Paper de les proteïnes AtKLC-1 i AtB" en la regulació de l'HMG-CoA reductasa d' "Arabidopsis thaliana"

[cat] L'enzim HMG-CoA reductasa (HMGR) catalitza la primera etapa limitant en la síntesi d'isoprenoides citosòlics. En plantes, és un enzim de membrana i la seva primera destinació subcel·lular és el reticle endoplasmàtic. Estructuralment, està formada per un domini amino-terminal (que inclou una regió amino-terminal citosòlica i dos fragments transmembrana) i un domini catalític altament conservat en tota l'escala evolutiva. En totes les espècies de plantes conegudes fins al moment, existeixen isoformes de l'HMGR codificades per diferents gens. Concretament, en Arabidopsis thaliana el gen hmg1 (expressat de forma majoritària) codifica per a les isoformes HMGR1S i HMGR1L, i el gen hmg2 (expressat a arrels, plàntules i inflorescències) codifica per a la isoforma HMGR2. A nivell d'estructura primària l'HMGR1L difereix de l'HMGR1S per la presència d'una regió extra de 50 residus aminoacídics a l'extrem amino-terminal. El domini amino-terminal confereix diferents destinacions de localització subcel·lular a la proteïna. En un treball anterior, es van identificar tres proteïnes que interaccionen amb les isoformes derivades del gen hmg1: AtB"α, AtB"β i AtKLC-1. Les dues primeres interaccionen específicament amb la regió amino-terminal de les isoformes HMGR1S i HMGR1L. Les proteïnes AtB"α i AtB"β són isoformes de la subunitat B" reguladora del complex proteïna fosfatasa 2A (PP2A). En el genoma d'A. thaliana hi ha cinc seqüències que codifiquen per a isoformes de la subunitat B" que comparteixen una gran homologia. En l'estructura primària de la subunitat B" s'han identificat motius EF-Hand (implicats en la unió a calci). S'ha demostrat que tant AtB"α com AtB"β uneixen calci. La tercera proteïna identificada, AtKLC-1, interacciona específicament amb la regió amino-terminal de l'HMGR1L. S'ha determinat per assaigs de doble híbrid en llevat i, posteriorment confirmats in vitro, que la PR65, proteïna estructural del complex PP2A, interacciona específicament amb l'AtKLC-1. La regió de la PR65 suficient per a la interacció amb la subunitat B" reguladora, AtB"α, i amb l'AtKLC-1 comprèn la mateixa seqüència aminoacídica. Per tant, l'AtB"α i l'AtKLC-1 podrien competir per a l'associació amb la PR65. Així, en la cèl·lula, la PP2A podria regular d'una forma particular les isoformes HMGR1S i HMGR1L. Assaigs in vivo i in vitro demostren que la PP2A és un regulador negatiu de l'enzim HMGR. Els ions calci inhibeixen també l'activitat HMGR. Per a què es dugui a terme la repressió de l'activitat HMGR tant per calci com per PP2A, es requereix el domini amino-terminal de la proteïna. Per tant, els resultats són consistents amb què les subunitats AtB" i/o AtKLC-1 duen a terme un paper mediador en la modulació de l'HMGR per la PP2A. El domini amino-terminal de l'HMGR1S està implicat en la morfogènesi de vesícules derivades del reticle endoplasmàtic. La subunitat AtB"α i la resta del complex PP2A participen en aquest procés. El domini amino-terminal de l'HMGR1L dirigeix la proteïna a la trama de reticle endoplasmàtic. La PP2A participa també en la localització subcel·lular d'aquesta isoforma. S'ha caracteritzat una línia mutant en el gen hmg1 d'A. thaliana que mostra absència del transcrit hmg1 i una reduïda activitat HMGR. Les plantes d'aquest mutant creixen normalment en un medi de cultiu estèril i manifesten severes alteracions del desenvolupament quan són cultivades en terra. L'addició de mevalonat, producte de la reacció catalitzada per l'HMGR, no reverteix el fenotip. Les dades indiquen que el gen hmg1 duu a terme una funció no metabòlica relacionada amb l'adaptació al medi. En aquestes condicions de creixement, les estructures vesiculars induïdes per l'HMGR1S podrien tenir alguna funció essencial relacionada amb la resposta a estrès.[eng] "ROLE OF THE PROTEINS AtKLC-1 AND AtB" IN THE REGULATION OF HMG-CoA REDUCTASE IN ARABIDOPSIS THALIANA". TEXT: The enzyme HMG-CoA reductase (HMGR) catalyses the formation of mevalonate in the first rate-limiting step of the mevalonate pathway for isoprenoid biosynthesis. All known plant HMGR isoforms are primarily targeted to the endoplasmic reticulum (ER).These proteins contain two domains: an N-terminal domain (which includes an N-terminal cytosolic region and two membrane-spanning sequences) and a conserved catalytic domain. In all plant species, there are a variety of HMGR isoforms encoded by a multigene family. In Arabidopsis thaliana, two genes (hmg1 and hmg2) encode three HMGR isoforms (HMGR1S, HMGR1L and HMGR2). Isoforms HMGR1S and HMGR1L are identical in sequence but HMGR1L is extended 50 amino acid residues at the N-terminal end. In a previous study, three proteins had been identified which specifically interact with the HMGR1L N-terminal end. Two of these proteins, designed AtB" and AtB", are regulatory B" subunits of protein phosphatase 2A (PP2A). They bind the N-terminal region of HMGR1S and HMGR1L. The third one, AtKLC-1, exclusively interact with the N-terminal region of HMGR1L. In vitro assays have revealed that PR65, PP2A scaffolding subunit, specifically interact with AtKLC-1. Genetic and pharmacological approaches demonstrate that PP2A exerts an inhibitory control over HMGR. These results indicate that AtB" or AtKLC-1 could have a mediating role in the HMGR regulation carried out by PP2A. The N-terminal domain of the HMGR1S isoform is involved in the biogenesis of vesicle structures derived from the ER membranes. The PP2A complex is concerned in this process and the subunit AtB" acts as an interceding factor. Furthermore, we have characterized an Arabidopsis thaliana insertion mutant for hmg1. This mutant exhibits no visible phenotype under sterile growth conditions but shows dwarfing and sterility when it is grown in a soil substrate. The addition of mevalonate doesn't rescue this phenotype. These data suggest that hmg1 performs a no-metabolic function related with environment adaptation. In this context, ER derived vesicles induced by HMGR1S could play an essential role related with stress response

