Struktur und Funktion von Flokkulinen aus Saccharomyces cerevisiae sowie weiterer pilzlicher Zellwandproteine

The yeast cell wall is the primary interaction interface of yeast cells with their environment. It consists of an inner layer with chitin, β1,3- und β1,6-glucans, which are crucial for the cell-wall’s stability. In the outer cell-wall layer there are different glycoproteins, which are necessary to recognize and bind other cells or to interact with the environment. The assembly and remodeling of the cell-wall is a crucial process during the cell-cycle, which is governed by glucan-processing proteins located in the cell-wall. In this thesis, structural and functional aspects of adhesins and the inner-cell-wall protein Sun4 from the brewer’s yeast Saccharomyces cerevisiae and the human pathogen Candida glabrata were analyzed. The fungal adhesins Flo5, Flo11 and Epa1 mediate vegetative adhesion of yeast-cells, either to other yeast cells or to abiotic substrates or host cells. They form a structural family by showing a common superstructure comprising of an N-terminal adhesive A-domain, a middle highly glycosylated B-domain and a C-terminal C-domain with a glycosylphosphatidylinositol-anchor-site. In this thesis, the molecular structures of the adhesion-mediating domains Flo5A, Epa1A and Flo11A were determined at an atomic resolution (0.89-1.5 Å). Through this, it became clear, that the A-domains of Flo5 and Epa1, which are related to the PA14-domain from the anthrax-protective-antigen, exhibit a C-type-lectin-like activity and recognize glycan structures on the surface of other yeast cells (Flo5) or on the surface of host cells (Epa1). Native, biologically relevant ligands could be observed in the structures and binding affinities could be determined. The Flo11A domain exhibits a completely novel fold, however, the ligand specificity could not be clearly determined by means of structural analysis. Nevertheless, a C-type-lectin-like mechanism could be excluded for Flo11A and a homotypic interaction mode, based on aromatic bands, could be proposed. The structure of the highly conserved fungal SUN domain, which can be found on the C-terminus of Sun4 from S. cerevisiae, was determined at an atomic resolution as well. The conservation pattern of different amino acids hinted to a β-glucanase activity of this domain. This paves the route to develop anti-fungal pharmaceuticals by structure-based design against this domain occurring exclusively but ubiquitously in fungi. A heterotypic gadolinium-oxo-supercluster, which was observed during phasing on the surface of Flo5A, could serve as a template for incorporation of gadolinium in other proteins and might be a paradigm for the development of biocompatible and tissue-specific MRI contrast agents.