GPCR-induced dissociation of G-protein subunits in early stage signal transduction

Abstract

G-protein coupled receptors (GPCRs) form a ternary complex of agonist, receptor and G-proteins during primary signal transduction at the cell membrane. Downstream signalling is thought to be preceded by the process of dissociation of Gα and Gβγ subunits, thus exposing new surfaces to interact with downstream effectors. We demonstrate here for the first time, the dissociation of heterotrimeric G-protein subunits (i.e., Gα and Gβγ) following agonist-induced GPCR (α2A-adrenergic receptor; α2A-AR) activation in a cell-free assay system. α2A-AR membranes were reconstituted with the G-proteins (±hexahistidine-tagged) Gαi1 and Gβ1γ2 and functional signalling was determined following activation of the reconstituted receptor:G-protein complex with the potent agonist UK-14304, and [35S]GTPγS. In the presence of Ni2+-coated agarose beads, the activated his-tagged Gαi1his-[35S]GTPγS complex was captured on the Ni2+-presenting surface. When his-tagged Gβ1γ2 (Gβ1γ2his) was used with Gαi1, the [35S]GTPγS-bound Gαi1 was not present on the Ni2+-coated beads, but rather, it was separated from the β1γ2(his)-beads, demonstrating receptor-induced dissociation of Gα and Gβγ subunits. Treatment of the reconstituted α2A-AR membranes containing Gβ1γ2his:Gαi1 with imidazole confirmed the specificity for the Ni2+:G-protein surface dissociation of Gαi1 from Gβ1γ2his. These data demonstrate for the first time, the complete dissociation of the G-protein subunits and extend observations on the role of G-proteins in the assembly and disassembly of the ternary complex in the primary events of GPCR signalling.