Two Novel GPCR-Type G Proteins Are Abscisic Acid Receptors in Arabidopsis

In plants, G proteins modulate signaling by the stress hormone, abscisic acid (ABA). We identify and characterize two novel Arabidopsis proteins that show homology to an orphan vertebrate GPCR (GPR89) and interact with the sole Arabidopsis G protein a subunit, GPA1, but also have intrinsic GTP-binding and GTPase activity. We have named these proteins GPCR-type G proteins (GTG1 and GTG2). Arabidopsis mutants lacking both GTG1 and GTG2 exhibit ABA hyposensitivity. GTG1 and GTG2 bind ABA specifically. The GDP-bound form of the GTGs exhibits greater ABA binding than the GTP-bound form, the GTPase activity of the GTGs is inhibited by GPA1, and gpa1 null mutants exhibit ABA-hypersensitive phenotypes. These results predict that, unusually, it is the GDP-bound, not the GTP-ound, form of the GTGs that actively relays the signal. We propose that GTG proteins function both as a new type of G protein and as a class of membrane-localized ABA receptors.