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.