Purification of a pituitary receptor for somatostatin. The utility of biotinylated somatostatin analogs.

A somatostatin (SRIF) receptor and its associated Gi regulatory proteins was purified from GH4C1 rat pituitary cells by: 1) saturation of the membrane-bound receptor with biotinyl-NH-[Leu8,D-Trp22,Tyr25] SRIF28 (bio-S28); 2) solubilization of receptor-ligand (R.L) complex with deoxycholate-lysophosphatidylcholine (D.L); 3) adsorption of solubilized receptor-ligand complex to immobilized streptavidin; and 4) elution of receptor and G-protein by GTP. The receptor, a glycoprotein with an average M(r) of 85,000, was then purified to substantial homogeneity on immobilized wheat germ agglutinin. The 85-kDa glycoprotein was identified as a SRIF receptor by several criteria. (a) It had the same size as the chemically cross-linked R.[125I]L complex. (b) Yield of the purified protein increased and plateaued in the same range of bio-S28 concentrations where specific high affinity binding reached saturation. (c) It was copurified with appropriate G-protein subunits. The 85-kDa receptor and two other proteins with M(r) values of 35,000 and 40,000, the sizes of G beta and G alpha, did not appear in eluates from control streptavidin columns done with SRIF receptors loaded with nonbiotinylated S14. The 40-kDa protein was identified as a Gi alpha by ADP-ribosylation from [32P]NAD catalyzed by pertussis toxin. (d) Both the chemically cross-linked R.[125I]L complex and SRIF receptor purified from [35S]methionine-labeled GH4C1 cells were reduced in size to about 38 kDa by endoglycosidase F. (e) Amino acid sequence from the purified receptor was nearly identical with that of a recently cloned SRIF receptor subtype.