New Papers

Plant Ubiquitin/26S Proteasome Papers in The Last Three Months

Updated on 10/5/2008

1: Plant Cell. 2008 Sep 30. [Epub ahead of print] Related Articles: Proteolysis-Independent Downregulation of DELLA Repression in Arabidopsis by the Gibberellin Receptor GIBBERELLIN INSENSITIVE DWARF1.

Ariizumi T, Murase K, Sun TP, Steber CM.

Department of Crop and Soil Science, Washington State University, Pullman, Washington 99164-6420.

This article presents evidence that DELLA repression of gibberellin (GA) signaling is relieved both by proteolysis-dependent and -independent pathways in Arabidopsis thaliana. DELLA proteins are negative regulators of GA responses, including seed germination, stem elongation, and fertility. GA stimulates GA responses by causing DELLA repressor degradation via the ubiquitin-proteasome pathway. DELLA degradation requires GA biosynthesis, three functionally redundant GA receptors GIBBERELLIN INSENSITIVE DWARF1 (GID1a, b, and c), and the SLEEPY1 (SLY1) F-box subunit of an SCF E3 ubiquitin ligase. The sly1 mutants accumulate more DELLA proteins but display less severe dwarf and germination phenotypes than the GA biosynthesis mutant ga1-3 or the gid1abc triple mutant. Interestingly, GID1 overexpression rescued the sly1 dwarf and infertility phenotypes without decreasing the accumulation of the DELLA protein REPRESSOR OF GA1-3. GID1 rescue of sly1 mutants was dependent on the level of GID1 protein, GA, and the presence of a functional DELLA motif. Since DELLA shows increasing interaction with GID1 with increasing GA levels, it appears that GA-bound GID1 can block DELLA repressor activity by direct protein-protein interaction with the DELLA domain. Thus, a SLY1-independent mechanism for GA signaling may function without DELLA degradation.

PMID: 18827182 [PubMed - as supplied by publisher]

2: EMBO J. 2008 Sep 25. [Epub ahead of print] Related Articles, Links: Structural and functional coupling of Hsp90- and Sgt1-centred multi-protein complexes.

Zhang M, Botër M, Li K, Kadota Y, Panaretou B, Prodromou C, Shirasu K, Pearl LH.

Section of Structural Biology, The Institute of Cancer Research, Chester Beatty Laboratories, London, UK.

Sgt1 is an adaptor protein implicated in a variety of processes, including formation of the kinetochore complex in yeast, and regulation of innate immunity systems in plants and animals. Sgt1 has been found to associate with SCF E3 ubiquitin ligases, the CBF3 kinetochore complex, plant R proteins and related animal Nod-like receptors, and with the Hsp90 molecular chaperone. We have determined the crystal structure of the core Hsp90-Sgt1 complex, revealing a distinct site of interaction on the Hsp90 N-terminal domain. Using the structure, we developed mutations in Sgt1 interfacial residues, which specifically abrogate interaction with Hsp90, and disrupt Sgt1-dependent functions in vivo, in plants and yeast. We show that Sgt1 bridges the Hsp90 molecular chaperone system to the substrate-specific arm of SCF ubiquitin ligase complexes, suggesting a role in SCF assembly and regulation, and providing multiple complementary routes for ubiquitination of Hsp90 client proteins.

PMID: 18818696 [PubMed - as supplied by publisher]
PMCID: PMC2556094

3: Plant Physiol. 2008 Sep 24. [Epub ahead of print] Related Articles, Links: Powdery Mildew Resistance Conferred by Loss of the EDR1 Protein Kinase is Suppressed by a Missense Mutation in KEG, a Regulator of ABA Signaling.

Wawrzynska A, Christiansen KM, Lan Y, Rodibaugh NL, Innes RW.

Department of Biology, Indiana University, Bloomington, IN 47405, USA.

Loss-of-function mutations in the Arabidopsis ENHANCED DISEASE RESISTANCE 1 (EDR1) gene confer enhanced resistance to infection by powdery mildew (Golovinomyces cichoracearum). EDR1 encodes a protein kinase, but its substrates, and the pathways regulated by EDR1 are unknown. To identify components of the EDR1 signal transduction pathway(s) we conducted a forward genetic screen for mutations that suppressed edr1-mediated disease resistance. Genetic mapping and cloning of one of these suppressor mutations revealed a recessive missense mutation in the KEEP ON GOING gene (KEG; At5g13530), which we designated keg-4. KEG encodes a multi-domain protein that includes a RING E3 ligase domain, a kinase domain, ankyrin repeats and HERC2-like repeats. The KEG protein has previously been shown to have ubiquitin ligase activity and to negatively regulate protein levels of the transcription factor ABCISIC ACID INSENSITIVE 5 (ABI5). KEG mRNA levels were found to be three fold higher in edr1 mutant plants compared to wild type. Loss-of-function mutations in KEG are seedling lethal and are hypersensitive to glucose and abscisic acid (ABA). The keg-4 mutation, in contrast, conferred resistance to 6% glucose, and suppressed edr1-mediated hypersensitivity to ABA, suggesting that the keg-4 mutation suppresses ABA signaling by altering KEG function. Several ABA-responsive genes were found to be further upregulated in the edr1 mutant following ABA treatment, and this upregulation was suppressed by the keg-4 mutation. We conclude that edr1-mediated resistance to powdery mildew is mediated, in part, by enhanced ABA signaling.

PMID: 18815384 [PubMed - as supplied by publisher]

4: J Biol Chem. 2008 Sep 24. [Epub ahead of print] Related Articles, Links: COP1 functions as a FoxO1 Ubiquitin E3 ligase to regulate FoxO1-mediated gene expression.

Kato S, Ding J, Psick E, Jhala US, Du K.

Tufts-New England Medical Center, Boston, MA 02111.

COP1 is a Ring-Finger E3 ubiquitin ligase that is involved in plant development, mammalian cell survival, growth and metabolism. Here, we report that COP1, whose expression is enhanced by insulin, regulates FoxO1 protein stability. We found that in Fao hepatoma cells, ectopic expression of COP1 decreased, whereas knockdown of COP1 expression increased the level of endogenous FoxO1 protein without impacting other factors such as C/EBPa and CREB. We further showed that COP1 binds FoxO1, enhances its ubiquitination and promotes its degradation via the ubiquitin-proteasome pathway. To determine the biological significance of COP1-mediated FoxO1 protein degradation, we have examined the impact of COP1 on FoxO1-mediated gene expression and found that COP1 suppressed FoxO1 reporter gene as well as FoxO1 target genes such as G6Pase and PEPCK, two key targets for FoxO1 in the regulation of gluconeogenesis, with corresponding changes of hepatic glucose production in Fao cells. We suggest that, by functioning as a FoxO1 E3 ligase, COP1 may play a role in the regulation of hepatic glucose metabolism.

