G protein activation without subunit dissociation depends on a Gαi-specific region

Abstract

G proteins transmit a variety of extracellular signals into intracellular responses. The Gα and Gβγ subunits are both known to regulate effectors. Interestingly, the Gα subunit also determines subtype specificity of Gβγ effector interactions. However, in light of the common paradigm that Gα and Gβγ subunits dissociate during activation, a plausible mechanism of how this subtype specificity is generated was lacking. Using a fluorescence resonance energy transfer (FRET)-based assay developed to directly measure mammalian G protein activation in intact cells, we demonstrate that fluorescent Gαi1,2,3, Gαz and Gβ1γ2 subunits do not dissociate during activation but rather undergo subunit rearrangement as indicated by an activation-induced increase in FRET. In contrast, fluorescent Gαo subunits exhibited an activation-induced decrease in FRET, reflecting subunit dissociation or, alternatively, a distinct subunit rearrangement. The αB/C-region within the α-helical domain, which is much more conserved within Gαi1,2,3 and Gαz as compared with that in Gαo, was found to be required for exhibition of an activation-induced increase in FRET between fluorescent Gα and Gβγ subunits. However, the αB/C-region of Gαi1 alone was not sufficient to transfer the activation pattern of Gαi to the Gαo subunit. Either residues in the first 91 amino acids or in the C-terminal remainder (amino acids 93-354) of Gαi1 together with the αB/C-helical region of Gαi1 were needed to transform the Gαo-activation pattern into a Gαi1-type of activation. The discovery of subtype-selective mechanisms of G protein activation illustrates that G protein subfamilies have specific mechanisms of activation that may provide a previously unknown basis for G protein signaling specificity. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.