Stable coordination of the inhibitory Ca2+ ion at MIDAS in integrin CD11b/CD18 by an antibody-derived ligand aspartate: Implications for integrin regulation and structure-based drug design

摘要

A central feature of integrin interaction with physiologic ligands is the monodentate binding of a ligand carboxylate to a Mg²⁺ ion hexacoordinated at the metal-ion-dependent-adhesion site (MIDAS) in the integrin A-domain. This interaction stabilizes the A-domain in the high-affinity state, which is distinguished from the default low-affinity state by tertiary changes in the domain that culminate in cell adhesion. Small molecule ligand-mimetic integrin antagonists act as partial agonists, eliciting similar activating conformational changes in the A-domain, which has contributed to paradoxical adhesion and increased patient mortality in large clinical trials. As with other ligand-mimetic integrin antagonists, the function-blocking monoclonal antibody (mAb) 107 binds MIDAS of integrin CD11b/CD18 A-domain (CD11bA), but in contrast, it favors the inhibitory Ca²⁺ ion over Mg²⁺ at MIDAS. We determined the crystal structures of the Fab fragment of mAb 107 complexed to the low- and high-affinity states of CD11bA. Favored binding of Ca²⁺ at MIDAS is caused by the unusual symmetric bidentate ligation of a Fab-derived ligand Asp to a heptacoordinated MIDAS Ca²⁺. Binding of Fab 107 to CD11bA did not trigger the activating tertiary changes in the domain or in the full-length integrin. These data show that denticity of the ligand Asp/Glu can modify divalent cation selectivity at MIDAS and hence integrin function. Stabilizing the Ca²⁺ ion at MIDAS by bidentate ligation to a ligand Asp/Glu may provide one approach for designing pure integrin antagonists.