Identification of G Protein α-Subunits in RINm5F Cells and Their Selective Interaction with Galanin Receptor

Galanin, an inhibitor of insulin secretion in pancreatic β-cells, exerts its multiple effects through mechanisms that are sensitive to pertussis toxin (PTX). G proteins have been characterized in RINm5F cells. By ADP ribosylation and immunoblotting, the α-subunits of G_i1, G_i2, G_i3, and two forms of G_o were identified, G_i12 being predominant. As expected from a G protein–linked receptor, GTP and its nonhydrolyzable analogue GTP-γ-S decreased tracer galanin binding to cell membranes. This resulted from a change in receptor affinity without any modification in the number of sites. Selective antibodies against the COOH-terminal decapeptide of the α-subunits of the G_i and G_o proteins were used to block G protein interaction before we studied galanin binding. Antibody AS, which selectively recognizes G_iα1 and G_iα2, decreased tracer galanin binding to membranes at concentrations where there were no effects of other antibodies specifically directed against G_iα3 or G_αo. These data suggest that G_i1 and/or G_i2 interact with the galanin receptor and probably mediate the effects of galanin in pancreatic β-cells.