4QKM:
reference: High-resolution structures of the M2 channel from influenza A virus reveal dynamic pathways for proton stabilization and transduction., Thomaston JL, Alfonso-Prieto M, Woldeyes RA, Fraser JS, Klein ML, Fiorin G, DeGrado WF, Proc Natl Acad Sci U S A. 2015 Nov 17;112(46):14260-5. doi:, 10.1073/pnas.1518493112. Epub 2015 Nov 2. PMID: 26578770

This OCA Structure page
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