Plant Aminopeptidases: Occurrence, Function and Characterization

Endopeptidases, exopeptidases, and glutamate decarboxylase in soybean water extract and their in vitro activity

The proteolytic activity of some soybean endogenous proteases have been clarified in the previous studies, but the information concerning the roles of these proteases and some other unknown ones during soybean processing are scarce. Herein, 16 endopeptidases, 13 exopeptidases, 24 inhibitors (two serpin-ZX and one subtilisin inhibitor firstly identified), and one glutamate decarboxylase were identified in the soybean water extract by the liquid chromatography tandem mass spectrometry analysis. Amongst the identified endopeptidases, just the aspartic endopeptidases (optimal at pH 2.5-3 and 35-45 °C) showed the detectable proteolytic activity by the tricine-sodium dodecyl sulphate-polyacrylamide gel electrophoresis and protease inhibitor assay analyses, whereas serine, cysteine, and metallo- endopeptidases (except P34 probable thiol protease) did not. Free amino acid analysis showed that the exopeptidases and glutamate decarboxylase were optimal at pH 6 and 45 °C, and by 6 h incubation, the free amino acids and γ-aminobutyric acid almost doubled.

Alterations in the Helicoverpa armigera midgut digestive physiology after ingestion of pigeon pea inducible leucine aminopeptidase

Jasmonate inducible plant leucine aminopeptidase (LAP) is proposed to serve as direct defense in the insect midgut. However, exact functions of inducible plant LAPs in the insect midgut remain to be estimated. In the present investigation, we report the direct defensive role of pigeon pea inducible LAP in the midgut of Helicoverpa armigera (Lepidoptera: Noctuidae) and responses of midgut soluble aminopeptidases and serine proteinases upon LAP ingestion. Larval growth and survival was significantly reduced on the diets supplemented with pigeon pea LAP. Aminopeptidase activities in larvae remain unaltered in presence or absence of inducible LAP in the diet. On the contrary, serine proteinase activities were significantly decreased in the larvae reared on pigeon pea LAP containing diet as compared to larvae fed on diet without LAP. Our data suggest that pigeon pea inducible LAP is responsible for the degradation of midgut serine proteinases upon ingestion. Reduction in the aminopeptidase activity with LpNA in the H. armigera larvae was compensated with an induction of aminopeptidase activity with ApNA. Our findings could be helpful to further dissect the roles of plant inducible LAPs in the direct plant defense against herbivory.

Recycling or regulation? The role of amino-terminal modifying enzymes

Post-translational modifications are essential for a variety of functions, such as the translocation, activation, regulation, and, ultimately, degradation of proteins. The amino-terminal (N-terminal) region is a particularly active area for such alterations. Three types of reactions predominate: limited proteolysis to remove one or more amino acids; modification of the alpha-amino group; and side-chain-specific changes. The N-terminal peptidases expose penultimate residues, providing new substrates for peptidase or transferase action. These enzymes can act sequentially or competitively to influence a protein's longevity, location or activity. N-terminal modifying enzymes (NTMEs) might target a protein for ubiquitination and degradation or protect a protein from rapid turnover. The N-terminal peptidases might also have important roles in processing the peptides that are released from the proteasome. Plant NTMEs have roles in senescence, meiosis and defense, and proposed roles in polar auxin transport.

Isolation and characterization of the neutral leucine aminopeptidase (LapN) of tomato

Tomatoes (Lycopersicon esculentum) express two forms of leucine aminopeptidase (LAP-A and LAP-N) and two LAP-like proteins. The relatedness of LAP-N and LAP-A was determined using affinity-purified antibodies to four LAP-A protein domains. Antibodies to epitopes in the most N-terminal region were able to discriminate between LAP-A and LAP-N, whereas antibodies recognizing central and COOH-terminal regions recognized both LAP polypeptides. Two-dimensional immunoblots showed that LAP-N and the LAP-like proteins were detected in all vegetative (leaves, stems, roots, and cotyledons) and reproductive (pistils, sepals, petals, stamens, and floral buds) organs examined, whereas LAP-A exhibited a distinct expression program. LapN was a single-copy gene encoding a rare-class transcript. A full-length LapN cDNA clone was isolated, and the deduced sequence had 77% peptide sequence identity with the wound-induced LAP-A. Comparison of LAP-N with other plant LAPs identified 28 signature residues that classified LAP proteins as LAP-N or LAP-A like. Overexpression of a His(6)-LAP-N fusion protein in Escherichia coli demonstrated distinct differences in His(6)-LAP-N and His(6)-LAP-A activities. Similar to LapA, the LapN RNA encoded a precursor protein with a molecular mass of 60 kD. The 5-kD presequence had features similar to plastid transit peptides, and processing of the LAP-N presequence could generate the mature 55-kD LAP-N. Unlike LapA, the LapN transcript contained a second in-frame ATG, and utilization of this potential initiation codon would yield a 55-kD LAP-N protein. The localization of LAP-N could be controlled by the balance of translational initiation site utilization and LAP-N preprotein processing.

