Agonist-regulated phosphorylation of the pancreatic cholecystokinin receptor

The present study was undertaken to determine if the cholecystokinin (CCK) receptor may be phosphorylated, and to gain insight into its regulation. For this, the ATP pool of rat pancreatic acini was prelabeled with 32P, and the cells were stimulated with various secretagogues. CCK receptors from treated cells were enriched by sequential fractionation to produce plasmalemma, and subsequent solubilization and lectin-affinity chromatography. This protocol detected a phosphorylated Mr = 85,000-95,000 plasma membrane glycoprotein with features similar to the CCK receptor. Phosphorylation of this protein occurred rapidly (less than 2 min) and in a concentration-dependent manner in response to CCK, and was inhibited by the CCK receptor antagonist L-364,718. Further evidence that this represented the CCK receptor included comigration of phosphorylated and CCK radioligand affinity-labeled proteins on sodium dodecyl sulfate-polyacrylamide gels, both in native forms and after endoglycosidase F deglycosylation, and the specific adsorption of the phosphoprotein to a CCK analogue affinity resin. Phosphorylation occurred predominantly on serine residues of the receptor protein. Phosphorylation of this protein was also enhanced in response to other secretagogues which, like CCK, stimulate a cascade leading to protein kinase C activation, and in response to direct activation of this enzyme by 12-O-tetradecanoylphorbol 13-acetate. Thus, the pancreatic CCK receptor is phosphorylated in a regulated manner, in response to both homologous and heterologous secretagogues, and to protein kinase C activation.