PMID: 18815134 [PubMed - as supplied by publisher]

5: Plant Cell. 2008 Sep 23. [Epub ahead of print] Related Articles, Links: Biochemical Characterization of Arabidopsis Complexes Containing CONSTITUTIVELY PHOTOMORPHOGENIC1 and SUPPRESSOR OF PHYA Proteins in Light Control of Plant Development.

Zhu D, Maier A, Lee JH, Laubinger S, Saijo Y, Wang H, Qu LJ, Hoecker U, Deng XW.

Peking-Yale Joint Center of Plant Molecular Genetics and Agrobiotechnology, College of Life Sciences, Peking University, Beijing 100871, China; National Institute of Biological Sciences, Zhongguancun Life Science Park, Beijing 102206, China; Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, Conecticut 06520-8104.

COP1 (for CONSTITUTIVELY PHOTOMORPHOGENIC1) and the four partially redundant SPA (for SUPPRESSOR OF PHYA) proteins work in concert to repress photomorphogenesis in Arabidopsis thaliana by targeting key transcription factors and phytochrome A for degradation via the 26S proteasome. Here, we report a detailed biochemical characterization of the SPA-COP1 complexes. The four endogenous SPA proteins can form stable complexes with COP1 in vivo regardless of light conditions but exhibit distinct expression profiles in different tissues and light conditions. The SPA proteins can self-associate or interact with each other, forming a heterogeneous group of SPA-COP1 complexes in which the exact SPA protein compositions vary depending on the abundance of individual SPA proteins. The four SPA proteins could be divided into two functional groups depending on their interaction affinities, their regulation of ELONGATED HYPOCOTYL5 degradation, and their opposite effects on COP1 protein accumulation. Loss-of-function mutations in a predominant SPA protein may cause a significant reduction in the overall SPA-COP1 E3 ligase activity, resulting in a partial constitutive photomorphogenic phenotype. This study thus provides an in-depth biochemical view of the SPA-COP1 E3 ligase complexes and offers new insights into the molecular basis for their distinct roles in the light control of plant development.

PMID: 18812498 [PubMed - as supplied by publisher]

6: Plant J. 2008 Sep 21. [Epub ahead of print] Related Articles, Links: THE BTB UBIQUITIN LIGASES ETO1, EOL1 AND EOL2 ACT COLLECTIVELY TO REGULATE ETHYLENE BIOSYNTHESIS IN ARABIDOPSIS BY CONTROLLING TYPE-2 ACC SYNTHASE LEVELS.

Christians MJ, Gingerich DJ, Hansen M, Binder BM, Kieber JJ, Vierstra RD.

Department of Genetics, 425-G Henry Mall, University of Wisconsin, Madison WI 53706-1574, USA.

Ethylene biosynthesis is directed by a family of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS) that convert S-adenosyl-L-methionine to the immediate precursor ACC. Members of the type-2 ACS subfamily are strongly regulated by proteolysis with various signals stabilizing the proteins to increase ethylene production. In Arabidopsis, this turnover is mediated by the ubiquitin/26S proteasome system, using a Broad Complex/Tramtrack/Bric-a-Brac (BTB) E3 assembled with the ETHYLENE OVERPRODUCER 1 (ETO)-1 BTB protein for target recognition. Here, we show that two Arabidopsis BTB proteins closely related to ETO1, designated ETO1-LIKE (EOL)-1 and EOL2, also negatively regulate ethylene synthesis via their ability to target ACSs for breakdown. Like ETO1, EOL1 interacts with type-2 ACSs (ACS4, 5 and 9) but not with type-1 or type-3 ACSs or type-2 ACS mutants that stabilize the corresponding proteins in planta. Whereas single and double mutants affecting EOL1 and EOL2 do not show an ethylene-related phenotype, they exaggerate the effects caused by inactivation of ETO1 and further increase ethylene production and the accumulation of ACS5 in eto1 plants. The triple eto1 eol1 eol2 mutant phenotype can be effectively rescued by the ACS inhibitor aminoethoxyvinylglycine and by silver, which antagonizes ethylene perception. Together with hypocotyl growth assays showing that the sensitivity and response kinetics to ethylene are normal, it appears that ethylene synthesis but not signaling is compromised in the triple mutant. Collectively, the data indicate that the Arabidopsis BTB E3s assembled with ETO1, EOL1 and EOL2 work together to negatively regulate ethylene synthesis by directing the degradation of type-2 ACS proteins.

PMID: 18808454 [PubMed - as supplied by publisher]

7: Nat Cell Biol. 2008 Oct;10(10):1208-16. Epub 2008 Sep 21. Related Articles, Links

Jade-1 inhibits Wnt signalling by ubiquitylating beta-catenin and mediates Wnt pathway inhibition by pVHL.

Chitalia VC, Foy RL, Bachschmid MM, Zeng L, Panchenko MV, Zhou MI, Bharti A, Seldin DC, Lecker SH, Dominguez I, Cohen HT.

Renal Section, Department of Medicine, Boston University School of Medicine, Boston, Massachusetts 02118, USA.

The von Hippel-Lindau protein pVHL suppresses renal tumorigenesis in part by promoting the degradation of hypoxia-inducible HIF-alpha transcription factors; additional mechanisms have been proposed. pVHL also stabilizes the plant homeodomain protein Jade-1, which is a candidate renal tumour suppressor that may correlate with renal cancer risk. Here we show that Jade-1 binds the oncoprotein beta-catenin in Wnt-responsive fashion. Moreover, Jade-1 destabilizes wild-type beta-catenin but not a cancer-causing form of beta-catenin. Whereas the well-established beta-catenin E3 ubiquitin ligase component beta-TrCP ubiquitylates only phosphorylated beta-catenin, Jade-1 ubiquitylates both phosphorylated and non-phosphorylated beta-catenin and therefore regulates canonical Wnt signalling in both Wnt-off and Wnt-on phases. Thus, the different characteristics of beta-TrCP and Jade-1 may ensure optimal Wnt pathway regulation. Furthermore, pVHL downregulates beta-catenin in a Jade-1-dependent manner and inhibits Wnt signalling, supporting a role for Jade-1 and Wnt signalling in renal tumorigenesis. The pVHL tumour suppressor and the Wnt tumorigenesis pathway are therefore directly linked through Jade-1.

