RP S19 C-terminal peptide trimer acts as a C5a receptor antagonist

We have demonstrated that ribosomal protein S19 (RP S19) polymer, when crosslinked between Lys122 and Gln137 by activated coagulation factor XIII, acts as a C5a receptor (C5aR) antagonist/agonist. Based on experimental data obtained using RP S19 analog peptide and recombinant protein monomer, we suggested that L131DR, I134AGQVAAAN and K143KH moieties in the RP S19 C-terminus act in, respectively, C5aR binding, penetration of the plasma membrane, and interaction with either an apoptosis-inducing molecule in neutrophils (delta lactoferrin) or a calcium channel-activating molecule (annexin A3) to induce the p38 MAPK pathway in macrophages. Recently, we observed RP S19 trimer in serum. To study the effects of this RP S19 trimer on C5aR, we prepared mutant RP S19 C-terminal peptide (RP S19122-145) dimer and trimer, and examined their chemotactic activities and signal transduction pathways in human C5aR-overexpressing squamous cell carcinoma HSC-1 (HSC-1C5aR) cells using 24 trans-well chamber and western blotting assays, respectively. HSC-1C5aR cells were attracted by RP S19122-145 dimer and vice versa by RP S19122-145 trimer. The RP S19122-145 dimer-induced attraction was competitively blocked by pre-treatment with RP S19122-145 trimer. Moreover, RP S19122-145 trimer-induced p38 MAPK phosphorylation was stronger than RP S19122-145 dimer-induced p38 MAPK phosphorylation. RP S19122-145 trimer appeared to act as a C5aR antagonist. The agonistic and antagonistic effects of RP S19122-145 dimers and trimers were reflected by monocytic, THP-1-derived macrophage-like cells. Unlike the C5aR agonist C5a, which acts at the inflammation phase of acute inflammation, RP S19 trimer might act as a C5aR antagonist at the resolution phase.