Identification, purification, and molecular cloning of N-1-naphthylphthalmic acid-binding plasma membrane-associated aminopeptidases from Arabidopsis

Polar transport of the plant hormone auxin is regulated at the cellular level by inhibition of efflux from a plasma membrane (PM) carrier. Binding of the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) to a regulatory site associated with the carrier has been characterized, but the NPA-binding protein(s) have not been identified. Experimental disparities between levels of high-affinity NPA binding and auxin transport inhibition can be explained by the presence of a low-affinity binding site and in vivo hydrolysis of NPA. In Arabidopsis, colocalization of NPA amidase and aminopeptidase (AP) activities, inhibition of auxin transport by artificial beta-naphthylamide substrates, and saturable displacement of NPA by the AP inhibitor bestatin suggest that PM APs may be involved in both low-affinity NPA binding and hydrolysis. We report the purification and molecular cloning of NPA-binding PM APs and associated proteins from Arabidopsis. This is the first report of PM APs in plants. PM proteins were purified by gel permeation, anion exchange, and NPA affinity chromatography monitored for tyrosine-AP activity. Lower affinity fractions contained two orthologs of mammalian APs involved in signal transduction and cell surface-extracellular matrix interactions. AtAPM1 and ATAPP1 have substrate specificities and inhibitor sensitivities similar to their mammalian orthologs, and have temporal and spatial expression patterns consistent with previous in planta histochemical data. Copurifying proteins suggest that the APs interact with secreted cell surface and cell wall proline-rich proteins. AtAPM1 and AtAPP1 are encoded by single genes. In vitro translation products of ATAPM1 and AtAPP1 have enzymatic activities similar to those of native proteins.

The induction of tomato leucine aminopeptidase genes (LapA) after Pseudomonas syringae pv. tomato infection is primarily a wound response triggered by coronatine

Tomato plants constitutively express a neutral leucine aminopeptidase (LAP-N) and an acidic LAP (LAP-A) during floral development and in leaves in response to insect infestation, wounding, and Pseudomonas syringae pv. tomato infection. To assess the physiological roles of LAP-A, a LapA-antisense construct (35S:asLapA1) was introduced into tomato. The 35S:asLapA1 plants had greatly reduced or showed undetectable levels of LAP-A and LAP-N proteins in healthy and wounded leaves and during floral development. Despite the loss of these aminopeptidases, no global changes in protein profiles were noted. The 35S:asLapA1 plants also exhibited no significant alteration in floral development and did not impact the growth and development of Manduca sexta and P. syringae pv. tomato growth rates during compatible or incompatible infections. To investigate the mechanism underlying the strong induction of LapA upon P. syringae pv. tomato infection, LapA expression was monitored after infection with coronatine-producing and -deficient P. syringae pv. tomato strains. LapA RNA and activity were detected only with the coronatine-producing P. syringae pv. tomato strain. Coronatine treatment of excised shoots caused increases in RNAs for jasmonic acid (JA)-regulated wound-response genes (LapA and pin2) but did not influence expression of a JA-regulated pathogenesis-related protein gene (PR-1). These results indicated that coronatine mimicked the wound response but was insufficient to activate JA-regulated PR genes.

Overexpression, purification and biochemical characterization of the wound-induced leucine aminopeptidase of tomato

Wounding of tomato leaves results in the accumulation of an exoprotease called leucine aminopeptidase (LAP-A). While the expression of LapA genes are well characterized, the specificity of the LAP-A enzyme has not been studied. The LAP-A preprotein and mature polypeptide were overexpressed in Escherichia coli. PreLAP-A was not processed and was inactive accumulating in inclusion bodies. In contrast, 55-kDa mature LAP-A subunits assembled into an active, 357-kDa enzyme in E. coli. LAP-A from E. coli cultures was purified to apparent homogeneity and characterized relative to its animal (porcine LAP) and prokaryotic (E. coli PepA) homologues. Similar to the porcine and E. coli enzymes, the tomato LAP-A had high temperature and pH optima. Mn2+ was a strong activator for all three enzymes, while chelators, zinc ion, and the slow-binding aminopeptidase inhibitors (amastatin and bestatin) strongly inhibited activities of all three LAPs. The substrate specificities of porcine, E. coli and tomato LAPs were determined using amino-acid-p-nitroanilide and -beta-naphthylamide substrates. The tomato LAP-A preferentially hydrolyzed substrates with N-terminal Leu, Met and Arg residues. LAP-A had substantially lower levels of activity on other chromogenic substrates. Several differences in substrate specificities for the animal, plant and prokaryotic enzymes were noted.

Leucine aminopeptidase RNAs, proteins, and activities increase in response to water deficit, salinity, and the wound signals systemin, methyl jasmonate, and abscisic acid

LapA RNAs, proteins, and activities increased in response to systemin, methyl jasmonate, abscisic acid (ABA), ethylene, water deficit, and salinity in tomato (Lycopersicon esculentum). Salicylic acid inhibited wound-induced increases of LapA RNAs. Experiments using the ABA-deficient flacca mutant indicated that ABA was essential for wound and systemin induction of LapA, and ABA and systemin acted synergistically to induce LapA gene expression. In contrast, pin2 (proteinase inhibitor 2) was not dependent on exogenous ABA. Whereas both LapA and le4 (L. esculentum dehydrin) were up-regulated by increases in ABA, salinity, and water deficit, only LapA was regulated by octadecanoid pathway signals. Comparison of LapA expression with that of the PR-1 (pathogenesis-related 1) and GluB (basic beta-1,3-glucanase) genes indicated that these PR protein genes were modulated by a systemin-independent jasmonic acid-signaling pathway. These studies showed that at least four signaling pathways were utilized during tomato wound and defense responses. Analysis of the expression of a LapA1:GUS gene in transgenic plants indicated that the LapA1 promoter was active during floral and fruit development and was used during vegetative growth only in response to wounding, Pseudomonas syringae pv tomato infection, or wound signals. This comprehensive understanding of the regulation of LapA genes indicated that this regulatory program is distinct from the wound-induced pin2, ABA-responsive le4, and PR protein genes.