title: 1.67-A X-ray structure of the B2 immunoglobulin-binding domain of streptococcal protein G and comparison to the NMR structure of the B1 domain.
abstract: 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 B1 domain of streptococcal protein G [Gronenborn, A. M., et al. (1991) Science 253, 657-661]. 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 beta-strands and a single helix lying diagonally across the beta-sheet, with a -1, +3 chi, -1 topology. This small structure is extensively hydrogen-bonded and has a relatively large hydrophobic core. These structural observations may account for the exceptional stability of protein G. A comparison of the B2 domain X-ray structure and the B1 domain NMR structure showed minor differences in the turn between strands and two and a slight displacement of the helix relative to the sheet. Hydrogen bonds between crystallographically related molecules account for most of these differences.
authors.affiliation: Protein Engineering Department, Enzon, Incorporated, Gaithersburg, Maryland 20877.
authors.forename: A
authors.initials: A
authors.lastname: Achari
authors.forename: S P
authors.initials: SP
authors.lastname: Hale
authors.forename: A J
authors.initials: AJ
authors.lastname: Howard
authors.forename: G M
authors.initials: GM
authors.lastname: Clore
authors.forename: A M
authors.initials: AM
authors.lastname: Gronenborn
authors.forename: K D
authors.initials: KD
authors.lastname: Hardman
authors.forename: M
authors.initials: M
authors.lastname: Whitlow
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001666:Binding Sites, Antibody; D006860:Hydrogen Bonding; D007074:Immunoglobulin G / Q000737:chemistry / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D013291:Streptococcus / Q000378:metabolism*; D014961:X-Ray Diffraction
chemical_list: D001426:Bacterial Proteins; C034284:IgG Fc-binding protein, Streptococcus; D007074:Immunoglobulin G