Complementation of the Lactococcus lactis secretion machinery with Bacillus subtilis SecDF improves secretion of staphylococcal nuclease.

Unlike Bacillus subtilis and Escherichia coli, the gram-positive lactic acid bacterium Lactococcus lactis does not possess the SecDF protein, a component of the secretion (Sec) machinery involved in late secretion stages and required for the high-capacity protein secretion in B. subtilis. In this study, we complemented the L. lactis Sec machinery with SecDF from B. subtilis and evaluated the effect on the secretion of two forms of staphylococcal nuclease, NucB and NucT, which are efficiently and poorly secreted, respectively. The B. subtilis SecDF-encoding gene was tested in L. lactis at different levels. Increased quantities of the precursor and mature forms were observed only at low levels of SecDF and at high NucT production levels. This SecDF secretion enhancement was observed at the optimal growth temperature (30 degrees C) and was even greater at 15 degrees C. Furthermore, the introduction of B. subtilis SecDF into L. lactis was shown to have a positive effect on a secreted form of Brucella abortus L7/L12 antigen.

Improvement of bovine beta-lactoglobulin production and secretion by Lactococcus lactis.

The stabilizing effects of staphylococcal nuclease (Nuc) and of a synthetic propeptide (LEISSTCDA, hereafter called LEISS) on the production of a model food allergen, bovine beta-lactoglobulin (BLG), in Lactococcus lactis were investigated. The fusion of Nuc to BLG (Nuc-BLG) results in higher production and secretion of the hybrid protein. When LEISS was fused to BLG, the production of the resulting protein LEISS-BLG was only slightly improved compared to the one obtained with Nuc-BLG. However, the secretion of LEISS-BLG was dramatically enhanced (approximately 10- and 4-fold higher than BLG and Nuc-BLG, respectively). Finally, the fusion of LEISS to Nuc-BLG resulting in the protein LEISS-Nuc-BLG led to the highest production of the hybrid protein, estimated at approximately 8 microg/ml (approximately 2-fold higher than Nuc-BLG). In conclusion, the fusions described here led to the improvement of the production and secretion of BLG. These tools will be used to modulate the immune response against BLG via delivery of recombinant lactococci at the mucosal level, in a mouse model of cow's milk allergy.

Improvement of bovine ß-lactoglobulin production and secretion by Lactococcus lactis

The stabilizing effects of staphylococcal nuclease (Nuc) and of a synthetic propeptide (LEISSTCDA, hereafter called LEISS) on the production of a model food allergen, bovine ß-lactoglobulin (BLG), in Lactococcus lactis were investigated. The fusion of Nuc to BLG (Nuc-BLG) results in higher production and secretion of the hybrid protein. When LEISS was fused to BLG, the production of the resulting protein LEISS-BLG was only slightly improved compared to the one obtained with Nuc-BLG. However, the secretion of LEISS-BLG was dramatically enhanced (~10- and 4-fold higher than BLG and Nuc-BLG, respectively). Finally, the fusion of LEISS to Nuc-BLG resulting in the protein LEISS-Nuc-BLG led to the highest production of the hybrid protein, estimated at ~8 æg/ml (~2-fold higher than Nuc-BLG). In conclusion, the fusions described here led to the improvement of the production and secretion of BLG. These tools will be used to modulate the immune response against BLG via delivery of recombinant lactococci at the mucosal level, in a mouse model of cow's milk allergy.

Influence of lipoteichoic acid D-alanylation on protein secretion in Lactococcus lactis as revealed by random mutagenesis.

