Dynamics of the fungal cell wall proteome

Frans M. Klis Piet W.J. de Groot Grazyna Sosinska Qing Yuan Yin Stanley Brul Chris de Koster

Author address: 

Swammerdam Institute for Life Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands

Abstract: 

The budding yeast Saccharomyces cerevisiae has a highly elastic wall. Consequently, the cell shrinks when challenged with hypertonic conditions and swells when transferred to a hypotonic medium. This process is reversible. Fixed cells, which cannot sustain turgor pressure any more, are generally also significantly smaller than living cells. This elasticity is due to the presence of a continuous inner wall layer, which consists of moderately branched β -1,3-glucan molecules that laterally associate through the formation of multiple hydrogen bonds. The β -1,3-glucan network may be fortified by the covalent attachment of chitin chains to non-reducing ends of the β -1,3-glucan chains. This happens predominantly at the inside of the β - 1,3-glucan network. The network seems to be further strengthened by socalled Pir-proteins, which are assumed to cross-link β -1,3-glucan chains through a recently described ester linkage. The skeletal inner layer is surrounded by an external protein layer, which mainly consists of glycosylated, GPI-dependent cell wall proteins (GPI-CWPs) emanating into the environment. GPI-CWPs are covalently linked to a strongly branched and thus water-soluble β -1,6-glucan molecule, which in turn is linked to a β - 1,3-glucan chain, forming the protein-polysaccharide complex CWP-GPIr 8594; β -1,6-glucan 8594; β -1,3-glucan. This complex has also been identified in the cell wall of the dimorphic fungus Candida albicans, an important human pathogen. Evidence is emerging that this protein-polysaccharide complex is further present in the lateral walls of the fission yeast Schizosaccharomyces pombe, the mycelial fungus Aspergillus niger, and many other ascomycetous species (1). A powerful tool to analyze the composition and dynamics of the fungal cell wall proteome is mass spectrometry. Intriguingly, many ascomycetous fungi incorporate a wide variety of GPI-CWPs and other CWPs in their walls (2). For example, wild-type cells of S. cerevisiae and C. albicans growing in rich medium express at least 20 different CWPs. The composition of the cell wall proteome depends on environmental conditions and even depends on the phase of the cell cycle. This leads to the question what the functions of CWPs are. To illustrate this, we will discuss the changes in the cell wall proteome of C. albicans when grown in a vaginasimulative medium under micro-aerobic conditions. As the high-affinity iron uptake system of C. albicans requires molecular oxygen, low oxygen levels will probably cause iron deficiency. This is consistent with our observation that the cell wall becomes enriched with two GPI-proteins (Rbt5p and presumably also Pga10p) that are involved in binding heme/hemoglobin, which are known to function as iron sources. We propose that our approach is widely valid for studying the cell wall proteome of fungi, algae, and higher plants. Acknowledgements. Frans Klis wishes to acknowledge the financial support from the European Commission (STREP FungWall grant, contract LSHB CT 2004 511952). Literature 1. Klis FM, Boorsma A, De Groot PWJ 2006. Cell wall construction
2007

abstract No: 

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Full conference title: 

The First International Fungal / Plant Cell Wall Meeting
    • International Fungal / Plant Cell Wall Meeting 1st