Glycosylation affects agonist binding and signal transduction of the rat somatostatin receptor subtype 3

The somatostatin receptor subtypes, sst1-sst5, bind their natural ligands, somatostatin-14, somatostatin-28 and cortistatin-17, with high affinity but do not much discriminate between them. Detailed understanding of the interactions between these receptors and their peptide ligands may facilitate the development of selective compounds which are needed to identify the biological functions of individual receptor subtypes. The influence of the amino-terminal domain and of the two putative N-linked glycosylation sites located in this region of rat sst3 was analysed. Biochemical studies in transfected cell lines suggested that the amino-terminus of sst3 is glycosylated at both sites. Mutation of the N-linked glycosylation site, Asn18Thr, had only a small effect on binding properties and inhibition of adenylyl cyclase. The double mutant Asn18Thr/Asn31Thr lacking both glycosylation sites showed a significant reduction in high affinity binding and inhibition of adenylyl cyclase while peptide selectivity was not affected. Truncation of the amino-terminal region by 32 amino acid residues including the two glycosylation sites caused similar but much stronger effects. Immunocytochemical analysis of receptor localisation revealed that the amino-terminal domain but not the carbohydrates appear to be involved in the transport of the receptor polypeptide to the cell surface.