Bidirectional allostery mechanism of catch-bond effect in cell adhesion

Catch-bonds, whereby noncovalent ligand-receptor interactions are counterintuitively reinforced by tensile forces, play a major role in cell adhesion under mechanical stress. A basic prerequisite for catch-bond formation is that force-induced remodeling of ligand binding interface occurs prior to bond rupture. However, what strategy receptor proteins utilize to meet such specific kinetic control is still unclear, rendering the mechanistic understanding of catch-bond an open question. Here we report a bidirectional allostery mechanism of catch-bond for the hyaluronan (HA) receptor CD44 which is responsible for rolling adhesion of lymphocytes and circulating tumor cells. Binding of ligand HA allosterically reduces the threshold force for unlocking of otherwise stably folded force-sensing element (i.e., forward allostery), so that much smaller tensile force can trigger the conformational switching of receptor protein to high binding-strength state via backward allosteric coupling before bond rupture. The effect of forward allostery was further supported by performing atomistic molecular dynamics simulations. Such bidirectional allostery mechanism fulfills the specific kinetic control required by catch-bond and is likely to be commonly utilized in cell adhesion. We also revealed a slip-catch-slip triphasic pattern in force response of CD44-HA bond arising from force-induced repartitioning of parallel dissociation pathways. The essential thermodynamic and kinetic features of receptor proteins for shaping the catch-bond were identified.