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

Identification of factors regulating the phosphorylation status of sucrose-phosphate synthase in vivo

The purpose of this study was to identify the factors that control sucrose-phosphate synthase (SPS)-kinase and SPS-protein phosphatase (SPS-PP) activity in situ, and thereby mediate the activation of SPS by light or mannose. Feeding mannose to excised spinach (Spinacia oleracea) leaves in darkness resulted in a general sequestration of cellular phosphate (as evidenced by accumulation of mannose-6-P and depletion of glucose-6-P [Glc-6-P] and fructose-6-P [Fru-6-P]) and a relatively slow activation of SPS (maximum activation achieved within 90 min). Supplying exogenous inorganic phosphate (Pi) with mannose reduced sequestration of cellular Pi (as evidenced by mannose-6-P accumulation without depletion of hexose-P) and substantially reduced mannose activation of SPS. Thus, depletion of cytoplasmic Pi may be required for SPS activation; accumulation of mannose-6-P alone is clearly not sufficient. It was verified that Glc-6-P, but not mannose-6-P, was an inhibitor of partially purified SPS-kinase, and that Pi was an inhibitor of partially purified SPS-PP. Total extractable activity of SPS-kinase did not vary diurnally, whereas a pronounced light activation of SPS-PP activity was observed. Pretreatment of leaves in the dark with cycloheximide blocked the light activation of SPS-PP (assayed in vitro) and dramatically reduced the rate of SPS activation in situ (in saturating light and carbon dioxide). We conclude that rapid activation of SPS by light involves reduction in cytosolic Pi, an inhibitor of SPS-PP, and light activation of SPS-PP, by a novel mechanism that may involve (directly or indirectly) a protein synthesis step. An increase in cytosolic Glc-6-P, an inhibitor of SPS-kinase, would also favor SPS activation. Thus, the signal transduction pathway mediating the light activation of SPS involves elements of "fine" and "coarse" control.

NO(3) Enhances the Kinase Activity for Phosphorylation of Phosphoenolpyruvate Carboxylase and Sucrose Phosphate Synthase Proteins in Wheat Leaves: Evidence from the Effects of Mannose and Okadaic Acid

The aim of this work was to determine which of the two reactions (i.e. phosphorylation or dephosphorylation) involved in the establishment of the phosphorylated status of the wheat leaf phosphoenolpyruvate carboxylase and sucrose phosphate synthase protein responds in vivo to NO(3) (-) uptake and assimilation. Detached mature leaves of wheat (Triticum aestivum L. cv Fidel) were fed with N-free (low-NO(3) (-) leaves) or 40 mm NO(3) (-) solution (high-NO(3) (-) leaves). The specific inhibition of the enzyme-protein kinase or phosphatase activities was obtained in vivo by addition of mannose or okadaic acid, respectively, in the uptake solution. Mannose at 50 mm, by blocking the kinase reaction, inhibited the processes of NO(3) (-)-dependent phosphoenolpyruvate carboxylase activation and sucrose phosphate synthase deactivation. Following the addition of mannose, the deactivation of phosphoenolpyruvate carboxylase and the activation of sucrose phosphate synthase, both due to the enzyme-protein dephosphorylation, were at the same rate in low-NO(3) (-) and high-NO(3) (-) leaves, indicating that NO(3) (-) had no effect per se on the enzyme-protein phosphatase activity. Upon treatment with okadaic acid, the higher increase of phosphoenolpyruvate carboxylase and decrease of sucrose phosphate synthase activities observed in high NO(3) (-) compared with low NO(3) (-) leaves showed evidence that NO(3) (-) enhanced the protein kinase activity. These results support the concept that NO(3) (-), or a product of its metabolism, favors the activation of phosphoenolpyruvate carboxylase and deactivation of sucrose phosphate synthase in wheat leaves by promoting the light activation of the enzyme-protein kinase(s) without affecting the phosphatase(s).

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.

Light activation of maize phosphoenolpyruvate carboxylase protein-serine kinase activity is inhibited by mesophyll and bundle sheath-directed photosynthesis inhibitors

