The phosphorylation of Sorghum leaf phosphoenolpyruvate carboxylase is a Ca++-calmodulin dependent process

Reduction of plant-specific arabinogalactan-type O-glycosylation by treating tobacco plants with ferrous chelator 2,2'-dipyridyl

Plant specific O-glycosylation of proteins includes the attachment of arabinogalactan to hydroxyproline (Hyp) residues. These Hyp residues are generated from peptidyl proline residues by the action of prolyl 4-hydroxylase which requires the ferrous ion. We investigated the effect of the ferrous chelator, 2,2'-dipyridyl on tobacco plants, and found that such treatment reduced the arabinogalactosylation of proteins.

Regulation of phosphoenolpyruvate carboxylase phosphorylation by metabolites and abscisic acid during the development and germination of barley seeds

During barley (Hordeum vulgare) seed development, phosphoenolpyruvate carboxylase (PEPC) activity increased and PEPC-specific antibodies revealed housekeeping (103-kD) and inducible (108-kD) subunits. Bacterial-type PEPC fragments were immunologically detected in denatured protein extracts from dry and imbibed conditions; however, on nondenaturing gels, the activity of the recently reported octameric PEPC (in castor [Ricinus communis] oil seeds) was not detected. The phosphorylation state of the PEPC, as judged by l-malate 50% inhibition of initial activity values, phosphoprotein chromatography, and immunodetection of the phosphorylated N terminus, was found to be high between 8 and 18 d postanthesis (DPA) and during imbibition. In contrast, the enzyme appeared to be in a low phosphorylation state from 20 DPA up to dry seed. The time course of 32/36-kD, Ca(2+)-independent PEPC kinase activity exhibited a substantial increase after 30 DPA that did not coincide with the PEPC phosphorylation profile. This kinase was found to be inhibited by l-malate and not by putative protein inhibitors, and the PEPC phosphorylation status correlated with high glucose-6-phosphate to malate ratios, thereby suggesting an in vivo metabolic control of the kinase. PEPC phosphorylation was also regulated by photosynthate supply at 11 DPA. In addition, when fed exogenously to imbibing seeds, abscisic acid significantly increased PEPC kinase activity. This was further enhanced by the cytosolic protein synthesis inhibitor cycloheximide but blocked by protease inhibitors, thereby suggesting that the phytohormone acts on the stability of the kinase. We propose that a similar abscisic acid-dependent effect may contribute to produce the increase in PEPC kinase activity during desiccation stages.

Modulation of phosphoenolpyruvate carboxylase in vivo by Ca2+ in Amaranthus hypochondriacus, a NAD-ME type C4 plant: possible involvement of Ca2+ in up-regulation of PEPC-protein kinase in vivo

The properties of phosphoenolpyruvate carboxylase (PEPC) were studied, with respect to calcium (Ca2+), in leaves of Amaranthus hypochondriacus, a C4 plant. Experiments were conducted in vitro (by adding Ca2+ during enzyme assay) or in vivo (by feeding Ca2+ to intact leaves through petiole). Inclusion of 10 microM Ca2+ during assay marginally increased (<30%) malate sensitivity of PEPC in extracts from dark-adapted leaves. The effect of Ca2+ was marginal on PEPC in extracts from illuminated leaves. Upon applying a low concentration of Ca2+ to leaves, the PEPC activity in leaves increased by 1.5-fold, while inhibition by malate decreased markedly. The light activation of PEPC in Ca2+-fed leaves was slightly higher than in the absence of Ca2+-ethyleneglycol-bis-(beta-aminoethyl ether) N,N,N',N'-tetra acetic acid (EGTA). To assess further the role of Ca2+, 5 mM EGTA (Ca2+ chelator) was either added during the enzyme assay or fed to leaves through petiole. EGTA had no effect on PEPC, when added during enzyme assay. Upon feeding EGTA, the PEPC activity in the dark-adapted leaf extracts increased by 30%, and the effect on malate sensitivity was marginal. However, there was a decrease in PEPC activity in illuminated extracts, resulting in a marked decrease in the extent of light activation of PEPC. The extent of phosphorylation of PEPC was much higher in Ca2+ or Ca2+-EGTA-fed leaves than in the control, but EGTA decreased the light-induced phosphorylation. Our results suggest that optimal alone concentration of Ca2+ is essential for PEPC in leaves of A. hypochondriacus, particularly in vivo. We suggest that Ca2+ regulates PEPC, at an upstream level, such as transcription, by modulating PEPC-protein kinase, thus facilitating the light activation of PEPC.

