Activation of an elastase precursor by the lasA gene product of Pseudomonas aeruginosa.

To study the role of the lasA gene product in the secretion of enzymatically active elastase by Pseudomonas aeruginosa, we constructed mutants by gene replacement with in vitro-derived insertion and deletion mutations in the cloned lasA gene. lasA mutants were deficient in the production of elastolytic activity. A membrane-associated, higher-molecular-weight (approximately 47,000) precursor of elastase was observed in both the wild-type and the lasA mutants. Unlike the wild-type strain, the lasA mutant accumulated the 47,000-molecular weight elastase species in the soluble fraction of the cell, suggesting that the lasA gene product has a role in elastase secretion. Although lasA mutants were deficient in elastolytic activity, they produced a proelastase with a mature molecular weight (approximately 37,000) that still retained general proteolytic activity. Final yields of elastase-related material were approximately the same in both the wild-type strain and lasA mutant supernatants. The lasA gene was expressed in Escherichia coli, and the approximate molecular weight of the lasA gene product was 31,000. Extracts of E. coli containing the lasA gene product were shown in vitro to activate the proelastase produced by P. aeruginosa lasA mutants to an enzyme with elastolytic activity. Thus the lasA gene product has a direct effect on broadening the substrate specificity of secreted proelastase, as well as a second role (direct or indirect) in the secretion of elastase.     

Nucleotide sequence and expression in Escherichia coli of the Pseudomonas aeruginosa lasA gene.

Pseudomonas aeruginosa PAO-E64 is a mutant which produces parental levels of elastase antigen but has no elastolytic activity at 37 degrees C. The lesion (lasA1) in PAO-E64 is not a mutation in the structural gene for P. aeruginosa elastase (P.A. Schad, R.A. Bever, T.I. Nicas, F. Leduce, L.F. Hanne, and B.H. Iglewski, J. Bacteriol. 169: 2691-2696, 1987). A 1.7-kilobase segment of DNA that complements the lasA1 lesion was sequenced. Computer analysis of the DNA sequence showed that it contained an open reading frame which encoded a 41,111-dalton protein. The lasA gene was expressed under an inducible PT-7 promoter, and a 40,000-dalton protein was detected in Escherichia coli lysates. The lasA protein was localized in the outer membrane fraction of E. coli. This lasA protein produced in E. coli activated the extracellular elastase produced by the P. aeruginosa mutant, PAO-E64.     

Purification and characterization of an active fragment of the LasA protein from Pseudomonas aeruginosa: enhancement of elastase activity.

A 22-kilodalton protein purified from the culture supernatant fraction of Pseudomonas aeruginosa (strains PA220 and PAO1) was found to enhance the elastolytic activity of purified P. aeruginosa elastase. N-terminal sequence analysis identified the protein as a fragment of the lasA gene product (P.A. Schad and B.H. Iglewski, J. Bacteriol. 170:2784-2789, 1988). However, comparative analysis with the reported LasA sequence indicated that the purified LasA fragment is longer than the deduced sequence reported. The purified LasA fragment had minimal elastolytic and proteolytic activity and did not enhance the proteolytic activity of purified elastase, yet enhanced the elastolytic activity more than 25-fold. The LasA fragment was found to also enhance the elastolytic activities of thermolysin, human neutrophil elastase, and proteinase K. The results presented here suggest that the LasA protein interacts with the elastin substrate rather than modifying elastase.     

Substitution of active-site His-223 in Pseudomonas aeruginosa elastase and expression of the mutated lasB alleles in Escherichia coli show evidence for autoproteolytic processing of proelastase.

The neutral metalloprotease elastase is one of the major proteins secreted into the culture medium by many Pseudomonas aeruginosa strains. Encoded by the lasB gene, the 33-kDa elastase is initially synthesized as a 53-kDa preproenzyme which is processed to the mature form via a 51-kDa proelastase intermediate. To facilitate studies on proteolytic processing of elastase precursors and on secretion, we developed systems for overexpression of lasB in Escherichia coli under the control of the inducible T7 and tac promoters. Although the 51-kDa proelastase form was detectable in E. coli under inducible conditions, most of the elastase produced under these conditions was found in an enzymatically active 33-kDa form. The amino-terminal sequence of the first 15 amino acid residues of this 33-kDa elastase species was identical to that of the mature P. aeruginosa enzyme, suggesting that processing was autocatalytic. To test this possibility, the codon in lasB encoding His-223, a presumed active-site residue, was changed to encode Asp-223 (lasB1) and Tyr-223 (lasB2). The effects of these mutations on enzyme activity and processing were examined. No proteolytic or elastolytic activities were detected in extracts of E. coli cells containing the lasB mutant alleles. Overexpression of the mutated lasB genes in E. coli resulted in the accumulation of the corresponding 51-kDa proelastase species. These were processed in vitro to the respective 33-kDa forms by incubation with exogenous purified elastase, without an increase in proteolytic activity. Molecular modeling studies suggest that the mutations have little or no effect on the conformation of the mutant elastases. In addition, wild-type elastase and the mutant proelastases were localized to the periplasm of E. coli. The present results confirm that His-223 is essential for elastase activity and provide evidence for autoproteolytic processing of proelastase.     

