Mechanism of polyamine inhibition of a leaf protease

Polyamines: Α bioenergetic smart switch for plant protection and development

The present review highlights the bioenergetic role of polyamines in plant protection and development and proposes a universal model for describing polyamine-mediated stress responses. Any stress condition induces an excitation pressure on photosystem II by reforming the photosynthetic apparatus. To control this phenomenon, polyamines act directly on the molecular structure and function of the photosynthetic apparatus as well as on the components of the chemiosmotic proton-motive force (ΔpH/Δψ), thus regulating photochemical (qP) and non-photochemical quenching (NPQ) of energy. The review presents the mechanistic characteristics that underline the key role of polyamines in the structure, function, and bioenergetics of the photosynthetic apparatus upon light adaptation and/or under stress conditions. By following this mechanism, it is feasible to make stress-sensitive plants to be tolerant by simply altering their polyamine composition (especially the ratio of putrescine to spermine), either chemically or by light regulation.

Coxsackievirus B3 Responds to Polyamine Depletion via Enhancement of 2A and 3C Protease Activity

Polyamines are small positively-charged molecules abundant in eukaryotic cells that are crucial to RNA virus replication. In eukaryotic cells, polyamines facilitate processes such as transcription, translation, and DNA replication, and viruses similarly rely on polyamines to facilitate transcription and translation. Whether polyamines function at additional stages in viral replication remains poorly understood. Picornaviruses, including Coxsackievirus B3 (CVB3), are sensitive to polyamine depletion both in vitro and in vivo; however, precisely how polyamine function in picornavirus infection has not been described. Here, we describe CVB3 mutants that arise with passage in polyamine-depleted conditions. We observe mutations in the 2A and 3C proteases, and we find that these mutant proteases confer resistance to polyamine depletion. Using a split luciferase reporter system to measure protease activity, we determined that polyamines facilitate viral protease activity. We further observe that the 2A and 3C protease mutations enhance reporter protease activity in polyamine-depleted conditions. Finally, we find that these mutations promote cleavage of cellular eIF4G during infection of polyamine-depleted cells. In sum, our results suggest that polyamines are crucial to protease function during picornavirus infection. Further, these data highlight viral proteases as potential antiviral targets and highlight how CVB3 may overcome polyamine-depleting antiviral therapies.

Polyamines interaction with thylakoid proteins during stress

The involvement of polyamines in plant responses to abiotic stresses is well investigated, while there has been few reports on the specific mode of action of polyamines on the photosynthetic apparatus. The objective of this review is thus to examine the mode of interaction of polyamines with proteins of photosystem II core and LHCII, including methylamine (monoamine) as a simplified model to better understand the mode of action of polyamines. Spectroscopic methods used to determine the binding mode of amines with PSII proteins showed that amines such as spermine, putrescine and methylamine interact with protein (H-bonding) through polypeptide C=O, C-N and N-H groups with major perturbations of protein secondary structure as the concentration of amines was raised. High concentration of amines added to PSII-enriched submembrane fractions causes a significant loss of PSII activity. However, at lower concentration, polyamines, especially spermine, improve the photosynthetic functions under stress. We concluded from this review that besides the conjugation of polyamines with LHC polypeptides, polyamines are likely to interact with extrinsic proteins and the hydrophilic part of intrinsic proteins of PSII by electrostatic interaction. This could stabilize the conformation of proteins under various stresses. However, at high concentration of polyamines a strong inhibition of PSII activity is observed.

A polyamine- and LHCII protease activity-based mechanism regulates the plasticity and adaptation status of the photosynthetic apparatus

In the present study we aim to dissect the basis of the polyamine mode of action in the structure and function of the photosynthetic apparatus. Although the modulating effects of polyamines in photosynthesis have been reported since long [K. Kotzabasis, A role for chloroplast-associated polyamines? Bot. Acta 109 (1996) 5-7], the underlying mechanisms remained until today largely unknown. The diamine putrescine was employed in this study, by being externally added to Scenedesmus obliquus cultures acclimated to either low or high light conditions. The results revealed the high efficiency by which putrescine can alter the levels of the major photosynthetic complexes in a concerted manner inducing an overall structure and function of the photosynthetic apparatus similar to that under higher light conditions. The revealed mechanism for this phenomenon involves alterations in the level of the polyamines putrescine and spermine which are bound to the photosynthetic complexes, mainly to the LHCII oligomeric and monomeric forms. In vitro studies point out to a direct impact of the polyamines on the autoproteolytic degradation of LHCII. Concomitantly to the reduction of the LHCII size, exogenously supplied putrescine, induces the reaction centers' density and thus the photosynthetic apparatus is adjusted as if it was adapted to higher light conditions. Thus polyamines, through LHCII, play a crucial role in the regulation of the photosynthetic apparatus' photoadaptation. The protective role of polyamines on the photosynthetic apparatus under various environmental stresses is also discussed in correlation to this phenomenon.

