Vasoactive intestinal peptide receptor on liver plasma membranes: characterization as a glycoprotein

Characterization of vasoactive intestinal peptide (VIP) receptors in mammalian lung

125I-VIP bound specifically to sites on human, rat, guinea pig, and rabbit lung membranes with a dissociation constant (KD) of 60-200 pM and binding site maxima of 200-800 fmol/mg of protein. The presence of a second lower affinity site was detected but not investigated further. High affinity 125I-VIP binding was reversible and displaced by structurally related peptides with an order of potency: VIP greater than rGRF greater than PHI greater than hGRF greater than secretin = Ac Tyr1 D Phe2 GRF. 125I-VIP has been covalently incorporated into lung membranes using disuccinimidyl suberate. Sodium dodecyl sulfate-polyacrilamide gel electrophoresis of labeled human, rat, and rabbit lung membranes revealed major 125I-VIP-receptor complexes of: Mr = 65,000, 56,000, and 64,000 daltons, respectively. Guinea pig lung membranes exhibited two 125I-VIP-receptor complexes of Mr = 66,000 and 60,000 daltons. This labeling pattern probably reflects the presence of differentially glycosylated forms of the same receptor since treatment with neuroaminidase resulted in a single homogeneous band (Mr = 57,000 daltons). Soluble covalently labeled VIP receptors from guinea pig and human lung bound to and were specifically eluted from agarose-linked wheat germ agglutinin columns. Our studies indicate that mammalian lung VIP receptors are glycoproteins containing terminal sialic acid residues.

Gastric inhibitory polypeptide receptor in hamster pancreatic beta cells. Direct cross-linking, solubilization and characterization as a glycoprotein

125I-labelled gastric inhibitory polypeptide (125I-GIP) is directly cross-linked to its specific receptor in hamster pancreatic beta cell membranes by using an ultraviolet irradiation procedure. This approach results in the identification of a GIP-protein complex of apparent Mr 64,000. The labelling of this protein species is specific since it is inhibited when incubating the membranes with increasing doses of native GIP (0.1 nM-1 microM) together with 125I-GIP, half-maximal inhibition being elicited by 5 nM peptide. Reduction of the GIP-protein complex by 100 mM dithiothreitol induces a decrease of the electrophoretic mobility of the complex. Alternatively pretreatment of membranes with dithiothreitol (up to 1 M) does not prevent the binding of 125I-GIP to its receptor. When prelabelled membranes are extracted by 0.5% Triton X-100 (v/v) and the extract is layered on a Sephadex G-50 column, a high peak of radioactivity is eluted with the void volume of the column. Treatment of this peak by 10 min ultraviolet irradiation followed by SDS-PAGE leads to identification of a major band of Mr 64,000. When the peak is further layered on Sephacryl S-200 it yields a single peak of radioactivity corresponding to a protein species with a Stokes radius of 3.2 nm and an apparent Mr of 65,000. The solubilized GIP-receptor complex is specifically adsorbed by Sepharose coupled to wheat germ agglutinin and concanavalin A and eluted from these lectins by their respective sugars. In conclusion the GIP receptor in pancreatic beta cells is a protein monomer of apparent Mr 59 000; its structure is maintained by intrachain disulfide bridges, these bonds being, however, not involved in the interaction of GIP with its receptor; the GIP receptor is a glycoprotein containing N-acetylglucosamine, mannose and probably sialic acid in its carbohydrate moiety.