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1.
Abstract

Plants uniquely have a family of proteins called extra-large G proteins (XLG) that share homology in their C-terminal half with the canonical Gα subunits; we carefully detail here that Arabidopsis XLG2 lacks critical residues requisite for nucleotide binding and hydrolysis which is consistent with our quantitative analyses. Based on microscale thermophoresis, Arabidopsis XLG2 binds GTPγS with an affinity 100 times lower than that to canonical Gα subunits. This means that given the concentration range of guanine nucleotide in plant cells, XLG2 is not likely bound by GTP in vivo. Homology modeling and molecular dynamics simulations provide a plausible mechanism for the poor nucleotide binding affinity of XLG2. Simulations indicate substantially stronger salt bridge networks formed by several key amino-acid residues of AtGPA1 which are either misplaced or missing in XLG2. These residues in AtGPA1 not only maintain the overall shape and integrity of the apoprotein cavity but also increase the frequency of favorable nucleotide-protein interactions in the nucleotide-bound state. Despite this loss of nucleotide dependency, XLG2 binds the RGS domain of AtRGS1 with an affinity similar to the Arabidopsis AtGPA1 in its apo-state and about 2 times lower than AtGPA1 in its transition state. In addition, XLG2 binds the Gβγ dimer with an affinity similar to that of AtGPA1. XLG2 likely acts as a dominant negative Gα protein to block G protein signaling. We propose that XLG2, independent of guanine nucleotide binding, regulates the active state of the canonical G protein pathway directly by sequestering Gβγ and indirectly by promoting heterodimer formation.

Communicated by Ramaswamy H. Sarma  相似文献   

2.
Heterotrimeric G proteins, consisting of Gα, Gβ, and Gγ subunits, play important roles in plant development and cell signaling. In Arabidopsis, in addition to one prototypical G protein α subunit, GPA1, there are three extra-large G proteins, XLG1, XLG2, and XLG3, of largely unknown function. Each extra-large G (XLG) protein has a C-terminal Gα-like region and a ~400 amino acid N-terminal extension. Here we show that the three XLG proteins specifically bind and hydrolyze GTP, despite the fact that these plant-specific proteins lack key conserved amino acid residues important for GTP binding and hydrolysis of GTP in mammalian Gα proteins. Moreover, unlike other known Gα proteins, these activities require Ca(2+) instead of Mg(2+) as a cofactor. Yeast two-hybrid library screening and in vitro protein pull-down assays revealed that XLG2 interacts with the nuclear protein RTV1 (related to vernalization 1). Electrophoretic mobility shift assays show that RTV1 binds to DNA in vitro in a non-sequence-specific manner and that GTP-bound XLG2 promotes the DNA binding activity of RTV1. Overexpression of RTV1 results in early flowering. Combined overexpression of XLG2 and RTV1 enhances this early flowering phenotype and elevates expression of the floral pathway integrator genes, FT and SOC1, but does not repress expression of the floral repressor, FLC. Chromatin immunoprecipitation assays show that XLG2 increases RTV1 binding to FT and SOC1 promoters. Thus, a Ca(2+)-dependent G protein, XLG2, promotes RTV1 DNA binding activity for a subset of floral integrator genes and contributes to floral transition.  相似文献   

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5.
Frizzled proteins, the receptors for Wnt ligands have seven hydrophobic transmembrane domains, a structural feature of G protein coupled receptors. Therefore a role for G proteins in the regulation of Wnt signaling has been proposed. Here I have used Xenopus oocytes to study the role of heterotrimeric G proteins in the regulation of GSK-3β and β-Catenin, two essential components of the canonical Wnt pathway. In these cells, general activators of G proteins such as GTPγ-S and AlF4 increase β-Catenin stability and decrease GSK-3β mediated phosphorylation of the microtubule associated protein, Tau. Among several members of Gα proteins tested, expression of a constitutively active mutant of Gαq (GαqQL) led to a significant increase in accumulation of β-Catenin. The stabilization of β-Catenin mediated by Gαq was reversed by a Gαq specific inhibitor, Gp-antagonist 2A, but not by a specific blocking peptide for Gαs. Expression of GαqQL also inhibited GSK-3β-mediated tau phosphorylation in Xenopus oocytes. These results support a role for the Gq class of G proteins in the regulation of Wnt/β-Catenin signal transduction.  相似文献   

