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

Background: Streptococcal protein G and staphylococcal protein A are b acterial 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 p rotein A, suggesting that the binding sites for protein A and protein G on Fc overlap, notwithstanding the Fact that they lack sequence or s tructural similarity. Results: To resolve this issue, the residues inv olved in the interaction between an IgG-binding domain of protein G (d omain II) and the Fc fragment of mouse IgG2a have been identified by u se of C-13 and N-15 NMR. Binding of protein G domain II selectively pe rturbed resonances from residues between the C(H)2 and C(H)3 domains o f Fc, whereas in domain II the residues affected are primarily those o n the alpha-helix and the third strand of the beta-sheet. This informa tion was used, together with the structures of the two uncomplexed pro teins, to construct a model of the complex, using Monte Carlo minimiza tion techniques. In this model, the alpha-helix of protein G lies in t he same position as helix 1 of protein A in the crystal structure of t he 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 ve rsatile set of protein-protein interactions. First, the IgC-binding do mains of protein A and protein G have quite different three-dimensiona l structures, but bind' to sites on the Fc Fragment that overlap exten sively. Secondly, protein G employs two quite difierent regions of its surface to bind to the Fab and Fc regions of IgG.