SOMATOSTATIN COMPETES WITH THE CENTRAL PORTION OF GASTRIN RELEASING PEPTIDE FOR BINDING TO A 120 KDA PROTEIN

[I-125]Gastrin releasing peptide (GRP) and [I-125]somatostatin were cross-linked with bis(sulfosuccinimidyl)suberate to a 120 kDa membrane protein in Triton extracts of different cell types. Labeling of the 120 kDa protein was blocked (IC50 approximately 10(-8) M) by unlabeled GRP, GRP(14-27) and somatostatin but not by 100 nM bombesin, [Tyr4]bombesin, AcGRP(20-27), three biologically active somatostatin analogs or peptides unrelated to GRP or somatostatin. Binding involves a tripeptide sequence consisting of aryl, nonpolar and positively charged moieties in residues 7-9 (Phe-Trp-Lys) of the receptor binding site of somatostatin and residues 15-17 (Tyr-Pro-Arg) of GRP, a region distinct from its biologically active carboxyl-terminus.