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 G subunit by binding of guanosine triphosphate causes dissociation of the and ß subunits. Fusion proteins between modified green fluorescent proteins and G_i1 and Gß₁ or G₂ were expressed in human embryonic kidney cells. Surprisingly, the FRET signal detectable with either the labeled and or ß and subunits increased upon stimulation of heterologously expressed adrenergic receptors. To confirm that the subunits underwent a rearrangement that caused the N termini of the ß subunits to move closer to the AB loop of G_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 ₂-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]