G PROTEINS Activation Without Dissociation

Using fluorescence resonance energy transfer (FRET), Bünemann et al. observed that the heterotrimeric guanine nucleotide-binding proteins (G proteins) did not dissociate upon activation. This is contrary to the dogma that states that activation of the Gα subunit by binding of guanosine triphosphate causes dissociation of the α and β subunits. Fusion proteins between modified green fluorescent proteins and Gα_i1 and Gβ₁ or G₂ were expressed in human embryonic kidney cells. Surprisingly, the FRET signal detectable with either the labeled and α or β and α subunits increased upon stimulation of heterologously expressed adrenergic receptors. To confirm that the subunits underwent a rearrangement that caused the N termini of the β subunits to move closer to the αAB loop of Gα_i, the authors constructed another fusion protein, cyan fluorescent protein attached to the C terminus of the G2, that showed a decrease in FRET when stimulated. The functional coupling of the fluorescent fusion proteins was tested, and all except the C-terminally tagged subunit G proteins were able to stimulate the G protein-activated inwardly rectifying K⁺ channel. Thus, at least for G_i heterotrimers activated by α₂-adrenergic receptors, G protein activation does not involve dissociation of the heterotrimer.

M. Bünemann, M. Frank, M. J. Lohse, G_i protein activation in intact cells involves subunit rearrangement rather than dissociation. Proc. Natl. Acad. Sci. U.S.A. 100, 16077-16082 (2003). [Abstract] [Full Text]