Maize leaf phosphoenolpyruvate carboxylase: phosphorylation of Ser 15 with a mammalian cyclic AMP-dependent protein kinase diminishes sensitivity to inhibition by malate

A conserved C-terminal peptide of sorghum phosphoenolpyruvate carboxylase promotes its proteolysis, which is prevented by Glc-6P or the phosphorylation state of the enzyme

MAIN CONCLUSION

A synthetic peptide from the C-terminal end of C4-phosphoenolpyruvate carboxylase is implicated in the proteolysis of the enzyme, and Glc-6P or phosphorylation of the enzyme modulate this effect. Phosphoenolpyruvate carboxylase (PEPC) is a cytosolic, homotetrameric enzyme that performs a variety of functions in plants. Among them, it is primarily responsible for CO2 fixation in the C4 photosynthesis pathway (C4-PEPC). Here we show that proteolysis of C4-PEPC by cathepsin proteases present in a semi-purified PEPC fraction was enhanced by the presence of a synthetic peptide containing the last 19 amino acids from the C-terminal end of the PEPC subunit (pC19). Threonine (Thr)944 and Thr948 in the peptide are important requirements for the pC19 effect. C4-PEPC proteolysis in the presence of pC19 was prevented by the PEPC allosteric effector glucose 6-phosphate (Glc-6P) and by phosphorylation of the enzyme. The role of these elements in the regulation of PEPC proteolysis is discussed in relation to the physiological context.

In Vivo Phosphorylation: Development of Specific Antibodies to Detect the Phosphorylated PEPC Isoform for the C4 Photosynthesis in Zea mays

Phosphoenolpyruvate carboxylases (PEPCs), mostly known as the enzymes responsible for the initial CO₂ fixation during C₄ photosynthesis, are regulated by reversible phosphorylation in vascular plants. The phosphorylation site on a PEPC molecule is conserved not only among isoforms but also across plant species. An anti-phosphopeptide antibody is a common and powerful tool for detecting phosphorylated target proteins with high specificity. We generated two antibodies, one against a peptide containing a phosphoserine (phosphopeptide) and the other against a peptide containing a phosphoserine mimetic, (S)-2-amino-4-phosphonobutyric acid (phosphonopeptide). The amino acid sequence of the peptide was taken from the site around the phosphorylation site near the N-terminal region of the maize C₄-isoform of PEPC. The former antibodies detected almost specifically the phosphorylated C₄-isoform of PEPC, whereas the latter antibodies had a broader specificity for the phosphorylated PEPC in various plant species. The following procedures are described herein: (1) preparation of the phosphopeptide and phosphonopeptide; (2) preparation and purification of rabbit antibodies; (3) preparation of cell extracts from leaves for analyses of PEPC phosphorylation with antibodies; and (4) characterization of the obtained antibodies. Finally, (5) two cases involving the application of these antibodies are presented.

Analysis of plant phosphoproteins

Chromatography supports to purify phosphorylated proteins (P-proteins) have become available recently, yet this has not been thoroughly investigated in the case of plant materials. In this study we used a commercial affinity matrix (Qiagen) and a test plant enzyme (phosphoenolpyruvate carboxylase PEPC). The malate test and gel blot experiments probed with a specific antibody (antiphosphorylated N-terminal domain) showed that the column efficiently binds P-PEPC from Sorghum with little or no contamination by non-P-PEPC. Similar results were obtained with the low-abundance PEPC of Arabidopsis leaves when a gel filtration step (Sephadex G-200) was performed prior to the chromatography. Three-dimensional mass spectrometry analysis of immunoprecipitated PEPC in Qiagen fractions confirmed this observation. Denaturing protein extraction by cold acetone/trichloroacetic acid of fixed material led to a complete, one-step separation of P-PEPC and non-P-PEPC. At a global scale, the column captured most of the (32)P-phosphate-labeled proteins in vivo (80%), the majority of which were subsequently found in the elution fraction (88%). This was also visualized by SDS-PAGE (1D and 2D gels) followed by Pro-Q diamond staining. Analysis of the P-protein fraction by 1D gels and liquid chromatography/tandem mass spectrometry allowed the identification of 250 proteins belonging to various functional categories. These results validate the method for in vitro/in vivo studies of native/denatured individual proteins/enzymes regulated by phosphorylation and for phosphorylome studies.