Multilevel Control of Arabidopsis 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase by Protein Phosphatase 2A[W]

HMG-CoA reductase has a key role in the regulation of the mevalonate pathway for isoprenoid biosynthesis and is modulated by many diverse endogenous and environmental stimuli. In this work, protein phosphatase 2A emerges as a positive and negative multilevel regulator of plant HMG-CoA reductase during normal development and in response to a variety of stress conditions

Multilevel control of arabidopsis 3-hydroxy-3-methylglutaryl coenzyme A reductase by protein phosphatase 2A

Plants synthesize a myriad of isoprenoid products that are required both for essential constitutive processes and for adaptive responses to the environment. The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes a key regulatory step of the mevalonate pathway for isoprenoid biosynthesis and is modulated by many endogenous and external stimuli. In spite of that, no protein factor interacting with and regulating plant HMGR in vivo has been described so far. Here, we report the identification of two B99 regulatory subunits of protein phosphatase 2A (PP2A), designated B99a and B99b, that interact with HMGR1S and HMGR1L, the major isoforms of Arabidopsis thaliana HMGR. B99a and B99b are Ca2+ binding proteins of the EF-hand type. We show that HMGR transcript, protein, and activity levels are modulated by PP2A in Arabidopsis. When seedlings are transferred to salt-containing medium, B99a and PP2A mediate the decrease and subsequent increase of HMGR activity, which results from a steady rise of HMGR1-encoding transcript levels and an initial sharper reduction of HMGR protein level. In unchallenged plants, PP2A is a posttranslational negative regulator of HMGR activity with the participation of B99b. Our data indicate that PP2A exerts multilevel control on HMGR through the fivemember B99 protein family during normal development and in response to a variety of stress conditions

Modulation of plant HMG-CoA reductase by protein phosphatase 2A: Positive and negative control at a key node of metabolism

The enzyme HMG-CoA reductase (HMGR) has a key regulatory role in the mevalonate pathway for isoprenoid biosynthesis, critical not only for normal plant development, but also for the adaptation to demanding environmental conditions. Consistent with this notion, plant HMGR is modulated by many diverse endogenous signals and external stimuli. Protein phosphatase 2A (PP2A) is involved in auxin, abscisic acid, ethylene and brassinosteroid signaling and now emerges as a positive and negative multilevel regulator of plant HMGR, both during normal growth and in response to a variety of stress conditions. The interaction with HMGR is mediated by B" regulatory subunits of PP2A, which are also calcium binding proteins. The new discoveries uncover the potential of PP2A to integrate developmental and calcium-mediated environmental signals in the control of plant HMGR