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.

Calcium and signal transduction in plants

Environmental and hormonal signals control diverse physiological processes in plants. The mechanisms by which plant cells perceive and transduce these signals are poorly understood. Understanding biochemical and molecular events involved in signal transduction pathways has become one of the most active areas of plant research. Research during the last 15 years has established that Ca2+ acts as a messenger in transducing external signals. The evidence in support of Ca2+ as a messenger is unequivocal and fulfills all the requirements of a messenger. The role of Ca2+ becomes even more important because it is the only messenger known so far in plants. Since our last review on the Ca2+ messenger system in 1987, there has been tremendous progress in elucidating various aspects of Ca(2+) -signaling pathways in plants. These include demonstration of signal-induced changes in cytosolic Ca2+, calmodulin and calmodulin-like proteins, identification of different Ca2+ channels, characterization of Ca(2+) -dependent protein kinases (CDPKs) both at the biochemical and molecular levels, evidence for the presence of calmodulin-dependent protein kinases, and increased evidence in support of the role of inositol phospholipids in the Ca(2+) -signaling system. Despite the progress in Ca2+ research in plants, it is still in its infancy and much more needs to be done to understand the precise mechanisms by which Ca2+ regulates a wide variety of physiological processes. The purpose of this review is to summarize some of these recent developments in Ca2+ research as it relates to signal transduction in 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.

Purification and characterization of wheat and pine small basic protein substrates for plant calcium-dependent protein kinase

A wheat basic protein (WBP) was purified to homogeneity from wheat germ by a protocol involving extraction, centrifugation, batchwise elution from carboxymethylcellulose (CM-52), acidification with trifluoroacetic acid, neutralization and HPLC on a SP5PW cation exchange column. WBP is a 10 kDa protein and is phosphorylated on serine residues by wheat germ Ca(2+)-dependent protein kinase (CDPK). [32P]phosphoWBP exactly comigrates with WBP on SDS-PAGE. WBP does not inhibit either wheat germ CDPK or calmodulin-dependent myosin light chain kinase. Apart from histone H1, WBP is the best endogenous substrate yet found for wheat embryo CDPK. A 12 kDa pine basic protein (PBP) was purified to homogeneity from seeds of stone pine (Pinus pinea L.) by a simple procedure involving batchwise elution from carboxymethylcellulose and cation exchange HPLC. PBP is also a good substrate for CDPK and is phosphorylated on Ser residues. N-terminal sequencing of WBP and PBP revealed that these proteins are homologous to a family of small basic plant proteins having a phospholipid transfer function.

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.

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.

Protein turnover as a component in the light/dark regulation of phosphoenolpyruvate carboxylase protein-serine kinase activity in C4 plants

