Neutral peptidases in the stroma of pea chloroplasts

Biochemical characterization of the triticale TsPAP1, a new type of plant prolyl aminopeptidase, and its impact on proline content and flowering time in transgenic Arabidopsis plants

Proline aminopeptidase (PAP, EC 3.4.11.5) is the only enzyme that effectively releases proline from the N-termini of peptides. The amino acid sequence of the PAP from Triticosecale, TsPAP1, comprises conserved regions, characteristic of the monomeric forms of PAP found in bacteria but not yet identified in plants. Therefore, we aimed to obtain and biochemically characterize the TsPAP1 protein. The recombinant TsPAP1 protein was received through heterologous expression of the TsPAP1 coding sequence in a bacterial expression system and purified with affinity chromatography. Gel filtration chromatography and SDS electrophoresis revealed that TsPAP1 is a monomer with a molecular mass of 37.5 kDa. TsPAP1 prefers substrates with proline at the N-terminus but is also capable of hydrolyzing β-naphthylamides of hydroxyproline and alanine. Among the peptides tested, the most preferred were di- and tripeptides, especially those with glycine in the Y position. The use of diagnostic inhibitors indicated that TsPAP1 is a serine peptidase; however, further characterization revealed that the SH residues are also important for maintaining its activity. To examine the role of TsPAP1 under physiological conditions, we developed transgenic Arabidopsis plants overexpressing TsPAP1. Compared with wild-type plants, the transgenic lines accumulated more proline, flowered an average of 3.5 days earlier, and developed more siliques than did untransformed controls. Our paper is the first to describe the biochemical properties of a novel monomeric plant PAP and contributes to the functional characterization of PAP proteins in plants.

Purification and partial characterization of aminopeptidase from barley (Hordeum vulgare L.) seeds

Aminopeptidases (EC 3.4.11) are proteolytic enzymes, which hydrolyze one amino acid from N-terminus of peptidic substrates. Inhibitors of plant aminopeptidases can find an application in agriculture as herbicides. Isolation and partial characterization of aminopeptidase from barley (Hordeum vulgare L.) seeds has been described. The enzyme was purified to molecular homogeneity using a six-step purification procedure (precipitation with (NH4)2SO4, followed by chromatography on Sephadex G-25, DEAE-Sepharose, Sephacryl HR 300, Macro-Prep Q and Phenyl-Sepharose HP columns). The enzyme was purified 365-fold with recovery above 18%. The molecular weight of the purified enzyme was determined by SDS-PAGE and gel filtration as 58 kDa, and was found to be a monomer. Its pH and temperature optima were 7.5 and 52 °C, respectively. The enzyme behaves as standard leucine aminopeptidase by preferring bulky amino acids at the N-terminus, with phenylalanine being of choice.

Biochemical characterisation of prolyl aminopeptidase from shoots of triticale seedlings and its activity changes in response to suboptimal growth conditions

Prolyl aminopeptidase (PAP) was isolated from the shoots of three-day-old triticale seedlings and was purified using a five-step purification procedure (acid precipitation, gel filtration, anion-exchange chromatography, hydrophobic chromatography and rechromatography). The enzyme was purified 460-fold with a recovery of 6%. Prolyl aminopeptidase appears to be a tetramer consisting of four subunits, each with a molecular weight of approximately 54kDa. Its pH and temperature optimum are pH 7.5 and 37°C, respectively. The enzyme prefers substrates with Pro and Hyp at the N-terminus, but is also capable of hydrolysing β-naphthylamides (β-NA) of Ala, Phe, and Leu. The K(m) value of PAP against Pro-β-NA was the lowest among the substrates tested and it was 1.47×10(-5)M. The activity of PAP was not inhibited by EDTA, 1,10-phenantroline, or pepstatin A. The most effective inhibitors were DFP, Pefabloc, and PMSF, which are serine protease inhibitors. However, significant inhibition was also observed in the presence of E-64, which modifies sulfhydryl groups. A significant increase of the aminopeptidase activity against Pro-β-NA was observed in shoots of triticale plants grown under salinity, drought stress, and in the presence of cadmium and aluminium ions in the nutrient solution.

SDS-dependent proteases induced by ABA and its relation to Rubisco and Rubisco activase contents in rice leaves

Protease activities and its relation to the contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Rubisco activase were investigated in detached leaves of rice (Oryza sativa L.) floated on the solutions containing abscisic acid (ABA) or benzyladenine (BA). Rubisco and Rubisco activase contents were decreased during the time course and the decreases were enhanced by ABA and suppressed by BA. The decrease in Rubisco activase was faster than that in Rubisco. SDS-dependent protease activities at 50-70 kDa (rice SDS-dependent protease: RSP) analyzed by the gelatin containing PAGE were significantly enhanced by ABA. RSPs were also increased in attached leaves during senescence. RSPs had the pH optimum of 5.5, suggesting that RSPs are vacuolar protease. Both decrease in Rubisco and Rubisco activase contents and increase in RSPs activities were suppressed by cycloheximide. These findings indicate that the activities of RSPs are well correlated with the decrease in these protein contents. Immunoblotting analysis showed that Rubisco in the leaf extracts was completely degraded by 5h at pH 5.5 with SDS where it was optimal condition for RSPs. However, the degradation of Rubisco did not proceed at pH 7.5 without SDS where it is near physiological condition for stromal proteins. Rubisco activase was degraded at similar rate under both conditions. These results suggest that RSPs can functions in a senescence related degradation system of chloroplast protein in rice leaves. Rubisco activase would be more susceptible to proteolysis than Rubisco under physiological condition and this could affect the contents of these proteins in leaves.