Proliferation and morphogenesis of the ER driven by 3-hydroxy-3-methylglutaryl coenzyme A reductase in plant cells

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) has a key regulatory role in the mevalonate pathway for isoprenoid biosynthesis and is composed of an endoplasmic reticulum (ER)-anchoring membrane domain with low sequence similarity among eukaryotic kingdoms and a conserved cytosolic catalytic domain. Organized smooth endoplasmic reticulum (OSER) structures are common formations of hypertrophied tightly packed ER membranes devoted to specific biosynthetic and secretory functions, the biogenesis of which remains largely unexplored. We show that the membrane domain of plant HMGR suffices to trigger ER proliferation and OSER biogenesis. The proliferating membranes become highly enriched in HMGR protein, but they do not accumulate sterols, indicating a morphogenetic rather than a metabolic role for HMGR. The N-terminal MDVRRRPP motif present in most plant HMGR isoforms is not required for retention in the ER, which was previously proposed, but functions as an ER morphogenic signal. Plant OSER structures are morphologically similar to those of animal cells, emerge from tripartite ER junctions, and mainly build up beside the nuclear envelope, indicating conserved OSER biogenesis in high eukaryotes. Factors other than the OSER-inducing HMGR construct mediate the tight apposition of the proliferating membranes, implying separate ER proliferation and membrane association steps. Overexpression of the membrane domain of Arabidopsis (Arabidopsis thaliana) HMGR leads to ER hypertrophy in every tested cell type and plant species, whereas the knockout of the HMG1 gene from Arabidopsis, encoding its major HMGR isoform, causes ER aggregation at the nuclear envelope. Our results show that the membrane domain of HMGR contributes to ER morphogenesis in plant cells

Multilevel control of arabidopsis 3-hydroxy-3-methylglutaryl coenzyme A reductase by protein phosphatase 2A

Plants synthesize a myriad of isoprenoid products that are required both for essential constitutive processes and for adaptive responses to the environment. The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes a key regulatory step of the mevalonate pathway for isoprenoid biosynthesis and is modulated by many endogenous and external stimuli. In spite of that, no protein factor interacting with and regulating plant HMGR in vivo has been described so far. Here, we report the identification of two B99 regulatory subunits of protein phosphatase 2A (PP2A), designated B99a and B99b, that interact with HMGR1S and HMGR1L, the major isoforms of Arabidopsis thaliana HMGR. B99a and B99b are Ca2+ binding proteins of the EF-hand type. We show that HMGR transcript, protein, and activity levels are modulated by PP2A in Arabidopsis. When seedlings are transferred to salt-containing medium, B99a and PP2A mediate the decrease and subsequent increase of HMGR activity, which results from a steady rise of HMGR1-encoding transcript levels and an initial sharper reduction of HMGR protein level. In unchallenged plants, PP2A is a posttranslational negative regulator of HMGR activity with the participation of B99b. Our data indicate that PP2A exerts multilevel control on HMGR through the fivemember B99 protein family during normal development and in response to a variety of stress conditions

Functional Domain Analysis of the Remorin Protein LjSYMREM1 in <em>Lotus japonicus</em>

<div><p>In legumes rhizobial infection during root nodule symbiosis (RNS) is controlled by a conserved set of receptor proteins and downstream components. MtSYMREM1, a protein of the Remorin family in <em>Medicago truncatula</em>, was shown to interact with at least three receptor-like kinases (RLKs) that are essential for RNS. Remorins are comprised of a conserved C-terminal domain and a variable N-terminal region that defines the six different Remorin groups. While both N- and C-terminal regions of Remorins belonging to the same phylogenetic group are similar to each other throughout the plant kingdom, the N-terminal domains of legume-specific group 2 Remorins show exceptional high degrees of sequence divergence suggesting evolutionary specialization of this protein within this clade. We therefore identified and characterized the MtSYMREM1 ortholog from <em>Lotus japonicus</em> (LjSYMREM1), a model legume that forms determinate root nodules. Here, we resolved its spatio-temporal regulation and showed that over-expression of <em>LjSYMREM1</em> increases nodulation on transgenic roots. Using a structure-function approach we show that protein interactions including Remorin oligomerization are mainly mediated and stabilized by the Remorin C-terminal region with its coiled-coil domain while the RLK kinase domains transiently interact <em>in vivo</em> and phosphorylate a residue in the N-terminal region of the LjSYMREM1 protein <em>in vitro</em>. These data provide novel insights into the mechanism of this putative molecular scaffold protein and underline its importance during rhizobial infection.</p> </div