CDNA CLONING OF A NOVEL G-PROTEIN-COUPLED RECEPTOR WITH A LARGE EXTRACELLULAR LOOP STRUCTURE

A cDNA designated as AZ3B has been isolated from a differentiated HL-6 0 cell cDNA library with a probe derived from the N-formyl peptide rec eptor gene. The 1.97-kb cDNA encodes a novel G protein-coupled recepto r (GPCR) with 482 amino acids. In addition to the predicted 7 transmem brane domains common to all GPCRs, the protein encoded by AZ3B contain s a large extracellular loop of similar to 172 amino acids between the fourth and the fifth transmembrane domains, a feature unique among th e hundreds of GPCRs identified to date. High sequence homology exists between the AZ3B protein and a number of chemoattractant receptors in the amino-terminal 170 residues and the carboxyl-terminal 150 residues . Northern and flow cytometric analyses suggested that the AZ3B messag e and protein are widely expressed in several differentiated hematopoi etic cell lines, in the lung, placenta, heart, and endothelial cells. We postulate that the AZ3B protein defines a distinct group of recepto rs within the GPCR superfamily.