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

Characterization of leucine aminopeptidase (LAP) activity in sweet pepper fruits during ripening and its inhibition by nitration and reducing events

KEY MESSAGE

Pepper fruits contain two leucine aminopeptidase (LAP) genes which are differentially modulated during ripening and by nitric oxide. The LAP activity increases during ripening but is negatively modulated by nitration. Leucine aminopeptidase (LAP) is an essential metalloenzyme that cleaves N-terminal leucine residues from proteins but also metabolizes dipeptides and tripeptides. LAPs play a fundamental role in cell protein turnover and participate in physiological processes such as defense mechanisms against biotic and abiotic stresses, but little is known about their involvement in fruit physiology. This study aims to identify and characterize genes encoding LAP and evaluate their role during the ripening of pepper (Capsicum annuum L.) fruits and under a nitric oxide (NO)-enriched environment. Using a data-mining approach of the pepper plant genome and fruit transcriptome (RNA-seq), two LAP genes, designated CaLAP1 and CaLAP2, were identified. The time course expression analysis of these genes during different fruit ripening stages showed that whereas CaLAP1 decreased, CaLAP2 was upregulated. However, under an exogenous NO treatment of fruits, both genes were downregulated. On the contrary, it was shown that during fruit ripening LAP activity increased by 81%. An in vitro assay of the LAP activity in the presence of different modulating compounds including peroxynitrite (ONOO-), NO donors (S-nitrosoglutathione and nitrosocyteine), reducing agents such as reduced glutathione (GSH), L-cysteine (L-Cys), and cyanide triggered a differential response. Thus, peroxynitrite and reducing compounds provoked around 50% inhibition of the LAP activity in green immature fruits, whereas cyanide upregulated it 1.5 folds. To our knowledge, this is the first characterization of LAP in pepper fruits as well as of its regulation by diverse modulating compounds. Based on the capacity of LAP to metabolize dipeptides and tripeptides, it could be hypothesized that the LAP might be involved in the GSH recycling during the ripening process.

From bitter to delicious: properties and uses of microbial aminopeptidases

Protein hydrolysates are easily digested and utilized by humans and animals, and are less likely to cause allergies. Protein hydrolysis caused by endopeptidases often leads to the exposure of hydrophobic amino acids at the ends of peptides, which consequently causes bitter taste. Microbial aminopeptidases remove the exposed hydrophobic amino acids at the ends of aminopeptides, which improves taste, allowing for easier production. This processe is attacking significant attention from industry and laboratories. Aminopeptidases selectively hydrolyze peptide bonds from the N-terminal of proteins or peptides to produce free amino acids. Aminopeptidases can be classified into leucine, lysine, methionine and proline aminopeptidases by hydrolyzed N-terminal residues; metallo-, serine- and cysteine- aminopeptidases by the reaction mechanisms; dipeptide and triphoptide enzymes by the released number of amino acid residues at the end of hydrolyzed peptides; or acidic, neutral and basic aminopeptidases by their optimal hydrolysis pH. Commercial aminopeptidases are generally produced by microbial fermentation, and are mainly applied in the debittering of protein hydrolysates, the deep hydrolysis of protein, and the production of condiments, cheese, and bioactive peptides, as well as for disease detection in the medical industry.

Proline Affects Flowering Time in Arabidopsis by Modulating FLC Expression: A Clue of Epigenetic Regulation?

The recent finding that proline-induced root elongation is mediated by reactive oxygen species (ROS) prompted us to re-evaluate other developmental processes modulated by proline, such as flowering time. By controlling the cellular redox status and the ROS distribution, proline could potentially affect the expression of transcriptional factors subjected to epigenetic regulation, such as FLOWERING LOCUS C (FLC). Accordingly, we investigated the effect of proline on flowering time in more detail by analyzing the relative expression of the main flowering time genes in p5cs1 p5cs2/P5CS2 proline-deficient mutants and found a significant upregulation of FLC expression. Moreover, proline-deficient mutants exhibited an adult vegetative phase shorter than wild-type samples, with a trichome distribution reminiscent of plants with high FLC expression. In addition, the vernalization-induced downregulation of FLC abolished the flowering delay of p5cs1 p5cs2/P5CS2, and mutants homozygous for p5cs1 and flc-7 and heterozygous for P5CS2 flowered as early as the flc-7 parental mutant, indicating that FLC acts downstream of P5CS1/P5CS2 and is necessary for proline-modulated flowering. The overall data indicate that the effects of proline on flowering time are mediated by FLC.

