Abstract
Heterotrimeric G proteins composed of α, β, and γ subunits are central signal transducers mediating the cellular response to multiple stimuli in most eukaryotes. Gγ subunits provide proper cellular localization and functional specificity to the heterotrimer complex. Plant Gγ subunits, divided into three structurally distinct types, are more diverse than their animal counterparts. Type B Gγ subunits, lacking a carboxyl-terminal isoprenylation motif, are found only in flowering plants. We present the functional characterization of type B Gγ subunit (SlGGB1) in tomato (Solanum lycopersicum). We show that SlGGB1 is the most abundant Gγ subunit in tomato and strongly interacts with the Gβ subunit. Importantly, the green fluorescent protein-SlGGB1 fusion protein as well as the carboxyl-terminal yellow fluorescent protein-SlGGB1/amino-terminal yellow fluorescent protein-Gβ heterodimer were localized in the plasma membrane, nucleus, and cytoplasm. RNA interference-mediated silencing of SlGGB1 resulted in smaller seeds, higher number of lateral roots, and pointy fruits. The silenced lines were hypersensitive to exogenous auxin, while levels of endogenous auxins were lower or similar to those of the wild type. SlGGB1-silenced plants also showed strong hyposensitivity to abscisic acid (ABA) during seed germination but not in other related assays. Transcriptome analysis of the transgenic seeds revealed abnormal expression of genes involved in ABA sensing, signaling, and response. We conclude that the type B Gγ subunit SlGGB1 mediates auxin and ABA signaling in tomato.
Highlights
Heterotrimeric G proteins (G proteins) consisting of Gα, Gβ and Gγ subunits are arguably the most important signaling mediators in eukaryotes
We demonstrate that the tomato type B Gγ subunit SlGGB1 interacts with the tomato Gβ subunit, but unlike any other known Gγ subunits, it localizes to the cytoplasm and nucleus in addition to the usual localization to the plasma membrane
BLAST searches of the tomato proteome (ITAG release 2.40) using Arabidopsis Gγ subunits as queries, identified four Gγ-like proteins
Summary
Heterotrimeric G proteins (G proteins) consisting of Gα, Gβ and Gγ subunits are arguably the most important signaling mediators in eukaryotes. G proteins bind to seventransmembrane-spanning G protein coupled receptors (GPCRs) at the plasma membrane. Upon perception of a stimulus GPCRs facilitate the replacement of GDP bound to the Gα subunit with. This exchange results in the activation of the heterotrimer. Gβγ dimer independently initiate specific signaling pathways. The intrinsic GTPase activity of Gα hydrolyzes GTP to GDP and thereby returns the heterotrimer to the steady-state mode (Gautam et al, 1998; McCudden et al, 2005)
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