title: Solution structure of the E-domain of staphylococcal protein A.
abstract: The E-domain of staphylococcal protein A is one of five homologous IgG-binding domains designated E, D, A, B, and C that comprise the extracellular portion of protein A. The E-domain binds tightly to Fc fragments of IgG and binds certain Fv fragments with micromolar affinity. To explore further the structural features of Fc binding by protein A, and as a first step in developing a structural understanding of E-domain/Fv complex formation, we have determined the solution structure of the uncomplexed E-domain using 2D homonuclear and heteronuclear NMR spectroscopy. Complete 1H and 15N resonance assignments were obtained, and the structure was determined from 383 NOE-derived distance restrains, 34 phi and 19 chi 1 dihedral angle restraints, and 54 restraints for 27 H-bonds. 3JH alpha-H beta coupling constants and long-range NOEs involving Phe11 indicate the side chain exists in more than one conformation with differing chi 1 values. NOE restraints that were incompatible with chi 1 = -60 degrees were removed from one set of structure calculations, and those incompatible with chi 1 = 180 degrees were removed from a second set to allow Phe11 to explore both rotamer wells. Thus, two sets of 20 final structures, having no distance or dihedral angle restraint violations greater than 0.12 A or 1.6 degrees, respectively, represent the solution structure of the E-domain. Backbone atomic rms differences with respect to the mean coordinates for each set of 20 structures for residues 8-53 averaged 0.41 +/- 0.06 and 0.35 +/- 0.06 A. No significant differences in the overall structure result from the different orientations of Phe11. The solution structure of the E-domain consists of three alpha-helices that pack together to form a compact helical bundle. A detailed comparison between the E-domain ensembles and the previously determined structure for the B-domain in complex with Fc indicates that only the 180 degrees chi 1 rotamer of Phe11 is competent for binding; the -60 degrees chi 1 rotamer must reorient to 180 degrees to create a cavity that is filled by Ile253 from the CH2 domain of Fc in the Fc-bound complex.
authors.affiliation: Department of Protein Engineering, Genentech, Inc., South San Francisco, California 94080, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Starovasnik
authors.forename: N J
authors.initials: NJ
authors.lastname: Skelton
authors.forename: M P
authors.initials: MP
authors.lastname: O'Connell
authors.forename: R F
authors.initials: RF
authors.lastname: Kelley
authors.forename: D
authors.initials: D
authors.lastname: Reilly
authors.forename: W J
authors.initials: WJ
authors.lastname: Fairbrother
mesh_terms: D000595:Amino Acid Sequence; D007141:Immunoglobulin Fc Fragments / Q000378:metabolism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D012984:Software; D013205:Staphylococcal Protein A / Q000737:chemistry* / Q000378:metabolism
chemical_list: D007141:Immunoglobulin Fc Fragments; D013205:Staphylococcal Protein A