Targeting and localization of wound-inducible leucine aminopeptidase A in tomato leaves

The constitutive and wound-inducible leucine aminopeptidases (LAP-N and LAP-A, respectively) of tomato encode 60-kDa proteins with 5-kDa presequences that resemble chloroplast-targeting peptides. Cell fractionation studies and immunoblot analyses of chloroplast and total proteins have suggested a dual location of the mature LAP-A proteins in the cytosol and the plastids. In this study, the subcellular localization of tomato LAPs was further investigated using in vitro chloroplast-targeting assays and immunocytochemical techniques at the light and TEM levels. In vitro-translated LAP-A1 and LAP-N preproteins were readily transported into pea chloroplasts and processed into mature proteins of 55 kDa indicating the presence of a functional chloroplast-targeting signal in the LAP-A1 and LAP-N protein precursors. In addition, a LAP polyclonal and a LAP-A-specific antisera were used to immunolocalize LAP proteins in leaves from healthy, wounded and methyl jasmonate (MeJA)-treated plants. Low levels of LAPs and/or LAP-like proteins were detected in leaves from unwounded plants. The LAP polyclonal antiserum, which detected LAP-A, LAP-N and LAP-like proteins, and the LAP-A specific antibodies, which detected only LAP-A, showed that LAP levels increased in leaf sections after wounding and MeJA treatments. LAP-A proteins were primarily detected within the chloroplasts of spongy and palisade mesophyll cells. The localization of LAP-A was distinct from the location of early wound-response proteins that are important in the biosynthesis of jasmonic acid or systemin and more similar to the late wound-response proteins with primary roles in defense. The importance of these findings relative to the potential roles of LAP-A in defense is discussed.

In vivo fragmentation of the large subunit of ribulose-1,5-bisphosphate carboxylase by reactive oxygen species in an intact leaf of cucumber under chilling-light conditions

Previous studies have demonstrated that the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase (Rubisco) is site-specifically cleaved by a hydroxyl radical (*OH) generated in the illuminated chloroplast lysates or by an artificial *OH-generating system. However, it is not known whether such cleavage of the LSU by reactive oxygen species (ROS) actually occurs in an intact leaf. When leaf discs of chilling-sensitive cucumber (Cucumis sativus L.) were illuminated at 4 degrees C, five major fragments of the LSU were observed. This fragmentation was completely inhibited by ROS scavengers, such as n-propyl gallate (for *OH) and 1,2-dihydroxybenzene-3,5-disulfonic acid (Tiron) (for superoxide). FeSO4 stimulated this fragmentation, whereas an iron-specific chelator, deferoxamine, suppressed it. Furthermore, such fragments were identical to those generated from the purified Rubisco by an *OH-generating system in vitro on two-dimensional PAGE. These results indicate that the direct fragmentation of the LSU by reactive oxygen species also occurs in an intact leaf.

Senescence-dependent degradation of Lhcb3 is mediated by a thylakoid membrane-bound protease

A novel proteolytic activity integrally associated with barley thylakoid membranes has been discovered and characterized. This enzymatic activity mediates senescence-dependent degradation of Lhcb3, one of the apoproteins of the major light-harvesting chlorophyll a/b protein complex of photosystem II. Once senescence of barley leaves is initiated by detachment and dark incubation, the degradation of Lhcb3 can proceed and be followed in vitro in an experimental system composed of thylakoids isolated from senescing leaves incubated in darkness in suitable medium at 25 degrees C. The protease involved is present in its active form and Lhcb3 is susceptible for proteolytic attack already in fresh leaves, although Lhcb3 degradation does not take place unless undefined extrinsic membrane proteins protecting Lhcb3 are removed in a senescence-dependent manner. It is thus concluded that senescence-dependent Lhcb3 degradation is regulated at the substrate availability level. The protease involved is ATP stimulated, has an optimum activity at pH 7.8, and requires 3 mM added Mg2+ (replicable by micromolar doses of Zn2+) for its proper activity. Studies using typical inhibitors of various classes of proteases indicate that the enzyme is a metalloprotease with disulfide linkage essential for its activity. Micromolar doses of Zn2+ were demonstrated to restore the activity of Lhcb3-degrading enzymes abolished by an ethylenediaminetetraacetic acid pretreatment of the thylakoids and it is inferred that the protease involved is a zinc-binding metalloprotease. Mg2+ was shown to be able to partially replace zinc as the bound ion.

The life of ribulose 1,5-bisphosphate carboxylase/oxygenase--posttranslational facts and mysteries

The life of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), from gene to protein to irreplaceable component of photosynthetic CO2 assimilation, has successfully served as a model for a number of essential cellular processes centered on protein chemistry and amino acid modifications. Once translated, the two subunits of Rubisco undergo a myriad of co- and posttranslational modifications accompanied by constant interactions with structurally modifying enzymes. Even after final assembly, the essential role played by Rubisco in photosynthetic CO2 assimilation is dependent on continuous conformation modifications by Rubisco activase. Rubisco is also continuously assaulted by various environmental factors, resulting in its turnover and degradation by processes that appear to be enhanced during plant senescence.