6.
Heterotrimeric G proteins composed of α, β and γ subunits regulate a number of fundamental processes concerned with growth and development in plants. In addition to the canonical heterotrimeric G proteins, plants also contain a small family of extra large G proteins (XLGs) that show significant similarity to the G-protein α subunit in their C-terminal regions. In this paper we show that one of the three XLG genes, XLG3 , and the Gβ subunit (AGB1) of the Arabidopsis G-protein heterotrimer are specifically involved in the regulation of a subset of root morphological and growth responses. Based on analysis of T-DNA insertional mutant phenotypes, XLG3 and AGB1 each positively regulate root waving and root skewing. Since these responses are regulated by physical as well as physiological cues, we assessed the roles of AGB1 and XLG3 in gravitropism, thigmotropism and hormonal responses. Our data show that mutants lacking either XLG3 or AGB1 genes are hypersensitive to ethylene and show growth responses consistent with alterations in auxin transport, while maintaining an essentially wild-type response to the physical cues of gravity and touch. These results suggest that XLG3 and AGB1 proteins regulate the hormonal determinants of root-waving and root-skewing responses in plants and possibly interact in a tissue-specific or signal-specific manner. Because plants harboring knockout mutations in the Gα subunit gene, GPA1 , exhibit wild-type root waving and skewing, our results may indicate that the AGB1 subunit functions in these processes without formation of a classic Gαβγ heterotrimer.  相似文献   

7.
Plants being sessile integrate information from a variety of endogenous and external cues simultaneously to optimize growth and development. This necessitates the signaling networks in plants to be highly dynamic and flexible. One such network involves heterotrimeric G‐proteins comprised of Gα, Gβ, and Gγ subunits, which influence many aspects of growth, development, and stress response pathways. In plants such as Arabidopsis, a relatively simple repertoire of G‐proteins comprised of one canonical and three extra‐large Gα, one Gβ and three Gγ subunits exists. Because the Gβ and Gγ proteins form obligate dimers, the phenotypes of plants lacking the sole or all genes are similar, as expected. However, Gα proteins can exist either as monomers or in a complex with Gβγ, and the details of combinatorial genetic and physiological interactions of different Gα proteins with the sole Gβ remain unexplored. To evaluate such flexible, signal‐dependent interactions and their contribution toward eliciting a specific response, we have generated Arabidopsis mutants lacking specific combinations of and genes, performed extensive phenotypic analysis, and evaluated the results in the context of subunit usage and interaction specificity. Our data show that multiple mechanistic modes, and in some cases complex epistatic relationships, exist depending on the signal‐dependent interactions between the Gα and Gβ proteins. This suggests that, despite their limited numbers, the inherent flexibility of plant G‐protein networks provides for the adaptability needed to survive under continuously changing environments.  相似文献   

8.
《Journal of molecular biology》2019,431(17):3302-3311
RGS6 and RGS7 are regulators of G protein signaling (RGS) proteins that inactivate heterotrimeric (αβγ) G proteins and mediate diverse biological functions, such as cardiac and neuronal signaling. Uniquely, both RGS6 and RGS7 can discriminate between Gαo and Gαi1—two similar Gα subunits that belong to the same Gi sub-family. Here, we show that the isolated RGS domains of RGS6 and RGS7 are sufficient to achieve this specificity. We identified three specific RGS6/7 “disruptor residues” that can attenuate RGS interactions toward Gα subunits and demonstrated that their insertion into a representative high-activity RGS causes a significant, yet non-specific, reduction in activity. We further identified a unique “modulatory” residue that bypasses this negative effect, specifically toward Gαo. Hence, the exquisite specificity of RGS6 and RGS7 toward closely related Gα subunits is achieved via a two-tier specificity system, whereby a Gα-specific modulatory motif overrides the inhibitory effect of non-specific disruptor residues. Our findings expand the understanding of the molecular toolkit used by the RGS family to achieve specific interactions with selected Gα subunits—emphasizing the functional importance of the RGS domain in determining the activity and selectivity of RGS R7 sub-family members toward particular Gα subunits.  相似文献   