C(4) phosphoenolpyruvate carboxylase (PEPC) is post-translationally regulated by reversible phosphorylation of a specific N-terminal seryl residue in response to light/dark transitions of the parent leaf tissue. The protein-serine kinase (PEPC-PK) that phosphorylates/activates this mesophyll-cytoplasm target enzyme is slowly, but strikingly, activated by high light and inactivated in darkness in vivo by a mechanism involving cytoplasmic protein synthesis/degradation as a primary component. In this report, evidence is presented indicating that the inhibition of Calvin cycle activity by a variety of mesophyll (3-(3,4-dichlorophenyl)-1,1-dimethylurea, isocil, methyl viologen) and bundle sheath (dl-glyceraldehyde)-directed photosynthesis inhibitors blocks the light activation of maize (Zea mays L.) PEPC-PK and the ensuing regulatory phosphorylation of its target enzyme in vivo. Based on these and related observations, we propose that the Calvin cycle supplies the C(4) mesophyll cell with (a) a putative signal (e.g. phosphorylated metabolite, amino acid) that interacts with the cytoplasmic protein synthesis event to effect the light activation of PEPC-PK and the concomitant phosphorylation of PEPC, and (b) high levels of known positive effectors (e.g. triose-phosphate, glucose-6-phosphate) that interact directly with the carboxylase. The combined result of this complex regulatory cascade is to effectively desensitize PEPC to feedback inhibition by the millimolar levels of l-malate required for rapid diffusive transport to the bundle sheath during high rates of C(4) photosynthesis.

Effect of Light and NO(3) on Wheat Leaf Phosphoenolpyruvate Carboxylase Activity: Evidence for Covalent Modulation of the C(3) Enzyme

Phosphoenolpyruvate carboxylase (PEPcase) activity was studied in excised leaves of wheat (Triticum aestivum L.) in the dark and in the light, in presence of either N-free (low-NO(3) (-) leaves) or 40 millimolar KNO(3) (high-NO(3) (-) leaves) nutrient solutions. PEPcase activity increased to 2.7-fold higher than that measured in dark-adapted tissue (control) during the first 60 minutes and continued to increase more slowly to 3.8-fold that of the control. This level was reached after 200 minutes exposure of the leaves to light and high NO(3) (-). In contrast, the lower rate of increase recorded for low-NO(3) (-) leaves ceased after 60 minutes of exposure to light at 2.3-fold the control level. The short-term NO(3) (-) effect increased linearly with the level of NO(3) (-) uptake. In immunoprecipitation experiments, the antibody concentration for PEPcase precipitation increased with the protein extracts from the different treatments in the order: control, illuminated low-NO(3) (-) leaves, illuminated high-NO(3) (-) leaves. This order also applied with regard to a decreasing sensitivity to malate and an increasing stimulation by okadaic acid (an inhibitor of P-protein phosphatases). Following these studies, (32)P labeling experiments were carried out in vivo. These showed that the light-induced change in the properties of the PEPcase was due to an alteration in the phosphorylation state of the protein and that this effect was enhanced in high-NO(3) (-) conditions. Based on the responses of PEPcase and sucrose phosphate synthase in wheat leaves to light and NO(3) (-), an interpretation of the role of NO(3) (-) as either an inhibitor of P-protein phosphatase(s) or activator of protein kinase(s) is inferred. In the presence of NO(3) (-), the phosphorylation state of both PEPcase and sucrose phosphate synthase is increased. This causes activation of the former enzyme and inhibition of the latter. We suggest that NO(3) (-) modulates the relative protein kinase/protein phosphatase ratio to favor increased phosphorylation of both enzymes in order to redirect carbon flow away from sucrose synthesis and toward amino acid synthesis.

In vivo regulatory phosphorylation site in c(4)-leaf phosphoenolpyruvate carboxylase from maize and sorghum

Reversible seryl-phosphorylation contributes to the light/dark regulation of C(4)-leaf phosphoenolpyruvate carboxylase (PEPC) activity in vivo. The specific regulatory residue that, upon in vitro phosphorylation by a maize-leaf protein-serine kinase(s), leads to an increase in catalytic activity and a decrease in malate-sensitivity of the target enzyme has been recently identified as Ser-15 in (32)P-phosphorylated/activated dark-form maize PEPC (J-A Jiao, R Chollet [1990] Arch Biochem Biophys 283: 300-305). In order to ascertain whether this N-terminal seryl residue is, indeed, the in vivo regulatory phosphorylation site, [(32)P]phosphopeptides were isolated and purified from in vivo(32)P-labeled maize and sorghum leaf PEPC and subjected to automated Edman degradation analysis. The results show that purified light-form maize PEPC contains 14-fold more (32)P-radioactivity than the corresponding dark-form enzyme on an equal protein basis and, more notably, only a single N-terminal serine residue (Ser-15 in maize PEPC and its structural homolog, Ser-8, in the sorghum enzyme) was found to be (32)P-phosphorylated in the light or dark. These in vivo observations, combined with the results from our previous in vitro phosphorylation studies (J-A Jiao, R Chollet [1989] Arch Biochem Biophys 269: 526-535; [1990] Arch Biochem Biophys 283: 300-305), demonstrate that an N-terminal seryl residue in C(4) PEPC is, indeed, the regulatory site that undergoes light/dark changes in phosphorylation-status and, thus, plays a major, if not cardinal role in the light-induced changes in catalytic and regulatory properties of this cytoplasmic C(4)-photosynthesis enzyme in vivo.