Topic 23.6

The Existence of G Protein-Coupled ABA Receptors Is Still Unresolved

A classic signal transduction pathway of animal cells begins with ligand binding to a receptor on the plasma membrane, leading to the activation of a heterotrimeric GTP-binding protein (G protein), composed of α, β, and γ subunits (see textbook Chapter 14). The Arabidopsis genome encodes single copies of the genes for the Gα and Gβ subunits, and two copies of the gene for the Gγ subunit. Mutations in the Gα and Gβ subunits result in defective ABA response, but this is seen as decreased response in stomatal regulation yet hypersensitivity in seed and seedling growth (Pandey et al. 2006). This indicates that G-protein signaling may either promote or inhibit a given ABA response, depending on the cell and tissue type.

Because both pharmacological and genetic evidence have implicated G-proteins in mediating ABA response, researchers have sought to identify an appropriate G-protein coupled receptor (GPCR) to link ABA to these responses. The first two candidates, the Arabidopsis genes G-protein coupled receptor(GCR)1 and GCR2, encode proteins with significant sequence similarity to nonplant GPCRs, but seem unlikely to be ABA receptors. Although GCR1 knockout mutants exhibit hypersensitivity to ABA, indicating that the protein functions as an inhibitor or negative regulator of Gα-mediated ABA responses, the ligand for GCR1 is unknown. On the other hand, GCR2 was initially reported to bind ABA, interact with the Gα subunit and mediate multiple ABA responses, but subsequent reports have contradicted both its role in ABA signaling and its identity as a GPCR (Risk et al.. 2009). Thus, although these GCRs may function in ABA response, they are probably not receptors.

The GPCR-type G (GTG) proteins, were identifed by an alternate bioinformatic approach searching for distantly related GPCRs (Pandey et al 2009). A novel feature of these GPCR-related proteins is their intrinsic GTPase activity. Functional characterization of mutant and overexpression lines demonstrated redundancy of the two GTG proteins, and a fairly mild effect on ABA responses such as inhibition of both germination and root growth. Preparations of recombinant GTG proteins specifically bind the naturally occuring (+)-ABA enantiomer, and this binding is more efficient in the presence of GDP than GTP.

Both GTG proteins are localized at the plasma membrane and interact with GPA1, the sole Gα subunit in Arabidopsis, but it is not clear whether they bind intracellular or extracellular ABA. The model proposed from these results is that GPA1 modulates the GTPase activity of the GTGs, thereby altering their ability to bind ABA and initiate a signaling cascade via unknown intermediates. This mechanism is unusual in that the GDP-bound form is more active than the GTP-bound GTGs, and the classic relationship between GPCR and G-protein is reversed: the G-protein regulates the receptors, not vice versa.