A conserved 19-amino acid synthetic peptide from the carboxy terminus of phosphoenolpyruvate carboxylase inhibits the in vitro phosphorylation of the enzyme by the calcium-independent phosphoenolpyruvate carboxylase kinase

Higher plant phosphoenolpyruvate carboxylase (PEPC) is subject to in vivo phosphorylation of a regulatory serine located in the N-terminal domain of the protein. Studies using synthetic peptide substrates and mutated phosphorylation domain photosynthetic PEPC (C4 PEPC) suggested that the interaction of phosphoenolpyruvate carboxylase kinase (PEPCk) with its target was not restricted to this domain. However, no further information was available as to where PEPCk-C4 PEPC interactions take place. In this work, we have studied the possible interaction of the conserved 19-amino acid C-terminal sequence of sorghum (Sorghum vulgare Pers cv Tamaran) C4 PEPC with PEPCk. In reconstituted assays, a C-terminal synthetic peptide containing this sequence (peptide C19) was found to inhibit the phosphorylation reaction by the partially purified Ca2+-independent PEPCk (50% inhibition of initial activity = 230 microm). This effect was highly specific because peptide C19 did not alter C4 PEPC phosphorylation by either a partially purified sorghum leaf Ca2+-dependent protein kinase or the catalytic subunit of mammalian protein kinase A. In addition, the Ca2+-independent PEPCk was partially but significantly retained in affinity chromatography using a peptide C19 agarose column. Because peptide C15 (peptide C19 lacking the last four amino acids, QNTG) also inhibited C4 PEPC phosphorylation, it was concluded that the amino acid sequence downstream from the QNTG motif was responsible for the inhibitory effect. Specific antibodies raised against peptide C19 revealed that native C4 PEPC could be in two different conformational states. The results are discussed in relation with the reported crystal structure of the bacterial (Escherichia coli) and plant (maize [Zea mays]) enzymes.

Photosynthesis with single-rooted Amaranthus leaves. II. Regulation of ribuelose-1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, NAD-malic enzyme and NAD-malate dehydrogenase and coordination between PCR and C4 photosynthetic metabolism in response to changes in the source-sink balance

We have studied source-sink relationships with a model consisting of single-rooted leaves without petioles. We previously reported that the rate of photosynthesis decreased when C4 model plants prepared from Amaranthus cruentus leaves were subjected to sink-limited conditions by exposure to continuous light for a few days. It was suggested that the inhibition is due to a coordinated decrease in the activity of ribulose-1,5-bisphosphate carboxylase (RuBPcase) and phosphoenol-pyruvate carboxylase (PEPcase), both essential enzymes for photosynthesis in C4 plants. We further investigated the mechanisms behind the decreased activity of RuBPcase, PEPcase, NAD-malic enzyme and NAD-malate dehydrogenase. The results suggested that (1) the initial activity of RuBPcase is suppressed by a lowering of the P(i) level in chloroplasts, (2) the inhibition of PEPcase is due to dephosphorylation of the enzyme via the inhibition of PEPcase kinase and PEPcase phosphatase, (3) the inhibition of NAD-malic enzyme and NAD-malate dehydrogenase is derived from the oxidation of these enzymes, and (4) some proteinous factor(s) may be involved in the inhibition of the activity of these latter three enzymes. The significance of a coordinated decrease in these enzymes in response to a change in the source-sink balance is discussed.

Thioredoxin-mediated reductive activation of a protein kinase for the regulatory phosphorylation of C4-form phosphoenolpyruvate carboxylase from maize

The activity of phosphoenolpyruvate carboxylase (PEPC, EC4.1.1.31) for the C4 photosynthesis is known to be regulated mainly in response to light/dark transitions through reversible phosphorylation by a specific protein kinase (PK). PEPC-PK with an M(r) of 30 kDa was purified about 1.4 million-fold to homogeneity from maize leaves and characterized. The purified PEPC-PK was readily inactivated under mild oxidative conditions, but the activity could be recovered by dithiothreitol (DTT). The recovery by DTT was strongly accelerated by thioredoxin (Trx) from E. coli. Trxs of plant origin such as Trx-m from spinach chloroplast and Trx-h from rice cytoplasm were also effective. These results suggest the possibility of PEPC-PK being redox-regulated via Trx in vivo.

