The I domain of integrin leukocyte function-associated antigen-1 is involved in a conformational change leading to high affinity binding to ligand intercellular adhesion molecule 1 (ICAM-1).
On T cells the leukocyte integrin leukocyte function-associated antigen-1 (LFA-1) (CD11a/CD18) can be induced to bind its ligand intercellular adhesion molecule 1 (ICAM-1) (CD54) either by increasing the affinity of the receptor with Mg2+ and EGTA or by receptor clustering following activation with phorbol ester. The existence of these two adhesion-inducing pathways implies that alternative mechanisms might exist by which LFA-1 engages ICAM-1. The LFA-1 alpha subunit I domain contains a major binding site for ICAM-1. In this study we show that soluble LFA-1 I domain blocks ICAM-1 binding of the high affinity Mg2+-induced form of LFA-1 but not the phorbol ester-induced form. Under conditions of Mg2+-activation, the soluble I domain also prevents expression of an activation dependent epitope on LFA-1, implying that it inhibits a conformational change necessary for conversion to the high affinity form of this integrin. In addition, the binding of Mg2+-activated LFA-1 to ICAM-1 is blocked by peptides covering the alpha4-beta3 loop, the beta3-alpha5 loop, and the alpha5 helix of the I domain, whereas none of the peptides tested blocks phorbol ester-mediated adhesion. The blocking peptides localize to the same face of the crystal structure of the LFA-1 I domain and define an area that, during activation, may be involved in association of the I domain with another region of LFA-1, potentially the beta-propeller domain. This is the first evidence linking a structural domain of an integrin, in this case the I domain, with a particular activation mechanism.
McDowall
A
A
Leukocyte Adhesion Laboratory, Imperial Cancer Research Fund, London WC2A 3PX, United Kingdom.
Leitinger
B
B
Stanley
P
P
Bates
P A
PA
Randi
A M
AM
Hogg
N
N
D000595:Amino Acid Sequence; D001665:Binding Sites; D001667:Binding, Competitive; D002448:Cell Adhesion / Q000187:drug effects; D000939:Epitopes; D018799:Intercellular Adhesion Molecule-1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008024:Ligands; D016169:Lymphocyte Function-Associated Antigen-1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008274:Magnesium / Q000494:pharmacology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000378:metabolism; D013601:T-Lymphocytes / Q000502:physiology*
D000939:Epitopes; D008024:Ligands; D016169:Lymphocyte Function-Associated Antigen-1; D010446:Peptide Fragments; D011994:Recombinant Proteins; D018799:Intercellular Adhesion Molecule-1; D015240:Phorbol 12,13-Dibutyrate; D008274:Magnesium