Cloning and expression of an Arabidopsis thaliana cDNA encoding a monofunctional aspartate kinase homologous to the lysine-sensitive enzyme of Escherichia coli

Proteomic profile of the germinating seeds reveals enhanced seedling growth in Arabidopsis rpp1a mutant

Ribosomal phosphoprotein P1 (RPP1) is an integral component of the P-protein stalk in the 60S subunit of eukaryotic ribosomes and is required for the efficient elongation of translation. Previously, Arabidopsis RPP1A was revealed to be involved in the regulation of seed size and seed storage protein accumulation. In this work, the seedling growth analysis shows that the knockout mutation of Arabidopsis RPP1A significantly promoted seedling growth, particularly in the shoots. The label-free quantitative proteomic analysis demonstrated that a total of 593 proteins were differentially accumulated between the germinating seeds of the wild-type Col-0 and rpp1a mutant. And these proteins were significantly enriched in the intracellular transport, nitrogen compound transport, protein transport, and organophosphate metabolic process. The abundance of proteins involved in the RNA and protein processing processes, including ncRNA processing and protein folding, were significantly increased in the rpp1a mutant. Mutation in RPP1A highlighted the effects on the ribosome, energy metabolism, and nitrogen metabolism. The abundance of enzymes involved in glycolysis and pyruvate mechanism was decreased in the germinating seeds of the rpp1a mutant. Whereas the processes of amino acid biosynthesis, protein processing in endoplasmic reticulum, and biosynthesis of cofactors were enhanced in the germinating seeds of the rpp1a mutant. Taken together, the lack of RPP1A triggered changes in other ribosomal proteins, and the higher amino acid contents in the seedlings of the rpp1a mutant probably contributed to enhanced biosynthesis, processing, and transport of proteins, resulting in accelerated growth. Our results show the novel role of a P-protein and shed new light on the regulatory mechanism of seedling growth.

Transcriptional control of aspartate kinase expression during darkness and sugar depletion in Arabidopsis: involvement of bZIP transcription factors

Initial steps of aspartate-derived biosynthesis pathway (Asp pathway) producing Lys, Thr, Met and Ile are catalyzed by bifunctional (AK/HSD) and monofunctional (AK-lys) aspartate kinase (AK) enzymes. Here, we show that transcription of all AK genes is negatively regulated under darkness and low sugar conditions. By using yeast one-hybrid assays and complementary chromatin immunoprecipitation analyses in Arabidopsis cells, the bZIP transcription factors ABI5 and DPBF4 were identified, capable of interacting with the G-box-containing enhancer of AK/HSD1 promoter. Elevated transcript levels of DPBF4 and ABI5 under darkness and low sugar conditions coincide with the repression of AK gene expression. Overexpression of ABI5, but not DPBF4, further increases this AK transcription suppression. Concomitantly, it also increases the expression of asparagines synthetase 1 (ASN1) that shifts aspartate utilization towards asparagine formation. However, in abi5 or dpbf4 mutant and abi5, dpbf4 double mutant the repression of AK expression is maintained, indicating a functional redundancy with other bZIP-TFs. A dominant-negative version of DPBF4 fused to the SRDX repressor domain of SUPERMAN could counteract the repression and stimulate AK expression under low sugar and darkness in planta. This effect was verified by showing that DPBF4-SRDX fails to recognize the AK/HSD1 enhancer sequence in yeast one-hybrid assays, but increases heterodimmer formation with DPBF4 and ABI5, as estimated by yeast two-hybrid assays. Hence it is likely that heterodimerization with DPBF4-SRDX inhibits the binding of redundantly functioning bZIP-TFs to the promoters of AK genes and thereby releases the repressing effect. These data highlight a novel transcription control of the chloroplast aspartate pathway that operates under energy limiting conditions.

