Characterization of theLactococcus lactis pepNgene encoding an aminopeptidase homologous to mammalian aminopeptidase N

Characterization of a Lactococcus lactis promoter for heterologous protein production

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Characterization of PTS-IIC, an endogenous constitutive promoter from L. lactis..

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Cellobiose enhances activity from PTS-IIC promoter.

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PTS-IIC promoter mediates protein expression in B. subtilis and E coli Nissle 1917.

Constitutively active promoter elements for heterologous protein production in Lactococcus lactis are scarce. Here, the promoter of the PTS-IIC gene cluster from L. lactis NZ3900 is described. This promoter was cloned upstream of an enhanced green fluorescent protein, GFPmut3a, and transformed into L. lactis. Transformants produced up to 13.5 μg of GFPmut3a per milliliter of log phase cells. Addition of cellobiose further increased the production of GFPmut3a by up to two-fold when compared to glucose. Analysis of mutations at two specific positions in the PTS-IIC promoter showed that a ‘T’ to ‘G’ mutation within the −35 element resulted in constitutive expression in glucose, while a ‘C’ at nucleotide 7 in the putative cre site enhanced promoter activity in cellobiose. Finally, this PTS-IIC promoter is capable of mediating protein expression in Bacillus subtilis and Escherichia coli Nissle 1917, suggesting the potential for future biotechnological applications of this element and its derivatives.

Production of a functional cell wall-anchored minicellulosome by recombinant Clostridium acetobutylicum ATCC 824

BACKGROUND

The use of fossil fuels is no longer tenable. Not only are they a finite resource, their use is damaging the environment through pollution and global warming. Alternative, environmentally friendly, renewable sources of chemicals and fuels are required. To date, the focus has been on using lignocellulose as a feedstock for microbial fermentation. However, its recalcitrance to deconstruction is making the development of economic processes extremely challenging. One solution is the generation of an organism suitable for use in consolidated bioprocessing (CBP), i.e. one able to both hydrolyse lignocellulose and ferment the released sugars, and this represents an important goal for synthetic biology. We aim to use synthetic biology to develop the solventogenic bacterium C. acetobutylicum as a CBP organism through the introduction of a cellulosome, a complex of cellulolytic enzymes bound to a scaffold protein called a scaffoldin. In previous work, we were able to demonstrate the in vivo production of a C. thermocellum-derived minicellulosome by recombinant strains of C. acetobutylicum, and aim to develop on this success, addressing potential issues with the previous strategy.

RESULTS

The genes for the cellulosomal enzymes Cel9G, Cel48F, and Xyn10A from C. cellulolyticum were integrated into the C. acetobutylicum genome using Allele-Coupled Exchange (ACE) technology, along with a miniscaffoldin derived from C. cellulolyticum CipC. The possibility of anchoring the recombinant cellulosome to the cell surface using the native sortase system was assessed, and the cellulolytic properties of the recombinant strains were assayed via plate growth, batch fermentation and sugar release assays.

CONCLUSIONS

We have been able to demonstrate the synthesis and in vivo assembly of a four-component minicellulosome by recombinant C. acetobutylicum strains. Furthermore, we have been able to anchor a minicellulosome to the C. acetobutylicum cell wall by the use of the native sortase system. The recombinant strains display an improved growth phenotype on xylan and an increase in released reducing sugar from several substrates including untreated powdered wheat straw. This constitutes an important milestone towards the development of a truly cellulolytic strain suitable for CBP.

An extracellular Serine/Threonine-rich protein from Lactobacillus plantarum NCIMB 8826 is a novel aggregation-promoting factor with affinity to mucin

Autoaggregation in lactic acid bacteria is directly related to the production of certain extracellular proteins, notably, aggregation-promoting factors (APFs). Production of aggregation-promoting factors confers beneficial traits to probiotic-producing strains, contributing to their fitness for the intestinal environment. Furthermore, coaggregation with pathogens has been proposed to be a beneficial mechanism in probiotic lactic acid bacteria. This mechanism would limit attachment of the pathogen to the gut mucosa, favoring its removal by the human immune system. In the present paper, we have characterized a novel aggregation-promoting factor in Lactobacillus plantarum. A mutant with a knockout of the D1 gene showed loss of its autoaggregative phenotype and a decreased ability to bind to mucin, indicating an adhesion role of this protein. In addition, heterologous production of the D1 protein or an internal fragment of the protein, characterized by its abundance in serine/threonine, strongly induced autoaggregation in Lactococcus lactis. This result strongly suggested that this internal fragment is responsible for the bioactivity of D1 as an APF. To our knowledge, this is the first report on a gene coding for an aggregation-promoting factor in Lb. plantarum.

Aminopeptidase N

The third edition of the Handbook of Proteolytic Enzymes aims to be a comprehensive reference work for the enzymes that cleave proteins and peptides, and contains over 850 chapters. Each chapter is organized into sections describing the name and history, activity and specificity, structural chemistry, preparation, biological aspects, and distinguishing features for a specific peptidase. The subject of Chapter 79 is Aminopeptidase N.

Keywords

Actinonin, amastatin, angiogenesis, angiotensin, bestatin, brush border, cancer, CD13, coronavirus, cysteinyl-glycinase, dipeptidyl peptidase IV, enkephalin, glutathione, neprilysin, puromycin, stem cells.

Tyrosine-containing peptides are precursors of tyramine produced by Lactobacillus plantarum strain IR BL0076 isolated from wine

BACKGROUND

Biogenic amines are molecules with allergenic properties. They are found in fermented products and are synthesized by lactic acid bacteria through the decarboxylation of amino acids present in the food matrix. The concentration of biogenic amines in fermented foodstuffs is influenced by many environmental factors, and in particular, biogenic amine accumulation depends on the quantity of available precursors. Enological practices which lead to an enrichment in nitrogen compounds therefore favor biogenic amine production in wine. Free amino acids are the only known precursors for the synthesis of biogenic amines, and no direct link has previously been demonstrated between the use of peptides by lactic acid bacteria and biogenic amine synthesis.

RESULTS

Here we demonstrate for the first time that a Lactobacillus plantarum strain isolated from a red wine can produce the biogenic amine tyramine from peptides containing tyrosine. In our conditions, most of the tyramine was produced during the late exponential growth phase, coinciding with the expression of the tyrDC and tyrP genes. The DNA sequences of tyrDC and tyrP in this strain share 98% identity with those in Lactobacillus brevis consistent with horizontal gene transfer from L. brevis to L. plantarum.

CONCLUSION

Peptides amino acids are precursors of biogenic amines for Lactobacillus plantarum strain IR BL0076.

