Research ArticleBiochemistry

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

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Science Signaling  05 Nov 2019:
Vol. 12, Issue 606, eaav9526
DOI: 10.1126/scisignal.aav9526

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Plant G proteins gain independence

Heterotrimeric guanine nucleotide–binding proteins (G proteins) are composed of α, β, and γ subunits. In animals, inactive G proteins contain GDP-bound Gα. Upon GDP-GTP exchange, the Gα becomes activated, dissociates from the βγ dimer, and stimulates effectors. The intrinsic GTPase activity of Gα returns the G protein to the resting state. Maruta et al. expressed wild-type Gα, a GTPase-deficient Gα mutant, or a Gα mutant unable to bind to guanine nucleotides in Arabidopsis plants deficient in the canonical Gα subunit AtGPA1. Some of the phenotypes caused by AtGPA1 loss, such as smaller flowers and rounder leaves, were rescued by the expression of either Gα mutant, suggesting that plant G proteins can function independently of guanine nucleotide exchange.

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.

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