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

The somatostatin receptor subtypes, sst1-sst5, bind their natural ligands, somatostalin-14, somatostatin-28 and cortistatin-17, with high affinity but do not much discriminate between them. Detailed understanding of the inter actions between these receptors and their peptide ligands may facilitate th e development of selective compounds which are needed to identify the biolo gical functions of individual receptor subtypes. The influence of the amino -terminal domain and of the two putative N-linked glycosylation sites locat ed in this region of rat sst3 was analysed. Biochemical studies in transfec ted cell lines suggested that the amino-terminus of sst3 is glycosylated at both sites. Mutation of the N-linked glycosylation site, Asn(18)Thr, had o nly a small effect on binding properties and inhibition of adenylyl cyclase . The double mutant Asn(18)Thr/Asn(31)Thr lacking both glycosylation sites showed a significant reduction in high affinity binding and inhibition of a denylyl cyclase while peptide selectivity was not affected. Truncation of t he amino-terminal region by 32 amino acid residues including the two glycos ylation sites caused similar but much stronger effects. Immunocytochemical analysis of receptor localisation revealed that the amino-terminal domain b ut not the carbohydrates appear to be involved in the transport of the rece ptor polypeptide to the cell surface. (C) 2000 Elsevier Science Ltd. Publis hed by Editions scientifiques et medicales Elsevier SAS.