Heterotrimeric G Proteins in Plants: Canonical and Atypical Gα Subunits

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.

Keywords: G protein activation; GDP-GTP exchange; GTPase; heterotrimeric G proteins; phosphorylation; plant biology; signal transduction.

Figure 1

The classic paradigm of heterotrimeric G protein signalling cycle. The heterotrimer consisting of GDP-bound Gα and the Gβγ dimer is associated with a 7TM-GPCR receptor at the plasma membrane in its resting state. Upon ligand binding, GPCR induces a conformational change in Gα, resulting in GDP release followed by GTP binding. GTP-bound Gα separates from Gβγ, and each interact with their cognate effectors to modulate downstream signalling. The intrinsic GTPase activity of Gα leads to GTP hydrolysis, thereby terminating signalling and returning the heterotrimer to the inactive state.

Figure 2

The structure of the Arabidopsis canonical Gα subunit (AtGPA1) in its GTPγS-bound form. (A) Overall tertiary structure of AtGPA1 (PDB: 2XTZ), showing the Ras domain and the helical domain. (B) Close-up view on the nucleotide-binding pocket with three switch regions highlighted in cyan and the G1 box (also known as the P-loop) highlighted in blue. Side chains of Ser 52 (P-loop) and Gln 222 (Switch II) are shown. The Mg²⁺ ion coordinates the side chain of Ser 52 and the β- and γ-phosphate moieties of GTP, indicated in red lines (dash). The side chain of Gln 222 hydrogen bonds with two water molecules providing hydrolysis of GTP.

Figure 3

Model for heterotrimeric G protein signalling in plants. (A) Nucleotide exchange-dependent cycle. The heterotrimer consisting of GDP-bound canonical Gα and the Gβγ dimer is associated with a 7TM-RGS and RLKs at the plasma membrane in its resting state. Upon ligand binding, RLKs phosphorylate Gα and RGS. Ligand binding is followed by receptor and RGS endocytosis and subsequent de-repression of Gα, which releases GDP and binds GTP. GTP-bound Gα does not necessarily separate from Gβγ, and each of the two components modulate downstream signalling cascades. Dephosphorylation (research is urgently needed) and binding to a new RGS leads to GTP hydrolysis terminating signalling and returning the heterotrimer to the inactive state. (B) Nucleotide exchange-independent cycle. The heterotrimer consisting of either canonical Gα or XLG and the Gβγ dimer is associated with a 7TM-RGS (the role of RGS here is yet to be determined), RLKs, and RLCKs at the plasma membrane in its resting state. Upon ligand binding, RLKs phosphorylate Gα/XLG, RGS, and RLCKs. Ligand binding is followed by receptor endocytosis. Here, activation could be caused by de-repression or phosphorylation-mediated activation of Gα/XLG or Gβγ (the activation mechanism is not established). In case of Arabidopsis, XLG2 dissociates from Gβγ upon perception of flg22 [85]. Both Gα/XLG and Gβγ modulate downstream signalling cascades. One example of effector activation is the BIK1/XLG2-mediated activation of RbohD [85]. Hypothetically, dephosphorylation (research is urgently needed) leads to signal termination and reassociation of the heterotrimer into the inactive state.