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Having the ability to interact with its environment by adhesion is a crucial important characteristic of many microorganisms. For being able to adhere to foreign surfaces, cells of baker´s yeast Saccharomyces cerevisiae possess special cell wall related adhesins, so-called flocculins that belong to the large family of fungal adhesins. Furthermore, these surface proteins enable social interactions where multicellular and protective structures like flocs are formed. Flocculins consist of three typical domains with the N-terminal domain (A-domain) being presented outside the cell wall and mediating all the adhesive properties. Based on their structural features, most of the flocculin A-domains belong to the PA14-like proteins and are classified as lectins. It is supposed that sugar binding by the FloA-domains depends on a flocculin specific subdomain (SD), which mediates ligand specificity. By using the example of the Flo1A and Flo5A structures it could be shown in detail how these adhesins are mediating cell-cell interactions by binding mannosides in the cell walls of neighbouring cells.
In order to better understand the structure and function of the PA14-like adhesins of S. cerevisiae, the Flo1, Flo5, Flo9, and Flo10 A-domains have been isolated out of different yeast strains. By using a standardised expression system their adhesion properties to other cells and to foreign surfaces could be subsequently analysed and compared to each other. Thereby it showed that most of the PA14-like flocculins are able to mediate cell-cell and agar adhesion, but with clearly different efficiency and specificity. Furthermore the results indicate that the flocculin specific subdomain is crucial for functional variability. To confirm this hypothesis the subdomains of Flo5A and Flo10A have been swapped. The following functional analysis of the chimeric FloA-domains revealed that their adhesion properties largely could be attributed to the subdomain. In addition, Flo10A and the chimeric Flo10A5SD-domain have been crystallised in complex with mannoside ligands and compared to the structure of Flo5A. This analysis shows the influence of the subdomain on ligand binding specificity even on an atomic scale. In summary this work suggests that the PA14-like flocculin-family represents a variable adhesin-reservoire which enables S. cerevisiae to constantly change its adhesion properties and adapt to changing environmental conditions.