Publication Types:

PMID: 18806787 [PubMed - in process]

8: Plant J. 2008 Sep 16. [Epub ahead of print] Related Articles, Links: The E2 ubiquitin-conjugating enzymes, AtUBC1 and AtUBC2, play redundant roles and are involved in activation of FLC expression and repression of flowering in Arabidopsis thaliana.

Xu L, Ménard R, Berr A, Fuchs J, Cognat V, Meyer D, Shen WH.

Institut de Biologie Moléculaire des Plantes (IBMP), Laboratoire Propre du CNRS (UPR2357) conventionné avec l'Université Louis Pasteur Strasbourg 1, 12 rue du Général Zimmer, 67084 Strasbourg Cedex, France.

Posttranslational modifications of proteins by addition of ubiquitin can regulate protein degradation and localization, protein-protein interaction and transcriptional activation. In the ubiquitylation system, substrate specificity is primarily determined by the E2 ubiquitin-conjugating enzyme (UBC) and the E3 ubiquitin-ligase. The Arabidopsis thaliana genome contains 37 genes encoding UBC homologues. Biological functions of these genes, however, remain largely uncharacterized. Here, we report reverse genetic characterization of AtUBC1 and AtUBC2. While loss-of-function single mutants Atubc1-1 and Atubc2-1 only show weak phenotypes, the double mutant Atubc1-1Atubc2-1 shows a dramatically reduced number of rosette leaves and an early-flowering phenotype. Consistently, transcript levels of the floral repressor genes FLOWERING LOCUS C (FLC), MADS ASSOCIATED FLOWERING4 (MAF4) and MAF5 are reduced in the double mutant. The loss-of-function mutants of HISTONE MONOUBIQUITINATION1 (HUB1) and HUB2, which were previously reported to encode an E3 involved in histone H2B ubiquitylation, also show an early-flowering phenotype and reduced levels of FLC, MAF4 and MAF5 transcripts. In both Atubc1-1Atubc2-1 and hub2-2 mutants, H2B mono-ubiquitylation is drastically reduced. Taken together, our results indicate that the E2 AtUBC1/AtUBC2 and the E3 HUB1/HUB2 together mediate H2B ubiquitylation, which is involved in the activation of floral repressor genes as well as in other processes as evidenced from pleiotropic phenotypes of the mutants.

PMID: 18798874 [PubMed - as supplied by publisher]

9: Biochem Soc Trans. 2008 Oct;36(Pt 5):843-7. Related Articles, Links: Do F-box proteins with a C-terminal domain homologous with the tobacco lectin play a role in protein degradation in plants?

Lannoo N, Peumans WJ, Van Damme EJ.

Laboratory of Biochemistry and Glycobiology, Department of Molecular Biotechnology, Ghent University, Coupure Links 653, 9000 Ghent, Belgium.

Protein turnover is a key post-translational event that regulates numerous cellular processes. It enables cells to respond rapidly to intracellular signals and changing environmental conditions by adjusting the levels of pivotal proteins. A major proteolytic pathway involves the ubiquitination of target proteins and subsequent targeting to the 26S proteasome for degradation. Many F-box proteins play a determining role in the substrate specificity of this degradation pathway. In most cases, selective recognition of the target proteins relies on protein-protein interactions mediated by the C-terminal domain of the F-box proteins. In mammals, the occurrence of F-box proteins with a C-terminal SBD (sugar-binding domain) that specifically interacts with high-mannose N-glycans on target glycoproteins has been documented. The identification and characterization of these sugar-binding F-box proteins demonstrated that F-box proteins do not exclusively use protein-protein interactions but also protein-carbohydrate interactions in the Ub (ubiquitin)/proteasome pathway. Recently, putative sugar-binding F-box proteins have been identified in plants. Genome analyses in Arabidopsis and rice revealed the presence of F-box proteins with a C-terminal lectin-related domain homologous with Nictaba, a jasmonate-inducible lectin from tobacco that was shown to interact with the core structure of high-mannose and complex N-glycans. Owing to the high similarity in structure and specificity between Nictaba and the SBD of the mammalian Fbs proteins, a similar role for the plant F-box proteins with a Nictaba domain in nucleocytoplasmic protein degradation in plant cells is suggested.

PMID: 18793148 [PubMed - in process]

10: Plant Physiol. 2008 Sep 10. [Epub ahead of print]
Related Articles, Links: Different hormonal regulation of cellular differentiation and function in nucellar projection and endosperm transfer cells - a microdissection-based transcriptome study of young barley grains.

Thiel J, Weier D, Sreenivasulu N, Strickert M, Weichert N, Melzer M, Czauderna T, Wobus U, Weber H, Weschke W.

Leibniz-Institut fur Pflanzengenetik und Kulturpflanzenforschung , Corrensstrasse 3, D-06466 Gatersleben, Germany.

Nucellar projection (NP) and endosperm transfer cells (ETC) are essential tissues in growing barley grains responsible for nutrient transfer from maternal to filial tissues, endosperm /embryo nutrition and grain development. A laser microdissection pressure catapulting (LMPC)-based transcriptome analysis was established to study NP and ETC separately using a barley 12k macroarray. A major challenge was to isolate high-quality mRNA from pre-embedded, fixed tissue, while maintaining tissue integrity. We show that probes generated from fixed and embedded tissue sections represent largely the transcriptome (>70%) of non-chemically treated and non-amplified references. In NP, the top-down gradient of cellular differentiation is reflected by expression of C3HC4-type ubiquitin ligases and different histone genes, cell-wall biosynthesis and expansin/extensin genes as well as genes involved in programmed cell death-related proteolysis coupled to N-remobilization, indicating distinct areas simultaneously undergoing mitosis, cell elongation and disintegration. Activated gene expression related to gibberellin synthesis and function suggests a regulatory role for gibberellins (GAs) in establishment of the differentiation gradient. Upregulation of plasmalemma-intrinsic protein- and tonoplast-intrinsic protein-genes indicates involvement in nutrient transfer and/or unloading. In ETC, AP2/EREBP-like transcription factors and ethylene functions are transcriptionally activated, a response possibly coupled to activated defence mechanisms. Transcriptional activation of nucleotide sugar metabolism may be attributed to ascorbate synthesis and/or cell-wall biosynthesis. These processes are potentially controlled by trehalose-6-phosphate synthase/phosphatase as suggested by expression of respective genes. Upregulation of amino acid permeases in endosperm transfer cells indicates important roles in active nutrient uptake from the apoplastic space into the endosperm.

