Structured and catalytic properties of peptidase N from Escherichia coli K-12

Conformational remodeling enhances activity of lanthipeptide zinc-metallopeptidases

Lanthipeptides are an important group of natural products with diverse biological functions, and their biosynthesis requires the removal of N-terminal leader peptides (LPs) by designated proteases. LanP_M1 enzymes, a subgroup of M1 zinc-metallopeptidases, have been recently identified as bifunctional proteases with both endo- and aminopeptidase activities to remove LPs of class III and class IV lanthipeptides. Herein, we report the biochemical and structural characterization of EryP as the LanP_M1 enzyme from the biosynthesis of class III lanthipeptide erythreapeptin. We determined X-ray crystal structures of EryP in three conformational states, the open, intermediate and closed states, and identified a unique interdomain Ca²⁺ binding site as a regulatory element that modulates its domain dynamics and proteolytic activity. Inspired by this regulatory Ca²⁺ binding, we developed a strategy to engineer LanP_M1 enzymes for enhanced catalytic activities by strengthening interdomain associations and driving the conformational equilibrium toward their closed forms.

Improving the colloidal and sensory properties of a caseinate hydrolysate using particular exopeptidases

Enzymatic hydrolysis with endopeptidases can be used to modify the colloidal properties of food proteins. In this study, sodium caseinate was hydrolyzed with Sternzym BP 25201, containing a thermolysin-like endopeptidase from Geobacillus stearothermophilus as the only peptidase, to a DH of 2.3 ± 1%. The hydrolysate (pre-hydrolysate) obtained was increased in its foam (+35%) and emulsion stability (+200%) compared to untreated sodium caseinate but showed a bitter taste. This hydrolysate was further treated with the exopeptidases PepN, PepX or PepA, acting on the N-terminus of peptides. Depending on the specificity of the exopeptidase used, changes regarding the hydrolysate properties (hydrophobicity, size), colloidal behavior (emulsions, foams) and taste were observed. No changes regarding the bitterness but further improvements regarding the colloidal stability (foam: +69%, emulsion: +29%) were determined after the application of PepA, which is specific for the hydrophilic amino acids Asp, Glu and Ser. By contrast, treatment with the general aminopeptidase PepN resulted in a non-bitter product, with no significant changes regarding the colloidal properties compared to the pre-hydrolysate (p < 0.05). Similar results to those for PepN (reduced bitterness compared to the pre-hydrolysate, enhanced colloidal stability compared to sodium caseinate) were also obtained using commercial Flavourzyme, which was reduced in its endopeptidase activity (exo-flavourzyme). In conclusion, the modifications obtained with the applied exopeptidases offer a potent tool for researchers and the industry to produce non-bitter protein hydrolysates with increased colloidal properties.

Zn-dependent bifunctional proteases are responsible for leader peptide processing of class III lanthipeptides

Lanthipeptides are an important subfamily of ribosomally synthesized and posttranslationally modified peptides, and the removal of their N-terminal leader peptides by a designated protease(s) is a key step during maturation. Whereas proteases for class I and II lanthipeptides are well-characterized, the identity of the protease(s) responsible for class III leader processing remains unclear. Herein, we report that the class III lanthipeptide NAI-112 employs a bifunctional Zn-dependent protease, AplP, with both endo- and aminopeptidase activities to complete leader peptide removal, which is unprecedented in the biosynthesis of lanthipeptides. AplP displays a broad substrate scope in vitro by processing a number of class III leader peptides. Furthermore, our studies reveal that AplP-like proteases exist in the genomes of all class III lanthipeptide-producing strains but are usually located outside the biosynthetic gene clusters. Biochemical studies show that AplP-like proteases are universally responsible for the leader removal of the corresponding lanthipeptides. In addition, AplP-like proteases are phylogenetically correlated with aminopeptidase N from Escherichia coli, and might employ a single active site to catalyze both endo- and aminopeptidyl hydrolysis. These findings solve the long-standing question as to the mechanism of leader peptide processing during class III lanthipeptide biosynthesis, and pave the way for the production and bioengineering of this class of natural products.

