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Reference.org
M2 proton channel
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<p>The Matrix-2 (M2) protein is a <a href="/facts/Proton/clCjepXP">proton</a>-selective <a href="/facts/Viroporin/QeE5YKvR">viroporin</a>, integral in the <a href="/facts/Viral_envelope/5P4eqVHn">viral envelope</a> of the <a href="/facts/Influenza/nWuFzh6L">influenza</a> A virus. The channel itself is a homotetramer (consists of four identical M2 units), where the units are helices stabilized by two <a href="/facts/Disulfide_bond/eDx5Xtlg">disulfide bonds</a>, and is activated by low pH. The M2 protein is encoded on the seventh RNA segment together with the <a href="/facts/M1_protein/4GuSnbEV">M1 protein</a>. Proton conductance by the M2 protein in influenza A is essential for viral replication. </p><p><a href="/facts/Influenza/nWuFzh6L">Influenza</a> B and C viruses encode proteins with similar function dubbed "BM2" and "CM2" respectively. They share little similarity with M2 at the sequence level, despite a similar overall structure and mechanism. </p>