A serine endopeptidase from cucumber leaves is inhibited by L-arginine, guanidino compounds and divalent cations

An endopeptidase was purified and characterized from green leaves of cucumber (Cucumis sativus L. suyo). The purified enzyme, a basic amino acid-specific endopeptidase with a pI of 5.0, was a monomeric protein of 80 kDa whose pH optimum was 9.5. Inhibitor analysis suggested that it was a serine endopeptidase and contained sulfhydryl groups essential for catalytic activity. Analysis of internal amino acid sequences of the endopeptidase showed no significant similarity to other proteins. Its activity was inhibited by L-Arg and guanidino compounds having high hydrophobicity, as well as divalent cations such as Mg2+ and Ca2+. The K(i) values of L-Arg and Mg2+, which are also likely in vivo inhibitors, were 3.5 and 10 mM, respectively. Inhibition by L-Arg and Mg2+ was additive, and more than 70% of the activity was reversibly inhibited under their physiologically significant concentrations. These results suggest that the enzyme is possibly regulated by L-Arg and/or guanidino compounds, and by divalent cations in vivo.

Purification and characterization of an aminopeptidase from the chloroplast stroma of barley leaves by chromatographic and electrophoretic methods

Aminopeptidases catalyze the cleavage of amino acids from the amino terminus of protein or peptide substrates. Although some aminopeptidase activities have been found in plant chloroplasts, the identity of these proteins remains unclear. In this work, we report the purification to apparent homogeneity of a soluble aminopeptidase from isolated barley chloroplasts which preferentially degraded alanyl-p-nitroanilide (Ala-pNA). After organelle isolation in a density gradient and precipitation of soluble proteins with ammonium sulfate, the proteins were purified in three consecutive steps including hydrophobic interaction, gel permeation and ion-exchange chromatographies. The purified enzyme appeared as a single band with a Mr of approximately 84000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. The Mr of the native enzyme was estimated to be approximately 93000 by gel permeation chromatography, suggesting that the protein is a monomer. Mass spectrometry analysis of tryptic digests indicates that the primary structure of the protein has not been reported previously. The enzyme was characterized as a metalloprotease as it could be totally inhibited by 1,10-phenanthroline. Strong inhibition could also be observed using the specific aminopeptidase inhibitors amastatin and bestatin. Besides Ala-pNA, the purified protein could also cleave with decreasing activity glycyl-pNA, leucyl-pNA, lysyl-pNA, methionyl-pNA and arginyl-pNA. The possible physiological role of this enzyme in the chloroplast stroma is discussed.

Rubisco small and large subunit N-methyltransferases. Bi- and mono-functional methyltransferases that methylate the small and large subunits of Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)is methylated at the alpha-amino group of the N-terminal methionine of the processed form of the small subunit (SS), and at the epsilon-amino group of lysine-14 of the large subunit (LS) in some species. The Rubisco LS methyltransferase (LSMT) gene has been cloned and expressed from pea and specifically methylates lysine-14 of the LS of Rubisco. We determine here that both pea and tobacco Rubisco LSMT also exhibit (alpha)N-methyltransferase activity toward the SS of Rubisco, suggesting that a single gene product can produce a bifunctional protein methyltransferase capable of catalyzing both (alpha)N-methylation of the SS and (epsilon)N-methylation of the LS. A homologue of the Rubisco LSMT gene (rbcMT-S) has also been identified in spinach that is closely related to Rubisco LSMT sequences from pea and tobacco. Two mRNAs are produced from rbcMT-S, and both long and short forms of the spinach cDNAs were expressed in Escherichia coli cells and shown to catalyze methylation of the alpha-amino group of the N-terminal methionine of the SS of Rubisco. Thus, the absence of lysine-14 methylation in species like spinach is apparently a consequence of a monofunctional protein methyltransferase incapable of methylating Lys-14, with activity limited to methylation of the SS.

Purification and properties of a novel chloroplast stromal peptidase. Processing of polyphenol oxidase and other imported precursors

Polyphenol oxidases (PPOs) are nuclear-encoded chloroplast proteins that are targeted to the thylakoid lumen by a bipartite presequence. The N-terminal part of this sequence is removed by a stromal processing peptidase (SPP), and the resulting intermediate is translocated across the thylakoid and processed to the mature protein. A 4800-fold-purified SPP processed a PPO precursor (pPPO) at a site identical to that occurring in organelle. The in vitro product of SPP action on pPPO was further processed and translocated by thylakoids. This SPP processed other precursors but was inactive toward those of light-harvesting chlorophyll binding proteins. The enzyme appeared to be a metalloendopeptidase, like previously reported SPPs. However, it differed in substrate specificity, apparent size, and, most significantly, cleavage site of pPPO. Whereas the processing sites of lumen proteins determined so far were relatively distant from the hydrophobic core of the thylakoid targeting domain, pPPO was cleaved immediately before this domain. Cleavage removed the twin arginine motif characteristic of thylakoid targeting domains of lumen proteins, which are translocated by the DeltapH-dependent pathway. The possible significance of these observations to PPO translocation mechanism is discussed. It is suggested that several SPPs may exist in chloroplasts with preferences for different subsets of precursors.