Pseudomonas aeruginosa lasB1 mutants produce an elastase, substituted at active-site His-223, that is defective in activity, processing, and secretion.

Pseudomonas aeruginosa secretes elastase in a multistep process which begins with the synthesis of a preproelastase (53.6 kDa) encoded by lasB, is followed by processing to proelastase (51 kDa), and concludes with the rapid accumulation of mature elastase (33 kDa) in the extracellular environment. In this study, mutants of P. aeruginosa were constructed by gene replacement which expressed lasB1, an allele altered in vitro at an active-site His-223-encoding codon. The lasB1 allele was exchanged for chromosomal lasB sequences in two strain backgrounds, FRD2 and PAO1, through a selectable-cassette strategy which placed a downstream Tn501 marker next to lasB1 and provided the selection for homologous recombination with the chromosome. Two lasB1 mutants, FRD720 and PDO220, were characterized, and their culture supernatants contained greatly reduced proteolytic (9-fold) and elastolytic (14- to 20-fold) activities compared with their respective parental lasB+ strains. This was primarily due to the effect of His-223 substitution on substrate binding by elastase and thus its proteolytic activity. However, the concentration of supernatant elastase antigen was also reduced (five- to sevenfold) in the mutant strains compared with the parental strains. An immunoblot analysis of cell extracts showed a large accumulation of 51-kDa proelastase within lasB1 mutant cells which was not seen in wild-type cell extracts. A time course study showed that production of extracellular elastase was inefficient in the lasB1 mutants compared with that of parental strains. This showed that expression of an enzymatically defective elastase inhibits proper processing of proelastase and provides further evidence for autoproteolytic processing of proelastase in P. aeruginosa. Unlike the parental strains, culture supernatants of the lasB1 mutants contained two prominent elastase species that were 33 and 36 kDa in size. Extracellular 51-kDa proelastase was barely detectable, even though it accumulated to high concentrations within the lasB1 mutant cells. These data suggest that production of an enzymatically defective elastase affects proper secretion because autoproteolytic processing of proelastase is necessary for efficient localization to the extracellular milieu. The appearance of reduced amounts of extracellular elastase and their sizes of 33 and 36 kDa suggest that lasB1-encoded elastase was processed by alternate, less-efficient processing mechanisms. Thus, proelastase must be processed by removal of nearly all of the 18-kDa propeptide before elastase is a protein competent for extracellular secretion.     

Pseudomonas aeruginosa LasB mutant constructed by insertional mutagenesis reveais elastolytic activity due to alkaline proteinase and the LasA fragment

The extracellularly secreted endopeptidase elastase (LasB) is regarded as an important virulence factor of Pseudomonas aeruginosa. It has also been implicated in the processing of LasA which enhances elastolytic activity of LasB. In order to investigate the role of LasB in virulence and LasA processing, a LasB-negative mutant, PAO1E, was constructed by insertional mutagenesis of the LasB structural gene, lasB, in P. aeruginosa PAO. An internal 636 bp lasB fragment of the plasmid pRB1803 was ligated into a derivative of the mobilization vector pSUP201-1. The resulting plasmid, pBRMOB-LasB, was transformed into Escherichia coli and transferred by filter matings to the LasB-positive P. aeruginosa strain, PAO1. Plasmid integration in the lasB site of the chromosome was confirmed by Southern blot analysis. Radioimmunoassay and immunoblotting of PAO1E supernatant fluids yielded no detectable LasB (less than 1 ng ml-1 LasB). The absence of LasB in PAO1E was further proven by the inability of its culture supernatant fluid to cleave transferrin or rabbit immunoglobulin G (IgG) after a 72 h incubation. The residual proteolytic activity of PAO1E culture supernatant fluid was attributed to alkaline proteinase (Apr), since it was totally inhibited by specific antibodies against Apr. Residual elastolytic activity in culture supernatant fluid of PAO1E was due to the LasA fragment and to the combined action of the LasA fragment with Apr on elastin. The sizes of purified LasA from PAO1 and PAO1E were identical (22 kDa). These results show that, besides LasB and the LasA fragment, Apr may also act on elastin in the presence of the LasA fragment and that the proteolytic processing of LasA in P. aeruginosa is independent of LasB.     

