"The influenza virus M2 protein was expressed in Xenopus laevis oocytes and shown to have an associated ion channel activity selective for monovalent ions."

"The anti-influenza virus drug amantadine hydrochloride significantly attenuated the inward current induced by hyperpolarization of oocyte membranes."

"Mutations in the M2 membrane-spanning domain that confer viral resistance to amantadine produced currents that were resistant to the drug."

"Analysis of the currents of these altered M2 proteins suggests that the channel pore is formed by the transmembrane domain of the M2 protein."

"The wild-type M2 channel was found to be regulated by pH."

"The wild-type M2 ion channel activity is proposed to have a pivotal role in the biology of influenza virus infection."

"The M2 protein from influenza A virus is a pH-activated proton channel that mediates acidification of the interior of viral particles entrapped in endosomes."

"Here we describe the crystal structure of the transmembrane-spanning region of the homotetrameric protein in the presence and absence of the channel-blocking drug amantadine."

"pH-dependent structural changes occur near a set of conserved His and Trp residues that are involved in proton gating."

"Binding of amantadine physically occludes the pore, and might also perturb the pK(a) of the critical His residue."

"The drug-binding site is lined by residues that are mutated in amantadine-resistant viruses."

"These results are converging to a model in which a histidine residue in the TM domain mediates proton relay with water, aided by microsecond conformational dynamics of the imidazole ring."

"This review will focus primarily on the structure and function of the TM domain of M2 (M2TM), which contains the proton-conducting residue, histidine 37 (His37), 12 and the channel-gating residue, tryptophan 41 (Trp41)."

"The TM domain reproduces most of the electrophysiological, pharmacological, and biophysical features of the full-length protein, such as low-pH activated proton conductivity, amantadine sensitivity of the proton current, and tetramerization of the protein."

"An early model envisioned a continuous aqueous channel that was gated by pH (shutter mechanism), 20 versus the currently accepted model in which protons diffuse along a water wire until reaching His37, where they are then "shuttled" by His37 through alternate protonation and deprotonation events"

"His37 residues interact with each other indirectly via water molecules in the dominant structure at equilibrium"

"Crystal structure of the transmembrane domain of the M2 protein from influenza A virus at 1.65 Å resolution, showing the homotetrameric bundle structure"

"Structure shows single amantadine molecule bound in the N-terminal pore lumen of the tetrameric channel"

"Structural visualization of the conserved histidine 37 and tryptophan 41 residues that control proton selectivity and pH activation"

"The influx of protons into virion interior is believed to disrupt interactions between the viral ribonucleoprotein (RNP), matrix protein 1 (M1), and lipid bilayers"

"M2 is expressed in the Golgi and pumps protons out of Golgi vesicles, raising the luminal pH. This prevents the premature conformational activation of HA during transport to the cell surface"

"Fluorescence microscopy studies in infected cells show that M2 actively recruits M1 to the membrane, initiating assembly of virion buds"

"M2's amphipathic helix in its cytoplasmic tail induces membrane curvature and scission during the final step of budding. M2 localizes to the budding neck (at the edge of lipid rafts where hemagglutinin (HA) and neuraminidase cluster) and its amphipathic helix inserts into the inner leaflet, deforming the membrane to pinch off the virion"

"His37 is the pH-activated switch, and Trp41 is the check-valve, together conferring M2's unique gating and selectivity properties"

"The first 9 residues of M2e are almost absolutely invariant among all influenza A isolates – this includes human, swine, equine, avian strains, and even the divergent bat influenza viruses"

"By the mid-2000s, S31N became nearly fixed in human H3N2 and H1N1 strains globally, such that >95% of circulating human influenza A viruses were amantadine-resistant"

"M2 has a conserved LC3-interacting region (LIR) in its cytosolic tail that engages the host autophagy machinery. M2 interacts with LC3 (a marker on autophagosome membranes), and this interaction was shown to redistribute autophagosomes to the plasma membrane where virus buds"

"the E3 ubiquitin ligase MARCH8 was recently shown to redirect M2 to lysosomes for degradation, thereby inhibiting influenza virion release (a host defense strategy)"