Figure 9.

Structure of the SdrG-Fg$\text{β}$ complex. (A) The crystal structure of the SdrG (blue): Fg$\text{β}$ (orange) complex. The locking strand (green) encloses the peptide in the binding pocket between the Ig-fold N2 (light blue) and N3 (dark blue) domains and a calcium (yellow) binding loop. The red arrows indicate the force applied to the molecular complex. (B) Schematic of the “bulgy plug” hypothesis. The bulky phenylalanine side chains (gray) of Fg$\text{β}$ (orange) are blocked by the locking strand (green). (C) Crystal structure showing the bulky phenylalanine side chains in van der Waals representation (gray spheres) of Fg$\text{β}$ (orange), which have to wiggle through a narrow constriction (cyan surface). This figure is adapted with permission from Milles et al. (2018).