Sci. Signal., 13 January 2009
Plant Biology G Protein–Included Receptors in Plants
L. Bryan Ray
Science, Science Signaling, AAAS, Washington, DC 20005, USA
Abscisic acid (ABA) is a key plant hormone that regulates responses to environmental conditions like temperature, water availability, and osmolarity. Intense investigations aimed at identifying the receptor have yielded candidates, but the true nature of ABA receptors in plants has remained unclear. Pandey et al. present strong evidence that two proteins with similarity to G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptors (GPCRs), but with some other very intriguing properties, appear to function as ABA receptors or subunits thereof. Pandey et al. identified GPCR-type G protein 1 and 2 (GTG1 and GTG2) by sequence analysis for their similarity to GPCRs. The authors used a split ubiquitin system that monitors interaction of membrane proteins to show that GTG1 and GTG2 interacted with the plant G protein subunit GPA1 much as well-known GPCRs do. What is different about the GTGs, though, is that they also have functional built-in G proteins. That is, they can bind guanine nucleotides and act as guanosine triphosphatases (GTPases). In vitro, recombinant GTG1 and GTG2 showed saturable, high-affinity binding to ABA. Surprisingly, ABA binding was more efficient by the guanosine diphosphate (GDP)–bound form of the receptor than the GTP-bound form, which indicates that the former is the active signaling form of the receptor. Interaction with the GTP-bound subunit GPA1 inhibited the GTPase activity of GTG1 and GTG2, presumably thus decreasing ABA binding. Mutant Arabidopsis plants lacking both GTG1 and GTG2 (but not plants lacking just one of the proteins) had diminished responses to ABA. Some ABA signaling remained, however, leaving open the possibility of other ABA signaling mechanisms as well. The authors note that the presence of GTG homologs in multiple phyla indicates that these unusual proteins that appear to combine hormone perception and initial signaling events of conventional GPCRs are of ancient origin. G protein activity was not detected in the human homolog, GPR89, but it remains to be determined whether other organisms retain the dual functions seen in plants. Christmann and Grill provide commentary.
S. Pandey, D. C. Nelson, S. M. Assmann, Two novel GPCR-type G proteins are abscisic acid receptors in Arabidopsis. Cell 136, 136–148 (2009). [PubMed]
A. Christmann, E. Grill, Are GTGs ABA's biggest fans? Cell 136, 21–23 (2009). [PubMed]
Citation: L. B. Ray, G Protein–Included Receptors in Plants. Sci. Signal. 2, ec18 (2009).
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