1.A.19.1.1
Matrix protein, M2, an acid activated drug-sensitive proton channel.  Transport involves binding to the four His-37s and transfer to water molecules on the inside of the channel (Acharya et al., 2010).  Functional properties and structure are known (Hong and Degrado 2012). The cytoplasmic tail facilitates proton conduction (Liao et al. 2015).  It is a dimer of dimers (Andreas et al. 2015).  The four TMSs flanking the channel lumen alone seem to determine the proton conduction mechanism (Liang et al. 2016). His-37 forms a planar tetrad in the configuration of the bundle accepting and translocating the incoming protons from the N terminal side, exterior of the virus, to the C terminal side, inside the virus (Kalita and Fischer 2017). The cholesterol binding site in M2 that mediates membrane scission in a cholesterol-dependent manner to cause virus budding and release has been identified (Elkins et al. 2017).Transport-related conformational changes coupled to water and H⁺ movements have been studied (Mandala et al. 2018). The L46P mutant confers a novel allosteric mechanism of resistance towards the influenza A virus M2 S31N proton channel blockers (Musharrafieh et al. 2019). The C-terminal domain of M2 may serve as a sensor that regulates how M2 participates in critical events in the viral infection cycle (Kim et al. 2019). The M2 proton channel protein  self-assembles into tetramers that retain the ability to bind to the drug amantadine, and the effects of phospholipid acyl chain length and cholesterol on the peptide association were investigated. Association of the helices depends on the thickness of the bilayer and cholesterol levels present in the phospholipid bilayer. The most favorable folding occurred when there was a good match between the width of the apolar region of the bilayer and the hydrophobic length of the transmembrane helix with tighter association upon inclusion of cholesterol in the lipid bilayer (Cristian et al. 2003).  The influenza A M2 homotetrameric channel consists of four transmembrane (TM) and four amphipathic helices (AHs). This viral proton channel is suggested to form clusters in the catenoid budding neck areas in raft-like domains of the plasma membrane, resulting in cell membrane scission and viral release.  holesterol-bridged M2 channels can exert a lateral force on the surrounding membrane to induce the necessary negative Gaussian curvature profile, which permits spontaneous scission of the catenoid membrane neck and leads to viral buds and scission (Kolokouris et al. 2025).