Identification of Angiotensin II-binding Domains in the Rat AT2 Receptor with Photolabile Angiotensin Analogs

To identify binding domains between angiotensin II (AngII) and its type 2 receptor (AT2), two different radiolabeled photoreactive analogs were prepared by replacing either the first or the last amino acid in the peptide with p-benzoyl-L-phenylalanine (Bpa). Digestion of photolabeled receptors with kallikrein revealed that the two photoreactive analogs label the amino-terminal part of the receptor within the first 182 amino acids. Digestion of 125I-[Bpa1]AngII·AT2 receptor complex with endoproteinase Lys-C produced a glycoprotein of 80 kDa. Deglycosylation of this 80-kDa product decreased its apparent molecular mass to 4.6 kDa and further cleavage of this 4.6-kDa product with V8 protease decreased its molecular mass to 3.6 kDa, circumscribing the labeling site of 125I-[Bpa1]AngII within amino acids 3-30 of AT2 receptor. Treatment of 125I-[Bpa8]AngII·AT2 receptor complex with cyanogen bromide produced two major receptor fragments of 3.6 and 2.6 kDa. Cyanogen bromide hydrolysis of a mutant AT2 receptor produced two major fragments of 12.6 kDa and 2.6 kDa defining the labeling site of 125I-[Bpa8]AngII within residues 129-138 of AT2 receptor. Our results indicate that the amino-terminal tail of the AT2 receptor interacts with the amino-terminal end of AngII, whereas the inner half of the third transmembrane domain of AT2 receptor interacts with the carboxyl-terminal end of AngII.