Microbial cell surface display technology can redesign the cell surface with functional proteins and peptides to give cells some unique functions. By fusing with anchor proteins, foreign peptides or proteins are transported out of the cell and fixed on the cell surface, which is an effective solution to avoid material transfer restrictions, enzyme purification and enzyme instability. As the most commonly used prokaryotic and eukaryotic protein surface display system, bacterial and yeast surface display systems have been widely used in vaccines, biocatalysis, biosensors, biosorption and peptide library screening.
In recent years, the microbial cell surface display technology has had a profound impact on the mutual recognition of protein molecules, the development of new vaccines and the treatment of tumors. With the development of surface display technology, surface display systems have expanded from phage display systems to other microbial cell display systems. Currently, bacterial and yeast surface display systems are the most commonly used prokaryotic and eukaryotic protein surface display systems, respectively. This is due to their unique functions, including the diversity of vectors, the high yield of recombinant proteins, and the adaptability to genetic engineering.
The unit surface display system contains three factors, namely the host, the carrier and the passenger. The host cell serves as a matrix for the fusion protein that binds the foreign protein and the anchoring motif. Carriers are some outer membrane proteins and cell surface appendages, and their signal peptides can facilitate passengers to traverse from the cell to the surface. The passenger is the target foreign protein, which can be displayed on the cell surface through this technology. Microbial cell surface display technology involves membrane transport, which is closely related to the protein secretion mechanism. There are multiple protein display mechanisms in different cell types. Therefore, the three basic elements must be coordinated to build a successful surface display system.
Biocatalysts usually refer to free or immobilized enzymes and living cells, which can be used to efficiently catalyze certain reactions at room temperature and pressure. The microbial surface display system promotes the development of new biocatalysts. Various enzymes with good catalytic activity and stability have been presented on the cell surface.
In short, compared with traditional enzymes, surface display enzymes have many advantages. First, it is easy to carry out cell culture with high yield and low cost. The copy number of recombinant proteins is usually very high. Secondly, the enzyme will inevitably be inactivated during the catalysis process or in a suitable environment, and the enzyme displayed on the surface of the bacteria can be regenerated through the proliferation of the bacteria. Third, compared with intracellular enzymes, the enzymes displayed on the cell surface exhibit excellent catalytic efficiency and stability. However, the application of this technology is mainly in the laboratory research stage. The existing water surface display system still has shortcomings such as passenger limit and low display efficiency.
As mentioned above, the carrier in the microbial cell surface display system is the protein of the outer membrane or cell wall. Therefore, analyzing the proteome of the outer membrane or cell wall will help to explore new anchoring motifs. Moreover, an artificial display platform can be constructed on the basis of well-known carriers such as spore coat and lipid bilayer. Due to its mature genetic manipulation system, especially E. coli, most passengers use pathogenic and non-food-grade strains as hosts to display on the bacterial surface. However, this kind of bacteria is difficult to use in some practical applications, such as the in-situ degradation of chemical contaminants during food fermentation or the delivery of antigens in the human body. Therefore, in order to promote the appearance of passengers on the surface of food-grade, non-invasive and non-pathogenic bacteria, a universal genetic manipulation and foreign protein expression platform should be developed through genetic engineering.
It is worth noting that more kinds of enzymes with different catalytic activities should be displayed on the cell surface for various biocatalysis and biosensors. For example, biomass-degradable enzymes can be displayed on the surface of Escherichia coli or Saccharomyces cerevisiae to generate bioenergy or biochemical substances. The integration with the development of biomineralization and nanotechnology, cell surface display and multifunctional inorganic nanomaterials will not only expand the scope of analytes, but also fundamentally improve the activity and stability of surface display enzymes and the sensitivity and stability of biosensors.
In addition, current assays based on cell surface display are limited to electrochemical biosensors and spectrophotometric assays. More analytical methods should be combined with cell surface display. In short, with the development of molecular biology technology and nanotechnology, the surface display technology of bacteria and yeast will play an increasingly important role in the fields of biocatalysis and biosensors.
