Conventional and Novel Gγ Protein Families Constitute the Heterotrimeric G-Protein Signaling Network in Soybean

Abstract

Heterotrimeric G-proteins comprised of Gα, Gβ and Gγ proteins are important signal transducers in all eukaryotes. The Gγ protein of the G-protein heterotrimer is crucial for its proper targeting at the plasma membrane and correct functioning. Gγ proteins are significantly smaller and more diverse than the Gα and Gβ proteins. In model plants Arabidopsis and rice that have a single Gα and Gβ protein, the presence of two canonical Gγ proteins provide some diversity to the possible heterotrimeric combinations. Our recent analysis of the latest version of the soybean genome has identified ten Gγ proteins which belong to three distinct families based on their C-termini. We amplified the full length cDNAs, analyzed their detailed expression profile by quantitative PCR, assessed their localization and performed yeast-based interaction analysis to evaluate interaction specificity with different Gβ proteins. Our results show that ten Gγ genes are retained in the soybean genome and have interesting expression profiles across different developmental stages. Six of the newly identified proteins belong to two plant-specific Gγ protein families. Yeast-based interaction analyses predict some degree of interaction specificity between different Gβ and Gγ proteins. This research thus identifies a highly diverse G-protein network from a plant species. Homologs of these novel proteins have been previously identified as QTLs for grain size and yield in rice.

Characterization of the Heterotrimeric G-Protein Complex and Its Regulator from the Green Alga Chara braunii Expands the Evolutionary Breadth of Plant G-Protein Signaling

“…The sequence identity between AtAGG1/AtAGG2 and CbGg is relatively modest (approximately 19% and 23% with AGG1 and AGG2, respectively), which is typical of most Gg proteins, given the large variability observed between them (Choudhury et al, 2011). However, signature motifs predicted to be essential for Gg function, namely the coiled-coil domain at the N terminus, the DPLL motif, and the sequences predicted to be involved in its high-affinity interaction with the Gb proteins, are mostly conserved in CbGg.…”

“…Moreover, one type of Gg protein seems to be specific to higher plants. The only plants that have multiple homologs of each of the subunits are the ones that have undergone recent whole-genome duplication events, such as soybean, which has four Ga, four Gb, and 10 Gg proteins (Bisht et al, 2011;Choudhury et al, 2011). A complete genome sequence assembly will be required to assess the existence of additional G-protein genes in the C. braunii genome.…”

“…However, an additional eight (e.g. in soybean) or 10 (in most monocots) amino acids preceding this Met are also common (Bisht et al, 2011). The significance of such modifications and whether they result in alternatively translated Ga proteins in planta remain unknown.…”

“…P/M represents the predicted sites for palmitoylation/myristoylation. B, The phylogenetic relationship of plant Ga proteins was inferred using the expression of higher plant G-protein genes, which also show a low but ubiquitous expression (Bisht et al, 2011;Choudhury et al, 2011). The relative expression levels of CbGa, CbGb, CbGg, and CbRGS were between 2% and 15% of the expression of the C. braunii ELONGATION FACTOR1a gene (CbEF1; GenBank accession no.…”

“…In plants, the analysis of G-protein functions in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and soybean (Glycine max) has emphasized the critical roles they play in plant growth, development, and stress responses (Li et al, 2012;Zhang et al, 2012;Urano et al, 2013); however, the genomes of most plants encode for a single canonical Ga and Gb protein and few Gg proteins (Urano et al, 2013), with the exception of plants where recent whole-genome duplications have resulted in their higher numbers (e.g. four Ga, four Gb, and 10 Gg proteins in soybean; Bisht et al, 2011;Choudhury et al, 2011;Pandey, 2011). Furthermore, at the biochemical level, the higher plant Ga proteins have significantly slower GTPase activity compared with the mammalian Ga proteins (Jones et al, 2011;Choudhury et al, 2012;Urano et al, 2012).…”