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

Oxvig, Claus; Springer, Timothy A.

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

Authors

Oxvig, Claus

Springer, Timothy A.

Publisher

National Academy of Sciences

Description

Integrins are large, heterodimeric surface molecules of wide importance in cell adhesion. The N-terminal half of all integrin α-subunits contains seven weak sequence repeats of ≈60 amino acids that are important in ligand binding and have been predicted to fold cooperatively into a single β-propeller domain with seven β-sheets. We provide evidence supporting this model with a mouse mAb to human Mac-1 (αMβ2, 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 β-sheet 5 and in the loop between strands 3 and 4 of β-sheet 6. The 1–2 loops of β-sheets 5–7 in integrins have EF hand-like Ca(2+)-binding motifs. CBRM1/20 binds to Mac-1 in the presence of Ca(2+) or Sr(2+) with an EC(50) of 0.2 mM. Mg(2+) or Mn(2+) cannot substitute. Antibodies to other epitopes on the Mac-1 β-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 β-propeller domain to which mAb CBRM1/20 binds does not bind ligand and, furthermore, cannot bind other integrin domains, such as those of the β-subunit.