Characterization of a Vibrio fischeri aminopeptidase and evidence for its influence on an early stage of squid colonization

Vibrio fischeri cells are the sole colonists of a specialized light organ in the mantle cavity of the sepiolid squid Euprymna scolopes. The process begins when the bacteria aggregate in mucus secretions outside the light organ. The cells eventually leave the aggregate, enter the light organ, and encounter a rich supply of peptides. The need to dissociate from mucus and presumably utilize peptides led us to hypothesize that protease activity is integral to the colonization process. Protease activity associated with whole cells of Vibrio fischeri strain ES114 was identified as the product of a putative cell membrane-associated aminopeptidase (PepN). To characterize this activity, the aminopeptidase was cloned, overexpressed, and purified. Initial steady-state kinetic studies revealed that the aminopeptidase has broad activity, with a preference for basic and hydrophobic side chains and k(cat) and K(m) values that are lower and smaller, respectively, than those of Escherichia coli PepN. A V. fischeri mutant unable to produce PepN is significantly delayed in its ability to colonize squid within the first 12 h, but eventually it establishes a wild-type colonization level. Likewise, in competition with the wild type for colonization, the mutant is outcompeted at 12 h postinoculation but then competes evenly by 24 h. Also, the PepN-deficient strain fails to achieve wild-type levels of cells in aggregates, suggesting an explanation for the initial colonization delay. This study provides a foundation for more studies on PepN expression, localization, and role in the early stages of squid colonization.

Proteomics as a tool for studying energy metabolism in lactic acid bacteria

Lactic acid bacteria (LAB) are very ancient organisms that can't obtain metabolic energy by respiration without external heme supplementation. Since the gain in ATP from lactic fermentation is inadequate to support efficient growth, they developed alternative strategies for energy production. Three main energy generating routes are present in LAB: amino acid decarboxylation, malate decarboxylation and arginine deimination (ADI pathway). These routes, apart from supplying energy, also play a role in pH control. Lactic fermentation, which leads to lactic acid accumulation, causes a pH decrease that amino acid decarboxylations, originating basic amines, and the ADI pathway, giving rise to ammonia, may partially contrast. In the present mini-review, the reciprocal relationships among these metabolic pathways are considered, on the basis of proteomic results obtained from four different LAB strains, all of which possess the ADI pathway, but express different amino acid decarboxylases. The strains have been isolated and selected from different habitats and the role of some inducing molecules as well as of the growth phases is discussed. The overall results have revealed that LAB are complex biosystems able to set up a sophisticated metabolic regulation through a complex network of proteins that also include stress responses, as well as protease activation or inhibition.

Thioredoxin reductase is a key factor in the oxidative stress response of Lactobacillus plantarum WCFS1

Background

Thioredoxin (TRX) is a powerful disulfide oxido-reductase that catalyzes a wide spectrum of redox reactions in the cell. The aim of this study is to elucidate the role of the TRX system in the oxidative stress response in Lactobacillus plantarum WCFS1.

Results

We have identified the trxB1-encoded thioredoxin reductase (TR) as a key enzyme in the oxidative stress response of Lactobacillus plantarum WCFS1.

Overexpression of the trxB1 gene resulted in a 3-fold higher TR activity in comparison to the wild-type strain. Subsequently, higher TR activity was associated with an increased resistance towards oxidative stress. We further determined the global transcriptional response to hydrogen peroxide stress in the trxB1-overexpression and wild-type strains grown in continuous cultures. Hydrogen peroxide stress and overproduction of TR collectively resulted in the up-regulation of 267 genes. Additionally, gene expression profiling showed significant differential expression of 27 genes in the trxB1-overexpression strain. Over expression of trxB1 was found to activate genes associated with DNA repair and stress mechanisms as well as genes associated with the activity of biosynthetic pathways for purine and sulfur-containing amino acids. A total of 16 genes showed a response to both TR overproduction and hydrogen peroxide stress. These genes are involved in the purine metabolism, energy metabolism (gapB) as well as in stress-response (groEL, npr2), and manganese transport (mntH2).

Conclusion

Based on our findings we propose that overproduction of the trxB1-encoded TR in L. plantarum improves tolerance towards oxidative stress. This response coincides with simultaneous induction of a group of 16 transcripts of genes. Within this group of genes, most are associated with oxidative stress response. The obtained crossover between datasets may explain the phenotype of the trxB1-overexpression strain, which appears to be prepared for encountering oxidative stress. This latter property can be used for engineering robustness towards oxidative stress in industrial strains of L. plantarum.

SpxB Regulates O-Acetylation-dependent Resistance of Lactococcus lactis Peptidoglycan to Hydrolysis

Endogenous peptidoglycan (PG)-hydrolyzing enzymes, the autolysins, are needed to relax the rigid PG sacculus to allow bacterial cell growth and separation. PGs of pathogens and commensal bacteria may also be degraded by hydrolases of animal origin (lysozymes), which act as antimicrobials. The genetic mechanisms regulating PG resistance to hydrolytic degradation were dissected in the Gram-positive bacterium Lactococcus lactis. We found that the ability of L. lactis to counteract PG hydrolysis depends on the degree of acetylation. Overexpression of PG O-acetylase (encoded by oatA) led to bacterial growth arrest, indicating the potential lethality of oatA and a need for its tight regulation. A novel regulatory factor, SpxB (previously denoted as YneH), exerted a positive effect on oatA expression. Our results indicate that SpxB binding to RNA polymerase constitutes a previously missing link in the multistep response to cell envelope stress, provoked by PG hydrolysis with lysozyme. We suggest that the two-component system CesSR responds to this stress by inducing SpxB, thus favoring its interactions with RNA polymerase. Induction of PG O-acetylation by this cascade renders it resistant to hydrolysis.