9.
Crystal structures of Gαi (and closely related family member Gαt) reveal much of what we currently know about G protein structure, including changes which occur in Switch regions. Gαt exhibits a low rate of basal (uncatalyzed) nucleotide exchange and an ordered Switch II region in the GDP‐bound state, unlike Gαi, which exhibits higher basal exchange and a disordered Switch II region in GαiGDP structures. Using purified Gαi and Gαt, we examined the intrinsic tryptophan fluorescence of these proteins, which reports conformational changes associated with activation and deactivation of Gα proteins. In addition to the expected enhancement in tryptophan fluorescence intensity, activation of GαGDP proteins was accompanied by a modest but notable red shift in tryptophan emission maxima. We identified a cation‐π interaction between tryptophan and arginine residues in the Switch II of Gαi family proteins that mediates the observed red shift in emission maxima. Furthermore, amino‐terminal myristoylation of Gαi resulted in a less polar environment for tryptophan residues in the GTPase domain, consistent with an interaction between the myristoylated amino terminus and the GTPase domain of Gα proteins. These results reveal unique insights into conformational changes which occur upon activation and deactivation of G proteins in solution.  相似文献   

10.
In this study, we report the functional characterization of heterotrimeric G-proteins from a nonvascular plant, the moss Physcomitrella patens. In plants, G-proteins have been characterized from only a few angiosperms to date, where their involvement has been shown during regulation of multiple signaling and developmental pathways affecting overall plant fitness. In addition to its unparalleled evolutionary position in the plant lineages, the P. patens genome also codes for a unique assortment of G-protein components, which includes two copies of and genes, but no canonical . Instead, a single gene encoding an extra-large Gα (XLG) protein exists in the P. patens genome. Here, we demonstrate that in P. patens the canonical Gα is biochemically and functionally replaced by an XLG protein, which works in the same genetic pathway as one of the Gβ proteins to control its development. Furthermore, the specific G-protein subunits in P. patens are essential for its life cycle completion. Deletion of the genomic locus of PpXLG or PpGβ2 results in smaller, slower growing gametophores. Normal reproductive structures develop on these gametophores, but they are unable to form any sporophyte, the only diploid stage in the moss life cycle. Finally, the mutant phenotypes of ΔPpXLG and ΔPpGβ2 can be complemented by the homologous genes from Arabidopsis, AtXLG2 and AtAGB1, respectively, suggesting an overall conservation of their function throughout the plant evolution.In all known eukaryotes, cellular signaling involves heterotrimeric GTP-binding proteins (G-proteins), which consist of Gα, Gβ, and Gγ subunits (Cabrera-Vera et al., 2003). According to the established paradigm, when Gα is GDP-bound, it forms a trimeric complex with the Gβγ dimer and remains associated with a G-protein coupled receptor. Signal perception by the receptor facilitates GDP to GTP exchange on Gα. GTP-Gα dissociates from the Gβγ dimer, and both these entities can transduce the signal by interacting with different effectors. The duration of the active state is determined by the intrinsic GTPase activity of Gα, which hydrolyzes bound GTP into GDP and inorganic phosphate (Pi), followed by the reassociation of the inactive, trimeric complex (Siderovski and Willard, 2005).In plants, G-protein signaling has been studied in only a few angiosperms to date at the functional level, although the proteins exist in the entire plant lineage (Hackenberg and Pandey, 2014; Urano and Jones, 2014; Hackenberg et al., 2016). Interestingly, while the overall biochemistry of the individual G-protein components and the interactions between them are conserved between plant and metazoan systems, deviations from the established norm are also obvious. For example, the repertoire of canonical G-proteins is significantly limited in plants; the human genome codes for 23 Gα, 5 Gβ, and 12 Gγ proteins, whereas most plant genomes, including those of basal plants, typically encode 1 canonical Gα, 1 Gβ, and three to five Gγ proteins (Urano and Jones, 2014). The only exceptions are some polyploid species, such as soybean, which have retained most of the duplicated G-protein genes (Bisht et al., 2011; Choudhury et al., 2011). Moreover, even in plants that possess only a single canonical Gα and Gβ protein, for example Arabidopsis (Arabidopsis thaliana) and rice, the phenotypes of plants lacking either one or both proteins are relatively subtle. The mutant plants exhibit multiple developmental and signaling defects but are able to complete the life cycle without any major consequences. These observations have questioned the significance of G-protein mediated signaling pathways in