IDENTIFICATION OF A 63-KDA SERUM-PROTEIN THAT BINDS SOMATOSTATIN AND GASTRIN-RELEASING PEPTIDE BUT NOT BOMBESIN

A 63-kDa serum protein was identified that bound somatostatin and gast rin-releasing peptide (GRP) but not bombesin. The 63-kDa protein was d etected by its ability to compete with the receptor for GRP in a recep tor binding assay. This interaction could be inhibited by the addition of somatostatin, producing a higher and more accurate calculated affi nity for the binding of GRP to its receptor. Somatostatin did not affe ct the affinity of the receptor for bombesin. Specificity of the 63-kD a protein for analogs of somatostatin, GRP, and bombesin was determine d by competition with I-125-GRP and I-125-somatostatin. A tripeptide m otif consisting of an aryl-hydrophobic-basic amino-acid structure in r esidues 15-17 of GRP (Tyr-Pro-Arg) and residues 7-9 of somatostatin (P he-Trp-Lys) was implicated in binding. This tripeptide binding motif i s not present in bombesin. That residues 15-17 of GRP are highly conse rved suggests that its interaction with the 63 kDa serum protein may b e physiological.