MAPPING THE INTERCELLULAR-ADHESION MOLECULE-1 AND MOLECULE-2 BINDING-SITE ON THE INSERTED DOMAIN OF LEUKOCYTE FUNCTION-ASSOCIATED ANTIGEN-1

By extensive mutagenic analysis of the inserted domain (I-domain) of t he alpha-chain (CD11a) of the leukocyte function-associated antigen-1 (LFA-1), we have defined a putative binding surface for intercellular adhesion molecules 1 and 2 (ICAM-1 and -2). This analysis showed that individually mutating Leu-205 or Glu-241 to alanine completely abolish ed LFA-1 binding to ICAM-1 or -2 without affecting I-domain structure, as assayed by antibody binding. Mutating Thr-243 to alanine also had a profound effect on LFA-1 binding to ICAM-1 and -2, as seen by comple te loss of binding to ICAM-1 and a significant reduction (70% decrease ) in binding to ICAM-2. Mutating Glu-146 to alanine reduced LFA-1 bind ing to ICAM-1 or -2 by 70%, and mutating His-264 or Glu-293 to alanine reduced binding to ICAM-1 or -2 by about 30-40%. Mutating Thr-175 to alanine reduced binding to ICAM-1 by about 30% and binding to ICAM-2 b y about 70%. Interestingly, mutating Lys-263 to alanine preferentially abolished LFA-1 binding to ICAM-2 Using these data, we have generated a model of the interface between the LFA-1 I-domain and residues in t he first domain of ICAM-1 that have been shown to be critical for this interaction. In addition, this model, together with the ICAM-2 crysta l structure, has been used to map residues that are likely to mediate LFA-1 I-domain binding to ICAM-2.