Gastric inhibitory polypeptide receptor in hamster pancreatic beta cells. Direct cross-linking, solubilization and characterization as a glycoprotein

Abstract

¹²⁵I‐labelled gastric inhibitory polypeptide (¹²⁵I‐GIP) is directly cross‐linked to its specific receptor in hamster pancreatic beta cell membranes by using an ultraviolet irradiation procedure. This approach results in the identification of a GIP‐protein complex of apparent M_r 64000. The labelling of this protein species is specific since it is inhibited when incubating the membranes with increasing doses of native GIP (0.1 nM–1 μM) together with ¹²⁵I‐GIP, half‐maximal inhibition being elicited by 5 nM peptide. Reduction of the GIP‐protein complex by 100 mM dithiothreitol induces a decrease of the electrophoretic mobility of the complex. Alternatively pretreatment of membranes with dithiothreitol (up to 1 M) does not prevent the binding of ¹²⁵I‐GIP to its receptor. When prelabelled membranes are extracted by 0.5% Triton X‐100 (v/v) and the extract is layered on a Sephadex G‐50 column, a high peak of radioactivity is eluted with the void volume of the column. Treatment of this peak by 10 min ultraviolet irradiation followed by SDS‐PAGE leads to identification of a major band of M_r 64000. When the peak is further layered on Sephacryl S‐200 it yields a single peak of radioactivity corresponding to a protein species with a Stokes radius of 3.2 nm and an apparent M_r of 65000. The solubilized GIP‐receptor complex is specifically adsorbed by Sepharose coupled to wheat germ agglutinin and concanavalin A and eluted from these lectins by their respective sugars. In conclusion (a) the GIP receptor in pancreatic beta cells is a protein monomer of apparent M_r 59000; (b) its structure is maintained by intrachain disulfide bridges, these bonds being, however, not involved in the interaction of GIP with its receptor; (c) the GIP receptor is a glycoprotein containing N‐acetylglucosamine, mannose and probably sialic acid in its carbohydrate moiety.