Structural analysis and functional role of the carbohydrate component of somatostatin receptors

SRIF receptors are membrane-bound glycoproteins. To structurally identify the carbohydrate components of SRIF receptors, solubilized rat brain SRIF receptors were subjected to lectin affinity chromatography. Solubilized SRIF receptors specifically bound to wheat germ agglutinin-lectin affinity columns but not to succinylated wheat germ agglutinin. This finding, as well as the ability of the solubilized receptor to interact with a Sambucus nigra L. lectin affinity column suggested that sialic acid residues are associated with SRIF receptors. The inability of the receptor to bind to concanavalin A, Dolichus biflorus agglutinin, Ulex europeaus I, and Jacalin lectin affinity columns suggests that high mannose, N-acetylgalactosamine, fucose, and O-linked carbohydrates are not associated with receptor. To investigate the functional role of the carbohydrate groups in brain SRIF receptors, specific sugars were selectively cleaved from SRIF receptors and the subsequent effect on the specific high affinity binding of the agonist [125I]MK 678 to SRIF receptors was determined. Treatment of the receptor with endoglycosidase D did not affect the specific binding of [125I] MK 678 to the solubilized SRIF receptors, consistent with the finding from lectin affinity chromatography that high mannose-type carbohydrate structures were not associated with SRIF receptors. Treatment of solubilized SRIF receptors with peptide-N-glycosidase F and endoglycosidases H and F reduced [125I]MK 678 binding to SRIF receptors indicating that either hybrid, or a combination of hybrid and complex N-linked carbohydrate structures, have a role in maintaining the receptor in a high affinity state for agonists. Treatment of solubilized SRIF receptors with neuraminidase from Vibrio cholera abolished high affinity agonist binding to the receptors, whereas treatment of the receptor with neuraminidase from Newcastle disease virus did not affect [125I]MK 678 binding to the receptor. These findings suggest that sialic acid residues in an alpha 2,6-configuration have a role in maintaining the SRIF receptor in a high affinity conformation for agonists. This is further indicated by studies on SRIF receptors in the pituitary tumor cell line, AtT-20. Treatment of AtT-20 cells in culture with neuraminidase (V. cholera) greatly reduces high affinity [125I] MK 678 binding sites, but did not alter the maximal ability of SRIF to inhibit forskolin-stimulated cAMP accumulation in intact AtT-20 cells. This finding suggests that the desialylated SRIF receptor is functionally active and remains coupled to GTP-binding proteins, but exhibits a reduced affinity for agonists. Treatment of AtT-20 cell membranes with neuraminidase from V. cholera was also able to greatly reduce the affinity of SRIF receptors for [125I]MK 678.(ABSTRACT TRUNCATED AT 400 WORDS)