Illumination increases the affinity of phosphoenolpyruvate carboxylase to bicarbonate in leaves of a C4 plant, Amaranthus hypochondriacus

Illumination increased markedly the affinity to bicarbonate of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) in leaves of Amaranthus hypochondriacus L., a C4 plant. When leaves were illuminated, the apparent Km for (HCO3-) of PEPC decreased by about 50% concurrent with a 2- to 5-fold increase in Vmax and 3- to 4-fold increase in Ki for malate. The inclusion of ethoxyzolamide, an inhibitor of carbonic anhydrase, during the assay had no effect on kinetic and regulatory properties of PEPC indicating that carbonic anhydrase was not involved during light-induced sensitization of PEPC to HCO3-. Pretreatment of leaf discs with cycloheximide (CHX), a cytosolic protein synthesis inhibitor, suppressed significantly the light-enhanced decrease in apparent Km (HCO3-). Further, in vitro phosphorylation of purified dark-form PEPC by protein kinase A (PKA) decreased the apparent Km (HCO3-) of the enzyme, in addition increasing Ki (malate) as expected. Such changes, due to in vitro phosphorylation of purified PEPC by PKA, occurred only with wild-type PEPC, but not in the mutant form of maize (S15D) which is already a mimic of the phosphorylated enzyme. These results suggest that phosphorylation of the enzyme is important during the sensitization of PEPC to HCO3- by illumination in C4 leaves. Since illumination is expected to increase the cytosolic pH and the availability of dissolved HCO3- in mesophyll cells, the sensitization by light of PEPC to HCO3- could be physiologically quite significant.

Immunological analysis of the phosphorylation state of maize C4-form phosphoenolpyruvate carboxylase with specific antibodies raised against a synthetic phosphorylated peptide

The phosphoenolpyruvate carboxylase (PEPC) isozyme involved in C4 photosynthesis is known to undergo reversible regulatory phosphorylation under illuminated conditions, thereby decreasing the enzyme's sensitivity to its feedback inhibitor, L-malate. For the direct assay of this phosphorylation in intact maize leaves, phosphorylation state-specific antibodies to the C4-form PEPC were prepared. The antibodies were raised in rabbits against a synthetic phosphorylated 15-mer peptide with a sequence corresponding to that flanking the specific site of regulatory phosphorylation (Ser15) and subsequently purified by affinity-chromatography. Specificity of the resulting antibodies to the C4-form PEPC phosphorylated at Ser15 was established on the basis of several criteria. The antibodies did not react with the recombinant root-form of maize PEPC phosphorylated in vitro. By the use of these antibodies, the changes in PEPC phosphorylation state were semi-quantitatively monitored under several physiological conditions. When the changes in PEPC phosphorylation were monitored during the entire day with mature (13-week-old) maize plants grown in the field, phosphorylation started before dawn, reached a maximum by mid-morning, and then decreased before sunset. At midnight dephosphorylation was almost complete. The results suggest that the regulatory phosphorylation of C4-form PEPC in mature maize plants is controlled not only by a light signal but also by some other metabolic signal(s). Under nitrogen-limited conditions the phosphorylation was enhanced even though the level of PEPC protein was decreased. Thus there seems to be some compensatory regulatory mechanism for the phosphorylation.

Three-dimensional structure of phosphoenolpyruvate carboxylase: a proposed mechanism for allosteric inhibition

The crystal structure of phosphoenolpyruvate carboxylase (PEPC; EC 4. 1.1.31) has been determined by x-ray diffraction methods at 2.8-A resolution by using Escherichia coli PEPC complexed with L-aspartate, an allosteric inhibitor of all known PEPCs. The four subunits are arranged in a "dimer-of-dimers" form with respect to subunit contact, resulting in an overall square arrangement. The contents of alpha-helices and beta-strands are 65% and 5%, respectively. All of the eight beta-strands, which are widely dispersed in the primary structure, participate in the formation of a single beta-barrel. Replacement of a conserved Arg residue (Arg-438) in this linkage with Cys increased the tendency of the enzyme to dissociate into dimers. The location of the catalytic site is likely to be near the C-terminal side of the beta-barrel. The binding site for L-aspartate is located about 20 A away from the catalytic site, and four residues (Lys-773, Arg-832, Arg-587, and Asn-881) are involved in effector binding. The participation of Arg-587 is unexpected, because it is known to be catalytically essential. Because this residue is in a highly conserved glycine-rich loop, which is characteristic of PEPC, L-aspartate seemingly causes inhibition by removing this glycine-rich loop from the catalytic site. There is another mobile loop from Lys-702 to Gly-708 that is missing in the crystal structure. The importance of this loop in catalytic activity was also shown. Thus, the crystal-structure determination of PEPC revealed two mobile loops bearing the enzymatic functions and accompanying allosteric inhibition by L-aspartate.