Threonine-insensitive homoserine dehydrogenase from soybean: genomic organization, kinetic mechanism, and in vivo activity

Aspartate kinase (AK) and homoserine dehydrogenase (HSD) function as key regulatory enzymes at branch points in the aspartate amino acid pathway and are feedback-inhibited by threonine. In plants the biochemical features of AK and bifunctional AK-HSD enzymes have been characterized, but the molecular properties of the monofunctional HSD remain unexamined. To investigate the role of HSD, we have cloned the cDNA and gene encoding the monofunctional HSD (GmHSD) from soybean. Using heterologously expressed and purified GmHSD, initial velocity and product inhibition studies support an ordered bi bi kinetic mechanism in which nicotinamide cofactor binds first and leaves last in the reaction sequence. Threonine inhibition of GmHSD occurs at concentrations (K(i) = 160-240 mM) more than 1000-fold above physiological levels. This is in contrast to the two AK-HSD isoforms in soybean that are sensitive to threonine inhibition (K(i) approximately 150 microM). In addition, GmHSD is not inhibited by other aspartate-derived amino acids. The ratio of threonine-resistant to threonine-sensitive HSD activity in soybean tissues varies and likely reflects different demands for amino acid biosynthesis. This is the first cloning and detailed biochemical characterization of a monofunctional feedback-insensitive HSD from any plant. Threonine-resistant HSD offers a useful biotechnology tool for manipulating the aspartate amino acid pathway to increase threonine and methionine production in plants for improved nutritional content.

Aspartate-Derived Amino Acid Biosynthesis in Arabidopsis thaliana

The aspartate-derived amino acid pathway in plants leads to the biosynthesis of lysine, methionine, threonine, and isoleucine. These four amino acids are essential in the diets of humans and other animals, but are present in growth-limiting quantities in some of the world's major food crops. Genetic and biochemical approaches have been used for the functional analysis of almost all Arabidopsis thaliana enzymes involved in aspartate-derived amino acid biosynthesis. The branch-point enzymes aspartate kinase, dihydrodipicolinate synthase, homoserine dehydrogenase, cystathionine gamma synthase, threonine synthase, and threonine deaminase contain well-studied sites for allosteric regulation by pathway products and other plant metabolites. In contrast, relatively little is known about the transcriptional regulation of amino acid biosynthesis and the mechanisms that are used to balance aspartate-derived amino acid biosynthesis with other plant metabolic needs. The aspartate-derived amino acid pathway provides excellent examples of basic research conducted with A. thaliana that has been used to improve the nutritional quality of crop plants, in particular to increase the accumulation of lysine in maize and methionine in potatoes.

Overexpression of mutated forms of aspartate kinase and cystathionine gamma-synthase in tobacco leaves resulted in the high accumulation of methionine and threonine

Methionine and threonine are two essential amino acids, the levels of which limit the nutritional quality of plants. Both amino acids diverge from the same branch of the aspartate family biosynthesis pathway; therefore, their biosynthesis pathways compete for the same carbon/amino substrate. To further elucidate the regulation of methionine biosynthesis and seek ways of increasing the levels of these two amino acids, we crossed transgenic tobacco plants overexpressing the bacterial feedback-insensitive aspartate kinase (bAK), containing a significantly higher threonine level, with plants overexpressing Arabidopsis cystathionine gamma-synthase (AtCGS), the first unique enzyme of methionine biosynthesis. Plants co-expressing bAK and the full-length AtCGS (F-AtCGS) have significantly higher methionine and threonine levels compared with the levels found in wild-type plants, but the methionine level does not increase beyond that found in plants expressing F-AtCGS alone. This finding can be explained through the feedback inhibition regulation mediated by the methionine metabolite on the transcript level of AtCGS. To test this assumption, plants expressing bAK were crossed with plants expressing two mutated forms of AtCGS in which the domains responsible for the feedback regulation have been deleted. Indeed, significantly higher methionine contents and its metabolites levels accumulated in the newly produced plants, and the levels of threonine were also significantly higher than in the wild-type plants. The transcript level of the two mutated forms of AtCGS significantly increased when there was a high content of threonine in the plants, suggesting that threonine modulates, probably indirectly, the transcript level of AtCGS.

