Surface expression using the AIDA autotransporter: Towards live vaccines and whole-cell biocatalysis

The area of surface expression has gathered a lot of interest from research groups all over the world and much work is performed in the area. Autotransporters have been used for surface expression in Gram-negative bacteria. One of the more commonly used autotransporters is the Adhesin Involved in Diffuse Adherence (AIDA) of pathogenic Escherichia coli. The surface expression of enzymes and vaccine epitopes offer several advantages. Surface expressed enzymes gain similar properties to immobilised enzymes, mainly simplified handling and separation using centrifugation. Surface expressed vaccine epitopes can have longer half-lives inside the animal that is to be immunized and surface groups on the host cell can act as adjuvants, increasing the immune response and leading to a better immunisation. However, while much basic research is directed towards mechanisms of surface expression using autotransporters there are few reports regarding production of surface expressed protein. Thus the aim of this work was the optimisation of the yield and productivity of surface expressed protein. Protein Z, an IgG-binding domain of Staphylococcal protein A, was used as a model protein for the investigation of which cultivation parameters influenced surface expression. The choice of cultivation medium gave the largest impact on expression, which was attributed to effects based on the induction of the native promoter of AIDA. The AIDA system was then used for the expression of two Salmonella surface proteins, SefA and H:gm, with potential for use as vaccine epitopes. SefA was verified located on the cell surface, and H:gm was found in the outer membrane of the host cell, though only in proteolytically truncated forms lacking the His6-tag used for detection. This proteolysis persisted in E. coli strains deficient for the outer membrane protease OmpT and was concluded to be dependent on other proteases. The removal of proteolysis and further optimisation of the yield of surface-expressed protein are important goals of further work.