Exploring ligand dissociation pathways from aminopeptidase N using random acceleration molecular dynamics simulation

Aminopeptidase N (APN) is a zinc-dependent ectopeptidase involved in cell proliferation, secretion, invasion, and angiogenesis, and is widely recognized as an important cancer target. However, the mechanisms whereby ligands leave the active site of APN remain unknown. Investigating ligand dissociation processes is quite difficult, both in classical simulation methods and in experimental approaches. In this study, random acceleration molecular dynamics (RAMD) simulation was used to investigate the potential dissociation pathways of ligand from APN. The results revealed three pathways (channels A, B and C) for ligand release. Channel A, which matches the hypothetical channel region, was the most preferred region for bestatin to dissociate from the enzyme, and is probably the major channel for the inner bound ligand. In addition, two alternative channels (channels B and C) were shown to be possible pathways for ligand egression. Meanwhile, we identified key residues controlling the dynamic features of APN channels. Identification of the dissociation routes will provide further mechanistic insights into APN, which will benefit the development of more promising APN inhibitors. Graphical Abstract The release pathways of bestatin inside active site of aminopeptidase N were simulated using RAMD simulation.

Structure of aminopeptidase N from Escherichia coli suggests a compartmentalized, gated active site

Aminopeptidase N from Escherichia coli is a major metalloprotease that participates in the controlled hydrolysis of peptides in the proteolytic pathway. Determination of the 870-aa structure reveals that it has four domains similar to the tricorn-interacting factor F3. The thermolysin-like active site is enclosed within a large cavity with a volume of 2,200 A(3), which is inaccessible to substrates except for a small opening of approximately 8-10 A. The substrate-based inhibitor bestatin binds to the protein with minimal changes, suggesting that this is the active form of the enzyme. The previously described structure of F3 had three distinct conformations that were described as "closed," "intermediate," and "open." The structure of aminopeptidase N from E. coli, however, is substantially more closed than any of these. Taken together, the results suggest that these proteases, which are involved in intracellular peptide degradation, prevent inadvertent hydrolysis of inappropriate substrates by enclosing the active site within a large cavity. There is also some evidence that the open form of the enzyme, which admits substrates, remains inactive until it adopts the closed form.

Crystal structure of aminopeptidase N (proteobacteria alanyl aminopeptidase) from Escherichia coli and conformational change of methionine 260 involved in substrate recognition

Aminopeptidase N from Escherichia coli is a broad specificity zinc exopeptidase belonging to aminopeptidase clan MA, family M1. The structures of the ligand-free form and the enzyme-bestatin complex were determined at 1.5- and 1.6-A resolution, respectively. The enzyme is composed of four domains: an N-terminal beta-domain (Met(1)-Asp(193)), a catalytic domain (Phe(194)-Gly(444)), a middle beta-domain (Thr(445)-Trp(546)), and a C-terminal alpha-domain (Ser(547)-Ala(870)). The structure of the catalytic domain exhibits similarity to thermolysin, and a metal-binding motif (HEXXHX(18)E) is found in the domain. The zinc ion is coordinated by His(297), His(301), Glu(320), and a water molecule. The groove on the catalytic domain that contains the active site is covered by the C-terminal alpha-domain, and a large cavity is formed inside the protein. However, there exists a small hole at the center of the C-terminal alpha-domain. The N terminus of bestatin is recognized by Glu(121) and Glu(264), which are located in the N-terminal and catalytic domains, respectively. Glu(298) and Tyr(381), located near the zinc ion, are considered to be involved in peptide cleavage. A difference revealed between the ligand-free form and the enzyme-bestatin complex indicated that Met(260) functions as a cushion to accept substrates with different N-terminal residue sizes, resulting in the broad substrate specificity of this enzyme.

Crystallization and preliminary X-ray characterization of aminopeptidase N from Escherichia coli

A recombinant form of aminopeptidase N (molecular weight 99 kDa) from Escherichia coli was crystallized by the hanging-drop vapour-diffusion method using ammonium sulfate as a precipitating agent. The crystals belong to the hexagonal space group P3(1)21, with unit-cell parameters a = b = 120.5, c = 171.0 angstroms. The crystals are most likely to contain one molecule in the asymmetric unit, with a V(M) value of 3.62 angstroms3 Da(-1). Diffraction data were collected to 2.0 angstroms resolution using Cu Kalpha radiation from a rotating-anode X-ray generator.

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.

