C5a is a potent chemoattractant for monocytes, neutrophils and other leukocytes. The receptor for human C5a is a member of the rhodopsin superfamily of G protein-coupled receptors and contains an aspartate residue (Asp82) within the putative second transmembrane domain conserved in all other G protein-linked receptors. We investigated the role of this residue and also the carboxy-terminal 23 residues of the C5a receptor in ligand binding and signal transduction by expressing wild-type and mutant receptors in the rat basophilic leukemia cell line RBL-2H3. Wild-type and truncated receptors coupled efficiently to effector systems, resulting in the C5a-dependent discharge of granule contents. In contrast RBL cells transfected with receptors in which Asp82 had been mutated to asparagine did not respond to human C5a by secretion despite binding human C5a with high affinity. We conclude therefore that Asp82 is not involved in the interaction with ligand but is essential for the proper transduction of the ligand binding signal.