Experimental support for a β-propeller domain in integrin α-subunits and a calcium binding site on its lower surface

Integrins are large, heterodimeric surface molecules of wide importance in cell adhesion. The N-terminal half of all integrin alpha-subunits contains seven weak sequence repeats of approximate to 60 amino acids that are important in ligand binding and have been predicted to fold cooperatively into a single beta-propeller domain with seven beta-sheets. We provide evidence supporting this model with a mouse mAb to human Mac-1 (alpha M beta 2, CD11b/CD18). This antibody, CBRM1/20, binds to amino acid residues that are in different repeats and are 94 residues apart in the primary structure in the loop between strands 1 and 2 of beta-sheet 5 and in the loop between strands 3 and 4 of beta-sheet 6. The 1-2 loops of beta-sheets 5-7 in integrins have EF hand-like Ca2+-binding motifs, CBRM1/20 binds to Mac-1 in the presence of Ca2+ or Sr2+ with an EC50 of 0.2 mM. Mg2+ or Mn2+ cannot substitute. Antibodies to other epitopes on the Mac-1 beta-propeller domain bind in the absence of calcium, mAb CBRM1/20 does not block ligand binding. Thus, the region on the lower surface of the beta-propeller domain to which mAb CBRM1/20 binds does not bind ligand and, furthermore, cannot bind other integrin domains, such as those of the beta-subunit.