Evidence for a slow-turnover form of the Ca2+-independent phosphoenolpyruvate carboxylase kinase in the aleurone-endosperm tissue of germinating barley seeds

Phosphoenolpyruvate carboxylase (PEPC) activity was detected in aleurone-endosperm extracts of barley (Hordeum vulgare) seeds during germination, and specific anti-sorghum (Sorghum bicolor) C4 PEPC polyclonal antibodies immunodecorated constitutive 103-kD and inducible 108-kD PEPC polypeptides in western analysis. The 103- and 108-kD polypeptides were radiolabeled in situ after imbibition for up to 1.5 d in 32P-labeled inorganic phosphate. In vitro phosphorylation by a Ca2+-independent PEPC protein kinase (PK) in crude extracts enhanced the enzyme's velocity and decreased its sensitivity to L-malate at suboptimal pH and [PEP]. Isolated aleurone cell protoplasts contained both phosphorylated PEPC and a Ca2+-independent PEPC-PK that was partially purified by affinity chromatography on blue dextran-agarose. This PK activity was present in dry seeds, and PEPC phosphorylation in situ during imbibition was not affected by the cytosolic protein-synthesis inhibitor cycloheximide, by weak acids, or by various pharmacological reagents that had proven to be effective blockers of the light signal transduction chain and PEPC phosphorylation in C4 mesophyll protoplasts. These collective data support the hypothesis that this Ca2+-independent PEPC-PK was formed during maturation of barley seeds and that its presumed underlying signaling elements were no longer operative during germination.

The interaction of shikimic acid and protein phosphorylation with PEP carboxylase from the C4 dicot Amaranthus viridis

Shikimic acid has been described as a potent competitive inhibitor of the activity of C4 phosphoenolpyruvate carboxylase (PEPC) from Amaranthus viridis. In the present study, the effects of shikimic acid were examined further with the dephospho (dark-form) and in vitro phosphorylated forms of homogeneous PEPC from A. viridis. Kinetic analysis showed that the inhibitor effect of shikimic acid was dependent on the phosphorylation state of the enzyme. Thus, the I50 value of shikimic acid for dark-form PEPC was six times lower than that for the phosphorylated enzyme (12 vs 71 microM, respectively). When Glc6P, an activator of C4 PEPC, was present in the assay medium, the I50 value increased 2- and 3-times with the phospho and dephospho PEPC-forms, respectively. Shikimic acid also markedly decreased 32P incorporation from Mg[gamma-32P]ATP into the dark-form of C4 PEPC, but not casein, catalyzed by protein kinase A. In this way, shikimic acid mimics the behaviour of L-malate, a well-known inhibitor of PEPC, in that it decreases both the enzyme's activity and phosphorylatability. Based on these data, a possible role for shikimic acid in the regulation of PEPC activity in plants is suggested.

Regulatory phosphorylation of plant phosphoenolpyruvate carboxylase: role of a conserved basic residue upstream of the phosphorylation site

In order to mimic regulatory phosphorylation of the Ser-15 of maize C4-form phosphoenolpyruvate carboxylase (PEPC), we replaced Ser-15 and Lys-12 with Asp (S15D) and Asn (K12N), respectively, by site-directed mutagenesis. Although both mutant enzymes were catalytically as active as the wild-type PEPC, they showed much less sensitivity to malate, an allosteric inhibitor, similarly to the phosphorylated wild-type PEPC. A maize protein kinase of 30 kDa which is known to be specific to PEPC (PEPC-PK), phosphorylated K12N as well as the wild-type PEPC but not S15D. The phosphorylation of K12N further diminished the sensitivity to malate. Thus, a positive charge of the conserved Lys-12 is not required for the recognition by PEPC-PK but contributes to the intrinsic sensitivity to malate inhibition.

The replacement of Lys620 by serine desensitizes Escherichia coli phosphoenolpyruvate carboxylase to the effects of the feedback inhibitors L-aspartate and L-malate

Chemical modification of Escherichia coli phosphoenolpyruvate carboxylase (P-pyruvate carboxylase) by 2,4,6-trinitrobenzene sulfonate, a specific reagent for amino groups, causes desensitization to allosteric inhibitors, L-aspartate and L-malate, as well as inactivation. When L-malate is included in the modification mixture, P-pyruvate carboxylase was markedly protected from both desensitization and inactivation [Naide, A., Izui, K., Yoshinaga, T. & Katsuki, H. (1979) J. Biochem. (Tokyo) 85, 423-432]. To determine the lysine residue(s) involved in allosteric inhibition, the lysine residues that were protected from modification by L-malate were investigated by analyzing trinitrophenylated peptides liberated by digestion with glutamyl endopeptidase (V8-protease). The identified residues were Lys491, Lys620, Lys650, and Lys773. Each of these residues was individually replaced with an alanine or serine residue by site-directed mutagenesis to produce mutant enzymes. The mutant enzyme whose lysine residue was replaced with serine ([Ser620]P-pyruvate carboxylase) showed a marked desensitization to L-aspartate and L-malate, while retaining almost the same maximal catalytic activity as the wild-type P-pyruvate carboxylase. Essentially no changes in enzymatic properties were observed for the [Ala491]- and [Ala650]P-pyruvate carboxylases, while for the [Ala620]- and [Ala773]P-pyruvate carboxylases the polypeptides of the expected size were not significantly accumulated in the transformed E. coli cells, presumably due to intracellular degradation.