Post-Proline Cleaving Enzymes (PPCEs): Classification, Structure, Molecular Properties, and Applications

Proteases or peptidases are hydrolases that catalyze the breakdown of polypeptide chains into smaller peptide subunits. Proteases exist in all life forms, including archaea, bacteria, protozoa, insects, animals, and plants due to their vital functions in cellular processing and regulation. There are several classes of proteases in the MEROPS database based on their catalytic mechanisms. This review focuses on post-proline cleaving enzymes (PPCEs) from different peptidase families, as well as prolyl endoprotease/oligopeptidase (PEP/POP) from the serine peptidase family. To date, most PPCEs studied are of microbial and animal origins. Recently, there have been reports of plant PPCEs. The most common PEP/POP are members of the S9 family that comprise two conserved domains. The substrate-limiting β-propeller domain prevents unwanted digestion, while the α/β hydrolase catalyzes the reaction at the carboxyl-terminal of proline residues. PPCEs display preferences towards the Pro-X bonds for hydrolysis. This level of selectivity is substantial and has benefited the brewing industry, therapeutics for celiac disease by targeting proline-rich substrates, drug targets for human diseases, and proteomics analysis. Protein engineering via mutagenesis has been performed to improve heat resistance, pepsin-resistant capability, specificity, and protein turnover of PPCEs for pharmacological applications. This review aims to synthesize recent structure-function studies of PPCEs from different families of peptidases to provide insights into the molecular mechanism of prolyl cleaving activity. Despite the non-exhaustive list of PPCEs, this is the first comprehensive review to cover the biochemical properties, biological functions, and biotechnological applications of PPCEs from the diverse taxa.

The dual-targeted prolyl aminopeptidase PAP1 is involved in proline accumulation in response to stress and during pollen development

Plant endosymbiotic organelles such as mitochondria and chloroplasts harbour a wide array of biochemical reactions. As a part of protein homeostasis to maintain organellar activity and stability, unwanted proteins and peptides need to be completely degraded in a stepwise mechanism termed the processing pathway, where at the last stage single amino acids are released by aminopeptidases. Here, we determined the molecular and physiological functions of a prolyl aminopeptidase homologue PAP1 (At2g14260) that is able to release N-terminal proline. Transcript analyses demonstrate that an alternative transcription start site gives rise to two alternative transcripts, generating two in-frame proteins PAP1.1 and PAP1.2. Subcellular localization studies revealed that the longer isoform PAP1.1, which contains a 51 residue N-terminal extension, is exclusively targeted to chloroplasts, while the truncated isoform PAP1.2 is located in the cytosol. Distinct expression patterns in different tissues and developmental stages were observed. Investigations into the physiological role of PAP1 using loss-of-function mutants revealed that PAP1 activity may be involved in proline homeostasis and accumulation, required for pollen development and tolerance to osmotic stress. Enzymatic activity, subcellular location, and expression patterns of PAP1 suggest a role in the chloroplastic peptide processing pathway and proline homeostasis.

Oilseed rape (Brassica napus): the importance of aminopeptidases in germination under normal and heavy metals stress conditions

BACKGROUND

Oilseed rape is one of the most important oilseed crops worldwide, crucial in the food and feed industries. Different environment and climatic conditions can influence its sustainable cultivation and crop yield. Aminopeptidases are crucial enzymes in many physiological processes in all organisms, including humans, so it is important to learn their behavior in food and feed sources. This study presents, for the first time, a detailed discussion on the importance of aminopeptidases, during the oilseed rape germination process, under standard and stress conditions.

RESULTS

During the germination of oilseed rape under standard conditions, a significant increase in aminopeptidases activity toward N-terminal amino acids - phenylalanine (Phe), alanine (Ala), glycine (Gly), leucine (Leu), proline (Pro), methionine (Met) - was observed. The change was substrate specific, with the highest increase being observed for Gly (3.2-fold), followed by Ala (2.9-fold), Pro (2.5-fold), Met (1.5-fold), and Phe (1.3-fold). Generally, N-terminal Phe was preferentially cleaved. Germination under stress conditions, caused by several heavy metal ions (e.g. divalent copper, zinc, cadmium, and lead ions), negatively influenced the plants' growth and quality, but significantly enhanced the expression of genes encoding aminopeptidases (or potentially activated aminopeptidases precursors), which was related to the dramatic increase of their activity.

