1.67-.ANG. X-ray structure of the B2 immunoglobulin-binding domain of streptococcal protein G and comparison to the NMR structure of the B1 domain

The structure of the B2 immunoglobulin-binding domain of streptococcal protein G has been determined at 1.67-A resolution using a combination of single isomorphous replacement (SIR) phasing and manual fitting of the coordinates of the NMR structure of B 1 domain of streptococcal protein G (Gronenborn, A. M., et al. (1991) Science 253, 657-6611, The final R value was 0.191 for data between 8.0 and 1.67 A. The structure described here has 13 residues preceding the 57-residue Ig-binding domain and 13 additional residues following it, for a total of 83 residues. The 57-residue binding domain is well-determined in the structure, having an average B factor of 18.0. Only residues 8-77 could be located in the electron density maps, with the ends of the structure fading into disorder. Like the B1 domain, the B2 domain consists of four 6-strands and a single helix lying diagonally across the s these globular domains are connected by extended linkers (Fahnestocket al., 1990). Protein G offersseveraladvantages over protein A for the binding of IgG immunoglobins. Protein G binds to all subclasses of human IgG, whereas protein A does not bind the IgG3 subclass (Reis et al., 1984). Protein G binds a number of animal IgGs to which protein A binds only weakly. Protein G has a number of useful applications, including the treatment of diseases and the detection and purification of IgG's. Protein G could be used in the removal of IgGs from plasma, using an extracorporeal column (Tenan et al., 1981). The detection of IgG's is important in screening for hybridoma clones, in quantitative studies of immune response in animals, and in detection of antigens by competition-binding