Force-Regulated Spontaneous Conformational Changes of Integrins α₅β₁ and α_Vβ₃

Integrins are cell surface nanosized receptors crucial for cell motility and mechanosensing of the extracellular environment, which are often targeted for the development of biomaterials and nanomedicines. As a key feature of integrins, their activity, structure and behavior are highly mechanosensitive, which are regulated by mechanical forces down to pico-Newton scale. Using single-molecule biomechanical approaches, we compared the force-modulated ectodomain bending/unbending conformational changes of two integrin species, α₅β₁ and α_Vβ₃. It was found that the conformation of integrin α₅β₁ is determined by a threshold head-to-tail tension. By comparison, integrin α_Vβ₃ exhibits bistability even without force and can spontaneously transition between the bent and extended conformations with an apparent transition time under a wide range of forces. Molecular dynamics simulations observed almost concurrent disruption of ∼2 hydrogen bonds during integrin α₅β₁ unbending, but consecutive disruption of ∼7 hydrogen bonds during integrin α_Vβ₃ unbending. Accordingly, we constructed a canonical energy landscape for integrin α₅β₁ with a single energy well that traps the integrin in the bent state until sufficient force tilts the energy landscape to allow the conformational transition. In contrast, the energy landscape of integrin α_Vβ₃ conformational changes was constructed with hexa-stable intermediate states and intermediate energy barriers that segregate the conformational change process into multiple small steps. Our study elucidates the different biomechanical inner workings of integrins α₅β₁ and α_Vβ₃ at the submolecular level, helps understand their mechanosignaling processes and how their respective functions are facilitated by their distinctive mechanosensitivities, and provides useful design principles for the engineering of protein-based biomechanical nanomachines.