Cre-lox-based system for multiple gene deletions and selectable-marker removal in Lactobacillus plantarum

The classic strategy to achieve gene deletion variants is based on double-crossover integration of nonreplicating vectors into the genome. In addition, recombination systems such as Cre-lox have been used extensively, mainly for eukaryotic organisms. This study presents the construction of a Cre-lox-based system for multiple gene deletions in Lactobacillus plantarum that could be adapted for use on gram-positive bacteria. First, an effective mutagenesis vector (pNZ5319) was constructed that allows direct cloning of blunt-end PCR products representing homologous recombination target regions. Using this mutagenesis vector, double-crossover gene replacement mutants could be readily selected based on their antibiotic resistance phenotype. In the resulting mutants, the target gene is replaced by a lox66-P(32)-cat-lox71 cassette, where lox66 and lox71 are mutant variants of loxP and P(32)-cat is a chloramphenicol resistance cassette. The lox sites serve as recognition sites for the Cre enzyme, a protein that belongs to the integrase family of site-specific recombinases. Thus, transient Cre recombinase expression in double-crossover mutants leads to recombination of the lox66-P(32)-cat-lox71 cassette into a double-mutant loxP site, called lox72, which displays strongly reduced recognition by Cre. The effectiveness of the Cre-lox-based strategy for multiple gene deletions was demonstrated by construction of both single and double gene deletions at the melA and bsh1 loci on the chromosome of the gram-positive model organism Lactobacillus plantarum WCFS1. Furthermore, the efficiency of the Cre-lox-based system in multiple gene replacements was determined by successive mutagenesis of the genetically closely linked loci melA and lacS2 in L. plantarum WCFS1. The fact that 99.4% of the clones that were analyzed had undergone correct Cre-lox resolution emphasizes the suitability of the system described here for multiple gene replacement and deletion strategies in a single genetic background.

Proteolytic systems of lactic acid bacteria

Lactic acid bacteria (LAB) have a very long history of use in the manufacturing processes of fermented foods and a great deal of effort was made to investigate and manipulate the role of LAB in these processes. Today, the diverse group of LAB includes species that are among the best-studied microorganisms and proteolysis is one of the particular physiological traits of LAB of which detailed knowledge was obtained. The proteolytic system involved in casein utilization provides cells with essential amino acids during growth in milk and is also of industrial importance due to its contribution to the development of the organoleptic properties of fermented milk products. For the most extensively studied LAB, Lactococcus lactis, a model for casein proteolysis, transport, peptidolysis, and regulation thereof is now established. In addition to nutrient processing, cellular proteolysis plays a critical role in polypeptide quality control and in many regulatory circuits by keeping basal levels of regulatory proteins low and removing them when they are no longer needed. As part of the industrial processes, LAB are challenged by various stress conditions that are likely to affect metabolic activities, including proteolysis. While environmental stress responses of LAB have received increasing interest in recent years, our current knowledge on stress-related proteolysis in LAB is almost exclusively based on studies on L. lactis. This review provides the current status in the research of proteolytic systems of LAB with industrial relevance.

Over-expression, purification, and characterization of aminopeptidase N from Escherichia coli

The gene from Escherichia coli encoding aminopeptidase N (PepN) was subcloned into pET-26b, and PepN was over-expressed in BL21(DE3) E. coli and purified using Q-Sepharose chromatography. This protocol yielded over 17 mg of purified, recombinant PepN per liter of growth culture under optimum conditions. Gel filtration chromatography revealed that recombinant PepN exists as a monomer. MALDI-TOF mass spectra showed that the enzyme has a molecular mass of 98,750 Da, and steady-state kinetic studies revealed that as-isolated, recombinant PepN exhibits a k(cat) of 354 +/- 11s(-1) and a K(m) of 376 +/- 39 microM when using L-alanine-p-nitroanilide as the substrate. Metal analyses demonstrated that as-isolated, recombinant PepN binds 0.5 and <0.1 equivalents of iron and zinc, respectively. The addition of Zn(II) to recombinant PepN inhibits catalytic activity, while the addition of iron causes a slight decrease or no change in activity. Further metal binding studies revealed that recombinant PepN tightly binds 5 equivalents of iron and <0.1 equivalents of Zn(II). By using this over-expression and purification system, E. coli PepN can now be obtained in quantities necessary for structural characterization and possibly inhibitor design efforts.

Inactivation of Bacillus stearothermophilus leucine aminopeptidase II by hydrogen peroxide and site-directed mutagenesis of methionine residues on the enzyme

Leucine aminopeptidases (LAPs) are exopeptidases that remove the N-terminal L-leucine from peptide substrates. Oxidative stability assay showed that the recombinant Bacillus stearothermophilus LAP II (rLAPII) was sensitive to oxidative damage by hydrogen peroxide at the elevated temperature. The H2O2-treated enzyme experienced obvious changes in the secondary structure when the oxidant concentration increased to 300 mM. To investigate the role of methionine residues on the oxidative inactivation, each of the five methionine residues in the rLAPII was replaced with leucine by site-directed mutagenesis. The mutant enzymes with an apparent Mr of approximately 44.5 kDa were overexpressed in Escherichia coli and were purified to homogeneity by nickel-chelate chromatography. The specific activities for Met82Leu, Met88Leu, Met254Leu, and Met382Leu were similar to that of the wild-type enzyme, whereas a reduced activity was observed in Met136Leu. The 50% decrease in the catalytic efficiency (kcat/Km) for Met136Leu was caused by 47% decrease in kcat value. As compared with the wild-type enzyme, all mutant proteins were more sensitive to the oxidant, implying that the methionine residues of B. stearothermophilus LAP II are important for the protection of the enzyme from oxidative inactivation.

Membrane alanyl aminopeptidase

This chapter focuses on the structural chemistry of membrane alanyl aminopeptidase (mAAP). The early history of mAAP relates to its role as Cys-Gly dipeptidase or cysteinyl-glycinase. It was proposed that this peptidase activity present in apparently purified RNA preparations contributed to polypeptide biosynthesis by acting in reverse in a sequential fashion. mAAP has a broad substrate specificity removing N-terminal amino acids (Xaa-Xbb-) from almost all unsubstituted oligopeptides and from an amide or arylamide. mAAP is a type II integral membrane protein located on the plasma membrane as an ectoenzyme. The pI is approximately 5. mAAP is widely distributed among species and tissues although it is of greatest abundance in brush border membranes of the kidney, in the mucosal cells of the small intestine and in the liver. It is also present in the lung where it is identical to the pI46 type II alveolar epithelial cell antigen and is located on endothelial cells in blood vessels. On polarized epithelial cells, mAAP is localized to the apical domain and is targeted there through an apical sorting signal thought to be located in the catalytic head group region of the protein.

