Structure and sequence data.

(A): Computer graphic representation of a part of the complex between a single staphylococcal protein A domain (domain B, closely related to the Z domain used in the present study) (yellow) and human IgG1 Fc (brown) (PDB file: 1FC2.pdb). The amino acid side chains corresponding to the seven positions addressed for substitutions in the Z domain are highlighted in cyan. Highlighted in purple, red and green, respectively, are three IgG1 Fc subregions in close contact with the B domain. (B): Alignment of amino acid sequences of Fc regions of human IgG1, mouse IgG1, mouse IgG2a and mouse IgG2b, respectively, corresponding to the three contact areas shown in (A), using the same colour code. (C): Amino acid sequence of the 58-residue Z domain, with the seven positions included in the engineering boxed. Indicated with red dots are the ten positions at which unique cysteine residues were introduced for site-specific labeling with a photoactivable maleimide benzophenone (MBP) group.