently achieve their function, while maintaining cellular integrity. Through formation of dynamic proton conduction pathways which mimic the water wires observed in dilute acid for proton diffusion, protons move rapidly and selectively along a hydrogen-bonding network composed of confined water and ionizable sidechains scattered within the lumen of proton channel proteins. One way proton channels mediate proton conductivity is through guide water wires, which are stable lumenal waters organized by polar protein groups. Guide water wires are well-studied as they are observed in high-resolution crystal structures, but whether they are mobile or static and how their dynamics affects proton conductivity remains unclear. Another way to modulate proton selectivity and conductivity is through transient water wires, which are thought to form and dissipate to allow for proton flux through well-packed apolar segments. While transient water wires have been hypothesized in molecular dynamics (MD) simulations, they are fundamentally difficult to test experimentally. Finally, proton channels also use proton shuttle mechanisms of protonation and deprotonation through an ionizable sidechain, such as His, Glu, and Asp, to tune proton conductance, but it is unclear the extent these sidechains mediate pore solvation, and whether the proton shuttle mechanism leads to a net transit of water. This work will address these mechanisms by which proton channel proteins mediate proton flux: the (1) seemingly stable hydrogen-bonding networks of guide water wires and protein polar groups, (2) transient water wires, and (3) proton shuttles composed of ionizable sidechains. Through our proposed study of a natural proton channel, the influenza A matrix protein 2 (M2), and de novo designed proton channels, we will test the hypotheses that (1) guide and transient water wires within proton channel proteins confer their selectivity and dictate their capacity to conduct protons, and (2) proton shuttles are not only necessary in defining the conduction rates of these proton channels, but also play critical roles in modulating proton and water permeability. In Aim 1, we will examine whether guide water wires are mobile or static by multidimensional infrared spectroscopy on M2 proton channels and the disease-relative mutants. Our measurements in the presence and absence of drugs will allow us to determine how the dynamics of these networks affect proton conductance, and how they change with drug binding and resistance mutations, which is critical to identifying new antiviral strategies. In Aim 2, we test the hypothesis of transient water wires through the de novo design and characterization of novel proton channels with varying lengths of apolar regions. In the R00 phase (Aim 3), we examine how ionizable sidechains potentiate pore hydration and investigate whether protonation/deprotonation events lead to the cotranslocation of protons and water.