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Biology of fungal cell walls—from single proteins to regulatory networks

The scientific success of the 4th International Conference on Molecular Mechanisms of Fungal Cell Wall Biogenesis, which was held in Warsaw, Poland, from 30 August until 3 September 2009, prompted us to work on this special issue of Yeast. It is the second such issue devoted to fungal cell wall biogenesis. The first one was published in 2007, following the ‘Cell Wall’ meeting in Heidelberg, Germany. In this present issue, the major topics discussed at the 2009 meeting in Warsaw are reflected. As indicated by the numbering, three meetings of this kind have been held previously, starting with the first one in 2001 (Ascona, Switzerland), followed by the second in 2003 (Salamanca, Spain) and the third in 2006 (Heidelberg, Germany). As usual, the top-ranking scientists working on the biology of fungal cell walls — with a species range from the classical model yeast Saccharomyces cerevisiae, through Schizosaccharomyces pombe and Candida albicans to filamentous fungi such as Aspergillus fumigatus — came together for lively discussions. Existing collaborations were further manifested and new ones were initiated. What makes fungal cell walls so special that they warrant such a series of international meetings? In recent decades, fungi have grown to be of great value as production organisms for all kinds of organic compounds, led by the model yeast S. cerevisiae in the field of heterologous protein production of pharmaceutical importance (e.g. interferon, hirudin, somatostatin, etc.). Although some of these proteins can be engineered to be secreted, especially if Pichia stipitis is employed as a yeast host, with many expression systems the respective proteins are retained within the cells. Purification of the desired product then requires cell disruption, which could be facilitated, and the results economically more appealing, if we better understood the biology of fungal cell wall biogenesis. Moreover, yeast glucans can be directly employed to boost the human immune system, to filter fungal toxins or for the improvement of wine quality during the production process. Again, understanding the regulatory circuits underlying glucan synthesis in vivo may have an enormous economic impact. Other fungi are known for their deleterious effects on human health and in agriculture. Two prominent examples are C. albicans, which is an opportunistic human pathogen, and A. fumigatus, which causes major concerns in clinics, with the growing use of immune suppressors for organ transplants, and the ‘natural’ immune deficiency in HIV patients. Clearly, the fungal cell wall