Expression of a structural domain of the beta 2 subunit essential for alpha M beta 2 ligand recognition.

The beta2 leukocyte integrins comprise a group of closely related adhesion receptors that mediate critical events during normal and inflammatory immune responses. Central to the understanding of beta2 integrin function is the basis of ligand recognition. Results from our laboratory and others indicate the presence of multiple ligand contact points in both the alpha and beta subunit. As an approach to identify and characterize regulatory domains of the beta2 subunit, we have generated two different subdomains of the beta2 subunit for expression on the surface of mammalian cells through a phosphatidyl-inositol glycan anchor. The first subdomain contains the putative beta2 MIDAS motif implicated in ligand binding [beta2(LB)], whereas the second beta2 subdomain contains the cysteine-rich region [beta2(CR)]. Cells expressing alphaM and beta2 constructs singly or cotransfected transiently in COS-7 cells were tested for the ability to bind to immobilized iC3b. Cells bearing the recombinant alphaMbeta2(LB) were capable of adhering to iC3b in a manner similar to that observed with the complete alphaMbeta2 heterodimer. In contrast, cells expressing alphaMbeta2(CR) failed to adhere to immobilized iC3b. Moreover, cells bearing singly transfected alpha or beta chains alone failed to adhere to immobilized iC3b. These results indicate that along with alphaM, the beta2(LB) subdomain contains the sufficient components within the beta2 subunit essential for ligand recognition. These findings support the hypothesis that the beta2 subunit cooperates with site(s) within the alphaM subunit in a receptor/cation/ligand complex resulting in high-affinity ligand interaction.