Evolution of the enzymatic characteristics of C4 phosphoenolpyruvate carboxylase--a comparison of the orthologous PPCA phosphoenolpyruvate carboxylases of Flaveria trinervia (C4) and Flaveria pringlei (C3)

C4 phosphoenolpyruvate (P-pyruvate) carboxylases have evolved from ancestral C3 P-pyruvate carboxylases during the evolution of C4 photosynthesis (Lepiniec et al., 1994). To meet the requirements of a new metabolic pathway, the C4 enzymes have gained distinct kinetic and regulatory properties compared to C3 enzymes. Our interest is to deduce the structure responsible for these C4-specific properties. As a model system, the orthologous ppcA P-pyruvate carboxylases of Flaveria trinervia (C4) and Flaveria pringlei (C3) were investigated by expressing them in Escherichia coli using their cDNAs. The K(m) (P-pyruvate) was about ten times higher for the C4 enzyme (650 microM) than for the C3 enzyme (60 microM). The activation by glucose 6-phosphate, which was shown by a decrease in the K(m) (P-pyruvate), was about twice for the C4 enzyme and three times for the C3 enzyme. The C3 enzyme showed a very high sensitivity to L-malate with an I(0.5) (50% inhibition) value of 80 microM malate, whereas the C4 enzyme was much less sensitive with a I(0.5) value of 1.2 mM malate. To locate the structural positions responsible for these differences in kinetic and regulatory properties, chimeras of these 95% identical enzymes were made. In this study, the first 437 residues of the 966-amino-acid protein were interchanged. The results showed that the N-terminal part of the enzyme was responsible for a small but significant part of the kinetic difference observed between these two isoenzymes. Additionally, the results suggest that the N-terminus was the site for glucose 6-phosphate activation and was also responsible for the observed difference in activation by this sugar phosphate. The difference in inhibition by L-malate, however, is suggested to originate mainly from the C-terminal part of the enzyme.

PHOSPHOENOLPYRUVATE CARBOXYLASE: A Ubiquitous, Highly Regulated Enzyme in Plants

Since plant phosphoenolpyruvate carboxylase (PEPC) was last reviewed in the Annual Review of Plant Physiology over a decade ago (O'Leary 1982), significant advances have been made in our knowledge of this oligomeric, cytosolic enzyme. This review highlights this exciting progress in plant PEPC research by focusing on the three major areas of recent investigation: the enzymology of the protein; its posttranslational regulation by reversible protein phosphorylation and opposing metabolite effectors; and the structure, expression, and molecular evolution of the nuclear PEPC genes. It is hoped that the next ten years will be equally enlightening, especially with respect to the three-dimensional structure of the plant enzyme, the molecular analysis of its highly regulated protein-Ser/Thr kinase, and the elucidation of its associated signal-transduction pathways in various plant cell types.

Regulatory phosphorylation of C4 phosphoenolpyruvate carboxylase from Sorghum: An immunological study using specific anti-phosphorylation site-antibodies

A peptide containing the N-terminal phosphorylation site (Ser-8) of Sorghum C4-phospho enolpyruvate carboxylase (PEPC) was synthesized, purified and used to raise an antiserum in rabbits. Affinity-purified IgGs prevented PEPC phosphorylation in a reconstituted in vitro assay and reacted with both the phosphorylated and dephosphorylated forms of either native or denatured PEPC in immunoblotting experiments. Saturation of dephospho-PEPC with these specific IgGs resulted in a marked alteration of its functional and regulatory properties that mimicked phosphorylation of Ser-8. A series of recombinant C4 PEPCs mutated in the N-terminal phosphorylation domain and a C3-like PEPC isozyme from Sorghum behaved similarly to their C4 counterpart with respect to these phosphorylation-site antibodies.

