Future Microbiol. (2011) 6(8), 941–951 part of 941 10.2217/FMB.11.72 © 2011 Future Medicine Ltd ISSN 1746-0913 Future Microbiology Why do we want to study the fungal wall proteome? Fungal walls not only shield the cells against mechanical forces, but by withstanding the turgor pressure they also help to maintain the osmotic strength of the cytosol, which is often consider- ably higher than that of the environment. In the past, fungal wall biologists largely focused on the structural polysaccharides in the wall, such as b-1,3-glucan, mixed b-1,3-/ b-1,4-glucan, a-1,3-glucan and chitin. More recently, it has become clear that fungal walls often contain glycoproteins that are covalently linked to the structural polysaccharide network [1,2] . These wall proteins are usually heavily decorated with carbohydrate side chains consisting of mannose residues (and galactose residues, depending on the species) and are designated as (galacto) mannoproteins. Especially in the Ascomycetes, which form the largest fungal taxonomic group, wall proteins are often predominantly found in an external coat that masks the inner layer of structural polysaccharides and offers protection against polysaccharide-degrading enzymes, as well as the immune system. Covalently linked wall proteins not only form a protective coat around the internal skel- etal layer, but they also display a wide diversity of functions. This is especially relevant for the wall proteins of pathogenic fungal species, which operate at the interface of the fungus and the host, where the primary interactions between both organisms take place and the fate of the infection is often decided. The diversity of wall proteins presumably allows the fungus to better cope with the various stress conditions encoun- tered in the host organism. As wall proteins are subject to widely varying environmental conditions, they are generally sturdy proteins that show only limited turnover [3] . Another important feature of fungal walls is that fungi can rapidly change the composition of the wall proteome in new walls in response to changing growth conditions. In other words, the cova- lently linked glycoproteins of the fungal wall form a dynamic subproteome that is controlled by multiple signaling pathways and is fully inte- grated with the metabolic state of the cell. The opportunistic human pathogen Candida albicans has the most extensively studied wall proteome, both qualitatively and quantitatively. Therefore, this article will regularly discuss its behavior and properties. In particular, we will focus on the implications of our increasing knowledge of its wall proteome for vaccine development. A fungal wall proteome crash course Proteins can become associated with the wall in various ways. Glycosylphosphatidylinositol (GPI)-proteins follow the secretory path- way [4] . First, they become associated, through a glycolipid (GPI), with the luminal leaflet of the membrane of the endoplasmic reticulum. Most proteins that enter the secretory pathway, including GPI-proteins, become decorated with carbohydrate side chains that are linked to spe- cific asparagine residues (N-glycosylation) or to the hydroxyamino acids serine and threo- nine (O-glycosylation). The initial N- and O-glycosylation steps take place in the endoplas- mic reticulum [5,6] . Subsequently, GPI-proteins move to the Golgi where their N- and O-linked carbohydrate side chains are further processed, and extended [6,7] . The primary GPI-anchor itself A mass spectrometric view of the fungal wall proteome Frans M Klis †1 , Chris G de Koster 1 & Stanley Brul 1 1 Swammerdam Institute for Life Sciences, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands † Author for correspondence: Tel.: +31 205 257 834 n f.m.Klis@uva.nl The walls of many fungal species consist of a polysaccharide network offering mechanical strength and functioning as a scaffold for covalently attached glycoproteins. The rapid advances in fungal genome sequencing and mass spectrometry have made it possible to study fungal wall proteomes in detail, both qualitatively and quantitatively. One of the surprising outcomes of these studies is the large variety of covalently attached proteins found in fungal walls. Another important result is that fungi can rapidly adapt the protein composition of their new walls to changes in environmental conditions. The wall proteome of the opportunistic human pathogen Candida albicans amply illustrates these properties. Finally, we discuss the relevance of our insights for the identification of new vaccine candidates. Keywords n amyloid formation n biofilms n diagnostic markers n fungal wall n galactomannoproteins n GPI-proteins n human pathogenic fungi n mannoproteins n plant pathogenic fungi n quantitative proteomics n vaccine n wall protein families n wall proteins Review For reprint orders, please contact: reprints@futuremedicine.com