MOLECULAR-CLONING OF 2 ISOFORMS OF THE GUINEA-PIG C3A ANAPHYLATOXIN RECEPTOR - ALTERNATIVE SPLICING IN THE LARGE EXTRACELLULAR LOOP

The anaphylatoxin C3a is released from C3 during complement activation , C3a is a potent spasmogen and has recently been described as an eosi nophil and mast cell chemotactic factor that mediates a number of infl ammatory reactions. Previously, we demonstrated the presence of a spec ific C3a receptor (C3aR) on guinea pig platelets, We report here the i solation of cDNA-clones encoding for two isoforms of guinea pig C3aR ( gpC3aR), Hydropathy analysis of the deduced amino acid sequence of bot h gpC3aR clones indicated seven transmembrane domains with a large ext racellular (EC) loop between the fourth and fifth transmembrane domain s, which is a known characteristic of the human C3aR, Northern blot an alysis revealed that the gpC3aR was abundantly expressed on macrophage s and in the spleen. A comparison of the deduced amino acid sequence o f the larger gpC3aR (gpC3aR-L) with the recently cloned human C3aR ind icated a 59.5% identity. The deduced amino acid sequence of the second , smaller cDNA clone was identical with gpC3aR-L, except that it lacke d 35 amino acids in the large EC loop. Our evidence indicates that alt ernative splicing occurred in the large EC loop that accounts for thes e two isoforms, L cells separately expressing one of these two isoform s of the gpC3aR showed similar high-affinity C3a binding. An RT-PCR an alysis documented that both forms of the C3aR were expressed in a vari ety of guinea pig tissues, The cloning and expression of these two nat ural forms of gpC3aR cDNA indicated that the deletion of the 35-residu e portion of the large EC loop of gpC3aR-L did not alter C3a binding.