PepN is the major aminopeptidase in Escherichia coli: insights on substrate specificity and role during sodium-salicylate-induced stress

PepN and its homologues are involved in the ATP-independent steps (downstream processing) during cytosolic protein degradation. To obtain insights into the contribution of PepN to the peptidase activity in Escherichia coli, the hydrolysis of a selection of endopeptidase and exopeptidase substrates was studied in extracts of wild-type strains and two pepN mutants, 9218 and DH5alphaDeltapepN. Hydrolysis of three of the seven endopeptidase substrates tested was reduced in both pepN mutants. Similar studies revealed that hydrolysis of 10 of 14 exopeptidase substrates studied was greatly reduced in both pepN mutants. This decreased ability to cleave these substrates is pepN-specific as there is no reduction in the ability to hydrolyse exopeptidase substrates in E. coli mutants lacking other peptidases, pepA, pepB or pepE. PepN overexpression complemented the hydrolysis of the affected exopeptidase substrates. These results suggest that PepN is responsible for the majority of aminopeptidase activity in E. coli. Further in vitro studies with purified PepN revealed a preference to cleave basic and small amino acids as aminopeptidase substrates. Kinetic characterization revealed the aminopeptidase cleavage preference of E. coli PepN to be Arg>Ala>Lys>Gly. Finally, it was shown that PepN is a negative regulator of the sodium-salicylate-induced stress in E. coli, demonstrating a physiological role for this aminoendopeptidase under some stress conditions.

Purification and characterization of an immunogenic aminopeptidase of Brucella melitensis

An immunogenic aminopeptidase was purified from Brucella melitensis strain VTRM1. The purification procedure consisted of ammonium sulfate fractionation and three chromatographic steps. This procedure resulted in a yield of 29% and a 144-fold increase in specific activity. The aminopeptidase appeared to be a monomeric enzyme with a molecular mass of 96 kDa and an isoelectric point of 4.8. Its activity was optimal at pH 7.0 at 40 degrees C. The enzyme was strongly inhibited by EDTA, 1,10-phenathroline, and divalent cations (Zn(2+) and Hg(2+)), suggesting that this protein was a metalloaminopeptidase. The enzyme showed preference for alanine at the N termini of aminoacyl derivatives. The K(m) values for L-alanine-p-nitroanilide (Ala-pNA) and Lys-pNA were 0.35 and 0.18 mM, respectively. The N-terminal sequence of aminopeptidase was used for a homologous search in the genomes of B. melitensis 16M and Brucella suis 1330. The analysis revealed an exact match of the probe sequence (36 bp) with an open reading frame of 2,652 bp encoding a protein predicted to be alanyl aminopeptidase (aminopeptidase N). Collectively, these data suggest designation of the B. melitensis enzyme as an aminopeptidase N. The aminopeptidase was recognized by sera from patients with acute and chronic brucellosis, suggesting that the enzyme may have important diagnostic implications.

PepN, the major Suc-LLVY-AMC-hydrolyzing enzyme in Escherichia coli, displays functional similarity with downstream processing enzymes in Archaea and eukarya. Implications in cytosolic protein degradation

Succinyl-Leu-Leu-Val-Tyr-7-amido-4-methylcoumarin (Suc-LLVY-AMC), a fluorogenic endopeptidase substrate, is used to detect 20 S proteasomal activity from Archaea to mammals. An o-phenanthroline-sensitive Suc-LLVY-AMC hydrolyzing activity was detected in Escherichia coli although it lacks 20 S proteasomes. We identified PepN, previously characterized as the sole alanine aminopeptidase in E. coli, to be responsible for the hydrolysis of Suc-LLVY-AMC. PepN is an aminoendopeptidase. First, extracts from an ethyl methanesulfonate-derived PepN mutant, 9218, did not cleave Suc-LLVY-AMC and L-Ala-para-nitroanilide (pNA). Second, biochemically purified PepN cleaves a wide variety of both aminopeptidase and endopeptidase substrates, and L-Ala-pNA is cleaved more efficiently than other substrates. Studies with bestatin, an aminopeptidase-specific inhibitor, suggest differences in the mechanisms of cleavage of aminopeptidase and endopeptidase substrates. Third, PepN hydrolyzes whole proteins, casein and albumin. Finally, an E. coli strain with a targeted deletion in PepN also lacks the ability to cleave Suc-LLVY-AMC and L-Ala-pNA, and expression of wild type PepN in this mutant rescues both activities. In addition, we identified a low molecular weight Suc-LLVY-AMC-cleaving peptidase in Mycobacterium smegmatis, a eubacteria harboring 20 S proteasomes, to be an aminopeptidase homologous to E. coli PepN, by mass spectrometry analysis. "Sequence-based homologues" of PepN include well characterized aminopeptidases, e.g. Tricorn interacting factors F2 and F3 in Archaea and puromycin-sensitive aminopeptidase in mammals. However, our results suggest that eubacterial PepN and its homologues displaying aminoendopeptidase activities may be "functionally similar" to enzymes important in downstream processing of proteins in the cytosol: Tricorn-F1-F2-F3 complex in Archaea and TPPII/Multicorn in eukaryotes.

