The Role of Backbone and Sidechain Dynamics on FimH Allostery

In adhesion proteins with catch-bond behavior, like FimH, one proposed mechanism is allostery governing two conformational states. In FimH, domain separation leads to a conformational change in the ligand-binding lectin domain, which switches from an inactive state to an active state with high affinity. To better understand allosteric inhibition in full-length FimH (H2 inactive), we study the lectin domain alone which has high affinity (HL active), and also the lectin domain with an R60P mutation on the putative allosteric pathway (HL mutant). The mutation stabilizes the low-affinity conformation while allowing allostery-like conformational change. To investigate the relative contributions of backbone and sidechain dynamics involved in the transmission of allostery across the lectin domain, we study H2 inactive, HL mutant, and HL active using molecular dynamics simulations. We find that HL mutant has larger backbone fluctuations than H2 inactive and HL active in the allosteric interdomain region and the binding pocket. In HL mutant, these regions also have greater amplitude for the low-frequency eigenmodes, suggesting collective motion consistent with the putative allosteric pathway. Using a network inference technique, we find that backbone motion dominates. Backbone dihedrals are coupled in a contact map pattern based on structure, while sidechain dihedral dynamics are coupled to the backbone. H2 inactive and HL mutant have matching lectin domain structure. However, in HL mutant, the larger fluctuations in the exposed interdomain region that are coupled to the binding pocket may contribute to conformational change. Additional mutations that stabilize these interdomain regions may conformationally lock the lectin domain in the inactive state. Understanding how to keep adhesion proteins in the inactive state may guide drug development to prevent adhesion and infection.