PMID: 18784282 [PubMed - as supplied by publisher]

11: J Proteome Res. 2008 Sep 9. [Epub ahead of print]
Related Articles, Links: Proteomic Analysis of Rice Endosperm Cells in Response to Expression of hGM-CSF.

Luo J, Ning T, Sun Y, Zhu J, Zhu Y, Lin Q, Yang D.

Center of Engineering Research of Plant Biotechnology and Germplasm Utilization, Ministry of Education, Department of Genetics, College of Life Sciences, Wuhan University, 430072, P.R. China, and UAlbany Proteomics Facility, Center for Functional Genomics, University at Albany, 1 Discovery Drive, Rensselaer, New York 12144 dyang@whu.edu.cn.

The accumulation of significant levels of transgenic products in plant cells is required not only for crop improvement, but also for molecular pharming. However, knowledge about the fate of transgenic products and endogenous proteins in grain cells is lacking. Here, we utilized a quantitative mass spectrometry-based proteomic approach for comparative analysis of expression profiles of transgenic rice endosperm cells in response to expression of a recombinant pharmaceutical protein, human granulocyte-macrophage colony stimulation factor (hGM-CSF). This study provided the first available evidence concerning the fate of exogenous and endogenous proteins in grain cells. Among 1883 identified proteins with a false positive rate of 5%, 103 displayed significant changes ( p-value < 0.05) between the transgenic and the wild-type endosperm cells. Notably, endogenous storage proteins and most carbohydrate metabolism-related proteins were down-regulated, while 26S proteasome-related proteins and chaperones were up-regulated in the transgenic rice endosperm. Furthermore, it was observed that expression of hGM-CSF induced endoplasmic reticulum stress and activated the ubiquitin/26S-proteasome pathway, which led to ubiquitination of this foreign gene product in the transgenic rice endosperm.

PMID: 18778094 [PubMed - as supplied by publisher]

12: Plant Physiol. 2008 Sep 5. [Epub ahead of print]
Related Articles, Links: F-box Gene Family is Expanded in Herbaceous Annual Plants Relative to Woody Perennial Plants.

Yang X, Kalluri UC, Jawdy S, Gunter LE, Yin T, Tschaplinski TJ, Weston DJ, Ranjan P, Tuskan GA.

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831.

F-box proteins are generally responsible for substrate recognition in the Skp1-Cullin-F-box complexes that are involved in protein degradation via the ubiquitin-26S proteosome pathway. In plants, F-box genes influence a variety of biological processes such as leaf senescence, branching, self-incompatibility and responses to biotic and abiotic stresses. The number of F-box genes in Populus ( approximately 320) is less than half that found in Arabidopsis ( approximately 660) or Oryza ( approximately 680), even though the total number of genes in Populus is equivalent to that in Oryza and 1.5 times that in Arabidopsis. We performed comparative genomics analysis between the woody perennial plant Populus and the herbaceous annual plants Arabidopsis and Oryza in order to explicate the functional implications of this large gene family. Our analyses reveal interspecific differences in genomic distribution, orthologous relationship, intron evolution, protein domain structure, and gene expression. The set of F-box genes shared by these species appear to be involved in core biological processes essential for plant growth and development; lineage-specific differences primarily occurred because of an expansion of the F-box genes via tandem duplications in Arabidopsis and Oryza. The number of F-box genes in the newly sequenced woody species Vitis (156) and Carica (139) is similar to that in Populus, supporting the hypothesis that F-box gene family is expanded in herbaceous annual plants relative to woody perennial plants. The present study provides insights into the relationship between the structure and composition of the F-box gene family in herbaceous and woody species and their associated developmental and physiological features.

PMID: 18775973 [PubMed - as supplied by publisher]

13: Plant Physiol. 2008 Sep 3. [Epub ahead of print]
Related Articles, Links: Protein tyrosine kinases and protein tyrosine phosphatases are involved in ABA-dependent processes in Arabidopsis thaliana seeds and suspension cells.

Ghelis T, Bolbach G, Clodic G, Habricot Y, Miginiac E, Sotta B, Jeannette E.

UPMC Universite Paris 06, UMR 7180, Laboratoire de Physiologie Cellulaire et Moleculaire des Plantes, 3 rue Galilee, 94200, Ivry sur Seine, France; CNRS, UMR 7180, Laboratoire de Physiologie Cellulaire et Moleculaire des Plantes, 3 rue Galilee, 94200, Ivry sur Seine, France; UPMC Universite Paris 06, IFR 83, Plate-Forme de Spectrometrie de Masse et Proteomique, 4 place Jussieu, 75005, Paris, France; CNRS, UMR 7613, Laboratoire de Structure et Fonction de Molecules Bioactives, 4 place Jussieu, 75005, Paris, France.

Protein tyrosine phosphorylation plays a central role in many signaling pathways leading to cell growth and differentiation in animals. Tyrosine phosphorylated proteins have been detected in higher plants and the roles of protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) in some physiological responses have been shown. We investigated the involvement of tyrosine phosphorylation events in abscisic acid (ABA) signaling using a pharmacological approach. Phenylarsine oxide (PAO), a specific inhibitor of PTP activity, abolished the ABA-dependent accumulation of RAB18 transcripts. PTK inhibitors like genistein, tyrphostin A23 and erbstatin, blocked the RAB18 expression induced by ABA in Arabidopsis. Stomatal closure induced by ABA was also inhibited by PAO and genistein. We studied the changes in the tyrosine phosphorylation levels of proteins in Arabidopsis seeds after ABA treatment. Proteins were separated by two-dimensional gel electrophoresis and those phosphorylated on tyrosine residues were detected using an anti-phosphotyrosine antibody by Western blot. Changes were detected in the tyrosine phosphorylation levels of 19 proteins after ABA treatment. Genistein inhibited the ABA-dependent tyrosine phosphorylation of proteins. The 19 proteins were analysed by MALDI-TOF-TOF mass spectrometry. Among the proteins identified were storage proteins like cruciferins, enzymes involved in the mobilization of lipid reserves like aconitase, enolase, aldolase and a lipoprotein, and enzymes necessary for seedling development like the large subunit of Rubisco. Additionally, the identification of three putative signaling proteins: a peptidyl-prolyl isomerase, a RNA-binding protein and a Small Ubiquitin-like MOdifier (SUMO) conjugating enzyme, enlightens how tyrosine phosphorylation might regulate ABA transduction pathways in plants.

