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.