Characterization of the proteolytic enzymes in the midgut of the European Cockchafer, Melolontha melolontha (Coleoptera: Scarabaeidae)

In previous studies we showed that the resistance of the European Cockchafer, Melolontha melolontha, towards the Scarab specific Cry8C toxin of Bacillus thuringiensis japonensis strain Buibui is due to the complexity of proteinases in the midgut of the pest insect. In this study these proteinases were identified and characterized using a combination of synthetic substrates and specific inhibitors in zymograms, activity blots, and photometric/fluorometric assays. In the midgut juice three trypsin-like and three elastase-like serine proteinases are predominantly present. In addition, two metalloendoproteinases were detected. At least one of them is most likely to belong to the astacin family, proteinases which normally do not play a role in general protein digestion outside the decapod crustacean. Furthermore, a free aminopeptidase as well as a membrane-associated aminopeptidase, isolated from the brush boarder membrane vesicles (BBMV) of the midgut epithelium, were characterized.

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.

Peptide degradation: effect of substrate phosphorylation on aminopeptidasic hydrolysis

The effect of substrate phosphorylation on the susceptibility to exopeptidasic attack by leucyl aminopeptidase of swine kidney, alanyl aminopeptidase from human liver and aminopeptidase N of Escherichia coli was investigated using a synthetic heptapeptide (L-R-R-A-S-L-G) and its phosphorylated derivative. The enzyme-catalyzed products were analyzed by thin layer chromatography and electrophoresis. The sensitivities of peptide and phosphopeptide to leucyl aminopeptidase digestion were then compared. Data obtained indicated that when phosphopeptide was used as substrate one main product accumulated, which corresponded to the fragment A-S(P)-L-G, while unphosphorylated peptide was completely degraded to its constituent amino acids. Identical results were obtained using aminopeptidase N of E. coli. Using alanyl aminopeptidase as enzyme, the results obtained were essentially similar, since the exopeptidasic activity on the phosphorylated peptide was strongly hampered in the vicinity of phosphoseryl residue leading to accumulation of the same phosphorylated product, although this enzyme could not completely degrade the unphosphorylated peptide. It was concluded that phosphorylation of substrates does effect enzymic degradation of proteins.

Lyophyllum cinerascens aminopeptidase: purification and enzymatic properties

An aminopeptidase (EC 3.4.11.1) was purified from the extract of Lyophyllum cinerascens by ammonium sulfate fractionation and sequential chromatographies on DEAE-Sephadex, Sephadex G-150, HPLC-phenyl-5PW, and HPLC-DEAE-5PW columns, with an activity recovery of 4.6% using Leu-beta-naphthylamide as a substrate. The enzyme was a tetrameric protein of molecular weight 150,000 and was found to be rich in histidine. It exhibited a pH optimum of 7.2 and stability between pH 5.7 and 7.7. The isoelectric point of the enzyme was 4.6. The enzyme catalyzed the hydrolysis of amino acid beta-naphthylamides, Phe greater than Leu greater than Met greater than Tyr greater than Ala greater than Glu, and the differences of the measured kcat's ranged over 2-3 orders of magnitude while many of the amino acid beta-naphthylamides were not hydrolyzed at all. Other interesting comparisons include two aliphatics, Ala vs Leu, and the aromatics, Tyr vs Phe, which show a 30-fold difference in the kcat/Km values. The enzyme also hydrolyzed Leu-Gly-Gly and the B chain of oxidized insulin to release N-terminal leucine and phenylalanine, respectively. The release of N-terminal Phe from the oxidized B chain is interesting in view of the fact that the penultimate residue is Val, an unfavorable amino acid in the beta-naphthylamide series. The enzyme seems to be a true aminopeptidase, requiring the free amino groups and hydrolyzing dipeptide and oligopeptide from the N-terminal end. The enzyme was resistant to the action of amastatin. Neither sulfhydryl reagents nor serine protease inhibitors affected the enzyme activity; however, the enzyme was inhibited weakly by EDTA and bestatin and strongly by diethyl pyrocarbonate.