PMID: 18768909 [PubMed - as supplied by publisher]

14: Plant Physiol. 2008 Aug 27. [Epub ahead of print]
Related Articles, Links: BAH1/NLA, a RING-type ubiquitin E3 ligase, regulates the accumulation of salicylic acid and immune responses to Pseudomonas syringae DC3000.

Yaeno T, Iba K.

Department of Biology, Faculty of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan.

Salicylic acid (SA) is a primary factor responsible for exerting diverse immune responses in plants, and is synthesized in response to attack by a wide range of pathogens. The Arabidopsis sid2 mutant is defective in a SA biosynthetic pathway involving isochorismate synthase 1 (ICS1) and consequently contains reduced levels of SA. However, the sid2 mutant as well as ICS-suppressed tobacco still accumulates a small but significant level of SA. These observations along with previous studies suggested that SA might also be synthesized by another pathway involving benzoic acid (BA). Here we isolated a benzoic acid hypersensitive 1-Dominant (bah1-D) mutant which excessively accumulated SA after application of BA from activation tagged lines. This mutant also accumulated higher levels of SA after inoculation with Pseudomonas syringae pv. tomato (Pst) DC3000. Analysis of the bah1-D sid2 double mutant suggested that the bah1-D mutation caused both ICS1-dependent and -independent accumulation. In addition, the bah1-D mutant showed SA-dependent localized cell death in response to Pst DC3000. The T-DNA insertional mutation which caused the bah1-D phenotypes resulted in the suppression of expression of the NLA gene, which encodes a RING-type ubiquitin E3 ligase. These results suggest that BAH1/NLA plays crucial roles in the ubiquitination-mediated regulation of immune responses, including BA- and pathogen-induced SA accumulation, and control of cell death.

PMID: 18753285 [PubMed - as supplied by publisher]
PMCID: PMC2556844

15: Trends Plant Sci. 2008 Aug 21. [Epub ahead of print]
Related Articles, Links: Membrane-bound transcription factors in plants.

Seo PJ, Kim SG, Park CM.

Molecular Signaling Laboratory, Department of Chemistry, Seoul National University, Seoul 151-742, Korea.

The ability to activate dormant transcription factors is an important molecular feature of the transcriptional regulatory networks that govern diverse cellular functions. An intriguing example is the controlled proteolytic activation of membrane-bound transcription factors (MTFs). Most MTFs are activated either by intramembrane proteases or by the ubiquitin-proteasome pathway. Recent studies have shown that several members of the bZIP and NAC families in Arabidopsis are membrane-associated and are activated by membrane-associated proteases during stress responses in the endoplasmic reticulum and when the plants experience environmental stresses. A genome-scale analysis shows that over 10% of all transcription factors are membrane bound, indicating that activation of MTFs occurs at the genomic level, allowing transcription to be regulated rapidly under stressful conditions.

PMID: 18722803 [PubMed - as supplied by publisher]

16: Mol Cell. 2008 Aug 22;31(4):607-13.
Related Articles, Links

Arabidopsis COP1/SPA1 complex and FHY1/FHY3 associate with distinct phosphorylated forms of phytochrome A in balancing light signaling.

Saijo Y, Zhu D, Li J, Rubio V, Zhou Z, Shen Y, Hoecker U, Wang H, Deng XW.

Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, CT 06520, USA.

Fine tuning of light signaling is crucial to plant development. Following light-triggered nuclear translocation, the photoreceptor phytochrome A (phyA) regulates gene expression under continuous far-red light and is rapidly destabilized upon red light irradiation by E3 ubiquitin ligases, including COP1. Here we provide evidence that the light signaling repressors SPA proteins contribute to COP1-mediated phyA degradation and that a COP1/SPA1 protein complex is tightly associated with phyA ubiquitination activity. Furthermore, a phosphorylated phyA form accumulates in the nucleus and preferentially associates with the COP1/SPA1 complex. In contrast, underphosphorylated phyA predominantly associates with the phyA-signaling intermediates FHY3 and FHY1. However, COP1 associates with underphosphorylated phyA in the absence of FHY3 or FHY1, suggesting that phyA associations with FHY3 and FHY1 protect underphosphorylated phyA from being recognized by the COP1/SPA complex. We propose that light-induced phyA phosphorylation acts as a switch controlling differential interactions of the photoreceptor with signal propagation or attenuation machineries.

Publication Types:

PMID: 18722184 [PubMed - indexed for MEDLINE]

17: BMC Mol Biol. 2008 Aug 18;9:75.
Related Articles, Links

Normalisation genes for expression analyses in the brown alga model Ectocarpus siliculosus.

Le Bail A, Dittami SM, de Franco PO, Rousvoal S, Cock MJ, Tonon T, Charrier B.

UPMC Univ Paris 6, UMR 7139 Végétaux marins et Biomolécules, Station Biologique, F 29682, Roscoff, France. lebail@sb-roscoff.fr

BACKGROUND: Brown algae are plant multi-cellular organisms occupying most of the world coasts and are essential actors in the constitution of ecological niches at the shoreline. Ectocarpus siliculosus is an emerging model for brown algal research. Its genome has been sequenced, and several tools are being developed to perform analyses at different levels of cell organization, including transcriptomic expression analyses. Several topics, including physiological responses to osmotic stress and to exposure to contaminants and solvents are being studied in order to better understand the adaptive capacity of brown algae to pollution and environmental changes. A series of genes that can be used to normalise expression analyses is required for these studies. RESULTS: We monitored the expression of 13 genes under 21 different culture conditions. These included genes encoding proteins and factors involved in protein translation (ribosomal protein 26S, EF1alpha, IF2A, IF4E) and protein degradation (ubiquitin, ubiquitin conjugating enzyme) or folding (cyclophilin), and proteins involved in both the structure of the cytoskeleton (tubulin alpha, actin, actin-related proteins) and its trafficking function (dynein), as well as a protein implicated in carbon metabolism (glucose 6-phosphate dehydrogenase). The stability of their expression level was assessed using the Ct range, and by applying both the geNorm and the Normfinder principles of calculation. CONCLUSION: Comparisons of the data obtained with the three methods of calculation indicated that EF1alpha (EF1a) was the best reference gene for normalisation. The normalisation factor should be calculated with at least two genes, alpha tubulin, ubiquitin-conjugating enzyme or actin-related proteins being good partners of EF1a. Our results exclude actin as a good normalisation gene, and, in this, are in agreement with previous studies in other organisms.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 18710525 [PubMed - indexed for MEDLINE]

PMCID: PMC2546422

18: Plant Physiol. 2008 Aug 13. [Epub ahead of print]
Related Articles, Links: The Rice ROOT ARCHITECTURE ASSOCIATED1 Binds the Proteasome Subunit RPT4 and Is Degraded in a D-box and Proteasome Dependent Manner.