Overexpression of peptidases in Lactococcus and evaluation of their release from leaky cells

Walker and Klaenhammer (2001) developed a novel expression system in Lactococcus lactis that facilitated the release of beta-galactosidase (117 kDa monomer) without the need for secretion or export signals. The system is based on the controlled expression of integrated prophage holin and lysin cassettes via a lactococcal bacteriophage phi31 transcriptional activator (Tac31A) that resides on a high-copy plasmid. Approximately 85% of beta-galactosidase activity was detected in the supernatant of leaky lactococci without evidence of hindered growth, cell lysis, or membrane damage. The objective of this study was to determine if intracellular peptidases were externalized from leaky lactococci. Five L. lactis peptidases (PepA, PepC, PepN, PepO and PepXP) and two Lactobacillus helveticus peptidases (PepN and PepO) were cloned and overexpressed on two high-copy vectors. The lactococcal peptidases were also cloned into the high-copy vector that contained the Tac31A transcriptional activator to determine if they were externalized from the leaky prophage-containing L. lactis subsp. lactis strain NCK203. Two of the lactococcal peptidases (PepA and PepO) required an additional strong promoter (Lactobacillus paracasei P144) and optimized assay conditions to detect enzyme activity. Results showed different levels of enzymatic overexpression associated with the cellular fraction (2 to 250-fold increases in activity) and negligible amounts of activity present within the supernatant fraction (0 to 6% of total peptidase activity). The lactococcal phage-based protein release mechanism did not facilitate the externalization of the lactococcal peptidases investigated in this study.

Purification, characterization, gene cloning, sequencing, and overexpression of aminopeptidase N from Streptococcus thermophilus A

The general aminopeptidase PepN from Streptococcus thermophilus A was purified to protein homogeneity by hydroxyapatite, anion-exchange, and gel filtration chromatographies. The PepN enzyme was estimated to be a monomer of 95 kDa, with maximal activity on N-Lys-7-amino-4-methylcoumarin at pH 7 and 37 degrees C. It was strongly inhibited by metal chelating agents, suggesting that it is a metallopeptidase. The activity was greatly restored by the bivalent cations Co2+, Zn2+, and Mn2+. Except for proline, glycine, and acidic amino acid residues, PepN has a broad specificity on the N-terminal amino acid of small peptides, but no significant endopeptidase activity has been detected. The N-terminal and short internal amino acid sequences of purified PepN were determined. By using synthetic primers and a battery of PCR techniques, the pepN gene was amplified, subcloned, and further sequenced, revealing an open reading frame of 2,541 nucleotides encoding a protein of 847 amino acids with a molecular weight of 96,252. Amino acid sequence analysis of the pepN gene translation product shows high homology with other PepN enzymes from lactic acid bacteria and exhibits the signature sequence of the zinc metallopeptidase family. The pepN gene was cloned in a T7 promoter-based expression plasmid and the 452-fold overproduced PepN enzyme was purified to homogeneity from the periplasmic extract of the host Escherichia coli strain. The overproduced enzyme showed the same catalytic characteristics as the wild-type enzyme.

A novel aminopeptidase associated with the 60 kDa chaperonin in the thermophilic archaeon Sulfolobus solfataricus

The chaperonins are high-molecular-weight protein complexes having a characteristic double-ring toroidal shape; they are thought to aid the folding of denatured or newly synthesized polypeptides. These proteins exist as two functionally similar but distantly related families, one including the bacterial and organellar chaperonins and the other (termed the CCT-TRiC family) including the chaperonins of the Archaea and the eukaryotes. The CCT-TRiC chaperonins, particularly their archeal members, are less well known than their bacterial counterparts, and their main cellular function is still doubtful. In this work, we report that the chaperonin of the thermophilic archaeon Sulfolobus solfataricus interacts with several polypeptides other than the two subunits that constitute the 18-mer double-ring structure. We have cloned and sequenced the gene encoding one 90 kDa chaperonin-associated protein and have shown, using biochemical assays, that the product is an enzyme belonging to the family of zinc-dependent aminopeptidases. The Sulfolobus protein shows maximal homology to eukaryotic (yeast and mouse) aminopeptidases. It contains a leucine zipper motif and can be phosphorylated by an unidentified kinase present in the cell extracts. The possible significance of an association between an aminopeptidase and a chaperonin is discussed.

Molecular cloning of a GPI-anchored aminopeptidase N from Bombyx mori midgut: a putative receptor for Bacillus thuringiensis CryIA toxin

An aminopeptidase N (APN) with a molecular weight of 110kDa was released from the midgut membrane of Bombyx mori by phosphatidylinositol-specific phospholipase C (PI-PLC), and purified to a homogeneous state. This 110-kDa APN was different from the 100-kDa APN that we previously reported, in chromatographic behaviors, substrate specificity, and N-terminal and internal amino acid sequences. However, the N-terminal sequence of 110-kDa APN, DPAFRLPTTTRPRHYQVTLT, was highly homologous with those of Manduca sexta and Heliothis virescens APNs, which were identified as a receptor for an insecticidal toxin of Bacillus thuringiensis. From a B. mori midgut cDNA library, we cloned the 110-kDa APN cDNA that possessed a 2958-bp open reading frame encoding a 111573-Da polypeptide of 986 residues. The sequence of the eicosa-peptide Asp42Thr61 deduced from the cDNA was completely matched with the N-terminal sequence of the mature 110-kDa APN. One potential N-glycosylation site, HEXXHXW zinc-binding motif and characteristic proline-rich repeats were observed in the ORF. Moreover, the primary sequence contained two hydrophobic peptides on N- and C-termini. The N-terminal peptide sequence showed characteristics of leader peptide for secretion and the C-terminal peptide contained a possible glycosylphosphatidylinositol (GPI) anchoring site. Taken together, the deduced amino acid sequence suggests that the 110-kDa APN is a GPI-anchored protein and a specific receptor protein for B. thuringiensis CryIA delta-endotoxin.

Cloning and characterization of Manduca sexta and Plutella xylostella midgut aminopeptidase N enzymes related to Bacillus thuringiensis toxin-binding proteins