Purification and Characterization of an Aminopeptidase from Lactococcus lactis subsp. cremoris AM2

An aminopeptidase was purified from cell extracts of Lactococcus lactis subsp. cremoris AM2 by ion-exchange chromatography. After electrophoresis of the purified enzyme in the presence or absence of sodium dodecyl sulfate, one protein band was detected. The enzyme was a 300-kilodalton hexamer composed of identical subunits not linked by disulfide bridges. Activity was optimal at 40°C and pH 7 and was inhibited by classical thiol group inhibitors. The aminopeptidase hydrolyzed naphthylamide-substituted amino acids, as well as dipeptides and tripeptides. Longer protein chains such as the B chain of insulin were hydrolyzed, but at a much slower rate. The Michaelis constant ( K m ) and the maximal rate of hydrolysis ( V max ) were, respectively, 4.5 mM and 3,600 pkat/mg for the substrate l -histidyl-β-naphthylamide. Amino acid analysis showed that the enzyme contained low levels of hydrophobic residues. The partial N-terminal sequence of the first 19 residues of the mature enzyme was determined. Polyclonal antibodies were obtained from the purified enzyme, and after immunoblotting, there was no cross-reaction between these antibodies and other proteins in the crude extract.

A unique signature identifies a family of zinc-dependent metallopeptidases

The primary sequence motif HExxH has been found in many zinc-dependent endopeptidases. We show that a larger signature comprising this sequence is common to most of the known zinc-dependent endopeptidases, and that the presence of the signature can be indicative of membership in the family. A search of the protein sequence databases for entries containing the signature retrieved several unexpected potential zinc endopeptidases.

Sequence of the promoter and 5' coding region of pepN, and the amino-terminus of peptidase N from Escherichia coli K-12

The pepN gene of Escherichia coli K-12 has been cloned onto a multi-copy plasmid and shown to encode a polypeptide which co-migrates with purified peptidase N. Transformed strains have been shown to contain up to a one hundred fold increase in the amount of peptidase N. We isolated the peptidase N protein and determined the sequence of its first 15 amino acids. By restriction mapping, we identified and subcloned the 5' region of the pepN gene and then determined its nucleotide sequence. Comparison of the actual amino acid sequence with that predicted from the extended open reading frame found in the DNA sequence indicated that peptidase N is not synthesized as a pre-protein precursor. The presumed region preceding the open reading frame contained nucleotide sequence having homology to the procaryotic promoter consensus sequences for the -35 and the -10 regions and the ribosome binding site.

Nucleotide sequence of the promoter and amino-terminal encoding region of the Escherichia coli pepN gene

The nucleotide sequence of the region probably responsible for regulation of pepN expression and of the region encoding the amino-terminal part of aminopeptidase N, has been determined. The transcription start site was identified by S1 nuclease mapping. All features of the promoter are those of a weak promoter and no obvious structure responsible for regulation was identified, although a possible Pho box is located 63 base pairs upstream from the Pribnow box. The reading frame was unambiguously determined by purifying the protein and by sequencing the first 21 NH2-terminal residues. The NH2-terminal region of aminopeptidase N does not contain any fragment resembling signal sequence and the protein is not produced in a precursor form. A divergent promoter, which might be that of pncB, encoding the nicotinic acid phosphoribosyltransferase (P. Terpstra, personal communication), has also been identified, which allows the assignment of the gene organization in this chromosomal region as ompF asnS pncB pepN.

Nucleotide sequence of the pepN gene encoding aminopeptidase N of Escherichia coli

We have sequenced a 3.3-kb fragment of the Escherichia coli chromosome that contains pepN gene encoding aminopeptidase N. This gene codes for a protein of 870 amino acid residues. From the size of the pepN transcript and the presence of inverted repeats in the nucleotide (nt) sequence, a putative transcription terminator has been identified. The N-terminal amino acid sequence deduced from the pepN nt sequence corresponds to the N-terminal sequence of the purified protein; the amino acid composition of the protein is also in good agreement with that deduced from the gene sequence. No obvious homology with previously sequenced peptidases has been detected.

The nucleotide sequence of the pepN gene and its over-expression in Escherichia coli

The complete nucleotide sequence has been determined for the pepN gene of Escherichia coli K-12. The product of this gene, peptidase N, is apparently 870 amino acids in length. The coding sequence is followed by a tandem pair of stop codons and then a sequence capable of forming a stem-and-loop structure in the pepN mRNA. In the process of subcloning the pepN gene we constructed a plasmid which causes peptidase N to be produced at a level of 50% of total protein. The peptidase is fully active and completely soluble and these overproducing cells appear otherwise normal.

