Structure of the bradykinin B-2 receptors' amino terminus

The peptide hormone bradykinin exerts important biological functions by binding to and activating bradykinin B-2 receptors. B-2 receptors belong to the seven transmembrane domain (7TM) receptor family. Cloning of the cDNA sequences for the rat, human, and mouse bradykinin B-2 receptor revealed several in-frame AUG triplets as potential initiation sites for translation. Due to ''Kozak-like'' consensus nucleotides, the AUG codon closest to transmembrane domain 1 was assumed the preferred initiation site for translation, but the real amino terminus of the B-2 receptor protein was unknown. The amino terminus of several 7TM receptors has been shown to be essentially involved in receptor activation and/or ligand binding. Therefore we determined the amino terminus of the human and of the rat B-2 receptor using domain-specific antipeptide antibodies, amino acid sequence analysis, and in vitro transcription/translation. We report that the human and rat B-2 receptor protein start with the methionine which is translated from the first in-frame AUG. This start site extends the known amino terminus of the human and rat B-2 receptors by 27 or 30 amino acid residues, respectively. Antibodies raised against a peptide of the initial 27 amino acids of the human B-2 receptor stained B-2 receptors on intact cells. This finding excludes the existence of a signal sequence for this receptor.