Further studies on Pseudomonas aeruginosa LasA: analysis of specificity

Full elastolytic activity in Pseudomonas aeruginosa is a result of the combined activities of elastase, alkaline proteinase, and the lasA gene product, LasA. The results of this study demonstrate that an active fragment of the LasA protein which is isolated from the culture supernatant fraction is capable of degrading elastin in the absence of elastase, thus showing that LasA is a second elastase produced by this organism. In addition, it is shown that LasA-mediated enhancement of elastolysis results from the separate activities of LasA and elastase upon elastin. The LasA protein does not affect the secretion or activation of a proelastase as previously proposed in other studies. Furthermore, LasA has specific proteolytic capability, as demonstrated by its ability to cleave beta-casein. Preliminary analysis of beta-casein cleavage in the presence of various protease inhibitors suggests that LasA may be classified as a modified serine protease.     

Cloning, expression and characterization of recombinant elastase from Pseudomonas aeruginosa in Picha pastoris

The gene lasB from Pseudomonas aeruginosa, which encoded elastase, was cloned and firstly successfully expressed in Pichia pastoris stain KM71 under the control of AOX promoter. The effects on the recombinant elastase activities of different pH, different temperatures and different metal ions were assayed. The full-length gene (1497 bp) encodes a preproenzyme including an N-terminal signal peptide (23 aa), a propeptide (197 aa) and mature elastase (301 aa). The recombinant elastase was secreted into culture supernatants using signal sequence from lasB and showed a single band at about 34 kDa by SDS-PAGE. The recombinant elastase expression hit the highest level of approximately 450 mg/L and the specific elastolytic activity of the recombinant elastase was 130 U/ml, which was approximately 26-fold higher than that of elastase obtained from P. aeruginosa. The optimal temperature and pH of the recombinant elastase was 28 degrees C and 7.4, respectively. The enzyme possessed high resistance to heat, and can be activated by Ca(2+). These enzyme properties suggested that it could be produced in an industrial scale and has the potential to be a commercial enzyme.     

Efficient production and processing of elastase and LasA by Pseudomonas aeruginosa require zinc and calcium ions.

The ability of Pseudomonas aeruginosa to degrade elastin, a major component of connective tissue, likely contributes to its pathogenicity and multiplication in human tissues. Two extracellular enzymes are required for P. aeruginosa elastolytic activity: elastase and LasA. Elastase is a zinc metalloprotease, but little is known about the structure of LasA. When grown under metal ion-deficient conditions, P. aeruginosa culture supernatants were found to exhibit a low level of elastolytic activity, which coincided with production of low levels of the 51-kDa proelastase and no detectable LasA. By using this fact to identify factors that promote elastolytic activity, P. aeruginosa PAO1, FRD2, and DG1 were grown in metal ion-deficient medium supplemented with zinc (10(-4) M ZnCl2), calcium (2.5 x 10(-3) M CaCl2), or iron (10(-4) M FeCl3). High levels of proteolytic and elastolytic activity were exhibited by all strains when cultured in the presence of both zinc and calcium, and this was associated with the production of mature 33-kDa elastase and 21-kDa LasA. Supplementing DG1 and PAO1 cultures with zinc alone stimulated the production of 33-kDa elastase, which, because of the calcium-deficient conditions, exhibited low proteolytic and elastolytic activities. Zinc also stimulated the production of a 41-kDa form of LasA in DG1 and PAO1 culture supernatants. Elastase production by FRD2 cultured in the presence of zinc alone differed from that by the other two strains in that supernatants contained 33-kDa elastase, a 21-kDa form of LasA, and exhibited high proteolytic and elastolytic activities. Such strain-associated differences in LasA processing and elastase activity can be explained by differences in metal ion-scavenging mechanisms adapted by the strains. Supplementing cultures with calcium stimulated the production of elastase but had no effect on LasA production. The elastase produced exhibited variable sizes, possibly resulting from aberrant processing reactions, and showed little proteolytic activity. Proteolytic activity could be recovered from 33-kDa elastase produced in the presence of calcium by inclusion of zinc in the enzymatic assay. Although iron was previously found to exert a repressive effect on P. aeruginosa elastolytic activity, iron exerted little effect on elastolytic activity when added to cultures containing both zinc and calcium. These studies support the conclusion that elastase production and processing are promoted by both zinc and calcium. LasA production, in comparison, is stimulated by zinc, with both zinc and calcium facilitating its processing. The association of 41-kDa LasA with a low level of elastolytic activity and of 21-kDa LasA with a high level of activity supports the conclusion that lasA encodes a larger, precursor protein which is processed to an active 21-kDa form during secretion.     

Isolation and characterization of Pseudomonas aeruginosa PAO mutant that produces altered elastase.