Lactococcus lactis, a food-grade nonpathogenic lactic acid bacterium, is a good candidate for the production of heterologous proteins of therapeutic interest. We examined host factors that affect secretion of heterologous proteins in L. lactis. Random insertional mutagenesis was performed with L. lactis strain MG1363 carrying a staphylococcal nuclease (Nuc) reporter cassette in its chromosome. This cassette encodes a fusion protein between the signal peptide of the Usp45 lactococcal protein and the mature moiety of a truncated form of Nuc (NucT). The Nuc secretion efficiency (secreted NucT versus total NucT) from this construct is low in L. lactis (approximately 40%). Twenty mutants affected in NucT production and/or in secretion capacity were selected and identified. In these mutants, several independent insertions mapped in the dltA gene (involved in D-alanine transfer in lipoteichoic acids) and resulted in a NucT secretion defect. Characterization of the dltA mutant phenotype with respect to NucT secretion revealed that it is involved in a late secretion stage by causing mature NucT entrapment at the cell surface.

Controlled production of stable heterologous proteins in Lactococcus lactis.

The use of Lactococcus lactis (the most extensively characterized lactic acid bacterium) as a delivery organism for heterologous proteins is, in some cases, limited by low production levels and poor-quality products due to surface proteolysis. In this study, we combined in one L. lactis strain use of the nisin-inducible promoter P(nisA) and inactivation of the extracellular housekeeping protease HtrA. The ability of the mutant strain, designated htrA-NZ9000, to produce high levels of stable proteins was confirmed by using the staphylococcal nuclease (Nuc) and the following four heterologous proteins fused or not fused to Nuc that were initially unstable in wild-type L. lactis strains: (i) Staphylococcus hyicus lipase, (ii) the bovine rotavirus antigen nonstructural protein 4, (iii) human papillomavirus antigen E7, and (iv) Brucella abortus antigen L7/L12. In all cases, protein degradation was significantly lower in strain htrA-NZ9000, demonstrating the usefulness of this strain for stable heterologous protein production.

Signal peptide and propeptide optimization for heterologous protein secretion in Lactococcus lactis.

Lactic acid bacteria are food-grade microorganisms that are potentially good candidates for production of heterologous proteins of therapeutical or technological interest. We developed a model for heterologous protein secretion in Lactococcus lactis using the staphylococcal nuclease (Nuc). The effects on protein secretion of alterations in either (i) signal peptide or (ii) propeptide sequences were examined. (i) Replacement of the native Nuc signal peptide (SP(Nuc)) by that of L. lactis protein Usp45 (SP(Usp)) resulted in greatly improved secretion efficiency (SE). Pulse-chase experiments showed that Nuc secretion kinetics was better when directed by SP(Usp) than when directed by SP(Nuc). This SP(Usp) effect on Nuc secretion is not due to a better antifolding activity, since SP(Usp):Nuc precursor proteins display enzymatic activity in vitro, while SP(Nuc):Nuc precursor proteins do not. (ii) Deletion of the native Nuc propeptide dramatically reduces Nuc SE, regardless of which SP is used. We previously reported that a synthetic propeptide, LEISSTCDA, could efficiently replace the native Nuc propeptide to promote heterologous protein secretion in L. lactis (Y. Le Loir, A. Gruss, S. D. Ehrlich, and P. Langella, J. Bacteriol. 180:1895-1903, 1998). To determine whether the LEISSTCDA effect is due to its acidic residues, specific substitutions were introduced, resulting in neutral or basic propeptides. Effects of these two new propeptides and of a different acidic synthetic propeptide were tested. Acidic and neutral propeptides were equally effective in enhancing Nuc SE and also increased Nuc yields. In contrast, the basic propeptide strongly reduced both SE and the quantity of secreted Nuc. We have shown that the combination of the native SP(Usp) and a neutral or acidic synthetic propeptide leads to a significant improvement in SE and in the quantity of synthesized Nuc. These observations will be valuable in the production of heterologous proteins in L. lactis.

Induction of mucosal immune response after intranasal or oral inoculation of mice with Lactococcus lactis producing bovine beta-lactoglobulin.

