title: Differential ligand binding specificities of recombinant CD11b/CD18 integrin I-domain.
abstract: The alpha subunits of leukocyte CD11/CD18 integrins contain an approximately 200-amino acid "inserted" domain (I-domain) that may be important for multivalent adhesive recognitions. A recombinant form of the I-domain of CD11b/CD18 was generated and analyzed directly for interaction with complementary integrin ligands. CD11b I-domain bound the activation-dependent monoclonal antibody 7E3, and the functionally blocking anti-CD11b monoclonal antibodies OKM9, 60.1, and LM2/1, but not OKM1 or M1/70. Fibrinogen or soluble intercellular adhesion molecule-1 associated with CD11b I-domain in a concentration-dependent manner. Binding of 125I-fibrinogen to recombinant CD11b I-domain was saturable, governed by a Kd of approximately 0.22 +/- 0.06 microM, and fully inhibited by molar excess of unlabeled fibrinogen, or by the P1 peptide (KY)GWTVFQKRLDGSV (IC50 approximately 2.5-5 microM), duplicating the fibrinogen gamma chain sequence Gly190-Val202. In contrast, 125I-factor X binding to CD11b I-domain was only partially inhibited (50-60%) by a molar excess of unlabeled factor X, and entirely unaffected by functionally blocking anti-CD11b monoclonal antibodies or by factor X-derived synthetic peptidyl antagonists. We conclude that the I-domain of CD11b participates in qualitative mechanisms of receptor activation and contains the binding site(s) for the CD11b/CD18 ligands fibrinogen and intercellular adhesion molecule-1, while it is only minimally implicated in the recognition of factor X.
authors.affiliation: R. W. Johnson Pharmaceutical Research Institute, Don Mills, Ontario, Canada.
authors.forename: L
authors.initials: L
authors.lastname: Zhou
authors.forename: D H
authors.initials: DH
authors.lastname: Lee
authors.forename: J
authors.initials: J
authors.lastname: Plescia
authors.forename: C Y
authors.initials: CY
authors.lastname: Lau
authors.forename: D C
authors.initials: DC
authors.lastname: Altieri
mesh_terms: D000595:Amino Acid Sequence; D015703:Antigens, CD / Q000737:chemistry* / Q000378:metabolism; D018821:CD18 Antigens; D015815:Cell Adhesion Molecules / Q000378:metabolism; D000939:Epitopes; D005170:Factor X / Q000737:chemistry / Q000378:metabolism; D005340:Fibrinogen / Q000378:metabolism; D006801:Humans; D018799:Intercellular Adhesion Molecule-1; D008024:Ligands; D016177:Macrophage-1 Antigen / Q000737:chemistry* / Q000378:metabolism; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D011994:Recombinant Proteins
chemical_list: D015703:Antigens, CD; D018821:CD18 Antigens; D015815:Cell Adhesion Molecules; D000939:Epitopes; D008024:Ligands; D016177:Macrophage-1 Antigen; D010455:Peptides; D011994:Recombinant Proteins; D018799:Intercellular Adhesion Molecule-1; D005170:Factor X; D005340:Fibrinogen