Electric properties of photoaffinity-labelled pancreatic A-subtype cholecystokinin

Purification of A-subtype pancreatic cholecystokinin receptor by immunoaffinity chromatography

So far, no efficient affinity chromatography for CCK receptor purification has been reported that prevented obtention of sequenceable amounts of purified receptor. In this work, 10% of plasma membrane receptor sites were specifically cross-linked with the photoreactive cleavable agonist 125I-ASD-[Thr28, Ahx31]-CCK-25-33, solubilized by NP-40, chromatographied on immobilized wheat germ agglutinin and further immunopurified using anti-CCK antibodies to an overall rate of 3000-3600-fold. Analysis of eluted material demonstrated a protein migrating at Mr 85,000-100,000 and the absence of 35S-labeled impurity. This single and efficient affinity chromatography should provide enough homogeneous receptor protein for microsequence determination and leads to consider immunoaffinity chromatography on immobilized anti-ligand antibodies as a potential tool for purification of membrane receptors.

Biochemical characterization of a subtype pancreatic cholecystokinin receptor and of its agonist binding domain

This study was undertaken in order to improve photoaffinity labelling efficiency of pancreatic cholecystokinin receptor by the cleavable probe 125I-ASD-(Thr28,Ahx31)-CCK-25-33 and to further characterize the denaturated receptor and its agonist binding domain. Membrane bound pancreatic cholecystokinin receptor was specifically labelled by 125I-ASD-(Thr28,Ahx31)-CCK-25-33 as a component of Mr approximately 85,000-100,000. The efficiency of the photolabelling was 3-4%. Performing photolysis on [125I-ASD-(Thr28,Ahx31)-CCK-25-33-receptor] complexes solubilized by CHAPS did not affect specificity of the labelling reaction but enhanced its efficiency so that up to 10% of the receptor site population could be cross-linked. Several lectins were tested for their ability to recognize and purify the cholecystokinin receptor denaturated by Nonidet P-40. Wheat germ agglutinin provided the best recovery and purification rate. The receptor was fully adsorbed on immobilized wheat germ agglutinin, while only a fraction was retained on ricin II (28%) and Ulex europaeus (58%), thus suggesting that the receptor is heterogeneously glycosylated. Finally, major labelled receptor fragments were generated by enzymatic digestion. There were: endoproteinase Glu-C----Mr approximately 34,000; endoproteinase Glu-C/trypsin----Mr approximately 12,000; chymotrypsin/endoproteinase Glu-C----Mr approximately 16,000 and 12,000. The fragment of Mr approximately 34,000 was deglycosylated to a component of Mr approximately 22,000 whereas the other fragments were insensitive to deglycosylation Such results strongly suggest that cholecystokinin binding occurs in a non-glycosylated domain of the cholecystokinin receptor protein.