Amino Acid Residues in the PSI Domain and Cysteine-rich Repeats of the Integrin β₂ Subunit that Restrain Activation of the Integrin α_xβ₂

The leukocyte integrin α_xβ₂ (p150,95) recognizes the iC3b complement fragment and functions as the complement receptor type 4. α_xβ₂ is more resistant to activation than other β₂ integrins and is inactive in transfected cells. However, when human α_x is paired with chicken or mouse β₂, α_xβ₂ is activated for binding to iC3b. Activating substitutions were mapped to individual residues or groups of residues in the N-terminal plexin/semaphorin/integrin (PSI) domain and C-terminal cysteine-rich repeats 2 and 3. These regions are linked by a long range disulfide bond. Substitutions in the PSI domain synergized with substitutions in the cysteine-rich repeats. Substitutions T4P, T22A, Q525S, and V526L gave full activation. Activation of binding to iC3b correlated with exposure of the CBR LFA-1/2 epitope in cysteine-rich repeat 3. The data suggest that the activating substitutions are present in an interface that restrains the human α_x/human β₂ integrin in the inactive state. The opening of this interface is linked to structural rearrangements in other domains that activate ligand binding.