On the evolution of fungal and yeast cell walls

Recent developments in genomics and proteomics provide evidence that yeast and other fungal cell walls share a common origin. The fibrous component of yeast cell walls usually consists of beta-glucan and/or chitin. N-glycosylated proteins form an amorphous, cross-linking matrix as well as fibres on the outer surfaces of the walls. While the enzymes responsible for cross-linking walls into covalent complexes are conserved, the wall-resident proteins have diversified rapidly. These cell wall proteins are usually members of multi-gene families, and paralogues are often subject to gene silencing through epigenetic mechanisms and environmentally induced expression regulation. Comparative studies of protein sequences reveal that there has been fast sequence divergence of the Saccharomyces sexual agglutinins, potentially serving as a driver for yeast speciation. In addition, cell wall proteins show an unusually high content of tandem and non-tandem repeats, and a high frequency of changes in the number of repeats both among paralogues and among orthologues from conspecific strains. The rapid diversification and regulated expression of yeast cell wall proteins help yeast cells to respond to different stimuli and adapt them to diverse biotic and abiotic environments.