MODEL FOR THE COMPLEX BETWEEN PROTEIN-G AND AN ANTIBODY FC FRAGMENT IN SOLUTION

Background: Streptococcal protein G and staphylococcal protein A are bacterial antibody-binding proteins, widely used as immunological tools, whose antibody-binding domains are structurally quite different. The binding of protein G to Fc fragments is competitive with respect to protein A, suggesting that the binding sites for protein A and protein G on Fc overlap, notwithstanding the Fact that they lack sequence or structural similarity. Results: To resolve this issue, the residues involved in the interaction between an IgG-binding domain of protein G (domain II) and the Fc fragment of mouse IgG2a have been identified by use of C-13 and N-15 NMR. Binding of protein G domain II selectively perturbed resonances from residues between the C(H)2 and C(H)3 domains of Fc, whereas in domain II the residues affected are primarily those on the alpha-helix and the third strand of the beta-sheet. This information was used, together with the structures of the two uncomplexed proteins, to construct a model of the complex, using Monte Carlo minimization techniques. In this model, the alpha-helix of protein G lies in the same position as helix 1 of protein A in the crystal structure of the protein A:Fc complex, but its orientation difers from the latter by 180 degrees. Conclusions: The interactions of the bacterial antibody-binding proteins with their 'target' immunoglobulins involve a very versatile set of protein-protein interactions. First, the IgC-binding domains of protein A and protein G have quite different three-dimensional structures, but bind' to sites on the Fc Fragment that overlap extensively. Secondly, protein G employs two quite difierent regions of its surface to bind to the Fab and Fc regions of IgG.