CONCLUSIONS

The activity/concentration of aminopeptidases in plants is adjusted to the needs at each stage of development and stress factors occurrence. The most significant increase of activity toward N-terminal Gly and Pro proved the key role of aminopeptidases in the defense mechanisms, by supplying the plants with osmoprotectants and organic nitrogen. The results provide new concepts of oilseed rape growth and cultivation under different conditions. © 2021 Society of Chemical Industry.

Induction of Proline Iminopeptidase by Nitric Oxide May Result in the Proline Accumulation in the Pea Roots

The nitric oxide (NO) donor sodium nitroprusside upregulated the proline iminopeptidase content in the pea seedling roots. It is assumed that NO activates deprolinization of the proline-rich proteins, as evidenced by an increase in the content of free proline, which is known to protect plants from the abiotic and biotic stressors.

Proline Concentration and Its Metabolism Are Regulated in a Leaf Age Dependent Manner But Not by Abscisic Acid in Pea Plants Exposed to Cadmium Stress

The accumulation of proline is one of the defense mechanisms of plants against the harmful effects of adverse environmental conditions; however, when pea plants were treated for 12 h with CdCl2, the proline concentration decreased in the youngest A (not expanded) and B1 (expanded) leaves, and did not change significantly in the B2 (mature, expanded) or C (the oldest) leaves. After 24 h of cadmium (Cd) stress, the proline concentration remained low in A and B1 leaves, while in B2 and C leaves, it increased, and after 48 h, an increase in the proline concentration in the leaves at each stage of development was observed. The role of proline in the different phases of plant response to the Cd treatment is discussed. Changes in proline accumulation corresponded closely with changes in the transcript levels of PsP5CS2, a gene encoding D1-pyrroline-5-carboxylate synthetase involved in proline synthesis, and PsPDH1, a gene encoding proline dehydrogenase engaged in proline degradation. CdCl2 application induced the expression of PsProT1 and PsProT2, genes encoding proline transporters, especially during the first 12 h of treatment in A and B1 leaves. When the time courses of abscisic acid (ABA) and proline accumulation were compared, it was concluded that an increase in the proline concentration in the leaves of Cd-treated pea plants was more related to a decrease in chlorophyll concentration (leaves B2 and C) and an increase in the malondialdehyde level (A and B1 leaves) than with an increase in ABA concentration alone. Exogenous application of ABA (0.5, 5, 50 µM) significantly increased the proline concentration in the A leaves of pea plants only, and was accompanied by an elevated and repressed expression of PsP5CS2 and PsPDH1 in these leaves, respectively. The presented results suggest that under Cd stress, the accumulation of proline in leaves of pea plants may take place independently of the ABA signaling.

Mutagenesis for Improvement of Activity and Stability of Prolyl Aminopeptidase from Aspergillus oryzae

In this study, the prokaryotic expression system of Escherichia coli was used to modify prolyl aminopeptidase derived from Aspergillus oryzae JN-412 (AoPAP) via random mutagenesis and site-directed saturation mutagenesis. A random mutant library with a capacity of approximately 3000 mutants was compiled using error-prone polymerase chain reaction, and nonconservative amino acids within 3 Å of the substrate L-proline-p-nitroaniline were selected as site-directed saturation mutagenesis sites via homologous simulation and molecular docking of AoPAP. Variants featuring high catalytic efficiency were screened by a high-throughput screening method. The specific activities of the variants of 3D9, C185V, and Y393W were 127 U mg^-1, 156 U mg^-1, and 120 U mg^-1, respectively, which were 27%, 56%, and 20% higher than those of the wild type, with a value of 100 U mg^-1. The half-life of thermostability of the mutant 3D9 was 4.5 h longer than that of the wild type at 50 °C. The mutant C185V improved thermostability and had a half-life 2 h longer than that of the wild type at a pH of 6.5. Prolyl aminopeptidase had improved stability within the acidic range and thermostability after modification, making it more suitable for a synergistic combination with various acidic and neutral endoproteases.