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

摘要

In plants, G proteins modulate signaling by the stress hormone, abscisic acid (ABA). We identify and char_acterize 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). Arabidop-sis 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 gpal null mutants exhibit ABA-hypersensitive phenotypes. These results predict that, unusually, it is the GDP-bound, not the GTP_bound, 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.