plants.Interestingly, plants also possess certain unique variants of the classical G-protein components such as the type III Cys-rich Gγ proteins and extra-large GTP-binding (XLG) proteins, which add to the diversity and expanse of the G-protein signaling networks (Roy Choudhury et al., 2011; Chakravorty et al., 2015; Maruta et al., 2015). The XLG proteins are almost twice the size of typical Gα proteins, with the C-terminal region that codes for Gα-like domain and an extended N-terminal region without any distinctive features. Plant XLGs are encoded by entirely independent genes and therefore are different from the mammalian extra-long versions of Gα proteins such as XLαs and XXLαs, which are expressed due to the use of alternate exons (Abramowitz et al., 2004). Three to five copies of XLG proteins are present in the genome of most angiosperms. At the functional level, the XLG proteins have been characterized only from Arabidopsis, to date, where recent studies suggest that the proteins compete with canonical Gα for binding with the Gβγ dimers and may form functional trimeric complexes (Chakravorty et al., 2015; Maruta et al., 2015). The XLG and Gβγ mutants of Arabidopsis seem to function in the same pathways during the regulation of a subset of plant responses, for example primary root length and its regulation by abscisic acid (ABA); the root waving and skewing responses; sensitivity to Glc, salt, and tunicamycin; and sensitivity to certain bacterial and fungal pathogens (Ding et al., 2008; Pandey et al., 2008; Chakravorty et al., 2015; Maruta et al., 2015). However, many of the phenotypes of Arabidopsis Gα and Gβγ mutants are also distinct from that of the xlg triple mutants. For example, compared to the wild-type plants, the canonical G-protein mutants exhibit altered response to gibberellic acid, brassinosteroids, and auxin and show changes in leaf shape, branching, flowering time, and stomatal densities (Ullah et al., 2003; Chen et al., 2004; Pandey et al., 2006; Zhang et al., 2008; Nilson and Assmann, 2010). The xlg triple mutants behave similarly to wild-type plants in all these aspects of development and signaling. Moreover, whether the XLG proteins are authentic GTP-binding and -hydrolyzing proteins and the extent to which they directly participate in G-protein-mediated signaling pathways remains confounding (Chakravorty et al., 2015; Maruta et al., 2015). Even in plants with a limited number of G-protein subunits such as Arabidopsis, one Gα and three XLGs potentially compete for a single Gβ protein, and the analysis of null mutants is not straightforward, that is, it is not possible to delineate whether the phenotypes seen in the Gα null mutants are truly due to the lack of Gα and/or because of an altered stoichiometry or availability of Gβ for the XLG proteins.As a bryophyte, Physcomitrella patens occupies a unique position in the evolutionary history of plants. It lacks vasculature but exhibits alteration between generations, which is dominated by a gametophytic (haploid) phase and a short sporophytic (diploid) phase (Cove et al., 2009). Many of the pathways related to hormone signaling, stress responses, and development are conserved between angiosperms and P. patens (Cove et al., 2009; Sun, 2011; Komatsu et al., 2013; Yasumura et al., 2015). It is also an intriguing example in the context of the G-protein signaling, because its fully sequenced genome does not encode a canonical Gα gene, although genes coding for the Gβ and Gγ proteins exist. A single gene for a potential XLG homolog also exists in the P. patens genome. This unique assortment of proteins predicts several alternative scenarios for G-protein signaling in P. patens. For example, the P. patens Gβγ proteins might be nonfunctional due to the loss of canonical Gα and are left in the genome as evolutionary artifacts. Alternatively, the Gβγ proteins of P. patens might maintain functionality regardless of the existence of a canonical Gα protein in pathways not regulated via classic G-protein signaling modes. Finally, a more likely scenario could be that the potential XLG protein can substitute for the Gα function in P. patens.To explore these possibilities and understand better the conserved and unique mechanisms of G-protein signaling pathways in plants and their significance, we examined the role of G-protein subunits in P. patens. We provide unambiguous evidence for the genetic coupling of XLG and Gβ proteins in controlling P. patens development. In contrast to all other plant species analyzed to date, where G-proteins are not essential for growth and survival, the XLG or one of the Gβ proteins is required for the sporophyte formation and life cycle completion in P. patens. Furthermore, one of the Arabidopsis XLG proteins, XLG2, and the canonical Gβ protein AGB1 can functionally complement the P. patens mutant phenotypes. These data provide new insights in the evolutionary breadth and the spectrum of signaling pathways regulated by G-proteins in plants.  相似文献   

