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 alpha-helices, helix I, helix II, and helix III [Gouda, H., Torigoe, H., Saito, A., Sato, 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 15N- and 15N/13C-labeled FB were prepared, and the backbone 13C 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 1H-15N 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.