A Plant Chloroplast Glutamyl Proteinase

A glutamyl proteinase was partially purified from Percoll gradient-purified spinach (Spinacia oleracea) chloroplast preparations and appeared to be predominantly localized in the chloroplast stroma. The enzyme degraded casein, but of the 11 synthetic endopeptidase substrates tested, only benzyloxycarbonyl-leucine-leucine-glutamic acid-[beta]-napthylamide was hydrolyzed at measurable rates. In addition, the enzyme cleaved the oxidized [beta]-chain of insulin after a glutamic acid residue. There was no evidence that native ribulose-1,5-bisphosphate carboxylase/oxygenase was cleaved by this proteinase. The apparent Km for benzyloxycarbonyl-leucine-leucine-glutamic acid-[beta]NA at the pH optimum of 8.0 was about 1 mM. Cl-ions were required for both activity and stability. Of the proteinase inhibitors covering all four classes of the endopeptidases, only 4-(2-aminoethyl)-benzenesulfonyl-fluoride HCl and L-1-chloro-3-[4-tosylamido]-4-phenyl-2-butanone significantly inhibited the proteinase. The partially purified enzyme had a molecular weight of about 350,000 to 380,000, based on size-exclusion chromatography. The enzyme has both similar and distinctive properties to those of the bacterial glutamyl proteinases. To our knowledge, this is the first description of a plant glutamyl proteinase found predominantly or exclusively in the chloroplast.

Proteolysis in plants: mechanisms and functions

Proteolysis is essential for many aspects of plant physiology and development. It is responsible for cellular housekeeping and the stress response by removing abnormal/misfolded proteins, for supplying amino acids needed to make new proteins, for assisting in the maturation of zymogens and peptide hormones by limited cleavages, for controlling metabolism, homeosis, and development by reducing the abundance of key enzymes and regulatory proteins, and for the programmed cell death of specific plant organs or cells. It also has potential biotechnological ramifications in attempts to improve crop plants by modifying protein levels. Accumulating evidence indicates that protein degradation in plants is a complex process involving a multitude of proteolytic pathways with each cellular compartment likely to have one or more. Many of these have homologous pathways in bacteria and animals. Examples include the chloroplast ClpAP protease, vacuolar cathepsins, the KEX2-like proteases of the secretory system, and the ubiquitin/26S proteasome system in the nucleus and cytoplasm. The ubiquitin-dependent pathway requires that proteins targeted for degradation become conjugated with chains of multiple ubiquitins; these chains then serve as recognition signals for selective degradation by the 26S proteasome, a 1.5 MDa multisubunit protease complex. The ubiquitin pathway is particularly important for developmental regulation by selectively removing various cell-cycle effectors, transcription factors, and cell receptors such as phytochrome A. From insights into this and other proteolytic pathways, the use of phosphorylation/dephosphorylation and/or the addition of amino acid tags to selectively mark proteins for degradation have become recurring themes.

Oxidative Stress Induces Partial Degradation of the Large Subunit of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase in Isolated Chloroplasts of Barley

The effects of oxidative stress on the degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were studied in isolated chloroplasts from barley (Hordeum vulgare L. cv Angora). Active oxygen (AO) was generated by varying the light intensity, the oxygen concentration, or the addition of herbicides or ADP-FeCl3-ascorbate to the medium. Oxidative treatments stimulated association of Rubisco with the insoluble fraction of chloroplasts and partial proteolysis of the large subunit (LSU). The most prominent degradation product of the LSU of Rubisco showed an apparent molecular mass of 36 kD. The data suggest that an increase in the amount of AO photogenerated by O2 reduction at photosystem I triggers Rubisco degradation. A possible relationship between AO-mediated denaturation of Rubisco and proteolysis of the LSU is discussed.

A Complex Array of Proteins Related to the Multimeric Leucine Aminopeptidase of Tomato

Leucine aminopeptidase (LAP) mRNAs are induced in response to mechanical wounding, pathogen infection, and insect infestation (V. Pautot, F.M. Holzer, B. Reisch, L.L. Walling [1993] Proc Natl Acad Sci USA 90: 9906-9910). Polyclonal antibodies to a glutathione S-transferase-LAP fusion protein and affinity-purified antibodies recognizing LAP antigenic determinants detected four classes of polypeptides in tomato (Lycopersicon esculentum) leaves. All four classes had multiple polypeptides in two-dimensional polyacrylamide gel electrophoresis immunoblots. Although antigenically related to the wound-induced tomato LAP proteins, the 77- and 66-kD LAP-like proteins accumulated in both healthy and wounded leaves. Two classes of 55-kD polypeptides with distinctive isoelectric points were designated as plant LAPs; only the acidic LAP proteins accumulated to high levels after mechanical wounding or Pseudomonas syringae pv tomato infection of tomato leaves. The temporal accumulation of LAP mRNAs was correlated with the increase in acidic LAP protein subunits. A slow-migrating LAP activity was detected using a native gel assay after wounding. The molecular mass of the native wound-induced LAP enzyme was 353 kD. The 55-kD acidic LAP proteins were associated with induced LAP activity, whereas the neutral LAPs and the LAP-like proteins were not associated with this exopeptidase. A second, fast-migrating aminopeptidase was detected in both healthy and wounded tomato leaves. Cell fractionation experiments revealed that wound-induced LAP is a soluble enzyme.

Immunological detection of proteins similar to bacterial proteases in higher plant chloroplasts

Despite numerous demonstrations of protein degradation in chloroplasts of higher plants, little is known about the identity of the proteases involved in these reactions. To identify chloroplast proteases by immunological means, we investigated two proteins: ClpP, a protein similar to the proteolytic subunit of the bacterial ATP-dependent Clp protease, for which a gene is found in the chloroplast genome [Maurizi, M.R., Clark, W.P., Kim, S. H. & Gottesman, S. (1990) J. Biol. Chem. 265, 12546-12552] and PrcA, a cyanobacterial Ca2+-stimulated protease [Maldener, I., Lockau, W., Cai, Y. & Wolk, P. (1991) Mol. & Gen. Genet. 225, 113-120]. We expressed the clpP gene from rice in Escherichia coli, purified its product, and generated antibodies against the product. Western blot analysis revealed the ClpP protein in different leaf extracts. Analysis of fractionated barley chloroplasts revealed that the protein was associated with the stromal fraction. The expression of ClpP is light independent and tissue specific, as it was found in green and etiolated barley leaves, but not in roots. A second protein, similar to the cyanobacterial protease PrcA, was also detected in chloroplasts. Antibody against this protease recognized proteins in various leaf extracts. When pea chloroplasts were fractionated, the antibody only recognized a stromal protein. The expression of this protein is regulated by light, as it was found in green leaves, but not in etiolated leaves. The tissue specificity of PrcA was similar to that of ClpP in that it could not be detected in root extracts.

