Electric properties of photoaffinity-labelled pancreatic A-subtype cholecystokinin

Although the isoelectric point of a protein is very important, electric focusing of intrinsic membrane proteins in polyacrylamide or agarose gels often fails. The recently introduced Bio-Rad Rotofor cell allowed isoelectric focusing of such a protein, cholecystokinin (CCK) receptor. Both the isoelectric point and the molecular weight (Mr) of pancreatic CCK receptor were determined. For this purpose, membrane CCK receptor was photoaffinity labelled by a cleavable agonist probe, subsequently prepurified on immobilized wheat germ agglutinin and analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis and isoelectrofocusing in solution in the presence of Nonidet P-40. CCK receptor was identified at Mr 85,000-100,000, whereas its deglycosylated product was shifted to Mr 42,000. Further, the isoelectric points of the glycosylated and deglycosylated forms of CCK receptor were pH 4.8 and 4.3, respectively. A knowledge of the isoelectric point should help in characterizing better CCK receptor heterogeneity and/or in purifying CCK receptor proteins.