Research ArticleG Proteins

Eukaryotic G Protein Signaling Evolved to Require G Protein–Coupled Receptors for Activation

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Science Signaling  21 May 2013:
Vol. 6, Issue 276, pp. ra37
DOI: 10.1126/scisignal.2003768

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Evolving New Interactions

Heterotrimeric G proteins become activated when GTP replaces GDP bound to the G protein α subunit; the GTPase activity of Gα hydrolyzes GTP to GDP to return the G protein to the inactive state. In animal cells, G protein–coupled receptors (GPCRs) activate cognate G proteins, whereas the Arabidopsis G protein is self-activating. Bradford et al. investigated the evolution of the G protein signaling network by performing phylogenetic analyses and in vitro characterization of the GTP binding and GTP hydrolysis rates of selected Gα proteins from divergent eukaryotic species. Gα proteins from Bikonts, including Arabidopsis and basal eukaryotes, which do not have GPCRs, exhibited self-activating properties, with rapid GTP binding and slow GTP hydrolysis rates, whereas Gα proteins from Unikonts, including fungi and humans, which have GPCRs, had opposing properties. These data indicate that in contrast to the GPCR-dominant system in higher eukaryotes, ancestral G proteins were not regulated at the activation step—and suggest that signaling molecules change their intrinsic properties as new interaction partners emerge.

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