11.
Several reports document the role of tumor necrosis factor alpha (TNF-α) and lipid metabolism in the context of acute inflammation as a causative factor in obesity-associated insulin resistance and as one of the causative parameter of type 2 diabetes mellitus (T2DM). Our aim was to investigate the association between −308G/A and −238G/A polymorphisms located in the promoter region of the TNF-α gene in T2DM in the Indian population with bioinformatics analysis of TNF-α protein networking with an aim to find new target sites for the treatment of T2DM. Demographics of 100 diabetes patients and 100 healthy volunteers were collected in a structured proforma and 3 ml blood samples were obtained from the study group, after approval of Institutional Ethics Committee of the hospital (IEC). The information on clinical parameters was obtained from medical records. Genomic DNA was extracted; PCR–RFLP was performed using TNF-α primers specific to detect the presence of SNPs. Various bioinformatics tools such as STRING software were used to determine its network with other associated genes. The PCR–RFLP studies showed that among the −238G/A types the GG genotype was 87%, GA genotype was 12% and AA genotype was 1%. Almost a similar pattern of results was obtained with TNF-α −308G/A polymorphism. The results obtained were evaluated statistically to determine the significance. By constructing TNF-α protein interaction network we could analyze ontology and hubness of the network to identify the networking of this gene which may influence the functioning of other genes in promoting T2DM. We could identify new targets in T2DM which may function in association with TNF-α. Through hub analysis of TNF-α protein network we have identified three novel proteins RIPK1, BIRC2 and BIRC3 which may contribute to TNF-mediated T2DM pathogenesis. In conclusion, our study indicated that some of the genotypes of TNF-α −308G/A, −238G/A were not significantly associated to type 2 diabetes mellitus, but TNF-α −308G/A polymorphism was reported to be a potent risk factor for diabetes in higher age (>45) groups. Also, the novel hub proteins may serve as new targets against TNF-α T2DM pathogenesis.  相似文献   