Han Y, Cao H, Jiang J, Xu Y, Du J, Wang X, Yuan M, Wang Z, Xu Z, Chong K.

Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China; State Key Laboratory of Plant Physiology and Biochemistry, Department of Plant Sciences, College of Biological Sciences, China Agricultural University, Beijing 100094, China; Department Plant Biology, Carnegie Institution of Washington and Department of Biological Sciences, Stanford University, Stanford CA 94305, USA; National Research Center for Plant Gene, Beijing 100093, China.

Root growth is mainly determined by cell division and subsequent elongation in the root apical area. Components regulating cell division in the root meristematic cells are largely unknown. Previous studies have identified OsRAA1 as a regulator in root development. Yet the function of OsRAA1 at the cellular and molecular level is unclear. Here, we show that OsRAA1 overexpressed transgenic rice showed reduced primary root growth, increased number of cells in metaphase, and reduced number of cells in anaphase, which suggests that OsRAA1 is responsible for limiting root growth by inhibiting the onset of anaphase. The expression of OsRAA1 in fission yeast also induced metaphase arrest, which is consistent with the fact that OsRAA1 functions through a conserved mechanism of cell cycle regulation. Moreover, co-localization assay has shown that OsRAA1 expresses predominantly at spindles during cell division. Yeast two-hybrid and pull-down, as well as bimolecular fluorescent complementation assays all have revealed that OsRAA1 interacts with a rice homolog of RPT4, a component that is involved in ubiquitin pathway. Treating transgenic rice with specific inhibitors of 26S proteasome blocked the degradation of OsRAA1 and increased the number of cells in metaphase. Mutation of a putative ubiquitination-targeting D-box (RGSLDLISL) in OsRAA1 interrupted the destruction of OsRAA1 in transgenic yeast. The results suggest that ubiquitination and proteasomic proteolysis are involved in the OsRAA1 degradation which is essential for the onset of anaphase and that OsRAA1 may modulate root development mediated by the ubiquitin-proteasome pathway as a novel regulatory factor of the cell cycle.

PMID: 18701670 [PubMed - as supplied by publisher]
PMCID: PMC2556835

19: Curr Opin Plant Biol. 2008 Oct;11(5):479-85. Epub 2008 Aug 7.
Related Articles, Links: Ethylene signaling: new levels of complexity and regulation.

Kendrick MD, Chang C.

Department of Cell Biology and Molecular Genetics, Bioscience Research Building, University of Maryland, College Park, MD 20742, USA.

The gaseous plant hormone ethylene plays important roles in plant growth and development. Recent discoveries have expanded our linear view of ethylene signaling by revealing an elaborate signaling network with multiple regulatory circuits. At the membrane, the ethylene receptors form heteromeric and higher order complexes providing enhanced sensitivity and fine-tuning of signaling. Ethylene sensitivity is further enhanced by the rapid degradation of ethylene receptors upon ethylene binding and by dependence on a novel protein REVERSION-TO-ETHYLENE SENSITIVITY1 (RTE1)/GREEN-RIPE (GR). In the nucleus, EIN3-BINDING F-BOX1 and 2 (EBF1/2) coordinately control 26S proteasome degradation of the critical transcription factors EIN3 and EIL1. EBF1/2 expression is repressed by ETHYLENE-INSENSITIVE5 (EIN5), which encodes the exoribonuclease XRN4. Additionally, EIN3 possesses two mitogen-activated protein kinase (MAPK) phosphorylation sites that have opposing effects on EIN3 stability.

PMID: 18692429 [PubMed - in process]

20: Plant Physiol. 2008 Jul 30. [Epub ahead of print]
Related Articles, Links: Comparative Transcriptomics of Arabidopsis thaliana Sperm Cells.

Borges F, Gomes G, Gardner R, Moreno N, McCormick S, Feijo JA, Becker JD.

Instituto Gulbenkian de Ciencia; Centro de Biologia do Desenvolvimento, Oeiras, 2780-901, Portugal; Plant Gene Expression Center, USDA/ARS-UC-Berkeley, 800 Buchanan St., Albany, CA, 94710; Depto. Biologia Vegetal, Faculdade de Ciencias da Universidade de Lisboa; Lisboa, Campo Grande, 1700, Portugal.

In flowering plants the two sperm cells are embedded within the cytoplasm of the growing pollen tube and as such are passively transported to the embryo sac, wherein double fertilization occurs upon their release. Understanding the mechanisms and conditions by which male gametes mature and take part in fertilization are crucial goals in the study of plant reproduction. Studies of gene expression in male gametes of maize and Plumbago, and in lily generative cells already showed that the previously held view of transcriptionally inert male gametes was not true, but genome-wide studies were lacking. Analyses in the model plant Arabidopsis thaliana were hindered because no method to isolate sperm cells was available. Here we used Fluorescence-activated cell sorting (FACS) to isolate sperm cells from Arabidopsis, allowing GeneChip analysis of the transcriptome of sperm cells at a genome-wide level. Comparative analysis of the sperm cell transcriptome with those of representative sporophytic tissues and of pollen showed that sperm has a distinct and diverse transcriptional profile. Functional classifications of genes with enriched expression in sperm cells showed that DNA repair, ubiquitin-mediated proteolysis and cell cycle progression are over-represented Gene Ontology categories. Moreover, analysis of the small RNA and DNA methylation pathways suggests that distinct mechanisms might be involved in regulating the epigenetic state of the paternal genome. We identified numerous candidate genes whose involvement in sperm cell development and fertilization can now be directly tested in Arabidopsis. These results provide a roadmap to decipher the role of sperm-expressed proteins.