Spinach chloroplast ATP-dependent endopeptidase: Ti-like protease

The soluble fraction of spinach chloroplast was used for purification and characterization of an ATP-dependent protease. Purification included Q Sepharose Fast Flow, hydroxylapatite and FPLC Superose 6 column chromatography. The isolated enzyme requires ATP and Mg2+ for stimulation and represents a ubiquitin independent serine protease, containing essential sulphydryl group(s). By using fluorogenic peptides a similarity of chloroplast protease to Escherichia coli Ti protease was observed. The chloroplast protease is immunochemically cross-reactive with the bacterial protease Ti.

Degradation of plastocyanin in copper-deficient Chlamydomonas reinhardtii. Evidence for a protease-susceptible conformation of the apoprotein and regulated proteolysis

In the green alga Chlamydomonas reinhardtii, the copper-dependent accumulation of plastocyanin is effected via the altered stability of the protein in copper-deficient versus copper-sufficient medium (t1/2) < 20 min versus several hours). To understand the mechanism of plastocyanin degradation in vivo, the purified apoprotein was characterized relative to the holoprotein with respect to conformation and protease susceptibility. Circular dichroism spectroscopy revealed that the apoprotein in solution did not display the characteristic secondary structure displayed by the native or reconstituted holoprotein. The apoprotein was also susceptible to digestion in vitro by chymotrypsin whereas the holoprotein was resistant. High ionic conditions, which stabilize the folded structure of apoplastocyanin, also inhibit its degradation by chymotrypsin. These results suggest that one explanation for plastocyanin degradation in copper-deficient cells in vivo might be the increased susceptibility of the apo form to a lumenal protease. Since apoplastocyanin is a normal biosynthetic intermediate for the formation of holoplastocyanin, the increased susceptibility of apoplastocyanin to proteolysis implies that degradative and biosynthetic activities would compete for the same substrate. However, characterization of an apoplastocyanin-accumulating mutant suggests that a plastocyanin-degrading protease is active only in copper-deficient cells. Thus, apoplastocyanin is rapidly degraded in copper-deficient cells, whereas its major fate in copper-supplemented cells is holoplastocyanin formation.

Purification and Characterization of a Novel Aminopeptidase, Plastidial Alanine-Aminopeptidase, from the Cotyledons of Etiolated Sugar Beet Seedlings

During prolonged dark growth of sugar beet (Beta vulgaris L.) seedlings, etioplasts, rapidly after the proplastid-etioplast transition, undergo a degenerative process characterized by ultrastructural modifications, protein loss, and the decrease of carotenoid and chlorophyll accumulation upon illumination. Two plastidial aminopeptidase activities were identified as early markers of this degenerative process (A. El Amrani, I. Couee, J.-P. Carde, J.-P. Gaudillere, P. Raymond [1994] Plant Physiology 106: 1555-1565). The present study focuses on one of these markers and describes the purification to homogeneity and characterization of plastidial alanine-aminopeptidase. This novel aminopeptidase was found to be a metallo-type naphthylamidase particularly active with alanyl, arginyl, and leucyl substrates. Its plastidial location was confirmed by immunofluorescence with polyclonal antibodies against the purified enzyme. Its physico-chemical and enzymic properties are discussed with respect to other higher plant aminopeptidases and to its potential functions during prolonged dark growth.

Purification and Developmental Analysis of an Extracellular Proteinase from Young Leaves of Soybean

A proteinase present in intercellular wash fluids from leaves of Glycine max has been purified 600-fold to electrophoretic homogeneity. The native protein is monomeric with a molecular mass of 60 kD, as estimated by denaturing gel electrophoresis, and has an isoelectric point of 7.7. The enzyme has a pH optimum of 9.5 when assayed with Azocoll as a substrate. The proteolytic activity is inhibited by p-chloromercuribenzoic acid and mercuric chloride and requires the presence of reducing agents. The enzyme activity is refractory to other classical sulfhydryl proteinases. The soybean leaf endoproteinase is present within the extracellular space of young leaves, and a portion is bound to the cell wall. Western blot analysis and activity measurements show that the enzyme is present only during the first 15 d postemergence of the leaf and is therefore under strict developmental control. We suggest that the enzyme may play a critical role in the extracellular milieu during rapid cell growth and leaf expansion.

