2GUF:
reference: In Meso Structure of the Cobalamin Transporter, BtuB, at 1.95 A Resolution., Cherezov V, Yamashita E, Liu W, Zhalnina M, Cramer WA, Caffrey M, J Mol Biol. 2006 Dec 8;364(4):716-34. Epub 2006 Sep 12. PMID: 17028020

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