PMID: 18667720 [PubMed - as supplied by publisher]
PMCID: PMC2556834 [Available on 10/01/09]

21: Plant Cell. 2008 Jul;20(7):1899-914. Epub 2008 Jul 29.
Related Articles, Links

Arabidopsis PUB22 and PUB23 are homologous U-Box E3 ubiquitin ligases that play combinatory roles in response to drought stress.

Cho SK, Ryu MY, Song C, Kwak JM, Kim WT.

Department of Biology, College of Life Science and Biotechnology, Yonsei University, Seoul 120-749, Korea.

Ubiquitination is involved in diverse cellular processes in higher plants. In this report, we describe Arabidopsis thaliana PUB22 and PUB23, two homologous U-box-containing E3 ubiquitin (Ub) ligases. The PUB22 and PUB23 genes were rapidly and coordinately induced by abiotic stresses but not by abscisic acid. PUB22- and PUB23-overexpressing transgenic plants were hypersensitive to drought stress. By contrast, loss-of-function pub22 and pub23 mutant plants were significantly more drought-tolerant, and a pub22 pub23 double mutant displayed even greater drought tolerance. These results indicate that PUB22 and PUB23 function as negative regulators in the water stress response. Yeast two-hybrid, in vitro pull-down, and in vivo coimmunoprecipitation experiments revealed that PUB22 and PUB23 physically interacted with RPN12a, a subunit of the 19S regulatory particle (RP) in the 26S proteasome. Bacterially expressed RPN12a was effectively ubiquitinated in a PUB-dependent fashion. RPN12a was highly ubiquitinated in 35S:PUB22 plants, but not in pub22 pub23 double mutant plants, consistent with RPN12a being a substrate of PUB22 and PUB23 in vivo. In water-stressed wild-type and PUB-overexpressing plants, a significant amount of RPN12a was dissociated from the 19S RP and appeared to be associated with small-molecular-mass protein complexes in cytosolic fractions, where PUB22 and PUB23 are localized. Overall, our results suggest that PUB22 and PUB23 coordinately control a drought signaling pathway by ubiquitinating cytosolic RPN12a in Arabidopsis.

Publication Types:

PMID: 18664614 [PubMed - in process]

PMCID: PMC2518226 [Available on 07/01/09]

22: Plant Cell Rep. 2008 Oct;27(10):1635-44. Epub 2008 Jul 29.
Related Articles, Links: Generation of selectable marker-free sheath blight resistant transgenic rice plants by efficient co-transformation of a cointegrate vector T-DNA and a binary vector T-DNA in one Agrobacterium tumefaciens strain.

Sripriya R, Raghupathy V, Veluthambi K.

Department of Plant Biotechnology, School of Biotechnology, Madurai Kamaraj University, Madurai, 625021, Tamil Nadu, India.

Co-transformation of Oryza sativa L. var. Pusa Basmati1 was done using an Agrobacterium tumefaciens strain harbouring a single-copy cointegrate vector and a multi-copy binary vector in the same cell. The T-DNA of the cointegrate vector pGV2260::pSSJ1 carried the hygromycin phosphotransferase (hph) and beta-glucuronidase (gus) genes. The binary vector pCam-chi11, without a plant selectable marker gene, harboured the rice chitinase (chi11) gene under maize ubiquitin promoter. Co-transformation of the gene of interest (chi11) with the selectable marker gene (hph) occurred in 4 out of 20 T(0) plants (20%). Segregation of hph from chi11 was accomplished in two (CoT6 and CoT23) of the four co-transformed plants in the T(1) generation. The selectable marker-free (SMF) lines CoT6 and CoT23 harboured single copies of chi11. Homozygous SMF T(2) plants were established in the lines CoT6 and CoT23. Northern and Western blot analysis of the homozygous SMF lines showed high level of transgene expression. In comparison to untransformed controls, chitinase specific activity was 66- and 22-fold higher in the homozygous SMF T(2) plants of lines CoT6 and CoT23, respectively. The lines CoT6 and CoT23 exhibited 38 and 40% reduction in sheath blight disease, respectively.

PMID: 18663452 [PubMed - in process]

23: Curr Opin Plant Biol. 2008 Aug;11(4):396-403. Epub 2008 Jul 24.
Related Articles, Links

Phytopathogen type III effector weaponry and their plant targets.

Block A, Li G, Fu ZQ, Alfano JR.

Center for Plant Science Innovation and Department of Plant Pathology, University of Nebraska, Lincoln, NE 68588-0660, USA.

Phytopathogenic bacteria suppress plant innate immunity and promote pathogenesis by injecting proteins called type III effectors into plant cells using a type III protein secretion system. These type III effectors use at least three strategies to alter host responses. One strategy is to alter host protein turnover, either by direct cleavage or by modulating ubiquitination and targeting the 26S proteasome. Another strategy involves alteration of RNA metabolism by transcriptional activation or ADP-ribosylation of RNA-binding proteins. A third major strategy is to inhibit the kinases involved in plant defence signaling, either by the removal of phosphates or by direct inhibition. The wide array of strategies that bacterial pathogens employ to suppress innate immunity suggest that circumvention of innate immunity is crucial for bacterial pathogenicity of plants.

Publication Types:

PMID: 18657470 [PubMed - indexed for MEDLINE]

24: J Cell Biochem. 2008 Oct 1;105(2):546-53.
Related Articles, Links

Quercetin suppresses hypoxia-induced accumulation of hypoxia-inducible factor-1alpha (HIF-1alpha) through inhibiting protein synthesis.

Lee DH, Lee YJ.

Department of Surgery and Pharmacology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15213, USA.

Quercetin, a ubiquitous bioactive plant flavonoid, has been shown to inhibit the proliferation of cancer cells and induce the accumulation of hypoxia-inducible factor-1alpha (HIF-1alpha) in normoxia. In this study, under hypoxic conditions (1% O(2)), we examined the effect of quercetin on the intracellular level of HIF-1alpha and extracellular level of vascular endothelial growth factor (VEGF) in a variety of human cancer cell lines. Surprisingly, we observed that quercetin suppressed the HIF-1alpha accumulation during hypoxia in human prostate cancer LNCaP, colon cancer CX-1, and breast cancer SkBr3 cells. Quercetin treatment also significantly reduced hypoxia-induced secretion of VEGF. Suppression of HIF-1alpha accumulation during treatment with quercetin in hypoxia was not prevented by treatment with 26S proteasome inhibitor MG132 or PI3K inhibitor LY294002. Interestingly, hypoxia (1% O(2)) in the presence of 100 microM quercetin inhibited protein synthesis by 94% during incubation for 8 h. Significant quercetin concentration-dependent inhibition of protein synthesis and suppression of HIF-1alpha accumulation were observed under hypoxic conditions. Treatment with 100 microM cycloheximide, a protein synthesis inhibitor, replicated the effect of quercetin by inhibiting HIF-1alpha accumulation during hypoxia. These results suggest that suppression of HIF-1alpha accumulation during treatment with quercetin under hypoxic conditions is due to inhibition of protein synthesis. (c) 2008 Wiley-Liss, Inc.

