THE I DOMAIN OF INTEGRIN LEUKOCYTE FUNCTION-ASSOCIATED ANTIGEN-1 IS INVOLVED IN A CONFORMATIONAL CHANGE LEADING TO HIGH-AFFINITY BINDING TOLIGAND INTERCELLULAR-ADHESION MOLECULE-1 (ICAM-1)

On T cells the leukocyte integrin leukocyte function-associated antige n-1 (LFA-1) (CD11a/CD18) can be induced to bind its ligand intercellul ar adhesion molecule 1 (ICAM-1) (CD54) either by increasing the affini ty of the receptor with Mg2+ and EGTA or by receptor clustering follow ing 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 me show that soluble LF A-1 I domain blocks ICAM-1 binding of the high affinity Mg2+-induced f orm of LFA-1 but not the phorbol ester-induced form. Under conditions of Mg2+-activation, the soluble I domain also prevents expression of a n activation dependent epitope on LFA-1, implying that it inhibits a c onformational change necessary for conversion to the high affinity for m of this integrin. In addition, the binding of Mg2+-activated LFA-1 t o ICAM-1 is blocked by peptides covering the alpha 4-beta 3 loop, the beta 3-alpha 5 loop, and the alpha 5 helix of the I domain, whereas no ne of the peptides tested blocks phorbol ester-mediated adhesion. The blocking peptides localize to the same face of the crystal structure o f the LFA-1 I domain and define an area that, during activation, may b e involved in association of the I domain with another region of LFA-1 , potentially the beta-propeller domain. This is the first evidence Li nking a structural domain of an integrin, in this case the I domain, w ith a particular activation mechanism.