Molecular biology of C4 phosphoenolpyruvate carboxylase: Structure, regulation and genetic engineering

Three to four families of nuclear genes encode different isoforms of phosphoenolpyruvate (PEP) carboxylase (PEPC): C4-specific, C3 or etiolated, CAM and root forms. C4 leaf PEPC is encoded by a single gene (ppc) in sorghum and maize, but multiple genes in the C4-dicot Flaveria trinervia. Selective expression of ppc in only C4-mesophyll cells is proposed to be due to nuclear factors, DNA methylation and a distinct gene promoter. Deduced amino acid sequences of C4-PEPC pinpoint the phosphorylatable serine near the N-terminus, C4-specific valine and serine residues near the C-terminus, conserved cysteine, lysine and histidine residues and PEP binding/catalytic sites. During the PEPC reaction, PEP and bicarbonate are first converted into carboxyphosphate and the enolate of pyruvate. Carboxyphosphate decomposes within the active site into Pi and CO2, the latter combining with the enolate to form oxalacetate. Besides carboxylation, PEPC catalyzes a HCO3 (-)-dependent hydrolysis of PEP to yield pyruvate and Pi. Post-translational regulation of PEPC occurs by a phosphorylation/dephosphorylation cascade in vivo and by reversible enzyme oligomerization in vitro. The interrelation between phosphorylation and oligomerization of the enzyme is not clear. PEPC-protein kinase (PEPC-PK), the enzyme responsible for phosphorylation of PEPC, has been studied extensively while only limited information is available on the protein phosphatase 2A capable of dephosphorylating PEPC. The C4 ppc was cloned and expressed in Escherichia coli as well as tobacco. The transformed E. coli produced a functional/phosphorylatable C4 PEPC and the transgenic tobacco plants expressed both C3 and C4 isoforms. Site-directed mutagenesis of ppc indicates the importance of His(138), His(579) and Arg(587) in catalysis and/or substrate-binding by the E. coli enzyme, Ser(8) in the regulation of sorghum PEPC. Important areas for further research on C4 PEPC are: mechanism of transduction of light signal during photoactivation of PEPC-PK and PEPC in leaves, extensive use of site-directed mutagenesis to precisely identify other key amino acid residues, changes in quarternary structure of PEPC in vivo, a high-resolution crystal structure, and hormonal regulation of PEPC expression.

Regulatory phosphorylation of phosphoenolpyruvate carboxylase in protoplasts from Sorghum mesophyll cells and the role of pH and Ca2+ as possible components of the light-transduction pathway

The light-dependent phosphorylation of the photosynthetic phosphoenolpyruvate carboxylase (PyrPC) was shown to occur in protoplasts from Sorghum mesophyll cells. It was accompanied by an increase in PyrPC protein-serine-kinase activity and conferred the target-specific functional properties, i.e. an increase in Vmax and apparent Ki for L-malate, as previously found with the whole leaf. The light-dependent regulatory phosphorylation of PyrPC was (a) specifically promoted by the weak bases NH4Cl and methylamine (agents which increase cytosolic pH), but not by KNO3, (b) inhibited by the cytosolic protein-synthesis inhibitor, cycloheximide, thus confirming that protein turnover is a component of the signal-transduction cascade, as reported in [4], (c) found to moderately decrease in the presence of EGTA and to be strongly depressed when the Ca(2+)-selective ionophore A23187 was added to the incubation medium together with EGTA. Addition of Ca2+, but not of Mg2+, to the Ca(2+)-depleted protoplasts partially, but significantly, relieved the inhibition. Calcium deprivation apparently affected the in-situ light-activation of the PyrPC protein kinase. These data indicated that both Ca2+ and an increase in cytosolic pH are required for the induction of PyrPC protein kinase activity/PyrPC phosphorylation in illuminated protoplasts from Sorghum mesophyll cells.

Regulatory protein phosphorylation of phosphoenolpyruvate carboxylase in the facultative crassulacean-acid-metabolism plant Mesembryanthemum crystallinum L

Phosphoenolpyruvate PyrP carboxylase (PyrPC) and PyrPC kinase were copurified from dark-adapted leaves of the common ice plant Mesembryanthemum crystallinum L. with crassulacean-acid metabolism (CAM). Purification by (NH4)2SO4 fractionation, chromatography on Fractogel-DEAE and hydroxylapatite resulted in a PyrPC preparation with a specific activity of 23-25 U/mg protein and a protein kinase activity of 255 mumol Pi.mol-1 PyrPC.s-1. After in vitro phosphorylation, the most prominently phosphorylated polypeptide was identified as PyrPC by immunoblotting and sequencing. Phosphorylation of PyrPC in vitro by incubation with 400 microM MgATP decreased its sensitivity towards malate. When purified in the absence of the protease inhibitor chymostatin, PyrPC lost an N-terminal sequence of 128 amino acids. Although the carboxylation reaction was unaffected, the truncated PyrPC could neither be phosphorylated in vitro nor inhibited by malate. This result and data obtained by limited proteolysis concur with the hypothesis [Jiao, J.A. & Chollet, R. (1989) Arch. Biochem. Biophys. 283, 300-305] that Ser11 is the phosphorylation site of the CAM PyrPC of M. crystallinum. At pH 7.0, the Km for ATP of the protein kinase was 25 microM; phosphorylation of PyrPC was maximal after 30 min at pH 7.0. The kinase showed also activity with histone III-S but not with dephosphorylated casein. It was inhibited by malate. The results show, that reversible protein phosphorylation is an important factor in the regulation of PyrPC in the facultative CAM plant M. crystallinum, similar to C4 and constitutive CAM plants.

