X-ray crystal structures of the influenza A M2 proton channel
The M2 proton channel is a drug target of the influenza A virus. It is also a model system for the study of the selective, unidirectional transport of protons across the membrane bilayer. The work presented here focuses on the structural study of the M2 proton channel using X-ray crystallography. High resolution (1.10 Å) X-ray crystal structures of the Inward(open) conformation of M2 reveal layers of ordered, carbonyl-associated waters in the M2 pore. These waters form hydrogen-bonded networks of "water wires" that potentially play a role in the transport of protons down to the channel's gating His37 residues. This crystal form was examined using three data collection techniques: cryogenic diffraction at a synchrotron source, room temperature diffraction at a synchrotron source, and room temperature diffraction at an XFEL source. The degree of solvent ordering observed within the M2 pore is dependent on pH, and is also sensitive to temperature and the effects of radiation damage. New structures of M2 bound to amantadine and rimantadine reveal that the adamantane drugs bind by forming hydrogen bonds with ordered waters in the M2 pore, interrupting the water wires that lead down to the His37 gate. These drugs take advantage of the channel's essential ability to stabilize hydronium; the drug ammonium group acts as a hydronium mimic, which positions the bulky adamantyl group to sterically block the diffusion of hydronium into the channel. Additionally, structural characterization of drug-resistant S31N and V27A mutants of M2 characterize the mechanisms through which mutation of pore-lining residues at the N-terminus of the channel disrupt the binding of the adamantane drugs. These structures provide information for the design of new drugs to target these adamantane-resistant M2 mutants that have become prevalent among currently circulating strains of influenza.