We report the purification, cloning and characterization of an aminopeptidase N from the midgut epithelium of Manduca sexta that binds Cry1Ab5, an insecticidal crystal protein [ICP] from Bacillus thuringiensis. Sequence information derived from this M. sexta aminopeptidase N was used for the cloning of an aminopeptidase N from the midgut brush-border membrane of Plutella xylostella, an insect species of which some populations acquired resistance against Cry1Ab5. Affinity chromatography on a Cry1Ab5 matrix was used to isolate a 120-kDa glycoprotein from the larval midgut of the lepidopteran M. sexta. On ligand blots the purified 120-kDa protein discriminates between the lepidopteran-specific Cry1Ab5 and the coleopteran-specific Cry3A delta-endotoxin. Internal amino acid sequences from the 120-kDa protein were used for the design of degenerate oligonucleotides. From a nested PCR with M. sexta midgut cDNA as template, a DNA fragment was obtained which shows similarity to prokaryotic and eukaryotic aminopeptidase N genes. This PCR fragment was used to screen cDNA libraries of larval midguts from M. sexta and P. xylostella. From the M. sexta midgut cDNA library a 2973-bp nucleotide sequence was cloned. The ORF of the sequence encodes a 942-residue aminopeptidase N (M. sexta Apn2) containing two hydrophobic regions. The NH2-terminal hydrophobic region corresponds to a secretory signal sequence and the COOH-terminal hydrophobic region is typical of glycosylphosphatidylinositol (glycosyl-PtdIns)-anchored proteins. Low-stringency hybridization of the P. xylostella midgut cDNA library with M. sexta apn2 probes enabled the isolation of a 3118-bp sequence with an ORF encoding a 946-residue preproprotein. This aminopeptidase N (P. xylostella Apn1) displays 61% amino acid identity to M. sexta Apn2 and contains a COOH-terminal signal peptide for glycosyl-PtdIns anchor addition. Both M. sexta Apn2 and P. xylostella Apn1 contain four Cys residues, which are highly conserved among eukaryotic aminopeptidase N molecules. Treatment of Sf9 cells expressing the P. xylostella apn1 gene with PtdIns-specific phospholipase C demonstrated that P. xylostella Apn1 is attached to the insect cell membrane by a glycosyl-PtdIns anchor.

The proteolytic systems of lactic acid bacteria

Proteolysis in dairy lactic acid bacteria has been studied in great detail by genetic, biochemical and ultrastructural methods. From these studies the picture emerges that the proteolytic systems of lactococci and lactobacilli are remarkably similar in their components and mode of action. The proteolytic system consists of an extracellularly located serine-proteinase, transport systems specific for di-tripeptides and oligopeptides (> 3 residues), and a multitude of intracellular peptidases. This review describes the properties and regulation of individual components as well as studies that have led to identification of their cellular localization. Targeted mutational techniques developed in recent years have made it possible to investigate the role of individual and combinations of enzymes in vivo. Based on these results as well as in vitro studies of the enzymes and transporters, a model for the proteolytic pathway is proposed. The main features are: (i) proteinases have a broad specificity and are capable of releasing a large number of different oligopeptides, of which a large fraction falls in the range of 4 to 8 amino acid residues; (ii) oligopeptide transport is the main route for nitrogen entry into the cell; (iii) all peptidases are located intracellularly and concerted action of peptidases is required for complete degradation of accumulated peptides.

Bacterial aminopeptidases: properties and functions

Aminopeptidases are exopeptidases that selectively release N-terminal amino acid residues from polypeptides and proteins. Bacteria display several aminopeptidasic activities which may be localised in the cytoplasm, on membranes, associated with the cell envelope or secreted into the extracellular media. Studies on the bacterial aminopeptide system have been carried out over the past three decades and are significant in fundamental and biotechnological domains. At present, about one hundred bacterial aminopeptidases have been purified and biochemically studied. About forty genes encoding aminopeptidases have also been cloned and characterised. Recently, the three-dimensional structure of two aminopeptidases, the methionine aminopeptidase from Escherichia coli and the leucine aminopeptidase from Aeromonas proteolytica, have been elucidated by crystallographic studies. Most of the quoted studies demonstrate that bacterial aminopeptidases generally show Michaelis-Menten kinetics and can be placed into either of two categories based on their substrate specificity: broad or narrow. These enzymes can also be classified by another criterium based on their catalytic mechanism: metallo-, cysteine- and serine-aminopeptidases, the former type being predominant in bacteria. Aminopeptidases play a role in several important physiological processes. It is noteworthy that some of them take part in the catabolism of exogenously supplied peptides and are necessary for the final steps of protein turnover. In addition, they are involved in some specific functions, such as the cleavage of N-terminal methionine from newly synthesised peptide chains (methionine aminopeptidases), the stabilisation of multicopy ColE1 based plasmids (aminopeptidase A) and the pyroglutamyl aminopeptidase (Pcp) present in many bacteria and responsible for the cleavage of the N-terminal pyroglutamate.

Detection and localization of peptidases in Lactococcus lactis with monoclonal antibodies

Monoclonal antibodies against peptidases of Lactococcus lactis were isolated and characterized: PEPN1-4 against a lysyl aminopeptidase PepN, PEPT1-5 against a tripeptidase PepT and PEPD1-3 against a dipeptidase PepD. These monoclonal antibodies reacted specifically with their respective antigens in crude cell extracts of Lc. lactis subspp. cremoris and lactis. A number of monoclonal antibodies cross reacted with proteins of other (lactic acid) bacteria. PEPT1, 2, 4 and 5 cross reacted weakly with a 35 kDa protein in Lactobacillus delbrueckii, while PEPT1 and PEPT2 reacted with proteins in the cell-free extract of Streptococcus thermophilus and Clostridium fervidus. Of the four isolated monoclonal antibodies against PEPN, only PEPN3 cross reacted weakly with a 90 kDa protein in Escherichia coli cell-free extract, and the other three antibody species against PEPN3 cross reacted with 80 kDa proteins of Lb. casei, Lb. delbrueckii, and Str. bovis, but not of Esch. coli. Of the three monoclonal antibodies against PepD, only PEPD1 and PEPD2 cross reacted with 40 kDa proteins of Lb. casei, Lb. delbrueckii and Str. bovis. All PEPN, PEPD and PEPT antibodies reacted with components in cell-free extracts of eleven different Lc. lactis strains, indicating that the peptidases of these strains were very similar to those of Lc. lactis subsp. cremoris WG2. However, Lc. lactis subsp. hordniae appeared to differ from the other Lc. lactis subspecies since only PEPT1, 2 and 5 reacted with a protein in the cell-free extract. Immunogold labelling of Lc. lactis WG2 with the isolated monoclonal antibodies revealed that PepN, PepD and PepT were located intracellularly. The intracellular location of these peptidases is discussed in relation to the supply of essential amino acids and peptides.

Multiple-peptidase mutants of Lactococcus lactis are severely impaired in their ability to grow in milk

To examine the contribution of peptidases to the growth of lactococcus lactis in milk, 16 single- and multiple-deletion mutants were constructed. In successive rounds of chromosomal gene replacement mutagenesis, up to all five of the following peptidase genes were inactivated (fivefold mutant): pepX, pepO, pepT, pepC, and pepN. Multiple mutations led to slower growth rates in milk, the general trend being that growth rates decreased when more peptidases were inactivated. The fivefold mutant grew more than 10 times more slowly in milk than the wild-type strain. In one of the fourfold mutants and in the fivefold mutant, the intracellular pools of amino acids were lower than those of the wild type, whereas peptides had accumulated inside the cell. No significant differences in the activities of the cell envelope-associated proteinase and of the oligopeptide transport system were observed. Also, the expression of the peptidases still present in the various mutants was not detectably affected. Thus, the lower growth rates can directly be attributed to the inability of the mutants to degrade casein-derived peptides. These results supply the first direct evidence for the functioning of lactococcal peptidases in the degradation of milk proteins. Furthermore, the study provides critical information about the relative importance of the peptidases for growth in milk, the order of events in the proteolytic pathway, and the regulation of its individual components.

