СЗа receptor (C3aR) is a G prolein-coupled receptor oi the rhodopsin superfamily. The receptor contains the characteristic seven transmembrane domains connected by intra- and extracellular loops, with the N-lerminus having an extracellular orientation and the C-terminus being intracellular and the region to which the G proteins bind. C3aR has a major distinguishing feature, which is an extraordinarily large extracellular loop between the fourth and fifth transmembrane helices. This loop in human C3aR contains 175 residues: in most G protein-coupled receptors the corresponding extracellular loop region is 30-40 residues in length.
The C3aR is widely distributed in both myeloid cells and nonmyeloid tissue cells. The C3aR on eosinophils and basophils is thought to be particularly important biologically since these cells are che-motactically activated by the ligand C3a. Neutrophils also express C3aR. but the ligand can only induce calcium mobilization in these cells and does not cause granular release, oxygen radical generation, or chemotaxis. C3aR expression has been shown to be upregulated on astrocytes and other brain cells during inflammation. Only one form of C3aR has been detected in human, rat. and mouse, but two isoforms of C3aR have been described in the guinea pig (C3aR-S and C3aR-L for small and large forms).