12.
Calmodulin (CaM) binds only oncogenic KRas, but not HRas or NRas, and thus contributes only to KRAS-driven cancers. How CaM interacts with KRas and how it boosts KRAS cancers are among the most coveted aims in cancer biology. Here we address this question, and further ask: Are there proteins that can substitute for CaM in HRAS- and NRAS-driven cancers? Can scaffolding protein IQGAP1 be one? Data suggest that formation of a CaM–KRas–PI3Kα ternary complex promotes full PI3Kα activation, and thereby potent PI3Kα/Akt/mTOR proliferative signaling. CaM binds PI3Kα at the cSH2 and nSH2 domains of its regulatory p85 subunit; the WW domain of IQGAP1 binds cSH2. This raises the question whether IQGAP1, together with an oncogenic Ras isoform, can partially activate PI3Kα. Activated, membrane-bound PI3Kα generates PIP3. CaM shuttles Akt to the plasma membrane; CaM's release and concomitant phosphoinositide binding stimulates Akt activation. Notably, IQGAP1 directly interacts with, and helps juxtapose, PI3Kα and Akt as well as mTOR. Our mechanistic review aims to illuminate CaM's actions, and help decipher how oncogenic Ras isoforms – not only KRas4B – can activate the PI3Kα/Akt/mTOR pathway at the membrane and innovate drug discovery, including blocking the PI3Kα–IQGAP1 interaction in HRAS- and NRAS-driven cancers.  相似文献   

13.
The pcdhα/CNR gene comprises a diverse array of neuronal cell-surface proteins of the cadherin superfamily, although very little is known about their role in neural development. Here we provide the first in-depth characterization of pcdh1α in zebrafish. Whole-mount immunocytochemistry demonstrates that a large proportion of endogenous cytoplasmic domain immunoreactivity is present in the nucleus, suggesting that endoproteolytic cleavage and nuclear translocation of the intracellular domain are important aspects of pcdh1α activity in vivo. Using whole-mount immunocytochemistry and BAC-based expression of Pcdh1α-GFP fusion proteins, we find that Pcdh1α does not appear to form stable, synaptic puncta at early stages of synaptogenesis. We also demonstrate that the presence of the Pcdh1α cytoplasmic domain is essential for normal function. Truncation of Pcdh1α proteins, using splice-blocking antisense morpholinos to prevent the addition of the common intracellular domain to the entire pcdh1α cluster, results in neuronal apoptosis throughout the developing brain and spinal cord, demonstrating an essential role for pcdh1α in early neural development. This cell death phenotype can be attenuated by the expression of a soluble Pcdh1α cytoplasmic domain.  相似文献   

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15.
The extra-large guanosine-5′-triphosphate (GTP)-binding protein 2, XLG2, is an unconventional Gα subunit of the Arabidopsis (Arabidopsis thaliana) heterotrimeric GTP-binding protein complex with a major role in plant defense. In vitro biochemical analyses and molecular dynamic simulations show that affinity of XLG2 for GTP is two orders of magnitude lower than that of the conventional Gα, AtGPA1. Here we tested the physiological relevance of GTP binding by XLG2. We generated an XLG2(T476N) variant with abolished GTP binding, as confirmed by in vitro GTPγS binding assay. Yeast three-hybrid, bimolecular fluorescence complementation, and split firefly-luciferase complementation assays revealed that the nucleotide-depleted XLG2(T476N) retained wild-type XLG2-like interactions with the Gβγ dimer and defense-related receptor-like kinases. Both wild-type and nucleotide-depleted XLG2(T476N) restored the defense responses against Fusarium oxysporum and Pseudomonas syringae compromised in the xlg2 xlg3 double mutant. Additionally, XLG2(T476N) was fully functional restoring stomatal density, root growth, and sensitivity to NaCl, but failed to complement impaired germination and vernalization-induced flowering. We conclude that XLG2 is able to function in a GTP-independent manner and discuss its possible mechanisms of action.