Publication Types:

PMID: 18655183 [PubMed - in process]

25: Curr Opin Plant Biol. 2008 Oct;11(5):486-94. Epub 2008 Jul 22.
Related Articles, Links: JAZ repressors set the rhythm in jasmonate signaling.

Chico JM, Chini A, Fonseca S, Solano R.

Departamento de Genética Molecular de Plantas, Centro Nacional de Biotecnología-CSIC, Campus Universidad Autónoma, 28049 Madrid, Spain.

Jasmonates (JAs) are essential hormones for plant defense and development. In spite of their importance, the molecular details of their signaling pathways remain largely unknown. A new family of regulators of JA signaling named JAZ, jasmonate ZIM-domain proteins, has recently been described. JAZ proteins repress of JA signaling and are targeted by the E3-ubiquitin ligase SCF(COI1) for proteasome degradation in response to JA. Hormone binding depends on a functional COI1 protein suggesting that COI1 is the JA receptor. MYC2, a positive regulator of JA-dependent responses, has been identified as a target of JAZ repressors. Interestingly, MYC2 and JAZ proteins are involved in a negative regulatory feedback loop, suggesting a model to explain how transcriptional reprogramming is turned on and off in response to JA. The discovery of JAZ repressors provides a new framework to understand JA-signaling pathways from hormonal perception to transcriptional activation.

PMID: 18653378 [PubMed - in process]

26: J Biol Chem. 2008 Sep 12;283(37):25200-8. Epub 2008 Jul 21.
Related Articles, Links

The three mitochondrial encoded CcmF proteins form a complex that interacts with CCMH and c-type apocytochromes in Arabidopsis.

Rayapuram N, Hagenmuller J, Grienenberger JM, Bonnard G, Giegé P.

Institut de Biologie Moléculaire des Plantes du CNRS, 12 rue du général Zimmer, Strasbourg, France.

Three reading frames called ccmF(N1), ccmF(N2), and ccmF(c) are found in the mitochondrial genome of Arabidopsis. These sequences are similar to regions of the bacterial gene ccmF involved in cytochrome c maturation. ccmF genes are always absent from animal and fungi genomes but are found in mitochondrial genomes of land plant and several evolutionary distant eukaryotes. In Arabidopsis, ccmF(N2) despite the absence of a classical initiation codon is not a pseudo gene. The 3 ccmF genes of Arabidopsis are expressed at the protein level. Their products are integral proteins of the mitochondrial inner membrane with in total 11 to 13 predicted transmembrane helices. The conserved WWD domain of CcmF(N2) is localized in the inter membrane space. The 3 CcmF proteins are all detected in a high molecular mass complex of 500 kDa by Blue Native PAGE. Direct interaction between CcmF(N2) and both CcmF(N1) and CcmF(C) is shown with the yeast two-hybrid split ubiquitin system, but no interaction is observed between CcmF(N1) and CcmF(C). Similarly, interaction is detected between CcmF(N2) and apocytochrome c but also with apocytochrome c(1). Finally, CcmF(N1) and CcmF(N2) both interact with CCMH previously shown to interact as well with cytochrome c. This strengthens the hypothesis that CcmF and CCMH make a complex that performs the assembly of heme with c-type apocytochromes in plant mitochondria.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 18644794 [PubMed - in process]

27: Plant J. 2008 Sep 19. [Epub ahead of print]
Related Articles, Links: ORTH/VIM proteins that regulate DNA methylation are functional ubiquitin E3 ligases.

Kraft E, Bostick M, Jacobsen SE, Callis J.

Department of Molecular and Cellular Biology, University of California-Davis, One Shields Avenue, Davis, CA 95616, USA.

Appropriate methylation of genomes is essential for gene regulation. Here, we describe the six-member ORTHRUS (ORTH) gene family of Arabidopsis thaliana that plays a role in DNA methylation in vivo. ORTH1- ORTH5 are predicted to encode proteins that contain one plant homeodomain (PHD), two really interesting new gene (RING) domains, and one set ring associated (SRA) domain, whereas ORTHlike-1 encodes a protein with only one RING and SRA domain. cDNAs for ORTH1, ORTH2, ORTH5 and ORTHlike-1 were isolated, and when expressed as glutathione-S-transferase (GST) fusion proteins, were capable of promoting ubiquitylation in vitro with the E2 AtUBC11. ORTH1 promotes ubiquitylation when paired with additional AtUBC8 family members. ORTH1 proteins with substitutions in metal-ligand binding residues in each ORTH1 RING domain individually, and ORTH1 truncation derivatives lacking one or both RING domains, were tested for their ability to catalyze ubiquitylation in vitro. In these assays, either ORTH1 RING domain is capable of promoting ubiquitylation. The PHD alone is not active as an E3 ligase, nor is it required for ligase activity. GFP-ORTH1 and GFP-ORTH2 are nuclear-localized in transgenic Arabidopsis plants. Overexpression of ORTH1 or ORTH2 in Arabidopsis leads to an altered flowering time. Inspection of DNA methylation at FWA and Cen180 repeats revealed hypomethylation when ORTH proteins were overexpressed. Once initiated, a late-flowering phenotype persisted in the absence of the ORTH transgene, consistent with epigenetic effects at FWA. We conclude that ORTH proteins are E3 ligases mediating DNA methylation status in vivo.

PMID: 18643997 [PubMed - as supplied by publisher]

28: Plant J. 2008 Jul 16. [Epub ahead of print]
Related Articles, Links: RING1 E3 ligase localizes to plasma membrane lipid raft to trigger FB1-induced programmed cell death in Arabidopsis.

Lin SS, Martin R, Mongrand S, Vandenabeele S, Chen KC, Jang IC, Chua NH.<