A Ca(2+)-dependent protein kinase phosphorylates phosphoenolpyruvate carboxylase in maize

In C4 plants the activity of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) is regulated by phosphorylation/dephosphorylation which is mediated by light/dark signals. The study using protein kinase inhibitors showed that the inhibition pattern of maize PEPC-protein kinase (PEPC-PK) is similar to that of myosin light chain kinase, a Ca(2+)-calmodulin-dependent PK. The kinase activity was also inhibited by EGTA and the inhibition was relieved by Ca2+. These results suggest that PEPC-PK is Ca(2+)-dependent in contrast with previous observations by other research groups.

Regulatory phosphorylation of Sorghum leaf phosphoenolpyruvate carboxylase. Identification of the protein-serine kinase and some elements of the signal-transduction cascade

The phosphoenolpyruvate (PPrv) carboxylase isozyme involved in C4 photosynthesis undergoes a day/night reversible phosphorylation process in leaves of the C4 plant, Sorghum. Ser8 of the target enzyme oscillates between a high (light) and a low (dark) phosphorylation status. Both in vivo and in vitro, phosphorylation of dark-form carboxylase was accompanied by an increase in the apparent Ki of the feedback inhibitor L-malate and an increase in Vmax. Feeding detached leaves various photosynthetic inhibitors, i.e. 3-(3,4-dichlorophenyl)-1,1-dimethylurea, gramicidin and DL-glyceraldehyde, prevented PPrv carboxylase phosphorylation in the light, thus suggesting that the cascade involves the photosynthetic apparatus as the light signal receptor, and presumably has the electron transfer chain and the Calvin-Benson cycle as components in the signal-transduction chain. Two protein-serine kinases capable of phosphorylating PPrv carboxylase in vitro have been partially purified from light-adapted leaves. One was isolated on a calmodulin-Sepharose column; it was calcium-dependent but did not require calmodulin for activity. The other was purified on a blue-dextran-agarose column and the only Me2+ required for activity was Mg2+. In reconstituted phosphorylation assays, only the latter caused the expected decrease in malate sensitivity of PPrv carboxylase suggesting that this protein is the genuine PPrv-carboxylase-kinase. Desalted extracts from light-adapted leaves possessed a considerably greater phosphorylation capacity with immunopurified dephosphorylated PPrv carboxylase as substrate than did dark extracts. This light stimulation was insensitive to type 2A protein phosphatase inhibitors, okadaic acid and microcystin-LR, which suggests that the kinase is a controlled step in the cascade which leads to phosphorylation of PPrv carboxylase. The higher phosphorylation capacity of light-adapted leaf tissue was nullified by pretreatment with the cytosolic protein synthesis inhibitor, cycloheximide. Thus, protein turnover is involved as part of the mechanism controlling the activity of the kinase purified on blue-dextran-agarose. However, no information is available with respect to the specific nature of the link between the above-mentioned light transducing steps and the protein kinase that achieves the physiological response. Finally, the in vivo phosphorylation site (Ser8) in the N-terminal region of the C4 type Sorghum PPrv carboxylase is also present in a non-photosynthetic form of the Sorghum enzyme (Ser7), as deduced by cDNA sequence analysis.

