G Protein γ Subunits Provide Functional Selectivity

Abstract

Heterotrimeric G protein signaling mediated by transmembrane G protein–coupled receptors (GPCRs) is ubiquitous among eukaryotes. The G protein heterotrimer consists of α, β, and γ subunits bound to specific GPCRs. Ligand binding to the GPCR induces a change in Gα and the exchange of bound GDP for GTP, which turns the Gα subunit and the Gβγ dimer into two functional signaling units. Intrinsic GTPase activity of Gα returns the heterotrimer back to the inactive form. Diversity and selectivity in G protein signaling in mammals is provided by the existence of gene families for each of the G protein subunits. Humans contain at least 23 Gα subunits, 6 Gβ subunits, and 12 Gγ subunits, which show selectivity in their interactions and differences in tissue specificity. By contrast, Arabidopsis and a number of other plants contain one Gα subunit, one Gβ subunit, and two Gγ subunits (some legumes have two Gα subunits). Evidence is emerging that Gα and Gβγ are involved in signaling in specific and independent pathways in plants. Trusov et al. (pages 1235–1250) show that the two Gγ subunits in Arabidopsis provide functional selectivity to Gβγ signaling. Genetic analyses reveal that the two Gγ subunits provide specificity to the Gβγ dimer action in at least three different signaling pathways: fungal resistance, glucose sensing, and auxin-mediated lateral root development. Figure 1 Distinct patterns of expression of the two Gγ genes, AGG1 and AGG2, in root tissue, relative to expression of AGB1, which encodes the Gβ subunit.