NMR study of the interaction between the B domain of staphylococcal protein A and the Fc portion of immunoglobulin G

The solution structure of the B domain of staphylococcal protein A (FB) complexed with the Fc fragment of immunoglobulin G (IgG) is reported. A previous NMR analysis has shown that in solution FB is composed of a bundle of three or-helices, helix I, helix II, and helix III [Gouda, H., Torigoe, H., Saito, A., Sate, M., Arata, Y., and Shimada, I. (1992) Biochemistry 31: 9665-9672]. In contrast, the crystal structure of FB in the FB-Fc complex lacks helix III. Uniformly N-15-and N-15/C-13-labeled FB were prepared, and the backbone C-13 resonances were assigned. The spectral data obtained in the present study indicated that in solution all three helices including helix III are preserved in the FB-Fc complex. The mode of interaction of FB with the Fc fragment was discussed on the basis of the combined data of hydrogen-deuterium exchange experiments and H-1-N-15 correlation spectroscopy. It was concluded that a contiguous surface shaped by F14, Y15, E16, L18, and H19 in helix I, and N29, Q33, L35, and K36 in helix II is responsible for the binding.