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 13C and 15N NMR. Binding of protein G domain II selectively perturbed resonances from residues between the CH2 and CH3 domains of Fc, whereas in domain II the residues affected are primarily those on the α-helix and the third strand of the β-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 α-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 differs from the latter by 180°. Conclusions: The interactions of the bacterial antibody-binding proteins with their 'target' immunoglobulins involve a very versatile set of protein-protein interactions. First, the IgG-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 different regions of its surface to bind to the Fab and Fc regions of IgG.