Nucleotide exchange–dependent and nucleotide exchange–independent functions of plant heterotrimeric GTP-binding proteins

Abstract

Heterotrimeric guanine nucleotide–binding proteins (G proteins), which are composed of α, β, and γ subunits, are versatile, guanine nucleotide–dependent, molecular on-off switches. In animals and fungi, the exchange of GDP for GTP on Gα controls G protein activation and is crucial for normal cellular responses to diverse extracellular signals. The model plant Arabidopsis thaliana has a single canonical Gα subunit, AtGPA1. We found that, in planta, the constitutively active, GTP-bound AtGPA1(Q222L) mutant and the nucleotide-free AtGPA1(S52C) mutant interacted with Gβγ1 and Gβγ2 dimers with similar affinities, suggesting that G protein heterotrimer formation occurred independently of nucleotide exchange. In contrast, AtGPA1(Q222L) had a greater affinity than that of AtGPA1(S52C) for Gβγ3, suggesting that the GTP-bound conformation of AtGPA1(Q222L) is distinct and tightly associated with Gβγ3. Functional analysis of transgenic lines expressing either AtGPA1(S52C) or AtGPA1(Q222L) in the gpa1-null mutant background revealed various mutant phenotypes that were complemented by either AtGPA1(S52C) or AtGPA1(Q222L). We conclude that, in addition to the canonical GDP-GTP exchange–dependent mechanism, plant G proteins can function independently of nucleotide exchange.

Heterotrimeric G-Protein Interactions Are Conserved Despite Regulatory Element Loss in Some Plants

“…Because the overexpression of a variant protein version with no GAP activity (RGS E319A ) did not show any effect, our data also confirm that the developmental phenotypes seen by the suppression of Gα or overexpression of RGS are indeed linked to classical G-protein activity in plants. This is especially important because under specific conditions, G-proteins have also been reported to regulate certain plant phenotypes independent of their classical activity (Maruta et al, 2019;Roy Choudhury et al, 2019).…”

Research Advances in Heterotrimeric G-Protein α Subunits and Uncanonical G-Protein Coupled Receptors in Plants

“…By studying the constitutive activity of the GTP-bound AtGPA1 (Q222L) mutant and the nucleotide-free AtGPA1 (S52C) mutant, Maruta found that they could interact with Gγβ dimers and the GTP-binding-impaired AtGPA1 (S52C) variant complemented some, but not all, gpa1-null mutant phenotypes. This means that in addition to the classic GDP-GTP exchange-dependent mechanism, plant G-proteins may also have mechanisms that function independently of nucleotide exchange [18]. Therefore, this review will mainly focus on research in the canonical GDP-GTP exchange-dependent mechanism of G-proteins.…”

Heterotrimeric G Proteins in Plants: Canonical and Atypical Gα Subunits

“…Functional diversification driven by sequence divergence within the superfamily resulted in saltational evolution of these atypical plant Gα subunits [11]. 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 also lack the N-terminal residues required for tethering to the plasma membrane, which potentially influences their sub-cellular localization and function.…”