Maize leaf phosphoenolpyruvate carboxylase [PEPC; orthophosphate:oxaloacetate carboxy-lyase (phosphorylating), EC 4.1.1.31] protein-serine kinase (PEPC-PK) phosphorylates serine-15 of its target enzyme, thus leading to an increase in catalytic activity and a concomitant decrease in malate sensitivity of this cytoplasmic C4 photosynthesis enzyme in the light. We have recently demonstrated that the PEPC-PK activity in maize leaves is slowly, but strikingly, increased in the light and decreased in darkness. In this report, we provide evidence that cycloheximide, an inhibitor of cytoplasmic protein synthesis, when fed to detached leaves of C4 monocots (maize, sorghum) and dicots (Portulaca oleracea) in the dark or light, completely prevents the in vivo light activation of PEPC-PK activity regardless of whether the protein kinase activity is assessed in vivo or in vitro. In contrast, chloramphenicol, an inhibitor of protein synthesis in chloroplasts, has no effect on the light activation of maize PEPC-PK. Similarly, treatment with cycloheximide did not influence the light activation of other photosynthesis-related enzymes in maize, including cytoplasmic sucrose-phosphate synthase and chloroplast stromal NADPH-malate dehydrogenase and pyruvate, Pi dikinase. These and related results, in which detached maize leaves were treated simultaneously with cycloheximide and microcystin-LR, a potent in vivo and in vitro inhibitor of the PEPC type 2A protein phosphatase, indicate that short-term protein turnover of the PEPC-PK itself or some other essential component(s) (e.g., a putative protein that modifies this kinase activity) is one of the primary levels in the complex and unique regulatory cascade effecting the reversible light activation/seryl phosphorylation of PEPC in the mesophyll cytoplasm of C4 plants.

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.

Reversible light activation of the phosphoenolpyruvate carboxylase protein-serine kinase in maize leaves

C4-leaf phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) undergoes reversible, light-induced increases in its activity-seryl phosphorylation-status in vivo. We now report that the PEPC-protein kinase activity in desalted crude extracts of light-adapted maize leaves is several-fold greater than that from the corresponding dark tissue when in vitro phosphorylation assays are performed with either endogenous or purified dark-form maize PEPC as substrate, both in the absence or presence of okadaic acid, a potent inhibitor of the PEPC type 2A protein phosphatase(s). These and related results indicate that the PEPC protein-serine kinase(s) per se is reversibly light activated in vivo by either covalent modification, protein turnover or, less likely, a tight-binding effector.

The cellular mechanism of circadian rhythms--a view on evidence, hypotheses and problems

A stable period length is a characteristic property of circadian oscillations. The question about whether higher frequency oscillators (0.5-8 hr) contribute to or establish the stable circadian periodicity cannot be answered at present. A sequential coupling of quantal subcycles appears possible on the basis of known "ultradian" oscillations. There is, however, no supporting evidence for such a concept. Phase response curves of the circadian clock derived from various perturbing pulses allow qualitative conclusions concerning the perturbed clock process. Deductions from computer simulations also allow conclusions about the phase of this oscillatory process. The distinction between processes (a) essential to the clock mechanism, (b) maintaining and controlling the clock (inputs) and (c) depending on the clock (outputs) on the basis of "oscillatory" and "change of psi or tau after perturbation" seems to be useful but not stringent. Protein synthesis may be an essential or input process. Oscillatory changes of this process may be due to periodic translational control or RNA-supply. Circadian changes in protein concentration and/or activity may depend on periodic synthesis, proteolysis, covalent modifications or aggregations. Specific essential proteins have not been identified conclusively. The large overlap between the group of agents and treatments that phase shift the clock and the group that induces stress proteins suggest that the latter may play a role in the controlling (input) or essential domain. The role of membranes in the clock mechanism is not clear: concepts assuming an essential function are based on circumstantial evidence. The membrane potential as well as Ca2+ may be involved in either input or essential function. Ca(2+)-calmodulin may also be important as concluded from inhibitor experiments. It is tempting to assume that a calmodulin-dependent kinase is part of a periodic protein phosphorylation process, yet it is not clear whether the periodic protein phosphorylation that has been observed is essential or is just another output process.

