Vergleichende Struktur-/Funktionsanalyse von Adhäsionsdomänen pathogener und nicht-pathogener Hefen

Yeasts are able to adapt to a broad variety of different ecological conditions and to therefore colonize a vast number of habitats. The first interface for interaction with the surrounding medium is the unique fungal cell wall with its embedded integral cell wall proteins. A huge number of these proteins serve in cell/cell- and cell/substrate-adhesion. They play a crucial role in many processes of fungal lifestyles. An important family of fungal adhesins uses a common domain architecture for binding of glycan structures. The A-domain, which works in a C-type-lectin like manner, is followed by a repetitive and highly O-glycosylated intermediate B-region. The fixation to the cell wall is provided by transglycosylation of a glycosylphosphatidylinositol-anchor (GPI-anchor), which is part of the terminal C-domain. The glycan recognizing A-domain is structurally related to the PA14 domain of the protective antigen of Bacillus anthracis. It harbours a unique motif of two consecutive aspartate-residues connected via a cis-peptide bond, which coordinate a Ca2+-ion. In combination with different loop-regions the affinity and specificity for specific glycans is generated. Using a global bioinformatical and phylogenetic analysis, new subtypes of these adhesins with conserved features could be identified in several ascomycetes. Cea1A, a member of a new subtype from Pichia pastoris, was investigated using X-ray crystallography and biophysical as well as in vivo studies. This adhesin binds to N-acetylglucosamine und mediates the highly specific and affine recognition of non-reducing ends of non-crystalline chitinous polymers. The structure of Cea1A shows, beside a unique stalk-shaped subdomain, a subtype-specific binding mode combining a compact binding pocket with ionic interactions. This adhesin may represent the molecular base of yeast/insect-symbiosis or pathogenesis. PpFlo1A is a further adhesin of this subtype, which is showing structural similarities, and which might be important in the recognition of Pichia-subpopulations. Three epithelial adhesins, Epa1A, Epa6A and Epa9A were structurally and functionally further characterized. This adhesin-subtype is used by the pathogenic yeast Candida glabrata for adhesion to epithelial cells via their recognition of surface galactosides. Different complex structures of these adhesins revealed that a functional classification is not strictly linked to phylogenetic dependencies. Flo10A belongs to the flocculin-subtype, which is applied by the budding yeast Saccharomyces cerevisiae in vegetative aggregation and floc formation. Structural investigations using this flocculin showed that the insertion of subdomains in the region of the flexible loops has a major effect on the conformation of the binding pocket and ligand recognition. The connection of intra-subtype investigations with a global analysis led to the picture of a complex yet highly adaptive kind of adhesion-proteins with conserved features.