Solution structure of an immunoactive peptide fragment of Staphylococcal protein-A

Staphylococcal protein-A (SpA) is known to bind the Fc fragment of immunogl obin G in vitro and induce a myriad of immunogenic responses in vivo. The l atter is ascribed to be due to the interaction of Fc and SpA. It has also b een proposed that in vivo proteolytically cleaved fragments of SpA may be f unctioning in the same manner. One such fragment (EQQNAFYEILHLPNLNEEQR), fr agment 8-27 of the B-domain (SpA-B), was recently shown to exhibit in vivo immunogenic response [Sinha, P., Sengupta, J., and Ray, P. K.. (1999) Bioch em. Biophys. Res. Commun. 258, 141-147]. As a first step towards understand ing the mode of interaction of this peptide with the Fc fragment, we have s tudied the solution conformation of this isolated peptide by CD and NMR. Th e peptide, with 7 contact residues in the crystal structure of the SpA-B/Fc complex and comprising of mostly helixI and part of helixII of the 3-helix bundle of SpA-B, was found to be present predominantly in extended structu re. However it showed nascent turn/helix like conformations around F14 & Y1 5. These two residues are known to play a vital role in SpA-B/Fc interactio n as deciphered from crystal structure and NMR studies of SpA-B/Fc complex and mutational studies. The implications of our results, especially the nas cent conformations found around F14 & Y15, in design of SpA-B mimetic small molecules are discussed.