A study of the substrate specificity of aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2

A systematic study was made of the ability of aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2 to hydrolyse different peptide substrates. The enzyme showed a marked preference for substrates containing arginine as the N-terminal residue but, to a lesser extent, was also capable of cleaving other residues such as lysine and leucine. There was a tendency for the activity to increase with the hydrophobicity index of the C-terminal residue of dipeptide substrates. It was also observed that the enzyme tended to have higher affinities but lower Vmax values for tripeptides with hydrophobic C-terminal residues. The values determined for Km and Vmax increased with chain length for oligopeptides of the general formula Lys-Phe-(Gly)n, the optimum, as determined from Vmax/Km, being when n = 4. Typical Km values for the most effective substrates were in the range 0.2-0.6 mM.

Immunological and electrophoretic study of the proteolytic enzymes from various Lactococcus and Lactobacillus strains

Cell extracts of various lactobacilli and two Lactococcus strains were investigated for their immunoresponse with monoclonal and polyclonal antibodies raised against various proteolytic enzymes from Lc. lactis. Except for Lactobacillus casei SBT 2233, none of the lactobacilli proteins showed immunoresponse with the monoclonal antibodies. With polyclonal antibodies raised against aminopeptidases N and C and endopeptidase of Lc. lactis an immunoresponse was observed. However, the molecular masses of the reactive bands on the blot were considerably different from those of the corresponding lactococcal peptidases, except for the band that reacted with polyclonal antibodies against aminopeptidase C. The polyclonal antibodies raised against X-prolyl-dipeptidyl aminopeptidase and tripeptidase did not show any immunoreaction. As a control, all antibodies reacted with the lactococcal proteins on the blot, with molecular masses corresponding to those reported for the proteinases and peptidases. The results clearly showed that most of the proteolytic enzymes of lactobacilli were immunologically different from those of lactococci. The proteolytic enzymes in the cell-free extracts were separated by non-denaturing PAGE and visualized by zymogram staining. The electrophoretic pattern of the proteolytic enzymes of lactobacilli was different from that of Lc. lactis. Both experiments indicate that the enzymes of the proteolytic system of lactobacilli are different from those of lactococci.

Sequence analysis, distribution and expression of an aminopeptidase N-encoding gene from Lactobacillus helveticus CNRZ32 [gene 155 (1995) 89-93]

Lactobacillus (Lb.) helveticus CNRZ32 possesses a 97-kDa metalloenzyme with aminopeptidase activity (PepN; EC 3.4.11.2). A 3.8-kb fragment encoding PepN was cloned into pIL253 and designated pSUW34. Transformation of Lactococcus (Lc.) lactis LM0230 with pSUW34 resulted in > 180-fold increase in general aminopeptidase (AP) activity using L-lysine-p-nitroanilide. Southern hybridization was conducted to determine the distribution of homology to the CNRZ32 pepN gene among lactic-acid bacteria (LAB). Hybridization was observed with strains of lactobacilli, pediococci, leuconostoc, streptococci and lactococci. The pepN gene was sequenced and found to encode a protein containing 844 amino acid (aa) residues. A comparison of Lb. helveticus CNRZ32 pepN to Lb. delbrueckii ssp. lactis DSM7290 pepN indicated 69.5% nucleotide (nt) identity and 71.8% aa identity, while comparison to pepN from Lc. lactis ssp. cremoris MG1363 indicated 61.1% nt identity and 49.2% aa identity. Alignment of peptidase aa sequences of LAB, Escherichia coli, yeast and mammalian origin display homology in the zinc-binding domain, as well as a conserved region upstream from the putative active site.

Cloning and characterization of a novel insulin-regulated membrane aminopeptidase from Glut4 vesicles

The insulin-regulated glucose transporter isotype GlutT4 expressed only in muscle and adipose cells is sequestered in a specific secretory vesicle. These vesicles harbor another major protein, referred to as vp165 (for vesicle protein of 165 kDa), that like GluT4 redistributes to the plasma membrane in response to insulin. We describe here the cloning of vp165 and show that it is a novel member of the family of zinc-dependent membrane aminopeptidases, with the typical large extracellular catalytic domain and single transmembrane domain but with a unique extended cytoplasmic domain. The latter contains two dileucine motifs, which may be critical for the specific trafficking of vp165, since this has been shown to be the case for this motif in GluT4. However, the tissue distribution of vp165 is much wider than that of GluT4; consequently, vp165 may also function in processes unrelated to insulin action and may serve as a ubiquitous marker for a specialized regulated secretory vesicle.

Sequence analysis, distribution and expression of an aminopeptidase N-encoding gene from Lactobacillus helveticus CNRZ32

Lactobacillus (Lb.) helveticus CNRZ32 possesses a 97-kDa metalloenzyme with aminopeptidase activity (PepN; EC 3.4.11.2). A 3.8-kb fragment encoding PepN was cloned into pIL253 and designated pSUW34. Transformation of lactococcus (Lc.) lactis LM0230 with pSUW34 resulted in > 180-fold increase in general aminopeptidase (AP) activity using L-lysine-p-nitroanilide. Southern hybridization was conducted to determine the distribution of homology to the CNRZ32 pepN gene among lactic-acid bacteria (LAB). Hybridization was observed with strains of lactobacilli, pediococci, leuconostoc, streptococci and lactococci. The pepN gene was sequenced and found to encode a protein containing 844 amino acid (aa) residues. A comparison of Lb. helveticus CNRZ32 pepN to Lb. delbrueckii ssp. lactis DSM7290 pepN indicated 69.5% nucleotide (nt) identity and 71.8% aa identity, while comparison to pepN from Lc. lactis ssp. cremoris MG1363 indicated 61.1% nt identity and 49.2% aa identity. Alignment of peptidase aa sequences of LAB, Escherichia coli, yeast and mammalian origin display homology in the zinc-binding domain, as well as a conserved region upstream from the putative active site.

