Fig. 4: The cryo-EM structure of BtuB1G1 is closed.

a Cryo-EM maps used to build BtuB1 and BtuG1 G domain (left panel) and BtuG1 H domain (right panel). BtuB1 in yellow, BtuG1 G-domain in blue and BtuG1 H-domain in salmon. b Cartoon model showing BtuG1 and BtuG1, colours as in a. The linker between the domains G and H is shown in red for spatial reference; note it has been modelled into very poor density (top close up view). The bottom close-up view shows an O-Glycosylation site⁷⁹ for BtuG1 at residue Thr164. c Representative 2D classification images from samples with and without B₁₂. White arrows denote the BtuG1 H-domain. d Close-up view of a cartoon model for BtuB1G1 showing the two B₁₂ binding sites occupied. B₁₂ has been placed by superimposing BtuH2-B₁₂ (7BIZ)²⁵ and BtuG2-B₁₂ onto BtuB1G1. Note the steric clashes of the modelled B₁₂ molecules with BtuB EL8 and with the BtuG1 G-domain.