Model for the complex between protein G and an antibody Fc fragment in solution

• Published in: Structure (London) Vol. 3; no. 1; pp. 79 - 85
• Main Authors: Kato, Koichi, Lian, Lu-Yun, Barsukov, Igor L, Derrick, Jeremy P, Kim, HaHyung, Tanaka, Runa, Yoshino, Atsuko, Shiraishi, Miki, Shimada, Ichio, Arata, Yoji, Roberts, Gordon CK
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 1995
• Subjects: antibody; Antigens, Bacterial - chemistry; Bacterial Proteins - chemistry; Bacterial Proteins - metabolism; Binding Sites; Immunoglobulin Fc Fragments - chemistry; Immunoglobulin Fc Fragments - metabolism; Magnetic Resonance Spectroscopy; Models, Molecular; Monte Carlo Method; NMR spectroscopy; protein A; protein G; Protein Structure, Secondary; protein–protein interactions; Solutions; Staphylococcal Protein A - chemistry; Staphylococcus; Streptococcus; protein–protein interactions; protein A; antibody; NMR spectroscopy; protein G
• ISSN: 0969-2126; 1878-4186
• DOI: 10.1016/S0969-2126(01)00136-8

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
13 C and
15 N 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
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°. .
Conclusion 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.