Europium-Labeled Synthetic C3a Protein as a Novel Fluorescent Probe for Human Complement C3a Receptor

Measuring ligand affinity for a G protein-coupled receptor is often a crucial step in drug discovery. It has been traditionally determined by binding putative new ligands in competition with native ligand labeled with a radioisotope of finite lifetime. Competing instead with a lanthanide-based fluorescent ligand is more attractive due to greater longevity, stability, and safety. Here, we have chemically synthesized the 77 residue human C3a protein and conjugated its N-terminus to europium diethylenetriaminepentaacetate to produce a novel fluorescent protein (Eu-DTPA-hC3a). Time-resolved fluorescence analysis has demonstrated that Eu-DTPA-hC3a binds selectively to its cognate G protein-coupled receptor C3aR with full agonist activity and similar potency and selectivity as native C3a in inducing calcium mobilization and phosphorylation of extracellular signal-regulated kinases in HEK293 cells that stably expressed C3aR. Time-resolved fluorescence analysis for saturation and competitive binding gave a dissociation constant (K_d) of 8.7 ± 1.4 nM for Eu-DTPA-hC3a and binding affinities for hC3a (pK_i of 8.6 ± 0.2 and K_i of 2.5 nM) and C3aR ligands TR16 (pK_i of 6.8 ± 0.1 and K_i of 138 nM), BR103 (pK_i of 6.7 ± 0.1 and K_i of 185 nM), BR111 (pK_i of 6.3 ± 0.2 and K_i of 544 nM) and SB290157 (pK_i of 6.3 ± 0.1 and K_i of 517 nM) via displacement of Eu-DTPA-hC3a from hC3aR. The macromolecular conjugate Eu-DTPA-hC3a is a novel nonradioactive probe suitable for studying ligand-C3aR interactions with potential value in accelerating drug development for human C3aR in physiology and disease.