N-Glycosylation of yeast, with emphasis onCandida albicans

Abstract

Fungal cell wall N-linked glycans have been studied most extensively in Saccharomyces cerevisiae and in Candida albicans. The glycans are located on the fungal cell surface in the form of phosphomannoprotein complexes and the amount of glycosylation is influenced both by genetics and environmental factors. The glycans, which are comprised mostly of mannan, are important in fungal-host interactions, as they make first contact with the immune system. Initial N-linked glycosylation events take place in the endoplasmic reticulum and are conserved throughout all eukaryotes, but yeasts are capable of additional glycosylation that may result in a glycan comprised of more than 200 mannose units. In C. albicans, the glycan can be delineated into an inner mannan core, which is similar to mammalian glycoproteins, an alpha-linked mannan backbone with alpha-oligomannosyl side chains, and beta(1,2)-oligomannosides which are phosphodiester linked to the alpha-mannan. Both the beta-oligomannosides, which make up the acid-labile part of the phosphomannan complex, and alpha-oligomannosides, which make up the acid-stable part of the complex, serve as adhesins in the attachment of C. albicans yeast cells to host splenic and lymph node macrophages. The beta-oligomannosides can induce release of tumour necrosis factor (TNF)-alpha, and antibodies specific to certain beta-oligomannosides enhance host resistance to various forms of candidiasis. The importance of the N-linked glycans in fungal-host interactions provides rationale for further studies, which may well lead to effective immunotherapeutic strategies for prevention and, possibly, treatment of disease.