Cellular activation of leukocyte function-associated antigen-1 and its affinity are regulated at the I domain allosteric site

The I domain of the integrin LFA-1 possesses a ligand binding interface tha t includes the metal ion-dependent adhesion site. Binding of the LFA-1 liga nd, ICAM-1 to the metal ion-dependent adhesion site is regulated by the I d omain allosteric site (IDAS). We demonstrate here that intracellular signal ing leading to activation of LFA-1 binding to ICAM-1 is regulated at the ID AS. Inhibitory mutations in or proximal to the IDAS are dominant to cytopla smic signals that activate binding to ICAM-1. In addition, mutational activ ation at the IDAS greatly increases the binding of lymphocyte-expressed LFA -1 to ICAM-1 in response to PMA, but does not result in constitutive bindin g. Binding of a novel CD18 activation epitope mAb to LFA-1 in response to s oluble ICAM-1 binding was also blocked by inhibitory and was enhanced by ac tivating IDAS mutations. Surface plasmon resonance using soluble wild-type LFA-1 and an IDAS mutant of LFA-1 indicate that the IDAS can regulate a 6-f old change in the K-d of ICAM-1 binding. The K-d of wild-type LFA-1 (1.2 x 10(-1) s(-1)) differed with that of the activating IDAS mutant (1.9 X 10(-2 ) s(-1)), but their K-a values were identical (2.2 x 10(5) M(-1)s(-1)). We propose that IDAS regulates the binding of LFA-1 to ICAM-1 activated by int racellular signals. IDAS can control the affinity state of LFA-1 with conco mitant I domain and CD18 conformational changes.