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Abstract
Heterotrimeric GTP-binding proteins (G proteins), consisting of Gα, Gβ and Gγ subunits, transduce signals from a diverse range of extracellular stimuli, resulting in the regulation of numerous cellular and physiological functions in Eukaryotes. According to the classic G protein paradigm established in animal models, the bound guanine nucleotide on a Gα subunit, either guanosine diphosphate (GDP) or guanosine triphosphate (GTP) determines the inactive or active mode, respectively. In plants, there are two types of Gα subunits: canonical Gα subunits structurally similar to their animal counterparts and unconventional extra-large Gα subunits (XLGs) containing a C-terminal domain homologous to the canonical Gα along with an extended N-terminal domain. Both Gα and XLG subunits interact with Gβγ dimers and regulator of G protein signalling (RGS) protein. Plant G proteins are implicated directly or indirectly in developmental processes, stress responses, and innate immunity. It is established that despite the substantial overall similarity between plant and animal Gα subunits, they convey signalling differently including the mechanism by which they are activated. This review emphasizes the unique characteristics of plant Gα subunits and speculates on their unique signalling mechanisms.
Highlights
The inactive, guanosine diphosphate (GDP)-bound Gα is associated with the obligate Gβγ dimer as a complex loosely coupled to a G protein-coupled receptors (GPCRs) at the intracellular side of the plasma membrane
Little was known of the function of this domain, until comparative structural studies of the helical domains of human Gαi1 and Arabidopsis AtGPA1 in domain swap experiments showed that this domain controls the intrinsic GDP-guanosine triphosphate (GTP) exchange rate and protein stability [59,60]
In both Gαi1 and AtGPA1, the guanine nucleotide is bound within the Ras domain and is buried under the helical domain, where mobility of the helical domain provides an opportunity for the nucleotide release
Summary
Heterotrimeric GTP-binding protein complexes, minimally comprising Gα, Gβ, and Gγ subunits, mediate the majority of signalling pathways in animals and regulate substantial signalling networks in plants. Their components are found in all major domains of eukaryotic life, placing the origin of the trimeric core structure in the common ancestor of Eukaryotes [1]. Deviation of the conserved catalytic motifs of some Gα subunits is found in Dictyostelium (slime moulds), Naegleria, and plants, suggesting reduced levels or complete loss of nucleotide-dependent activity [1,12,13,14] Some of these Gα subunits lack the N-terminal residues required for tethering to the plasma membrane, which potentially influences their sub-cellular localization and function. A fitness pressure for rapid adaptation to the new harsh terrestrial environment drove the early evolution of XLG subunits
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