Modifications of Etioplasts in Cotyledons during Prolonged Dark Growth of Sugar Beet Seedlings (Identification of Etiolation-Related Plastidial Aminopeptidase Activities)

We studied the effects of prolonged dark growth on proplastids and etioplasts in cotyledons of sugar beet (Beta vulgaris L.) seedlings. Differentiation of proplastids into etioplasts occurred between d 4 and d 6 after imbibition, with the typical characteristics of increased synthesis of plastidial proteins, protein and carotenoid accumulation, size increase, development of plastid membranes and of the prolamellar body, and increase of the greening capacity. However, this situation of efficient greening capacity was short-lived. The greening capacity started to decline from d 6 after imbibition. This decline was due in part to reserve depletion and glucose limitation and also to irreversible damage to plastids. Indeed, electron microscopy observations in situ showed some signs of plastidial damage, such as accumulation of plastoglobuli and membrane alterations. The biochemical characterization of purified plastids also showed a decrease of proteins per plastid. Aminopeptidase activities, and to a lesser extent, neutral endopeptidase activities, were found to increase in plastids during this degenerative process. We identified two plastidial aminopeptidases showing a sharp increase of activity at the onset of the degenerative process. One of them, an alanyl aminopeptidase, was shown to be inactivated by exposure to light or addition of exogenous glucose, thus confirming the relationship with prolonged dark growth and indicating a relationship with glucose limitation.

Gene expression during leaf senescence

Leaf senescence is a hiphly-controlled sequence of events comprising the final stage of development. Cells remain viable during the process and new gene expression is required. There is some similarity between senescence in plants and programmed cell death in animals. In this review, different classes of senescence-related genes are defined and progress towards isolating such genes is reported. A range of internal and external factors which appear to cause leaf senescence is considered and various models for the mechanism of senescence- initiation are described. The current understanding of senescence at the wrganelle and molecular levels is presented. Finally, same ideas are mooted as to why senescence occurs and why it should be studied further. Contents Summary 419 I. Introduction 420 II. Internal factors that cause senescence 423 III. External factors that cause senescence 427 IV. What is the mechanism of senescence initiation? 428 V. Progress in the understanding of organelle senescence 431 VI. Progress in the understanding of senescence at the molecular level 434 VII. The control of senescence in animals and plants 440 VIII. Why is senescence necessary? 441 IX. Why study senescence? 441 References 442.

A Purified Zinc Protease of Pea Chloroplasts, EP1, Degrades the Large Subunit of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase

A previously reported endopeptidase (EP1) from pea chloroplasts was purified over 11,000-fold using a four-step protocol involving ultrafiltration, sucrose gradient centrifugation, isoelectric focusing, and high performance liquid chromatography gel filtration. The enzyme was determined to be a metalloprotease requiring bound Zn2+ and added Mg2+ or Ca2+ for proper activity. Its localization in the stroma of pea chloroplasts was confirmed by demonstrating its insensitivity to thermolysin when the envelope was intact. A contaminating serine protease that attacks EP1 was found. The contaminating protease was inhibited by 4-(2-aminoethyl)-benzenesulfonyl fluoride, but not by o-phenanthroline, whereas EP1 sensitivities were the reverse. EP1 is able to hydrolyze the large subunit of native ribulose-1,5-bisphosphate carboxylase/oxygenase under physiological conditions.

Effects of light and external solutes on the catabolism of nuclear-encoded stromal proteins in intact chloroplasts isolated from pea leaves

The catabolism of nuclear-encoded stromal proteins was investigated in intact chloroplasts isolated mechanically from pea (Pisum sativum) leaves. Glutamine synthetase, phosphoribulokinase, and nitrite reductase (quantified by immunoblotting) were more rapidly degraded in the light than in the dark. Furthermore, the degradation rates depended on exogenously supplied metabolites. For example, 2-oxoglutarate accelerated the catabolism of all three enzymes in chloroplasts incubated in the light, whereas oxaloacetate stabilized glutamine synthetase and at the same time destabilized the other two enzymes.

Purification and Characterization of a Membrane-Bound Protease from Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii y-1, newly synthesized chlorophyll a/b-binding apoproteins are degraded when chlorophylls are not present for assembly of stable light-harvesting complexes. A protease was purified from the membrane fraction of degreened y-1 cells, which digested chlorophyll a/b-binding proteins in membranes from C. reinhardtii pg-113, a protease-deficient strain. This protease was active with p-nitroanilides of nonpolar amino acids (Leu and Phe), but not of basic amino acids (Lys and Arg). The apparent molecular weight of the enzyme is 38,000 +/- 2,000 as determined by electrophoresis in the presence of sodium dodecyl sulfate. Typical inhibitors of the major classes of proteases were ineffective with this enzyme. Protease activity was constant from pH 7.5 to 9; a plot of log V versus pH suggested that deprotonation of an ionizable group with a pK value of 6.0 to 6.5 is required for activity. The protease was inactivated by diethylpyrocarbonate and by photooxidation sensitized by rose bengal. These results suggested that a histidyl residue is required for catalysis. Although very sensitive to photodynamic conditions in vitro, the enzyme was not inactivated in vivo when cells were exposed to light.

Characterization of a prolyl endopeptidase from spinach thylakoids

A prolyl endopeptidase (PEPase, EC 3.4.21.26) that specifically cleaves the 18-kDa protein of photosystem II was extracted from photosystem II membranes with 1 M NaCl. Proteolytic activity measured with artificial substrates was less than a quarter of that with the protein. Studies on inhibition of the proteolysis by an artificial substrate suggested that the protease recognizes the scissile prolyl bond. The protease was inhibited by CuCl2, but not by diisopropyl fluorophosphate or p-chloromercuriphenylsulfonic acid. These findings suggest that the protease represents a new class of PEPase. The specificity of the enzyme is discussed in relation to the structure of the 18-kDa protein.

