Affinity Purification of Pancreastatin Receptor–Gq/11 Protein Complex from Rat Liver Membranes

Abstract

Pancreastatin, a chromogranin A derived peptide, exerts a glycogenolytic effect on the hepatocyte. This effect is initiated by binding to membrane receptors which are coupled to pertussis toxin insensitive G proteins belonging to the Gq/11 family. We have recently solubilized active pancreastatin receptors from rat liver membranes still functionally coupled to G proteins. Here, we have purified pancreastatin receptors by a two-step procedure. First, pancreastatin receptors with their associated Gq/11 regulatory proteins were purified from liver membranes by lectin absorption chromatography on wheat germ agglutinin immobilized on agarose. A biotinylated rat pancreastatin analog was tested for binding to liver membranes before using it for affinity purification. Unlabeled biotinylated rat pancreastatin competed for 125I-labeled [Tyr0]PST binding to solubilized receptors with a Kd = 0.27 nM, comparable to that of native pancreastatin. The biotinylated analog was immobilized on streptavidin-coated Sepharose beads and used to further affinity purify wheat germ agglutinin eluted receptor material. Specific elution at low pH showed that the receptor protein was purified as an 80-kDa protein in association with a G protein of the q/11 family, as demonstrated by specific immunoblot analysis. The specificity of the receptor band was assessed by chemical cross-linking of the purified material followed by SDS–PAGE and autoradiography. In conclusion, we have purified pancreastatin receptor as a glycoprotein of 80 kDa physically associated with a Gq/11 protein.