Amino Acid Residues in the PSI Domain and Cysteine-rich Repeats of the Integrin β2 Subunit That Restrain Activation of the Integrin αxβ2

The leukocyte integrin alpha(X)beta(2) (p150,95) recognizes the iC3b complement fragment and functions as the complement receptor type 4. alpha(X)beta(2) is more resistant to activation than other beta(2) integrins and is inactive in transfected cells. However, when human alpha(X) is paired with chicken or mouse beta(2), alpha(X)beta(2) 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 alpha(X)/human beta(2) integrin in the inactive state. The opening of this interface is linked to structural rearrangements in other domains that activate ligand binding.