Regulation of phosphoenolpyruvate carboxylase: an example of signal transduction via protein phosphorylation in higher plants

There is now good evidence that the malate sensitivity of PEPc is regulated by phosphorylation/dephosphorylation in the leaf tissue of C4 and CAM plants. This statement is based on the assessment of the phosphorylation state of PEPc in [32P]-labeled intact tissue by immunoprecipitation and the correlation between phosphorylation state and malate sensitivity that has been observed during incubation of purified PEPc in vitro with protein kinases or protein phosphatases. The phosphorylation of PEPc in the CAM plant B. fedtschenkoi is controlled by an endogenous rhythm whereas that of PEPc in the C4 plant maize is triggered directly by light. In neither case has the mechanism of signal transduction been identified. It is hoped that further work on the protein kinases and protein phosphatases involved will reveal the nature of the signalling systems. Preliminary work suggests that plant protein phosphatases are very similar to their mammalian counterparts. It is also noteworthy that higher plant genes very similar to the genes encoding the cyclic nucleotide-dependent protein kinases and the protein kinase C family have recently been identified. It is interesting to speculate that the protein kinases and phosphatases involved in signal transduction systems in plants may prove to be closely related to well-studied mammalian enzymes.

Identification of high levels of type 1 and type 2A protein phosphatases in higher plants

Extracts of Brassica napus (oilseed rape) seeds contain type 1 and type 2A protein phosphatases whose properties are indistinguishable from the corresponding enzymes in mammalian tissues. The type 1 activity dephosphorylated the beta-subunit of phosphorylase kinase selectively and was inhibited by the same concentrations of okadaic acid [IC50 (concentration causing 50% inhibition) approximately 10 nM], mammalian inhibitor 1 (IC50 = 0.6 nM) and mammalian inhibitor 2 (IC50 = 2.0 nM) as the rabbit muscle type 1 phosphatase. The plant type 2A activity dephosphorylated the alpha-subunit of phosphorylase kinase preferentially, was exquisitely sensitive to okadaic acid (IC50 approximately 0.1 nM), and was unaffected by inhibitors 1 and 2. As in mammalian tissues, a substantial proportion of plant type 1 phosphatase activity (40%) was particulate, whereas plant type 2A phosphatase was cytosolic. The specific activities of the plant type 1 and type 2A phosphatases were as high as in mammalian tissue extracts, but no type 2B or type 2C phosphatase activity was detected. The results demonstrate that the improved procedure for identifying and quantifying protein phosphatases in animal cells is applicable to higher plants, and suggests that okadaic acid may provide a new method for identifying plant enzymes that are regulated by reversible phosphorylation.

Regulatory seryl-phosphorylation of C4 phosphoenolpyruvate carboxylase by a soluble protein kinase from maize leaves

A reconstituted system composed of purified phosphoenolpyruvate carboxylase (PEP-Case) and a soluble protein kinase (PK) from green maize leaves was developed to critically assess the effects of in vitro protein phosphorylation on the catalytic and regulatory (malate sensitivity) properties of the target enzyme. The PK was partially purified from light-adapted leaf tissue by ammonium sulfate fractionation (0-60% saturation fraction) of a crude extract and blue dextran-agarose affinity chromatography. The resulting preparation was free of PEPCase. This partially purified protein kinase activated PEPCase from dark-adapted green maize leaves in an ATP-, Mg2+-, time-, and temperature-dependent fashion. Concomitant with these changes in PEPCase activity was a marked decrease in the target enzyme's sensitivity to feedback inhibition by L-malate. The PK-mediated incorporation of 32P from [gamma-32P]ATP into the protein substrate was directly correlated with these changes in PEPCase activity and malate sensitivity. The maximal molar 32P-incorporation value was about 0.25 per 100-kDa PEPCase subunit (i.e., 1 per holoenzyme). Phosphoamino acid analysis of the 32P-labeled target enzyme by two-dimensional thin-layer electrophoresis revealed the exclusive presence of phosphoserine. These in vitro results, together with our recent studies on the light-induced changes in phosphorylation status of green maize leaf PEPCase in vivo (J. A. Jiao and R. Chollet (1988) Arch. Biochem. Biophys. 261, 409-417), collectively provide the first unequivocal evidence that the seryl-phosphorylation of the dark-form enzyme by a soluble protein kinase is responsible for the changes in catalytic activity and malate sensitivity of C4 PEPCase observed in vivo during dark/light transitions of the parent leaf tissue.