Site-directed mutagenesis of the conserved histidine residue of phosphoenolpyruvate carboxylase. His138 is essential for the second partial reaction

Histidine residues have previously been suggested to be essential for the activity of phosphoenolpyruvate carboxylase as demonstrated by chemical modification of these residues. Although the location of these residues on the primary structure is not known, a comparison of nine phosphoenolpyruvate (P-pyruvate) carboxylases sequenced recently revealed that there are only two conserved histidine residues (His138 and His579, coordinates from the E. coli enzyme). Site-directed mutagenesis of these residues were undertaken with the E. coli P-pyruvate carboxylase and the properties of purified mutant enzymes were investigated. Mutation of His138 to asparagine (H138N) produced a protein which did not show carboxylase activity. However, this mutant enzyme catalyzed the bicarbonate-dependent dephosphorylation (Vmax = 1.4 mumol.min-1.mg-1) of the P-pyruvate. Since this reaction is due to one of the two partial reactions proposed for this enzyme, the results indicate that His138 is obligatory for the second-step reaction, i.e. the carboxylation of the enolate form of pyruvate by carboxyphosphate. Mutation of His579 to asparagine (H579N) produced an enzyme which had 69% of the wild-type carboxylase activity, but its affinity for P-pyruvate was decreased by 24-fold.

Illumination increases the phosphorylation state of maize leaf phosphoenolpyruvate carboxylase by causing an increase in the activity of a protein kinase

Illumination of maize leaves increases the phosphorylation state of phosphoenolpyruvate carboxylase and reduces the sensitivity of the enzyme to feedback inhibition by malate. Red, white and blue light were each found to be equally potent, and the effect of light was blocked by 3(3,4-dichlorophenyl)-1,1-dimethylurea. A phosphoenolpyruvate carboxylase kinase was partially purified from illuminated maize leaves by a three-step procedure. Phosphorylation of phosphoenolpyruvate carboxylase by this protein kinase reached 0.7-0.8 molecules/subunit and correlated with a 3- to 4-fold increase in Ki for malate. The protein kinase was inhibited by L-malate, but was insensitive to a number of other potential regulators. Freshly prepared and desalted extracts of darkened maize leaves contained very little kinase activity, but the activity appeared when leaves were illuminated for 30-60 min before extraction. The catalytic subunit of protein phosphatase 2A from rabbit skeletal muscle, but not that of protein phosphatase 1, could dephosphorylate phosphoenolpyruvate carboxylase. The protein phosphatases 1 and 2A activities of maize leaves were not affected by illumination. It is suggested that the major means by which light stimulates the phosphorylation of phosphoenolpyruvate carboxylase is by an increase in the activity of the protein kinase.

Isolation and sequence of an active-site peptide from maize leaf phosphoenolpyruvate carboxylase inactivated by pyridoxal 5'-phosphate

An active-site peptide from maize (Zea mays L.) phosphoenolpyruvate carboxylase has been isolated, sequenced and identified in the primary structure following chemical modification/inactivation of the enzyme by pyridoxal 5'-phosphate and reduction with sodium borohydride. The amino acid sequence of the purified dodecapeptide is Val-Gly-Tyr-Ser-Asp-Ser-Gly-L*ys-Asp-Ala-Gly-Arg, which corresponds exactly to residues 599-610 in the deduced primary sequence of the maize-leaf enzyme. Comparative analysis of the deduced amino acid sequences of the enzyme from Escherichia coli, Anacystis nidulans and C3, C4 and Crassulacean acid metabolism plants indicates that they all contain this specific lysyl group, as well as a high degree of sequence homology flanking this species-invariant residue. This observation suggests a critical role for Lys-606 during catalysis by maize phosphoenolpyruvate carboxylase. This represents the first identification of a specific, species-invariant active-site residue in the enzyme.

Regulatory phosphorylation of serine-15 in maize phosphoenolpyruvate carboxylase by a C4-leaf protein-serine kinase

We have recently reported that the light-induced changes in the enzymatic and regulatory properties of maize leaf phosphoenolpyruvate carboxylase are attributed to the regulatory seryl phosphorylation of this C4-photosynthesis enzyme. In the present study, the darkform target enzyme was phosphorylated/activated in vitro by a maize leaf protein-serine kinase, and the 32P-labeled regulatory site phosphopeptide was purified from a tryptic digest by metal-ion affinity and reversed-phase chromatography. Automated Edman degradation analysis by covalent protein sequencing technology revealed that the amino acid sequence of this phosphoseryl peptide is His-His-Ser(P)-Ile-Asp-Ala-Gln-Leu-Arg. This nonapeptide, which corresponds exactly to residues 13-21 in the deduced primary sequence of the maize leaf carboxylase, is far removed from recently identified active-site cysteine (Cys-553) and lysine (Lys-606) residues in the C-terminal region of the primary structure. Comparative analysis of the deduced N-terminal sequences of C3-, C4-, and Crassulacean acid metabolism (CAM)-leaf phosphoenolpyruvate carboxylases suggests that the motif of Lys/Arg-X-X-Ser is an important structural requirement of the C4- and CAM-leaf protein-serine kinases.