Figure 6.

(A and B) Comparison of the (A) Apo- and (B) ${\text{Ca}}^{2+}$-bound states of BtuB in the simulated OM systems. Orientation differences observed in loop 3/4 are emphasized by the red circle. (A) Apo-state BtuB (orange ribbon) aligned to the CBL-bound crystal structure (transparent gray ribbon) from Chimento et al. (25). Residues within 4 Å of CBL in the CBL-bound crystal structure are highlighted as blue (Apo1-OM) and purple (crystal structure) sticks. (B) ${\text{Ca}}^{2+}$-bound BtuB (orange ribbon) aligned to the CBL-bound crystal structure (transparent gray ribbon). Calcium ions are shown as cyan spheres and the green sticks represent the CBL-binding residues in the Ca1-OM system. Replicates from the second OM system can be seen in Fig. S13. To see this figure in color, go online.