Folding and function of I domain-deleted Mac-1 and lymphocyte function-associated antigen-1

In those integrins that contain it, the I domain is a major ligand recognit ion site. The I domain is inserted between beta-sheets 2 and 3 of the predi cted beta-propeller domain of the integrin alpha subunit, We deleted the I domain from the integrin alpha(M) and alpha(L) subunits to give I-less Mac- 1 and lymphocyte function-associated antigen-1 (LFA-1), respectively. The I -less alpha(M) and alpha(L) subunits were expressed in association with the wild-type beta(2) subunit on the surface of transfected cells and bound to all the monoclonal antibodies mapped to the putative beta-propeller and C- terminal regions of the a, and a, subunits, suggesting that the folding of these domains is independent of the I domain. I-less Mac-1 bound to the lig ands iC3b and factor X, but this binding was reduced compared with wild-typ e Mac-1. In contrast, I-less Mac-1 did not bind to fibrinogen or denatured bovine serum albumin. Binding to iC3b and factor X by I-less Mac-1 was inhi bited by the function-blocking antibody CBRM1/32, which binds to the beta-p ropeller domain of the a, subunit, I-less LFA-1 did not bind its ligands in tercellular adhesion molecule-1 and -3. Thus, the I domain is not essential for the folding, heterodimer formation, and surface expression of Mac-1 an d LFA-1 and is required for binding to some ligands, but not others.