The structure of the C5a receptor-blocking domain of chemotaxis inhibitory protein of Staphylococcus aureus is related to a group of immune evasive molecules

The chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is a 121 residue excreted virulence factor. It acts by binding the C5a- (C5aR) and formylated peptide receptor (FPR) and thereby blocks specific phagocyte responses. Here, we report the solution structure of a CHIPS fragment consisting of residues 31-121 (CHIPS31-121). CHIPS31-121 has the same activity in blocking the C5aR compared to full-length CHIPS, but completely lacks FPR antagonism. CHIPS31-121 has a compact fold comprising an alpha-helix (residues 38-51) packed onto a four-stranded anti-parallel beta-sheet. Strands beta2 and beta3 are joined by a long loop with a relatively well-defined conformation. Comparison of CHIPS31-121 with known structures reveals striking homology with the C-terminal domain of staphylococcal superantigen-like proteins (SSLs) 5 and 7, and the staphyloccocal and streptococcal superantigens TSST-1 and SPE-C. Also, the recently reported structures of several domains of the staphylococcal extracellullar adherence protein (EAP) show a high degree of structural similarity with CHIPS. Most of the conserved residues in CHIPS and its structural homologues are present in the alpha-helix. A conserved arginine residue (R46 in CHIPS) appears to be involved in preservation of the structure. Site-directed mutagenesis of all positively charged residues in CHIPS31-121 reveals a major involvement of arginine 44 and lysine 95 in C5aR antagonism. The structure of CHIPS31-121 will be vital in the further unraveling of its precise mechanism of action. Its structural homology to S.aureus SSLs, superantigens, and EAP might help the design of future experiments towards an understanding of the relationship between structure and function of these proteins.