Posttranslational modifications in the amino- terminal region of the large subunit of ribulose- 1,5-bisphosphate carboxylase/oxygenase from several plant species

A combination of limited tryptic proteolysis, reverse phasehigh performance liquid chromatography, Edman degradative sequencing, amino acid analysis, and fast-atom bombardment mass-spectrometry was used to remove and identify the first 14 to 18 N-terminal amino acid residues of the large subunit of higher plant-type ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from Chlamydomonas reinhardtii, Marchantia polymorpha, pea (Pisum sativum), tomato (Lycopersicon esculentum), potato (Solanum tuberosum), pepper (Capsicum annuum), soybean (Glycine max), petunia (Petunia x hybrida), cowpea (Vigna sinensis), and cucumber (Cucumis sativus) plants. The N-terminal tryptic peptide from acetylated Pro-3 to Lys-8 of the large subunit of Rubisco was identical in all species, but the amino acid sequence of the penultimate N-terminal tryptic peptide varied. Eight of the 10 species examined contained a trimethyllysyl residue at position 14 in the large subunit of Rubisco, whereas Chlamydomonas and Marchantia contained an unmodified lysyl residue at this position.

Accumulation of Chlorophyll a/b-Binding Polypeptides in Chlamydomonas reinhardtii y-1 in the Light or Dark at 38 degrees C : Evidence for Proteolytic Control

The kinetics of accumulation of light harvesting chlorophyll (Chl) a/b-binding polypeptides (LHCPs) in thylakoid membranes were analyzed during greening of Chlamydomonas reinhardtii y-1 at 38 degrees C. Initial accumulation of LHCPs in thylakoid membranes was linear; LHCP precursors or polypeptides in transit within the chloroplast stroma were not detected. The rate of accumulation in the light was at least five-fold greater than that in the dark. The relatively small amount of LHCPs that accumulated in the dark was integrated properly in the membrane, as judged by the pattern of cleavage in vitro by exogenous proteases, and did not turn over at a significant rate in vivo. The kinetic data suggested that in y-1 cells either translation of LHCP mRNA was inhibited in the dark or newly synthesized polypeptides were degraded concurrently with transport into the chloroplast unless rescued by Chl. LHCPs accumulated in cells of the Chl b-deficient strain pg-113 at the same rate in the dark or the light at 38 degrees C, an indication that light did not affect translation of LHCP mRNA. Membrane-associated LHCPs in pg-113 cells were completely degraded, in contrast to those in y-1 cells, by exogenous proteases, which suggested that pg-113 cells are deficient in a proteolytic activity. A peptidase was recovered from y-1 cells in a membrane fraction with a buoyant density slightly less than that of thylakoid membranes. Although a role for this activity in degradation of LHCPs has not been established, the specific activity of this peptidase in pg-113 cells was only 10 to 15% of the level in y-1 cells.

Proteolytic activity during senescence of plants

Although information has rapidly developed concerning the intracellular localization of plant proteins, relatively few reports concern the intracellular location of endo- and exo-proteolytic activities. Relatively few proteases have been purified, characterized, and associated with a specific cellular location. With the exception of the processing proteases involved in transport of proteins across membranes, little progress has yet been made concerning determination of in vivo products of specific proteases. Information on the turnover of individual proteins and the assessment of rate-limiting steps in pathways as proteins are turned over is steadily appearing. Since chloroplasts are the major site of both protein synthesis and, during senescence, degradation, it was important to show unambiguously that chloroplasts can degrade their own constituents. Another important contribution was to obtain evidence that the chloroplasts contain proteases capable of degrading their constituents. This work has been more tenuous because of the low activities found and the possibility of contamination by vacuolar enzymes during the isolation of organelles. The possible targeting of cytoplasmic proteins for degradation by facilitating their transport into vacuoles is a field which hopefully will develop more rapidly in the future. Information on targeting of proteins for degradation via the ubiquitin (Ub) degradation pathway is developing rapidly. Future research must determine how much unity exists across the different eukaryotic systems. At present, it has important implications for protein turnover in plants, since apparently Ub is involved in the degradation of phytochrome. Little information has been developed regarding what triggers increased proteolysis with the onset of senescence, although it appears to involve protein synthesis. Thus far, the evidence indicates that the complement of proteases prior to senescence is sufficient to carry out the observed protein degradation. This field of study has great practical implications, e.g. maintaining photosynthesis during seed-fill in order to obtain greater crop yields. The current use of stay green' variants in the populations of several crop plants to produce increased yields shows the potential for future development. The near future should see exciting discoveries in these areas of research that will have far reaching effects on the construction of transgenic plants for future research accomplishments and agricultural use.

Proteolytic Activity at Alkaline pH in Oat Leaves, Isolation of an Aminopeptidase

Proteolytic activity in oat leaf extracts was measured with both azocasein and ribulose bisphosphate carboxylase (Rubisco) as substrates over a wide range of pH (3.0-9.2). With either azocasein or Rubisco activity peaks appeared at pH 4.8, 6.6, and 8.4. An aminopeptidase (AP) which hydrolyzes leucine-nitroanilide was partially purified. Purification consisted of a series of six steps which included ammonium sulfate precipitation, gel filtration, and two ionic exchange chromatographies. The enzyme was purified more than 100-fold. The apparent K(m) for leucine-nitroanilide is 0.08 millimolar at its pH optimum of 8.4. AP may be a cystein protease since it is inhibited by heavy metals and activated by 2-mercaptoethanol. Isolated chloroplasts were also able to hydrolyze leucine-nitroanilide at a pH optimum of 8.4, indicating that AP could be localized inside the photosynthetic organelles.