Effect of NaCl and mannitol iso-osmotic stresses on proline and free polyamine levels in embryogenic Fraxinus angustifolia callus

With the aim to differentiate the ionic and osmotic components of salt stress, short and long-term changes in free polyamines and proline induced by iso-osmotic concentrations of NaCl (0.1 mol/L and 0.2 mol/L) and mannitol (0.2 mol/L and 0.4 mol/L) were determined in Fraxinus angustifolia callus. The peculiarities of the short-term responses were: i) a very early (30 min) and temporary increase in Putrescine (Pu) and Spermine (Spm) as a consequence of salt treatment, and ii) a continuous accumulation of Spermidine (Spd) and Spm in response to mannitol. The changes of Proline (Pro) were quite limited both in the short and in the long term, and generally occurred later than Polyamine (PAs) changes took place, suggesting a regulatory mechanism of PAs metabolism on Pro biosynthesis. In the long-term, no drastic accumulations of Pro or PAs in response to NaCl and mannitol were observed, suggesting that their physiological role is unlikely to be that of osmo-compatible solutes in this plant system. The salt induced a higher callus growth inhibition effect than did mannitol and this inhibition was associated with the reduction of endogenous levels of PAs, especially Pu. However, while a diverging time course was observed under lethal salt concentration (0.2 mol/L NaCl), a high parallelism in the endogenous changes of Pro and Pu was observed under all non-lethal conditions (control--0.2 and 0.4 mol/L mannitol--0.1 mol/L NaCl). Therefore the synchronous changes of Pro and Pu can be considered as a physiological trait associated with cell survival. These results indicate a strong metabolic co-ordination between PAs and Pro pathways and suggest that the metabolic fluxes through these pathways start competing only when the stress level is high enough to be lethal for cells.

Spermidine and related-metabolic inhibitors modulate sugar and amino acid levels in Vitis vinifera L.: possible relationships with initial fruitlet abscission

The relationships between free polyamines (PAs), sugar and amino acid status were investigated in cuttings from two cultivars of Vitis vinifera L., Pinot noir (PN), a low abscising cultivar and Merlot (MRT), a high abscising one. In both cultivars free PAs decreased in inflorescences, but more drastically in MRT plants. Upon anthesis, this was associated with a decreased sugar content, especially sucrose, and an increase in total free amino acids. Thereafter, sucrose and amino acids showed opposite trends. In addition, darkening the PN plants at full flowering resulted in a dramatic decrease of PAs and sugars in inflorescences, but an increase in amino acid content, followed by high abscission. The concept that initial fruitlet abscission might be regulated by free PAs through changes in primary metabolites was hypothesized. Hence, the application of exogenous spermidine (Spd), but not putrescine (Put), prior to flowering markedly inhibits abscission. The Spd treatment also increased soluble sugar content but reduced amino acids in both leaves and inflorescences, while Put had no significant effect. By contrast, cyclohexylamine and beta-hydroxyethylhydrazine, as potent inhibitors of Spd synthase and PA-oxidases, respectively, exerted inverse effects on sugar, amino acid and abscission levels. Sucrose and free proline seemed to be highly sensitive to these treatments. This study suggests that Spd could regulate fruitlet abscission in grapevine by modulating, in a reverse way, the levels of sugars and amino acids in inflorescences.

Transglutaminase activity during senescence and programmed cell death in the corolla of tobacco (Nicotiana tabacum) flowers

Corolla life span of undetached flowers of Nicotiana tabacum was divided into stages from the closed corolla (stage 1) through anthesis (stage 5) to death (stage 9). Senescence began around stage 6 in the proximal part, concomitantly with DNA laddering. Nuclear blebbing, DNA laddering, cell wall modification, decline in protein, water, pigment content and membrane integrity were observed during senescence and PCD. Transglutaminase activity was measured as mono- and bis-derivatives of putrescine (mono-PU; bis-PU) and bis-derivatives of spermidine (bis-SD). Bis-derivatives decreased with the progression of senescence, while mono-PU increased during early senescence; derivatives were present in different amounts in the proximal and distal parts of the corolla. In excised flowers, exogenous spermine delayed senescence and PCD, and caused an increase in free and acid-soluble conjugated PA levels. Bis-PU was the most abundant PA-derivative before DNA laddering stage; thereafter, bis-PU generally decreased and mono-PU became the most abundant derivative.

