Two functional states of the CD11b A-domain: Correlations with key features of two Mn2+-complexed crystal structures

In the presence of bound Mn2+, the three-dimensional structure of the ligan d-binding A-domain from the integrin CR3 (CD11b/CD18) is shown to exist in the "open" conformation previously described only for a crystalline Mg2+ co mplex. The open conformation is distinguished from the "closed" form by the solvent exposure of F302, a direct T209-Mn2+ bond, and the presence of a g lutamate side chain in the MIDAS site. Approximately 10% of wild-type CD11b A-domain is present in an "active" state (binds to activation-dependent li gands, e.g., iC3b and the mAb 7E3). In the isolated domain and in the holor eceptor, the percentage of the active form can be quantitatively increased or abolished in F302W and T209A mutants, respectively. The iC3b-binding sit e is located on the MIDAS face and includes conformationally sensitive resi dues that undergo significant shifts in the open versus closed structures. We suggest that stabilization of the open structure is independent of the n ature of the metal ligand and that the open conformation may represent the physiologically active form.