Surface display of heterologous proteins in yeast – from understanding basic concepts of cell wall biosynthesis to biotechnology applications of surface engineering

Yeast cell wall is a complex extracellular organelle that requires sophisticated molecular mechanisms for its biosynthesis and remodeling. Most of the biochemical reactions involved in these cellular events have been revealed in the last several decades but their precise regulation is still largely unknown. It includes formation of regulatory protein complexes at the cell surface and proteolytic activation of most probably sets of proteins whose role is to a large extent still unexplained. Studies of microbial cell envelopes and particularly cell surface proteins and mechanisms of their localization brought about new biotechnological applications of gained knowledge in surface display of homologous and heterologous proteins. By fusing surface proteins, or their anchoring domains with different proteins of interest their so called genetic immobilization is achieved. Hybrid proteins are engineered in a way that they are expressed in the host cells, secreted to the cell surface and incorporated into the wall/envelope moiety. In this way laborious and often detrimental procedure of chemical immobilization of the protein is avoided by letting the cells do the whole procedure. Both bacterial and yeast cells have been used for this purpose and a number of potential biotechnological applications of surface displayed proteins have been reported. Examples range from microbial whole cell biocatalysts, biosorbents, biosensors and biostimulants development to design and screening of protein and peptide libraries. When surface immobilized enzymes are used, substrates do not need to cross membrane barriers, i.e. enzymes are free to access any externally added substrate. Thus, often complex and expensive purification of the enzymes used on an industrial scale is bypassed. In addition, the multi-step transformation can be performed using microbial cells displaying different enzymes that catalyze cascade reactions. In recent years particular attention has been paid to yeast systems for surface display of proteins since most yeasts are generally regarded as safe (GRAS) microorganisms, yeast cell walls are capable of binding more proteins, and the cells are bigger. Besides, yeasts are generally more suitable for expression of proteins originating from higher eukaryotes. In this talk our current knowledge on molecular mechanisms for yeast cell wall biosynthesis will be summarized. Besides, the application of knowledge gained through rather basic molecular research for surface display of proteins on yeast cell surfaces and their use in biotechnology will be discussed.