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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 2017Kalita and Fischer 2017).

M2 of influenza virus type A

Matrix protein M2 of 96 aas and 1 TMS.  Forms a proton-selective ion channel that is necessary for the efficient release of the viral genome during virus entry. After attaching to the cell surface, the virion enters the cell by endocytosis. Acidification of the endosome triggers M2 ion channel activity. The influx of protons into the virion interior is believed to disrupt interactions between the viral ribonucleoprotein (RNP), matrix protein 1 (M1), and lipid bilayers, thereby freeing the viral genome from interaction with viral proteins and enabling RNA segments to migrate to the host cell nucleus, where influenza virus RNA transcription and replication occur. Also plays a role in viral proteins secretory pathway.

M2 of Influenza A virus (A/flat-faced bat/Peru)

Matrix protein 2, M2, of 80 aas and 1 TMS

M2 of Influenza A virus (A/swine)

M2 protein of 95 aas and 1 TMS, AM2.  This protein shows 28% identity, 50% similarity and 10% gaps with BM2 (TC# 1.B.58.1.1) with residues 8 - 65 aligning with residues 26 - 81.

  AM2 of Influenza A virus (A/herring gull/Newfoundland)

Protein of 489 aas with C-terminal region resembling the M2 protein (33% identity with no gaps).

Protein of Apis mellifera filamentous virus