Abstract
G-proteins are universal signal transducers mediating many cellular responses. Plant G-protein signaling has been modeled on the well-established animal paradigm but accumulated experimental evidence indicates that G-protein-dependent signaling in plants has taken a very different evolutionary path. Here we review the differences between plant and animal G-proteins reported over past two decades. Most importantly, while in animal systems the G-protein signaling cycle is activated by seven transmembrane-spanning G-protein coupled receptors, the existence of these type of receptors in plants is highly controversial. Instead plant G-proteins have been proven to be functionally associated with atypical receptors such as the Arabidopsis RGS1 and a number of receptor-like kinases. We propose that, instead of the GTP/GDP cycle used in animals, plant G-proteins are activated/de-activated by phosphorylation/de-phosphorylation. We discuss the need of a fresh new look at these signaling molecules and provide a hypothetical model that departs from the accepted animal paradigm.
Highlights
Heterotrimeric G-proteins (G-proteins) are universal signal transducing proteins that, in animals, mediate signaling from G-protein coupled receptors (GPCRs)
Activation of the 7-transmembrane-spanning GPCRs, promote the exchange of GDP for GTP in Gα, causing a conformational change that leads to activation of the heterotrimer accompanied or not by dissociation of the Gα subunit from the
The first candidate GPCR reported in plants was the Arabidopsis GCR1 with claims that it was a cytokinin receptor (Plakidou-Dymock et al, 1998), an assertion that was promptly disputed (Humphrey and Botella, 2001) and eventually led to a retraction by the authors (Kanyuka et al, 2001). Another GPCR, GCR2 was identified as an abscisic acid (ABA) receptor (Liu et al, 2007) but was strongly contested (Gao et al, 2007; Johnston et al, 2007b). Of their possible roles as hormonal receptors, plant GPCR candidates have been mostly identified through bio-informatics analysis using structural characteristics such as having seven-transmembrane-spanning (7TM) domains, instead of more important functional attributes such as having guanine nucleotide exchange factor (GEF) activity (Chung et al, 2011)
Summary
Heterotrimeric G-proteins (G-proteins) are universal signal transducing proteins that, in animals, mediate signaling from G-protein coupled receptors (GPCRs). AGG3 homologs had been known in rice for some time given the radical differences with their canonical animal counterparts, they had not been effectively identified as G-protein subunits (Chakravorty et al, 2011).
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