Affinity purification of a somatostatin receptor-G-protein complex demonstrates specificity in receptor-G-protein coupling.

The inhibitory neuropeptide somatostatin (SRIF) initiates many of its physiological effects by binding to membrane receptors which are coupled to pertussis toxin-sensitive G-protein(s). We have solubilized such a SRIF receptor-G-protein complex and purified it using a biotinylated SRIF analog and guanine nucleotide-dependent affinity chromatography. Following [125I-Tyr11]SRIF binding to membranes from AR4-2J pancreatic acinar cells, only two detergents, dodecyl-beta-D-maltoside (D beta M) and sucrose monolaureate, extracted greater than 70% of the prebound peptide in association with receptor. The D beta M-solubilized ligand-receptor complex was extremely stable: the half-time (t1/2) for dissociation was 11 days at 4 degrees C. However, guanosine 5'-3-O-(thio)triphosphate (10 microM) elicited rapid dissociation of the [125I-Tyr11]SRIF-receptor complex (t1/2 < 30 s), and this effect was concentration-dependent (ED50 = 4.0 + 0.3 nM). [125I-Tyr11]SRIF dissociation was also stimulated by GDP (ED50 = 4.1 +/- 0.3 microM), and the potency of GDP was increased 4-fold by 30 microM AlF4-. Thus, the solubilized receptor was functionally associated with G-proteins. Cross-linking of the soluble [125I-Tyr11]SRIF-receptor complex resulted in the covalent labeling of a 70-90-kDa band, the same band that was specifically labeled in membranes. Affinity purification of the SRIF receptor-G-protein complex was accomplished by prebinding a biotinylated SRIF analog, [N-biotinyl-Leu8,D-Trp22,Tyr25]SRIF28, to membranes followed by solubilization of the ligand-receptor-G-protein complex, adsorption to streptavidin-agarose, and specific elution with 100 microM GDP, 30 microM AlF4-. G-proteins were identified in the eluate by immunoblotting with specific antipeptide antisera. Using this protocol, the G-protein subunits alpha i, alpha i-3, and beta 36 were shown to be specifically associated with the AR4-2J cell SRIF receptor. Thus, we have developed a new, generally applicable, procedure for the efficient solubilization and affinity purification of a stable SRIF receptor-G-protein complex and have characterized the specific G-protein subunits associated with pancreatic SRIF receptors.