Purification and some properties of an aminopeptidase from endosperms of ×Haynaldoticum sardoum Meletti et Onnis

Aminopeptidase activity was detected in dry and germinating seeds of x Haynaldoticum sardoum. The enzyme was partly purified from germinating seeds and preliminary characterization was carried out. The aminopeptidase showed a pH optimum of 8.0, a molecular weight estimated at 89 000 by Sephadex G-100 gel filtration and an isoelectric point of 4.5. The enzyme hydrolysed p-nitroanilides, but was unable to break down exogenous native proteins. Activity was severely inhibited by p-chloromercuribenzoate, p-hydroxy-mercuribenzoate and bestatin, slightly inhibited by iodoacetamide and ethylenediaminetetraacetic acid and not influenced by 2-mercaptoethanol, phenylmethanesulphonyl fluoride and polyamines. A possible physiological role for the enzyme in the final stages of storage protein degradation is suggested.

Endopeptidases in the stroma and thylakoids of pea chloroplasts

Three endopeptidases (EP2, EP3, and EP4) were identified after fractionation of pea (Pisum sativum, var Feltham First) chloroplast stromal extracts. All three were identified by their ability to cleave in vitro-synthesized preplastocyanin to lower molecular weight forms. EP2 is inhibited by phenylmethylsulfonylfluoride, and both EP2 and EP3 are inhibited by the heavy metal chelators 1,10-phenanthroline and EDTA. A further endopeptidase, EP5, was identified in Triton X-100 extracts of thylakoid membranes. Experiments involving contrifugation through a sucrose pad indicate that EP5 either has a high molecular weight or is associated with a thylakoid protein complex. EP5 is effectively inhibited by phenylmethylsulfonylfluoride, iodoacetate, and 1,10-phenanthroline, but not by EGTA. The implications of these results for the analysis of chloroplast protein maturation are discussed, and an improved protocol for the purification of the stromal processing peptidase is described which ensures the removal of EP2, the most active of the stromal peptidases analysed in this study.

Senescence-induced, thylakoid-bound diisopropylfluorophosphate-binding protein in spinach : induction pattern, localization, and some properties

Changes in diisopropylfluorophosphate (DFP)-binding proteins during development and senescence of spinach (Spinacia oleracea) leaves were followed using [(3)H]DFP and sodium dodecylsulfate-polyacrylamide gel electrophoresis-fluorography. Experiments using a series of aging stages of leaves attached to plants and ones with detached leaves stored in the dark both showed that a protein of 38 kilodaltons was the only major DFP-binding protein in the membrane fraction and that its DFP-binding increased markedly as senescence proceeded, corresponding with the degradation of leaf protein. DFP binding to the 38-kilodalton protein was not affected by membrane solubilization with Triton X-100, and gradually decreased upon preservation of the membranes. The DFP binding was inhibited completely by phenylmethane-sulfonyl fluoride and slightly by p-chloromercuribenzoic acid, suggesting a serine protease-like character of the protein and a possible contribution of SH residues to the binding. Both differential and Percoll-gradient centrifugation indicated that the 38-kilodalton protein was localized in thylakoid membranes. The sedimentation behavior of the detergent-solubilized protein indicated that it belongs to a complex different from photosystem I, photosystem II, or coupling factor 1 of the ATP-synthesizing complex.

Light-Harvesting Chlorophyll a/b Protein : Membrane Insertion, Proteolytic Processing, Assembly into LHC II, and Localization to Appressed Membranes Occurs in Chloroplast Lysates

The apoprotein of the light-harvesting chlorophyll a/b protein (LHCP) is a major integral thylakoid membrane protein that is normally complexed with chlorophyll and xanthophylls and serves as the antenna complex of photosystem II. LHCP is encoded in the nucleus and synthesized in the cytosol as a higher molecular weight precursor that is subsequently imported into chloroplasts and assembled into thylakoids. In a previous study it was established that the LHCP precursor can integrate into isolated thylakoid membranes. The present study demonstrates that under conditions designed to preserve thylakoid structure, the inserted LHCP precursor is processed to mature size, assembled into the LHC II chlorophyll-protein complex, and localized to the appressed thylakoid membranes. Under these conditions, light can partially replace exogenous ATP in the membrane integration process.

Characterization and subcellular localization of aminopeptidases in senescing barley leaves

Four aminopeptidases (APs) were separated using native polyacrylamide gel electrophoresis of cell-free extracts and the stromal fractions of isolated chloroplasts prepared from primary barley (Hordeum vulgare L., var Numar) leaves. Activities were identified using a series of aminoacyl-beta-naphthylamide derivatives as substrates. AP1, 2, and 3 were found in the stromal fraction of isolated chloroplasts with respective molecular masses of 66.7, 56.5, and 54.6 kilodaltons. AP4 was found only in the cytoplasmic fraction. No AP activity was found in vacuoles of these leaves. It was found that 50% of the L-Leu-beta-naphthylamide and 25% of the L-Arg-beta-naphthylamide activities were localized in the chloroplasts. Several AP activities were associated with the membranes of the thylakoid fraction of isolated chloroplasts. AP1, 2, and 4 reacted against a broad range of substrates, whereas AP3 hydrolyzed only L-Arg-beta-naphthylamide. Only AP2 hydrolyzed L-Val-beta-naphthylamide. Since AP2 and AP3 were the only ones reacting against Val-beta-naphthylamide and Arg-beta-naphthylamide, respectively, several protease inhibitors were tested against these substrates using a stromal fraction from isolated chloroplasts as the source of the two APs. Both APs were sensitive to both metallo and sulfhydryl type inhibitors. Although AP activity decreased as leaves senesced, no new APs appeared on gels during senescence and none disappeared.