Lysine enhances methionine content by modulating the expression of S-adenosylmethionine synthase

Lysine and methionine are two essential amino acids whose levels affect the nutritional quality of cereals and legume plants. Both amino acids are synthesized through the aspartate family biosynthesis pathway. Within this family, lysine and methionine are produced by two different branches, the lysine branch and the threonine-methionine branch, which compete for the same carbon/amino substrate. To elucidate the relationship between these biosynthetic branches, we crossed two lines of transgenic tobacco plants: one that overexpresses the feedback-insensitive bacterial enzyme dihydrodipicolinate synthase (DHPS) and contains a significantly higher level of lysine, and a second that overexpresses Arabidopsis cystathionine gamma-synthase (AtCGS), the first unique enzyme of methionine biosynthesis. Significantly higher levels of methionine and its metabolite, S-methylmethionine (SMM), accumulated in the newly produced plants compared with plants overexpressing AtCGS alone, while the level of lysine remained the same as in those overexpressing DHPS alone. The increased levels of methionine and SMM were correlated with increases in the mRNA and protein levels of AtCGS and a reduced mRNA level for the genes encoding S-adnosylmethionine (SAM) synthase, which converts methionine to SAM. Reduction in SAMS expression level leads most probably to the reduction of SAM found in plants that feed with lysine. As SAM is a negative regulator of CGS, this reduction leads to higher expression of CGS and consequently to an increased level of methionine. Elucidating the relationship between lysine and methionine synthesis may lead to new ways of producing transgenic crop plants containing increased methionine and lysine levels, thus improving their nutritional quality.

Allosteric monofunctional aspartate kinases fromArabidopsis

Plant monofunctional aspartate kinase is unique among all aspartate kinases, showing synergistic inhibition by lysine and S-adenosyl-l-methionine (SAM). The Arabidopsis genome contains three genes for monofunctional aspartate kinases. We show that aspartate kinase 2 and aspartate kinase 3 are inhibited only by lysine, and that aspartate kinase 1 is inhibited in a synergistic manner by lysine and SAM. In the absence of SAM, aspartate kinase 1 displayed low apparent affinity for lysine compared to aspartate kinase 2 and aspartate kinase 3. In the presence of SAM, the apparent affinity of aspartate kinase 1 for lysine increased considerably, with K(0.5) values for lysine inhibition similar to those of aspartate kinase 2 and aspartate kinase 3. For all three enzymes, the inhibition resulted from an increase in the apparent K(m) values for the substrates ATP and aspartate. The mechanism of aspartate kinase 1 synergistic inhibition was characterized. Inhibition by lysine alone was fast, whereas synergistic inhibition by lysine plus SAM was very slow. SAM by itself had no effect on the enzyme activity, in accordance with equilibrium binding analyses indicating that SAM binding to aspartate kinase 1 requires prior binding of lysine. The three-dimensional structure of the aspartate kinase 1-Lys-SAM complex has been solved [Mas-Droux C, Curien G, Robert-Genthon M, Laurencin M, Ferrer JL & Dumas R (2006) Plant Cell18, 1681-1692]. Taken together, the data suggest that, upon binding to the inactive aspartate kinase 1-Lys complex, SAM promotes a slow conformational transition leading to formation of a stable aspartate kinase 1-Lys-SAM complex. The increase in aspartate kinase 1 apparent affinity for lysine in the presence of SAM thus results from the displacement of the unfavorable equilibrium between aspartate kinase 1 and aspartate kinase 1-Lys towards the inactive form.

The aspartic acid metabolic pathway, an exciting and essential pathway in plants

Aspartate is the common precursor of the essential amino acids lysine, threonine, methionine and isoleucine in higher plants. In addition, aspartate may also be converted to asparagine, in a potentially competing reaction. The latest information on the properties of the enzymes involved in the pathways and the genes that encode them is described. An understanding of the overall regulatory control of the flux through the pathways is undisputedly of great interest, since the nutritive value of all cereal and legume crops is reduced due to low concentrations of at least one of the aspartate-derived amino acids. We have reviewed the recent literature and discussed in this paper possible methods by which the concentrations of the limiting amino acids may be increased in the seeds.

