5OVW:
reference: Structural basis of nanobody-mediated blocking of BtuF, the cognate substrate-binding protein of the Escherichia coli vitamin B12 transporter BtuCD., Mireku SA, Sauer MM, Glockshuber R, Locher KP, Sci Rep. 2017 Oct 30;7(1):14296. doi: 10.1038/s41598-017-14512-8. PMID: 29084999

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