Characterization and expression of the pepN gene encoding a general aminopeptidase from Lactobacillus helveticus

An aminopeptidase N (pepN) gene was detected by DNA hybridization from an industrially important Lactobacillus helveticus strain using part of the L. helveticus CNRZ32 pepN gene as the probe. One of five hybridization positive clones was characterized in more detail. A subcloned 3.7 kb fragment, positive in hybridization and encoding aminopeptidase activity, was sequenced and analyzed. Only one open reading frame (ORF) of 2532 base pairs with a coding capacity for a 95.9 kDa protein could be found. The deduced amino acid sequence of the 95.9 kDa protein showed homology to PepN proteins from other lactic acid bacteria and carried the conserved catalytic and zinc binding sites of the neutral zinc metallo-peptidase family confirming the identity of the pepN gene. A 2.75 kb transcript and two transcription start sites were identified with mRNA analyses. Expression of pepN in L. helveticus, studied as the function of growth, revealed a high level of pepN transcripts throughout the growth, in contrast to the steady state levels of other peptidase mRNAs from L. helveticus analyzed in our laboratory.

Aminopeptidase N from Streptococcus salivarius subsp. thermophilus NCDO 573: purification and properties

A 96 kDa aminopeptidase was purified from Streptococcus salivarius subsp. thermophilus NCDO 573. The enzyme had similar properties to aminopeptidases isolated from lactococci and lactobacilli and showed a high degree of N-terminal amino acid sequence homology to aminopeptidase N from Lactococcus lactis subsp. cremoris. It catalysed the hydrolysis of a range of aminoacyl 4-nitroanilides and 7-amido-4-methylcoumarin derivatives, dipeptides, tripeptides and oligopeptides. In common with aminopeptidases from other lactic acid bacteria, the enzyme from Strep. salivarius subsp. thermophilus showed highest activity with lysyl derivatives but was also very active with arginyl and leucyl derivatives. Relative activity with alanyl, phenylalanyl, tyrosyl, seryl and valyl derivatives was considerably lower and with glycyl, glutamyl and prolyl derivatives almost negligible. The aminopeptidase also catalysed the hydrolysis of dipeptides and tripeptides but mostly at rates much less than that with L-lysyl-4-nitroanilide and oligopeptides. The enzyme catalysed the successive hydrolysis of various amino acid residues from the N-terminus of several oligopeptides but it was unable to cleave peptide bonds on the N-terminal side of a proline residue.

Gene cloning and characterization of PepC, a cysteine aminopeptidase from Streptococcus thermophilus, with sequence similarity to the eucaryotic bleomycin hydrolase

Streptococcus thermophilus CNRZ 302 contains at least three general aminopeptidases able to hydrolyze Phe-beta-naphthylamide substrate. The gene encoding one of these aminopeptidases was cloned from a total DNA library of S. thermophilus CNRZ 302 constructed in Escherichia coli TG1 using pBluescript plasmid. The wild-type TG1 strain, although not deficient in aminopeptidase activity, is unable to hydrolyze the substrate Phe-beta-naphthylamide, and thus the library could be screened with an enzymic plate assay using this substrate. One clone was selected which was shown to express an aminopeptidase, identified as a PepC-like enzyme on the basis of cross-reactivity with polyclonal antibodies directed against the lactococcal PepC cysteine aminopeptidase. The gene was further subcloned and sequenced. A complete open reading frame coding for a 445-residue (50414 Da) polypeptide was identified. 70% identity was found between the deduced amino acid sequence and the sequence of PepC from Lactococcus lactis subspecies cremoris, confirming the identity of the cloned gene. High sequence similarity (38% identity) was also found with an eucaryotic enzyme, bleomycin hydrolase. In addition, the predicted amino acid sequence of the streptococcal PepC showed a region of strong similarity to the active site of cysteine proteinases with conservation of the residues involved in the catalytic site. The product of the cloned pepC gene was overproduced in E. coli and was purified from a cellular extract. Purification to homogeneity was achieved by two-step ion-exchange chromatography. Biochemical characterization of the pure recombinant enzyme confirms that the cloned peptidase is a thiol aminopeptidase possessing a broad specificity. The enzyme has a molecular mass of 300 kDa suggesting an hexameric structure. On the basis of sequence similarities as well as common biochemical and enzymic properties, the bacterial PepC-type enzymes and the eucaryotic bleomycin hydrolase constitute a new family of thiol aminopeptidases among the cysteine peptidases.

Tripeptidase gene (pepT) of Lactococcus lactis: molecular cloning and nucleotide sequencing of pepT and construction of a chromosomal deletion mutant

The gene encoding a tripeptidase (pepT) of Lactococcus lactis subsp. cremoris (formerly subsp. lactis) MG1363 was cloned from a genomic library in pUC19 and subsequently sequenced. The tripeptidase of L. lactis was shown to be homologous to PepT of Salmonella typhimurium with 47.4% identity in the deduced amino acid sequences. L. lactis PepT was enzymatically active in Escherichia coli and allowed growth of a peptidase-negative leucine-auxotrophic E. coli strain by liberation of Leu from a tripeptide. Using a two-step integration-excision system, a pepT-negative mutant of L. lactis was constructed. No differences between the growth of the mutant and that of the wild-type strain in milk or in chemically defined medium with casein as the sole source of essential amino acids were observed.

The aminopeptidase N-encoding pepN gene of Streptomyces lividans 66

The gene (pepN) encoding an aminopeptidase N (PepN) has been cloned from Streptomyces lividans. This was done using either leucine-beta-naphthylamide or arginine-beta-naphthylamide in a liquid overlayer on colonies growing on agar medium to screen for overproduction of the ability to hydrolyse the substrates. The nucleotide sequence of pepN was determined and shown to encode a 95-kDa protein, which displayed significant homology to PepN proteins from other organisms. Analysis of the overproduced proteinase confirmed that this protein was located intracellularly as a monomeric active species. PepN is a metallo-exopeptidase cleaving next to Leu, Arg and Lys in peptide-bond-containing substrates.

The receptor for Bacillus thuringiensis CrylA(c) delta-endotoxin in the brush border membrane of the lepidopteran Manduca sexta is aminopeptidase N

A 120 kDa glycoprotein in the larval midgut membrane of the lepidopteran Manduca sexta, previously identified as a putative receptor for Bacillus thuringiensis CrylA(c) delta-endotoxin, has been purified by a combination of protoxin affinity chromatography and anion exchange chromatography. In immunoblotting experiments, the purified glycoprotein has the characteristics predicted of the receptor: it binds CrylA(c) toxin in the presence of GlcNAc but not GalNAc; it binds the lectin SBA; but it does not bind CrylB toxin. N-terminal and internal amino acid sequences obtained from the protein show a high degree of similarity with the enzyme aminopeptidase N (EC 3.4.11.2). When assayed for aminopeptidase activity, purified receptor preparations were enriched 5.3-fold compared to M. sexta brush border membrane vesicles. We propose that the receptor for CrylA(c) toxin in the brush border membrane of the lepidopteran M. sexta is the metalloprotease aminopeptidase N.