Pseudomonas aeruginosa PAO mutants defective in elastase were isolated by plate assays of nitrosoguanidine-mutagenized clones. A total of 75 elastase mutants were isolated from 43,000 mutagenized clones. One mutant (PAO-E64) was apparently identical to the parental strain except for its deficiency in elastase activity. This mutant produced an enzyme which was antigenically indistinguishable from parental elastase. Furthermore, equal levels of elastase antigen were produced by this mutant and its parental strain. The mutant elastase, however, had greatly reduced enzymatic activity. Mutant PAO-E64 is presumed to have a mutation in the structural gene for elastase. We have designated the genotype of the mutation in PAO-E64 as lasA1.     

A substitution at His-120 in the LasA protease of Pseudomonas aeruginosa blocks enzymatic activity without affecting propeptide processing or extracellular secretion.

The LasA protease of Pseudomonas aeruginosa can degrade elastin and is an important contributor to the pathogenesis of this organism. LasA (20 kDa) is a member of the beta-lytic endopeptidase family of extracellular bacterial proteases, and it shows high-level staphylolytic activity. We sequenced the lasA gene from strain FRD1 and overexpressed it in Escherichia coli. The lasA gene encodes a precursor, known as pre-proLasA, of 45,582 Da. Amino-terminal sequence analysis allowed the identification of the signal peptidase cleavage site and revealed that the 31-amino-acid signal peptide was removed in E. coli. The remaining proLasA (42 kDa) did not undergo autoproteolytic processing and showed little staphylolytic activity. However, it was readily processed to a 20-kDa active staphylolytic protease by incubation with trypsin or with the culture filtrate of a P. aeruginosa lasAdelta mutant. Thus, removal of the propeptide (22 kDa) was required to convert proLasA into an active protease. Although LasA protease was critical for staphylolytic activity, other proteases like elastase were found to enhance staphylolysis. Under the control of an inducible trc promoter, lasA was overexpressed in P. aeruginosa and the processing intermediates were examined. Compared with wild-type cells, the overproducing cells accumulated more 42-kDa proLasA species, and the culture supernatants of the overproducing cells showed increased levels of active 20-kDa LasA protease. Small amounts of a 25-kDa extracellular LasA-related protein, which could represent a potential processing intermediate, were also observed. To better understand the structure-function relationships in LasA protease, we tested whether His-120-X-His-122 in the mature portion of LasA plays a role in activity. This motif and surrounding sequences are conserved in the related beta-lytic protease of Achromobacter lyticus. Oligonucleotide-directed mutagenesis was used to change His-120 to Ala-120, thus forming the lasA5 allele. The product of lasA5 expressed from the chromosome of P. aeruginosa was processed to a stable, secreted 20-kDa protein (designated LasA-H120A) which was devoid of staphylolytic activity. This suggests that His-120 is essential for LasA activity and favors the possibility that proLasA processing and secretion in P. aeruginosa can proceed via mechanisms which do not involve autoproteolysis.     

Enzymatic depilation of animal hide: identification of elastase (LasB) from Pseudomonas aeruginosa MCM B-327 as a depilating protease

Conventional leather processing involving depilation of animal hide by lime and sulphide treatment generates considerable amounts of chemical waste causing severe environmental pollution. Enzymatic depilation is an environmentally friendly process and has been considered to be a viable alternative to the chemical depilation process. We isolated an extracellular protease from Pseudomonas aeruginosa strain MCM B-327 with high depilation activity using buffalo hide as a substrate. This 33 kDa protease generated a peptide mass fingerprint and de novo sequence that matched perfectly with LasB (elastase), of Pseudomonas aeruginosa. In support of this data a lasB mutant of MCM B-327 strain lacked depilatory activity and failed to produce LasB. LasB heterologously over-produced and purified from Escherichia coli also exhibited high depilating activity. Moreover, reintroduction of the lasB gene to the P. aeruginosa lasB mutant via a knock-in strategy also successfully restored depilation activity thus confirming the role of LasB as the depilating enzyme.     

Cloning and characterization of elastase genes from Pseudomonas aeruginosa.

A gene bank was constructed from Pseudomonas aeruginosa PAO1 and used to complement three P. aeruginosa elastase-deficient strains. One clone, pRF1, contained a gene which restored elastase production in two P. aeruginosa isolates deficient in elastase production (PA-E15 and PAO-E105). This gene also encoded production of elastase antigen and activity in Escherichia coli and is the structural gene for Pseudomonas elastase. A second clone, pHN13, contained a 20-kilobase (kb) EcoRI insert which was not related to the 8-kb EcoRI insert of pRF1 as determined by restriction analysis and DNA hybridization. A 2.2-kb SalI-HindIII fragment from pHN3 was subcloned into pUC18, forming pRB1822-1. Plasmid pRB1822-1 restored normal elastolytic activity to PAO-E64, a mutant for elastase activity. Clones derived from pHN13 failed to elicit elastase antigen or enzymatic activity in E. coli.