Salt bridge interactions within the β integrin α helix mediate force-induced binding and shear resistance ability.

The functional performance of the αI domain α helix in β integrin activation depends on the allostery of the α helix, which axially slides down; therefore, it is critical to elucidate what factors regulate the allostery. In this study, we determined that there were two conservative salt bridge interaction pairs that constrain both the upper and bottom ends of the α helix. Molecular dynamics (MD) simulations for three β integrin members, lymphocyte function-associated antigen-1 (LFA-1; α β ), macrophage-1 antigen (Mac-1; α β ) and α β , indicated that the magnitude of the salt bridge interaction is related to the stability of the αI domain and the strength of the corresponding force-induced allostery. The disruption of the salt bridge interaction, especially with double mutations in both salt bridges, significantly reduced the force-induced allostery time for all three members. The effects of salt bridge interactions of the αI domain α helix on β integrin conformational stability and allostery were experimentally validated using Mac-1 constructs. The results demonstrated that salt bridge mutations did not alter the conformational state of Mac-1, but they did increase the force-induced ligand binding and shear resistance ability, which was consistent with MD simulations. This study offers new insight into the importance of salt bridge interaction constraints of the αI domain α helix and external force for β integrin function.