Structural characterization of the somatostatin receptor in rat anterior pituitary membranes

To structurally characterize the somatostatin receptor in the anterior pituitary, the chemical cross-linking reagent N-5-azido-nitrobenzoyloxysuccinimide was used to attach covalently [125I-Tyr11]somatostatin-14 to its receptor in pituitary membranes. Rat anterior pituitary membranes were incubated with [125I-Tyr11]somatostatin-14, washed, and then treated with 100 microM cross-linker, which was activated by exposure to UV light. Gel electrophoresis followed by autoradiography revealed a broad band centered at 88,000 mol wt. The appearance of this band was unaffected by dithiothreitol. Competitive inhibition of binding by unlabeled somatostatin resulted in a parallel inhibition of labeling of the 88,000 mol wt protein. The addition of guanine nucleotides in concentrations that inhibit binding similarly inhibited cross-linking. The cross-linked membranes were solubilized in Zwittergent 3-12, a nondenaturing detergent, and the glycosylation pattern of the labeled protein was investigated by incubation with various lectins coupled to agarose. The cross-linked protein was selectively adsorbed by wheat germ agglutinin, and this interaction was blocked by the addition of N,N',N"-triacetylchitotriose, indicating that the rat anterior pituitary somatostatin receptor is a glycoprotein containing polymeric beta-1-4 linked N-acetylglucosamine groups. The results of this study show that the rat anterior pituitary somatostatin receptor is a glycoprotein of 88,000 mol wt containing no disulfide-linked subunits.