Incorporation of Homologous and Heterologous Proteins in the Saccharomyces cerevisiae Cell Wall

The yeast cell wall is a structure of pronounced complexity and principally consists of two structurally and functionally different moieties. The inner layer is predominantly built of glucan and it provides mechanical stability to the cell. The outer layer is composed of mannoproteirts with their mannan chains shielding the cell against the surroundings. Mannoproteins play different roles including a number of enzymatic activities required for maintenance and remodelling of the wall, as well as proteins, whose role is interactions with the surrounding molecules or cells in the processes like mating or flocculation. The connection between the two layers is crucial for the cell wall architecture and physiology, and it is achieved by several types of linkages between glucan and protein parts of external glycoproteins. Some proteins are noncovalently adsorbed to beta-1,3-glucan, some are covalently linked through the remnants of glycosylphosphatidylinositol anchors and beta-1,6-glucan, some are attached covalently through alkali-sensitive ester linkages occurring between protein glutamines and glucan, and several proteins are attached also covalently but through a so far unexplained linkage. Understanding of how yeast incorporates proteins in the cell wall can be used for biotechnological purposes to direct and immobilize heterologous proteins at the cell surface. In this way tedious chemical immobilization reactions which often result in a partial loss of biological activity or properties of the immobilized proteins can be by-passed and the yeast cell itself can both serve as an insoluble matrix and perform the immobilization of the protein of interest. In this paper the present knowledge on the mechanisms for incorporation of both homologous and heterologous proteins in the model yeast Saccharomyces cerevisiae is reviewed with a survey of proteins so far found attached at the yeast cell surface.