Structure-function of the putative I-domain within the integrin β2 subunit

The central region (residues 125-385) of the integrin beta (2) subunit is postulated to adopt an I-domain-like fold (the beta I-2-domain) and to play a critical role in ligand binding and heterodimer formation. To understand structure-function relationships of this region of beta (2), a homolog-scanning mutagenesis approach, which entails substitution of nonconserved hydrophilic sequences within the beta I-2-domain with their homologous counterparts of the beta I-1-domain, has been deployed. This approach is based on the premise that beta (1) and beta (2) are highly homologous, yet recognize different ligands. Altogether, 16 segments were switched to cover the predicted outer surface of the beta I-2-domain. When these mutant beta (2) subunits were transfected together with wild-type alpha (M) in human 293 cells, all 16 beta (2) mutants were expressed on the cell surface as heterodimers, suggesting that these 16 sequences within the beta I-2-domain are not critically involved in heterodimer formation between the alpha (M), and beta (2) subunits. Using these mutant alpha (M)beta (2) receptors, we have mapped the epitopes of nine beta I-2-domain specific mAbs, and found that they all recognized at least two noncontiguous segments within this domain. The requisite spatial proximity among these non-linear sequences to form the mAb epitopes supports a model of an I-domain-like fold for this region. In addition, none of the mutations that abolish the epitopes of the nine function-blocking mAbs, including segment Pro(192)-Glu(197), destroyed ligand binding of the alpha (M)beta (2) receptor, suggesting that these function-blocking mAbs inhibit alpha (M)beta (2) function allosterically. Given the recent reports implicating the segment equivalent to Pro(192)-Glu(197) in ligand binding by beta (3) integrins, these data suggest that ligand binding by the beta (2) integrins occurs via a different mechanism than beta (3). Finally, both the conformation of the beta I-2-domain and C3bi binding activity of alpha (M)beta (2) were dependent on a high affinity Ca2+ binding site (K-d = 105 muM), which is most likely located within this region of beta (2).