Experimental support for a beta-propeller domain in integrin alpha-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 approximately 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 (alphaM beta2, 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.
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Department of Pathology, The Center for Blood Research and Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.
Springer
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D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D018604:Epitope Mapping; D006801:Humans; D016023:Integrins / Q000378:metabolism / Q000648:ultrastructure*; D016177:Macrophage-1 Antigen / Q000378:metabolism / Q000648:ultrastructure*; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009504:Neutrophils / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D016415:Sequence Alignment
D000911:Antibodies, Monoclonal; D016023:Integrins; D016177:Macrophage-1 Antigen; D002118:Calcium