Arabidopsis extra-large GTP-binding proteins have nucleotide-independent functions.  相似文献   

16.
17.
Elongation factor 1α (EF-1α) and elongation factor-like protein (EFL) are considered to be functionally equivalent proteins involved in peptide synthesis. Eukaryotes can be fundamentally divided into ‘EF-1α-containing’ and ‘EFL-containing’ types. Recently, EF-1α and EFL genes have been surveyed across the diversity of eukaryotes to explore the origin and evolution of EFL genes. Although the phylum Cercozoa is a diverse group, gene data for either EFL or EF-1α are absent from all cercozoans except chlorarachniophytes which were previously defined as EFL-containing members. Our survey revealed that two members of the cercozoan subphylum Filosa (Thaumatomastix sp. and strain YPF610) are EFL-containing members. Importantly, we identified EF-1α genes from two members of Filosa (Paracercomonas marina and Paulinella chromatophora) and a member of the other subphylum Endomyxa (Filoreta japonica). All cercozoan EFL homologues could not be recovered as a monophyletic group in maximum-likelihood and Bayesian analyses, suggesting that lateral gene transfer was involved in the EFL evolution in this protist assemblage. In contrast, EF-1α analysis successfully recovered a monophyly of three homologues sampled from the two cercozoan subphyla. Based on the results, we postulate that cercozoan EF-1α genes have been vertically inherited, and the current EFL-containing species may have secondarily lost their EF-1α genes.  相似文献   

18.
As in mammalian systems, heterotrimeric G proteins, composed of alpha, beta and gamma subunits, are present in plants and are involved in the regulation of development and cell signaling. Besides the sole prototypical G protein alpha subunit gene, GPA1, the Arabidopsis thaliana genome has three extra-large GTP-binding protein (XLG)-encoding genes: XLG1 (At2g23460), XLG2 (At4g34390) and XLG3 (At1g31930). The C-termini of the XLGs are Galpha domains that are homologous to GPA1, whereas their N-termini each contain a cysteine-rich region and a putative nuclear localization signal (NLS). GFP fusions with each XLG confirmed nuclear localization. All three XLG genes are expressed in essentially all plant organs, with strong expression in vascular tissues, primary root meristems and lateral root primordia. Analysis of single, double and triple T-DNA insertional mutants of the XLG genes revealed redundancy in XLG function. Dark-grown xlg1-1 xlg2-1 xlg3-1 triple mutant plants showed markedly increased primary root length compared with wild-type plants. This phenotype was not observed in dark-grown xlg single mutants, and was suppressed upon complementation of the xlg triple mutant with each XLG. Root cell sizes of the xlg triple mutant and root morphology were highly similar to those of wild-type roots, suggesting that XLGs may regulate cell proliferation. Dark-grown roots of the xlg triple mutants also showed altered sensitivity to sugars, ABA hyposensitivity and ethylene hypersensitivity, whereas seed germination in xlg triple mutants was hypersensitive to osmotic stress and ABA. As plant-specific proteins, regulatory mechanisms of XLGs may differ from those of conventional Galphas.  相似文献   

19.
20.
Modulation of the active versus inactive forms of the Gα protein is critical for the signaling processes mediated by the heterotrimeric G‐protein complex. We have recently established that in Arabidopsis, the regulator of G‐protein signaling (RGS1) protein and a lipid‐hydrolyzing enzyme, phospholipase Dα1 (PLDα1), both act as GTPase‐activity accelerating proteins (GAPs) for the Gα protein to attenuate its activity. RGS1 and PLDα1 interact with each other, and RGS1 inhibits the activity of PLDα1 during regulation of a subset of responses. In this study, we present evidence that this regulation is bidirectional. Phosphatidic acid (PA), a second messenger typically derived from the lipid‐hydrolyzing activity of PLDα1, is a molecular target of RGS1. PA binds and inhibits the GAP activity of RGS1. A conserved lysine residue in RGS1 (Lys259) is directly involved in RGS1–PA binding. Introduction of this RGS1 protein variant in the rgs1 mutant background makes plants hypersensitive to a subset of abscisic acid‐mediated responses. Our data point to the existence of negative feedback loops between these two regulatory proteins that precisely modulate the level of active Gα, consequently generating a highly controlled signal–response output.  相似文献   

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