A novel strategy to engineer functional fluorescent inhibitory G-protein alpha subunits

Signaling studies in living cells would be greatly facilitated by the development of functional fluorescently tagged G-protein alpha subunits. We have designed G(i/o)alpha subunits fused to the cyan fluorescent protein and assayed their function by studying the following two signal transduction pathways: the regulation of G-protein-gated inwardly rectifying K(+) channels (Kir3.0 family) and adenylate cyclase. Palmitoylation and myristoylation consensus sites were removed from G(i/o) alpha subunits (G(i1)alpha, G(i2)alpha, G(i3)alpha, and G(oA)alpha) and a mutation introduced at Cys(-4) rendering the subunit resistant to pertussis toxin. This construct was fused in-frame with cyan fluorescent protein containing a short peptide motif from GAP43 that directs palmitoylation and thus membrane targeting. Western blotting confirmed G(i/o)alpha protein expression. Confocal microscopy and biochemical fractionation studies revealed membrane localization. Each mutant G(i/o) alpha subunit significantly reduced basal current density when transiently expressed in a stable cell line expressing Kir3.1 and Kir3.2A, consistent with the sequestration of the Gbetagamma dimer by the mutant Galpha subunit. Moreover, each subunit was able to support A1-mediated and D2S-mediated channel activation when transiently expressed in pertussis toxin-treated cells. Overexpression of tagged G(i3)alpha and G(oA)alpha alpha subunits reduced receptor-mediated and forskolin-induced cAMP mobilization.