Solubilization of the liver vasoactive intestinal peptide receptor. Hydrodynamic characterization and evidence for an association with a functional GTP regulatory protein

Vasoactive intestinal peptide (VIP) receptors were solubilized using the nondenaturing detergent Triton X-100 after occupancy of rat liver membrane-bound receptors with 125I-VIP. Gel filtration and ultracentrifugation on sucrose density gradients revealed the existence in the soluble macromolecular fraction of two labeled components: a major (80%) heavy component and a minor (20%) light one. The two components exhibit the following hydrodynamic parameters: Stokes radius, 5.8 nm: s/sub 20,w,/ 5.98 s; M/sub r/, 150,000; frictional ratio, 1.52 for the major; and Stokes radius, 3.0 nm: s/sub 20,w,/ 3.98 s; M/sub r/= 52,000; frictional ratio, 1.12 for the minor component. The labeling of these components was specific in that it dramatically decreased when unlabeled VIP was added together with 125I-VIP. The pharmacological specificity was also assessed by using 10 nM histidylisoleucineamide (a VIP agonist). Many lines of evidence indicate that the light component (M/sub r/ = 52,000) is the VIP-receptor complex while the heavy component (M/sub r/ = 150,000) is a ternary complex consisting of VIP, the receptor, and a guanine nucleotide regulatory protein, probably N/sub s/. In conclusion, these data represent initial reports on the successful solubilization of functional VIP-receptor complexes and provide evidence for an interaction between liver VIP-receptor complexes and a GTP-binding protein