Interactions between the
complement degradation product
C3bi and leukocyte
integrin alpha(M)beta(2) are critical for host defense against foreign pathogens and in
tumor cell surveillance. To gain insight into the mechanism by which the alpha(M)I-domain of the
integrin interacts with
C3bi, detailed mapping of the
C3bi binding site was undertaken. Previous mutagenesis studies had implicated five small structural segments within the alpha(M)I-domain in recognition of this
ligand. Sets of three
amino acids within the five implicated segments were mutated to the corresponding alpha(L)I-domain residues. Then, within the affected mutants, single point mutations were introduced to precisely define the requisite residues. Ultimately, H148, F150, Q204, L205, R208, T211, T213, I256, P257 were identified as being critical for
C3bi binding. A synthetic
peptide approach confirmed the involvement of the specified residues with the complex midsegment, Q204-I215, in
C3bi recognition. Furthermore, the alpha(D)I-domain, which has a low intrinsic affinity for
C3bi, acquired high affinity for the
ligand when the implicated residues were inserted. The residues necessary to engage
C3bi reside on or adjacent to the
cation binding MIDAS site of the alpha(M)I-domain. The
amino acids involved in
C3bi binding are distinct from those involved in interaction of previously mapped
ligands with the alpha(M)I-domain. This divergence supports a mosaic model, in which different
ligands engage different
amino acids to bind to alpha(M)I-domain, accounting for the broad recognition capacity of
integrin alpha(M)beta(2).