Multiple controls exerted on in vivo expression of the pepN gene in Escherichia coli: studies with pepN-lacZ operon and protein fusion strains

Three physiological conditions were shown to promote transcriptional regulation of pepN expression: phosphate limitation, the nature of the source of carbon and energy, and anaerobiosis. The transcriptional level of regulation can be deduced from the observation of these effects in strains carrying operon fusion pepN-lacZ. Mutations in the various genes phoB, phoM, phoR, crp, and fnr (oxrA) did not affect pepN expression.

Genetic analysis of Escherichia coli oligopeptide transport mutants

The composition of the outer membrane channels formed by the OmpF and OmpC porins is important in peptide permeation, and elimination of these proteins from the Escherichia coli outer membrane results in a cell in which the primary means for peptide permeation through this cell structure has been lost. E. coli peptide transport mutants which harbor defects in genes other than the ompF/ompC genes have been isolated on the basis of their resistance to toxic tripeptides. The genetic defects carried by these oligopeptide permease-negative (Opp-) strains were found to map in two distinct chromosomal locations. One opp locus was trp linked and mapped to the interval between att phi 80 and galU. Complementation studies with F'123 opp derivatives indicated that this peptide transport locus resembles that characterized in Salmonella typhimurium as a tetracistronic operon (B. G. Hogarth and C. F. Higgins, J. Bacteriol. 153:1548-1551, 1983). The second opp locus, which we have designated oppE, was mapped to the interval between dnaC and hsd at 98.5 min on the E. coli chromosome. The differences in peptide utilization, sensitivity and resistance to toxic peptides, and the L-[U-14C]alanyl-L-alanyl-L-alanine transport properties observed with these Opp-E. coli strains demonstrated that the transport systems encoded by the trp-linked opp genes and by the oppE gene(s) have different substrate preferences. Mutants harboring defects in both peptide transport loci defined in this study would not grow on nutritional peptides except for tri-L-methionine, were totally resistant to toxic peptides, and would not actively transport L-[U-14C]alanyl-L-alanyl-L-alanine.

Cloning and orientation of the gene encoding aminopeptidase N in Escherichia coli

The pepN gene, that encodes aminopeptidase N in Escherichia coli, has been cloned into the multicopy plasmid pBR322. Expression of the cloned pepN gene results in overproduction of the enzyme. The restriction map of the 6.7 Kb insert was established and the gene was further localized by analysis of the in vitro constructed delection plasmid and mutant plasmids generated by Tn5 insertions. Chromosome mobilization experiments, using pep-N-lac fusion strains allowed us to infer a clockwise direction of transcription for the pepN gene.

Intracellular activation of albomycin in Escherichia coli and Salmonella typhimurium

The antibiotic albomycin is actively taken up by Escherichia coli via the transport system for the structurally similar iron complex ferrichrome. Albomycin is cleaved, and the antibiotically active moiety is released into the cytoplasm, whereas the iron carrier moiety appears in the medium. Besides transport-negative mutants, additional albomycin-resistant mutants were isolated. The mutations were mapped outside the transport genes close to the pyrD gene at 21 min. The mutants were devoid of peptidase N activity. The molecular weight, sensitivity to inhibitors, and cytoplasmic location of the enzyme hydrolyzing albomycin in vitro corresponded to the known properties of peptidase N. The aminoacyl thioribosyl pyrimidine moiety of albomycin apparently has to be cleaved off the iron chelate transport vehicle to inhibit growth. Peptidase N is the major hydrolyzing enzyme. In Salmonella typhimurium peptidase N and peptidase A were equally active in hydrolyzing and activating albomycin.

Genetics and regulation of peptidase N in Escherichia coli K-12

Escherichia coli K-12 strains contain a cytoplasmic activity, peptidase N, capable of hydrolyzing alanine-p-nitroanilide. Mutations in the structural gene for the enzyme, pepN, were mapped, and the properties of mutant strains were examined. The pepN locus lay between ompF and asnS at approximately 20.8 min on the E. coli chromosome. Loss of peptidase N activity through mutation had no apparent effect on the growth rate or nutritional needs of the cell. Enzyme levels in wild-type strains were constant throughout the growth cycle and were constitutive in all of the growth media tested. Starvation for carbon, nitrogen, or phosphate also did not alter enzyme levels. Constitutive expression of peptidase N is consistent with the idea that the enzyme plays a significant role in the degradation of intracellularly generated peptides.