Changes in free and conjugated polyamines during starvation of sugarcane juices as analyzed by high-performance liquid chromatography

Changes of the polyamines (PAs) titer in high- and mid-molecular-mass carbohydrates (HMMCs and MMMCs, respectively) obtained from sugarcane juices stored for 72 h at pH 5.2 or clarified at pH 8.0 have been studied. Cadaverine (CAD) is the most abundant free (S) PA in the MMMC fraction from juices at pH 5.2, whereas putrescine (PUT) was revealed as the main PA at pH 8.0. A slight increase in the free PUT titer can be noted at pH 5.2 for 72 h of juice starvation. PAs from MMMC were mainly conjugated to acid-insoluble (PH) molecules. Accumulation of PH-PAs with the time of starvation was especially significant for PUT and CAD. However, CAD has also been detected in the acid-soluble (SH) fraction and its concentration increases with the time of starvation at pH 5.2. The accumulation pattern of free and conjugated PAs from HMMCs is similar to that found for MMMCs although some differences can be observed. For instance, the increase in free PUT with the time at pH 8.0 was 2.7-times higher in the HMMC fraction than in the MMMC fraction. Conjugated PAs associated to acid-soluble macromolecules (SH fraction) achieved a level in HMMC fractions higher than that observed in the MMMC fraction. Moreover, the reported increase with time that was observed in PH-CAD from the MMMC fraction was not observed in the HMMC fraction, and, finally, the increase in PH-PUT with the time was lower for the HMMC fraction than for the MMMC fraction.

Effects of polyamine levels on the degradation of short-lived and long-lived proteins in cultured L-132 human lung cells

Biogenic polyamines have important regulatory functions in various biological processes and it has also been suggested that they could modulate intracellular protein degradation. For an overall assessment of the role of polyamines in this process, we have investigated the effect that the decrease in intracellular polyamine levels caused by inhibitors of polyamine biosynthesis brings about on the degradation of the pools of short- and long-lived proteins in cultured L-132 human lung cells. Treatment of cells with 100 microM (2R,5R)-delta-methyl acetylenic putrescine (MAP), a potent enzyme-activated irreversible inhibitor of ornithine decarboxylase, or with 100 microM MAP plus 50 microM N-butyl 1,3-diaminopropane, a specific inhibitor of spermine synthase, caused a similar decrease (65-70% of control) in the total intracellular levels of polyamines, although they affected the concentrations of spermidine and spermine differently. The effect of the two treatments on protein degradation was essentially the same. In polyamine-depleted cells we observed an inhibition of degradation in long-lived proteins of 16% (P<0.05), with a significant increase in the half-life (t12) of this pool from 100.5 to 120.1 h. This was concomitant with an increase of 26% (P<0. 05) in degradation in short-lived proteins, with a significant decrease in the t12 of this pool from 0.85 to 0.67 h. Recovery of polyamine levels by the addition of 50 microM spermidine to polyamine-depleted cells resulted in a restoration of the degradation rates in both pools of proteins. The way(s) by which polyamines could modulate proteolysis are discussed.

The inhibition of a leaf proteinase by L-lysine homopolymers

The role of interlinked positively charged amino acids in the mechanism of inhibition of a monomeric trypsin-like proteinase has been investigated using high molecular mass L-lysine homopolymers ranging from 3.8 to 109 kDa. The data show that the degree of polymerization enhances the inhibitory efficiency which is maximal for homopolymers with more than eighteen interlinked lysine residues. The inhibition is cooperative and, under the maximal inhibition conditions, nine lysine residues of the polymer are involved in the electrostatic binding to the enzyme. A limited conformational change of the protein molecule accompanies the transition from a fully active to a fully inactivated enzyme.

Separation of arginase isoforms by capillary zone electrophoresis and isoelectric focusing in density gradient column

Four major arginase isoforms, I, II, III and IV, have been detected in Evernia prunastri thallus. They differ in terms of both physical and biochemical properties. The isoelectric point (pI) of these proteins has been determined by both isoelectric focusing in density gradient column and high-performance capillary electrophoresis (HPCE). Isoelectric focusing revealed charge microheterogeneity for isoforms II and IV whereas arginases I and II had the same pI value of 5.8. HPCE separation confirmed this charge microheterogeneity for isoform IV but not for isoform III, and provided evidence of microheterogeneity for isoforms I and II. The effect of various electrolyte buffers and running conditions on the HPCE separation of arginase isoform were investigated. Addition of 0.5 mM spermidine (SPD) to the running buffer reduced the electroosmotic flow (EOF) and permitted discriminating between the native proteins and protein fragments.

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.

Polyamines inhibit the ATP-dependent proteolytic pathway in rabbit reticulocyte lysates

Reticulocytes contain a soluble nonlysosomal proteolytic pathway that requires ATP and ubiquitin. Polyamines at physiological concentrations were found to inhibit rapidly the ATP-dependent proteolytic system in reticulocyte lysates; spermidine and putrescine inhibited this process by 26-72% and spermine by 71-96%. Spermine had little effect on the ATP-independent breakdown of oxidant-treated hemoglobin. By fractionating the ATP-dependent system, we show that polyamines inhibit the ATP-dependent degradation of ubiquitin-protein conjugates.