Overproduction, purification, and characterization of recombinant aspartate semialdehyde dehydrogenase from Arabidopsis thaliana

In plant and microorganisms, aspartate semialdehyde dehydrogenase (ASDH) produces the branch point intermediate between the lysine and threonine/methionine pathways. In this study, we report the first cDNA cloning, purification, and characterization of a plant ASDH. The Arabidopsis thaliana ASDH is an homodimeric enzyme composed of subunits of 36 kDa. The plant enzyme exhibited a specific activity of 26 micromol NADPH oxidized min(-1) mg(-1) of protein with a K(M) value for NADPH of 92 microM. ASDH showed cooperative behavior for aspartyl phosphate with a K(0.5) value of 37 microM.

Overproduction, purification, and characterization of recombinant bifunctional threonine-sensitive aspartate kinase-homoserine dehydrogenase from Arabidopsis thaliana

In plant, the first and the third steps of the synthesis of methionine and threonine are catalyzed by a bifunctional enzyme, aspartate kinase-homoserine dehydrogenase (AK-HSDH). In this study, we report the first purification and characterization of a highly active threonine-sensitive AK-HSDH from plants (Arabidopsis thaliana). The specific activities corresponding to the forward reaction of AK and reverse reaction of HSDH of AK-HSDH were 5.4 micromol of aspartyl phosphate produced min(-1) mg(-1) of protein and 18.8 micromol of NADPH formed min(-1) mg(-1) of protein, respectively. These values are 200-fold higher than those reported previously for partially purified plant enzymes. AK-HSDH exhibited hyperbolic kinetics for aspartate, ATP, homoserine, and NADP with K(M) values of 11.6 mM, 5.5 mM, 5.2 mM, and 166 microM, respectively. Threonine was found to inhibit both AK and HSDH activities by decreasing the affinity of the enzyme for its substrates and cofactors. In the absence of threonine, AK-HSDH behaved as an oligomer of 470 kDa. Addition of the effector converted the enzyme into a tetrameric form of 320 kDa.

Identification of a monofunctional aspartate kinase gene of Arabidopsis thaliana with spatially and temporally regulated expression

We screened a gene trap library of Arabidopsis thaliana and isolated a line in which a gene encoding a homologue of monofunctional aspartate kinase was trapped by the reporter gene. Aspartate kinase (AK) is a key enzyme in the biosynthsis of aspartate family amino acids such as lysine, threonine, isoleucine, and methionine. In plants, two types of AK are known: one is AK which is sensitive to feedback inhibition by threonine and carries both AK and homoserine dehydrogenase (HSD) activities. The other one is monofunctional, sensitive to lysine and synergistically S-adenosylmethionine, and has only AK activity. We concluded that the trapped gene encoded a monofunctional aspartate kinase and designated as AK-lys3, because it lacked the HSD domain and had an amino acid sequence highly similar to those of the monofunctional aspartate kinases ofA. thaliana. AK-lys3 was highly expressed in xylem of leaves and hypocotyls and stele of roots. Significant expression of this gene was also observed in trichomes after bolting. Slight expression of AK-lys3 was detected in vascular bundles and mesophyll cells of cauline leaves, inflorescence stems, sepals, petals, and stigmas. These results indicated that this aspartate kinase gene was not expressed uniformly but in a spatially specific manner.

Arabidopsis loss-of-function mutant in the lysine pathway points out complex regulation mechanisms

In plants, the amino acids lysine, threonine, methionine and isoleucine have L-aspartate-beta-semialdehyde (ASA) as a common precursor in their biosynthesis pathways. How this ASA precursor is dispersed among the different pathways remains vague knowledge. The proportional balances of free and/or protein-bound lysine, threonine, isoleucine and methionine are a function of protein synthesis, secondary metabolism and plant physiology. Some control points determining the flux through the distinct pathways are known, but an adequate explanation of how the competing pathways share ASA in a fine-tuned amino acid biosynthesis network is yet not available. In this article we discuss the influence of lysine biosynthesis on the adjacent pathways of threonine and methionine. We report the finding of an Arabidopsis thaliana dihydrodipicolinate synthase T-DNA insertion mutant displaying lower lysine synthesis, and, as a result of this, a strongly enhanced synthesis of threonine. Consequences of these cross-pathway regulations are discussed.