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(iα2) 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.