Mechanical Strength of Catch Bond Forming FimH and Mannose

Abstract

Adhesive interactions must withstand mechanical forces occurring in an organism. Forces have influenced some proteins such as integrins, clotting factors, and bacterial adhesive proteins to develop catch bond properties, in which the lifetime between a biomolecule and ligand increases with applied force. One of the most well studied catch bond behaviors is found between the E. coli adhesive protein, FimH, and its ligand, mannose. FimH has two distinct conformations with significantly different mannose affinities. The low affinity conformation predominates the initial FimH-mannose interactions, as this state, characterized with an open binding pocket, facilitates fast kinetics. A force pulling on the FimH-mannose interaction elongates FimH, inducing a transition into the high mannose affinity state, which includes a closed mannose binding pocket. While in this force-stabilized high affinity conformation, FimH has a remarkably long-lived lifetime that has never been fully measured before. We suspect the closed binding pocket is responsible for this lifetime, and mannose dissociation occurs when the pocket spontaneously opens. If the opening of this pocket occurs independent of the strength of the applied force, mannose unbinding would thus represent a force independent, or ideal bond, interaction. To explore this possibility, we measured the lifetimes of the mannose-FimH interaction at various forces with FimH in the high affinity state. We utilized a magnetic tweezer apparatus, to apply various constant forces to mannose-coated magnetic beads bound to immobilized FimH. While true ideal bond behavior cannot be concluded with certainty, the FimH-mannose bond often outlasted the 15-minute collection period, even at high forces relative to other catch bond forming proteins. Understanding the relationship between force and FimH's incredibly stable binding pocket, would grant further insight into the catch bond mechanism associated with FimH.