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Chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is excreted by the majority of S. aureus strains and is a potent inhibitor of C5a- and formylated peptide-mediated chemotaxis of neutrophils and monocytes. Recently, we reported that CHIPS binds to the C5a receptor (C5aR) and the formylated peptide receptor, thereby blocking activation by C5a and formylated peptides, respectively. The anaphylatoxin C5a plays an important role in host immunity and pathological inflammatory processes. For C5a a two-site binding model is proposed in which C5a initially binds the C5aR N terminus, followed by interaction of the C5a C-terminal tail with an effector domain on the receptor. We have shown here that CHIPS does not affect activation of the C5aR by a peptide mimic of the C5a C terminus. Moreover, CHIPS was found to bind human embryonic kidney 293 cells expressing only the C5aR N terminus. Deletion and mutation experiments within this C5aR N-terminal expression system revealed that the binding site of CHIPS is contained in a short stretch of 9 amino acids (amino acids 10-18), of which the aspartic acid residues at positions 10, 15, and 18 plus the glycine at position 12 are crucial. Binding studies with C5aR/C3aR and C5aR/IL8RA chimeras confirmed that CHIPS binds only to the C5aR N terminus without involvement of its extracellular loops. CHIPS may provide new strategies to block the C5aR, which may lead to the development of new C5aR antagonists. [ABSTRACT FROM AUTHOR]