Experimental Support for a β -propeller Domain in Integrin α -subunits and a Calcium Binding Site on Its Lower Surface
Abstract
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 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 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.
- Publication:
-
Proceedings of the National Academy of Science
- Pub Date:
- April 1998
- DOI:
- 10.1073/pnas.95.9.4870
- Bibcode:
- 1998PNAS...95.4870O