M2 Proton Channel: Difference between revisions

The M2 proton channel from influenza A is 97 amino acid residues and forms a 24-residue N-terminal extracellular domain, a 19-residue trans-membrane domain, and a 54-residue C-terminal cytoplasmic domain.<ref name="Wu" /> The 19-residue TM domain forms the highly selective proton channel.<ref name="Takeuchi">PMID:12972149 </ref> Circular dichroism spectra has shown the TM domain to form an <scene name='User:Sarah_Henke/Sandbox_1/Momomer/2'>α-helix</scene> that spans the membrane.<ref name="Wu" /> By analytical ultracentrifugation, the TM domain is found to form <scene name='User:Sarah_Henke/Sandbox_1/Alpha_hlix/1'>homotetramers</scene> which contains four identical α-helices.<ref name="Takeuchi" /> Secondary structure is color coded by the following, if present: {{Template:ColorKey_Helix}}, {{Template:ColorKey_Strand}}, and {{Template:ColorKey_Turn}}. When viewed in the <scene name='User:Sarah_Henke/Sandbox_1/N_to_c/1'>N->C color coding</scene> the <FONT COLOR="blue">'''N-terminus'''</FONT> (see scale below) is located near the external side of the membrane while the <FONT COLOR="red">'''C-terminus'''</FONT> is located near the internal side of the membrane, closest to the virion. This tetrameric bundle of the TM domain is found by NMR to be tilted by 25-38° from the channel axis.<ref name="Takeuchi" /> The tetrameric helices form a left-handed bundle that resembles a truncated cone.<ref name="Stouffer" /> The TM helicies are arranged around the channel pore with an approximate four-fold rotational symmetry.<ref name="Takeuchi" />

{{Template:ColorKey_N2CRainbow}}

== Central Cavity ==

The hydrophilic residues in each α-helix monomer are oriented towards the pore lumen.<ref name="Wu" /> The <scene name='User:Sarah_Henke/Sandbox_1/Hydrophobic/2'>hydrophobic</scene> residues will be in contact with the membrane (Color code= {{Template:ColorKey_Hydrophobic}} or {{Template:ColorKey_Polar}}). Most of the residues in the M2 channel are hydrophobic except Ser31 Gly34, and His37.<ref name="Wu" /> The central cavity of the M2 proton channel is most constricted near residue <scene name='User:Sarah_Henke/Sandbox_1/Val27/1'>Val27</scene>.<ref name="Stouffer" />  After this residue, the cavity opens to a water-filled pore that is lined with residues <scene name='User:Sarah_Henke/Sandbox_1/Pore/1'>Ala30, Ser31, and Gly34</scene>.<ref name="Stouffer" />  Mutagenesis studies have found that the residues facing the pore are Val27, Ala30, Ser31, Gly34, His37, Leu38, and Trp41.<ref name="Wu" />  The central cavity also constricts at residues His37 and Trp41.<ref name="Stouffer" />  Residues <scene name='User:Sarah_Henke/Sandbox_1/His_37/1'>His37</scene> and <scene name='User:Sarah_Henke/Sandbox_1/Trp20/1'>Trp41</scene> play a key role in the gating mechanism.<ref name="Wu" /> In the closed state, the <scene name='User:Sarah_Henke/Sandbox_1/Histrp/1'>His37 and Trp41</scene> residues block the channel, preventing proton conductance. <ref name="Lear">PMID:12972146 </ref>

== pH Gating ==

The M2 channel is low-pH gated and has a 50-fold increase in proton conductance when the pH drops from 8.2 down to 4.2.<ref name="Wu" /> The His side chain, a five-membered ring, has two nitrogen atoms.<ref name="Takeuchi" /> At a pH of 7 or higher, only one nitrogen is protonated in the neutral imidazole form.<ref name="Takeuchi" /> However, at a pH of about 5.8, the second nitrogen in one or two helicies becomes protonated and forms the cationic imidazolium form.<ref name="Takeuchi" /> This protonation causes the His37 residues to move away from each other due to electrostatic repulsion.<ref name="Takeuchi" /><ref name="Lear" /><ref name="Wu" /> The bi-protonated His37 residues are then stabilized by the Trp41 residues by a cation-π interaction.<ref name="Wu" /> This allows the channel to open and allow water from the pore to penetrate.<ref name="Lear" /><ref name="Wu" /> This forms what is called a proton-conductive water wire through the gate.<ref name="Wu" />