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 receptor gene. The 1.97-kb cDNA encodes a novel G protein-coupled receptor (GPCR) with 482 amino acids. In addition to the predicted 7 transmembrane domains common to all GPCRs, the protein encoded by AZ3B contains a large extracellular loop of approximately 172 amino acids between the fourth and the fifth transmembrane domains, a feature unique among the 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 message and protein are widely expressed in several differentiated hematopoietic cell lines, in the lung, placenta, heart, and endothelial cells. We postulate that the AZ3B protein defines a distinct group of receptors within the GPCR superfamily.