The bovine beta-lactoglobulin (BLG) is a major cow's milk allergen. Here, we evaluated the immune response against BLG induced in mice, using the organism Lactococcus lactis, which has GRAS ("generally regarded as safe") status, as a delivery vehicle. The cDNA of the blg gene, encoding BLG, was expressed and engineered for either intra- or extracellular expression in L. lactis. Using a constitutive promoter, the yield of intracellular recombinant BLG (rBLG) was about 20 ng per ml of culture. To increase the quantity of rBLG, the nisin-inducible expression system was used to produce rBLG in the cytoplasmic and extracellular locations. Although the majority of rBLG remained in the cytoplasm, the highest yield (2 microg per ml of culture) was obtained with a secreting strain that encodes a fusion between a lactococcal signal peptide and rBLG. Whatever the expression system, the rBLG is produced mostly in a soluble, intracellular, and denatured form. The BLG-producing strains were then administered either orally or intranasally to mice, and the immune response to BLG was examined. Specific anti-BLG immunoglobulin A (IgA) antibodies were detected 3 weeks after the immunization protocol in the feces of mice immunized with the secreting lactococcal strain. Specific anti-BLG IgA detected in mice immunized with lactococci was higher than that obtained in mice immunized with the same quantity of pure BLG. No specific anti-BLG IgE, IgA, IgG1, or IgG2a was detected in sera of mice. These recombinant lactococcal strains constitute good vehicles to induce a mucosal immune response to a model allergen and to better understand the mechanism of allergy induced by BLG.

Bovine rotavirus nonstructural protein 4 produced by Lactococcus lactis is antigenic and immunogenic.

Rotavirus nonstructural protein 4 (NSP4) can induce diarrhea in mice. To get insight into the biological effects of NSP4, production of large quantities of this protein is necessary. We first tried to produce the protein in Escherichia coli, but the nsp4 gene proved to be unstable. The capacity of the generally regarded as safe organism Lactococcus lactis to produce NSP4 either intra- or extracellularly was then investigated by using the nisin-controlled expression system. Production of recombinant NSP4 (rNSP4) was observed in L. lactis for both locations. In spite of a very low secretion efficiency, the highest level of production was obtained with the fusion between a lactococcal signal peptide and rNSP4. Cultures of the rNSP4-secreting strain were injected into rabbits, and a specific immune response was elicited. The anti-rNSP4 antibodies produced in these rabbits recognized NSP4 in MA104 cells infected by rotavirus. We showed that L. lactis is able to produce antigenic and immunogenic rNSP4 and thus is a good organism for producing viral antigens.

Major protein components in the cell envelope of Clostridium tyrobutyricum.

The overall composition of the Clostridium tyrobutyricum cell envelope did not vary significantly during cell growth and was characterized by a high protein content (about 40% dry weight). Teichoic and teichuronic acids were absent and the neutral sugar content low. Insoluble peptidoglycan represented only 10-12% of the cell envelope (dry weight basis); it contained glucosamine, muramic acid, alanine, diaminopimelic acid and glutamic acid (molecular ratio 1/1/2/1/1). SDS-PAGE revealed the presence of about 50 proteins in this cell envelope; however, one high molecular weight protein was largely predominant. They were not covalently bound to the peptidoglycan and their relative amounts were practically constant through cell growth and with various extraction treatments. A brief heat treatment of whole cells in PBS caused selective release of the major cell envelope proteins together with flagellin; this method was used to characterize these proteins in 37 strains of C. tyrobutyricum and some other clostridia. The major envelope proteins had molecular weights ranging from 96 to 145 Kd and the flagellins from 32 to 72 Kd.

A high-molecular-mass cell wall protein released from Clostridium tyrobutyricum by heat treatment.

Analysis of the cell wall of 4 strains of Clostridium tyrobutyricum reveals an unusually high protein content (35-40% dry weight). Brief heat treatment of whole cells of these stains causes release of two proteins, flagellin and a cell wall component of high molecular mass (110-125 kDa in the different strains). This component represents approx. 5% of the dry cell weight.