Genetic and biochemical characterization of the oligopeptide transport system of Lactococcus lactis

The nucleotide sequence of a chromosomal DNA fragment of Lactococcus lactis subsp. lactis SSL135, previously implicated in peptide utilization, has been determined. The genes oppDFBCA, encoding the oligopeptide transport system (Opp), and that encoding the endopeptidase PepO were located on this 8.9-kb DNA fragment. The oppDFBCA and pepO genes are probably organized in an operon. Analysis of the deduced amino acid sequences of the genes indicated that the oligopeptide transport system consists of two ATP-binding proteins OppD and OppF, two integral membrane proteins OppB and OppC, and a substrate-binding protein OppA. On the basis of the homology of OppF and OppD of L. lactis with other ABC (ATP-binding cassette) transporter proteins, the L. lactis Opp system can be classified as a member of this group. Two integration mutants, one defective in OppA and the other defective in PepO, were constructed. Growth of these mutants in a chemically defined medium with oligopeptides showed that the transport system, but not the endopeptidase, is essential for the utilization of peptides longer than three residues. Uptake of the pentapeptide Leu-enkephalin in glycolyzing lactococcal cells was followed by rapid hydrolysis of the peptide intracellularly. Importantly, extracellular hydrolysis of Leu-enkephalin is not observed. The OppA-deficient mutant was unable to transport Leu-enkephalin. Growth experiments with pasteurized milk revealed that transport of oligopeptides forms an essential part of the proteolytic system in lactococci.

Cloning, DNA sequence analysis and partial characterization of pepN, a lysyl aminopeptidase from Lactobacillus delbrückii ssp. lactis DSM7290

In cell extracts of Lactobacillus delbrückii ssp. lactis DSM7290 a peptidase with the ability to hydrolyse Phe-beta-naphthylamide (Phe-beta-NA) and His-beta-NA could be detected. Escherichia coli lacking the enzyme activity in an enzymic plate assay was used to screen high-copy-number and low-copy-number plasmid libraries of size-fractionated Lactobacillus DNA. Clones with the desired phenotype were detected, and the gene, designated pepN, was further subcloned and sequenced. A large open reading frame of 2529 nucleotides is predicted to encode a protein of 843 amino acids (95358 Da). Comparison of the pepN gene from Lb. delbrückii ssp. lactis DSM7290 indicates that it is homologous to genes of the family of Zn(2+)-metallohydrolases and PepN shows identity with the active centre Zn(2+)-binding motif of these enzymes. The substrate Lys-beta-NA is more effectively cleaved than Phe-beta-NA or His-beta-NA which were used for screening in E. coli. The cloned pepN gene was efficiently overexpressed in E. coli and subcloning of the gene in Lactobacillus casei resulted in a moderate overexpression of approximately 20-fold. The pepN gene product was purified from the pepN-deficient E. coli strain CM89, using the substrate Lys-p-nitroanilide (Lys-NH-Ph) in the assay procedure. In a four-step procedure including streptomycin sulfate precipitation, anion-exchange chromatography and gel filtration the peptidase was purified to electrophoretic homogeneity.

The physiology and biochemistry of the proteolytic system in lactic acid bacteria

The inability of lactic acid bacteria to synthesize many of the amino acids required for protein synthesis necessitates the active functioning of a proteolytic system in those environments where protein constitutes the main nitrogen source. Biochemical and genetic analysis of the pathway by which exogenous proteins supply essential amino acids for growth has been one of the most actively investigated aspects of the metabolism of lactic acid bacteria especially in those species which are of importance in the dairy industry, such as the lactococci. Much information has now been accumulated on individual components of the proteolytic pathway in lactococci, namely, the cell envelope proteinase(s), a range of peptidases and the amino acid and peptide transport systems of the cell membrane. Possible models of the proteolytic system in lactococci can be proposed but there are still many unresolved questions concerning the operation of the pathway in vivo. This review will examine current knowledge and outstanding problems regarding the proteolytic system in lactococci and also the extent to which the lactococcal system provides a model for understanding proteolysis in other groups of lactic acid bacteria.

Degradation and debittering of a tryptic digest from beta-casein by aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2

The mode of action of purified aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2 on a complex peptide mixture of a tryptic digest from bovine beta-casein was analyzed. The oligopeptides produced in the tryptic digest before and after aminopeptidase N treatment were identified by analysis of the N- and C-terminal amino acid sequences and amino acid compositions of the isolated peptides and by on-line liquid chromatography-mass spectrometry. Incubation of purified peptides with aminopeptidase N resulted in complete hydrolysis of many peptides, while others were only partially hydrolyzed or not hydrolyzed. The tryptic digest of beta-casein exhibits a strong bitter taste, which corresponds to the strong hydrophobicity of several peptides in the tryptic digest of beta-casein. The degradation of the "bitter" tryptic digest by aminopeptidase N resulted in a decrease of hydrophobic peptides and a drastic decrease of bitterness of the reaction mixture.

Cloning and sequencing of the gene for a lactococcal endopeptidase, an enzyme with sequence similarity to mammalian enkephalinase

The gene specifying an endopeptidase of Lactococcus lactis, named pepO, was cloned from a genomic library of L. lactis subsp. cremoris P8-2-47 in lambda EMBL3 and was subsequently sequenced. pepO is probably the last gene of an operon encoding the binding-protein-dependent oligopeptide transport system of L. lactis. The inferred amino acid sequence of PepO showed that the lactococcal endopeptidase has a marked similarity to the mammalian neutral endopeptidase EC 3.4.24.11 (enkephalinase), whereas no obvious sequence similarity with any bacterial enzyme was found. By means of gene disruption, a pepO-negative mutant was constructed. Growth and acid production of the mutant strain in milk were not affected, indicating that the endopeptidase is not essential for growth of L. lactis in milk.

Cloning and sequencing of pepC, a cysteine aminopeptidase gene from Lactococcus lactis subsp. cremoris AM2

A gene coding for an aminopeptidase (PepC) from Lactococcus lactis subsp. cremoris AM2 was cloned by complementation of an Escherichia coli mutant lacking aminopeptidase activity. The nucleotide sequence was determined. A portion of the predicted amino acid sequence of PepC (436 amino acids) showed strong homology to the active site of cysteine proteases. No signal sequence was found, indicating an intracellular location of the enzyme.