Overlapping, but not identical, sites are involved in the recognition of C3bi, neutrophil inhibitory factor, and adhesive ligands by the α(M)β2 integrin

Abstract

The α(M)β2 (CD11b/CD18, Mac-1) integrin receptor binds numerous ligands, including neutrophil inhibitory factor (NIF), C3bi, and certain immobilized protein substrates, represented by denatured ovalbumin. These ligands share no obvious structural similarities, yet their interactions with receptor are inhibited by NIF and involve the I domain, a stretch of ~200 amino acids in the α(M) subunit. Recombinant wild-type and mutant forms of α(M)β2 have been used to compare the recognition requirements of these ligands. The various constructs were expressed efficiently on the surface of human embryonic kidney 293 cells and formed αβ heterodimeric complexes. The wild-type transfectants bound the three ligands in a similar fashion to naturally occurring α(M)β2. NIF inhibited these interactions, and deletion of the D248PLGY from within the I domain abolished binding of all three ligands, suggesting an overlapping recognition specificity. A single point mutation of Ser138 to Ala in the β2 subunit abolished C3bi binding and cell adhesion but did not affect NIF binding. A switch of the R281QELNTI sequence in helix 6 of the α(M) I domain to the corresponding sequence in the I domain of the α(L) (QETLHKF) subunit completely abrogated adhesion while not affecting C3bi and NIF binding. The two mutant receptors also did not support activation-dependent adhesion to fibrinogen. Thus, the contact sites for NIF, C3bi, and adhesive proteins, represented by denatured ovalbumin and fibrinogen, in α(M)β2 are overlapping but not identical.