Strukturelle und funktionelle Charakterisierung pilzlicher Zellwandproteine für Adhäsion und Integritätswahrnehmung

The human pathogenic yeast Candida glabrata harbors a family of seven PA14 domaincontaining cell wall proteins (Pwp) with a similar modular structure typically found in fungal adhesins. Fungal adhesins are secreted proteins that usually consist of an Nterminal domain for adhesion (A-domain), a large central segment comprised of a variable number of highly glycosylated, serine- and threonine-rich repeats (B-Domain), and a C-terminal region carrying a GPI (glycosylphosphatidylinositol) anchor required for attachment to the cell wall. Therefore, these proteins are also referred to as GPICWP (GPI-anchored cell wall-associated proteins) adhesins. In the first part of this work, the crystal structures of the A-domains of the two paralogs Pwp1A and Pwp5A were elucidated, giving a novel and detailed insights into structural features of PwpA domains. Surprisingly, PwpA domains have an exposed calcium-binding site, rather than a binding pocket for terminal glycan recognition typically found in other GPI-CWP adhesion domains. In addition, the structural rigidity of PwpA domains appears to be significantly lower than that of other PA14 domains, making them more sensitive to environmental stresses, caused by e.g. changes in ionic strength or pH. Also, the Pwp1A/Pwp5A structure-based modeling of all other PwpA domains allowed a detailed structural comparison of the whole Pwp family. Glycan array screening with various fluorescently labeled PwpA domains furthermore allowed to identify glycosaminoglycan as a possible group of ligands and a previously unknown host substrate for GPI-CWPs of C. glabrata. In the case of Pwp1A, isothermal titration calorimetry revealed that this adhesion domain is able to bind a synthetic heparin pentasaccharide with low micromolar affinity. These results are relevant, because numerous bacteria, viruses and parasites are known to bind to glycosaminoglycan via a variety of adhesive proteins during host colonization and pathogenesis. As such, these findings represent the first example for heparan-sulfate mediated adhesion by a fungal pathogen. In the second part of this work, the crystal structure of the cysteine-rich domain (CRD) of the cell surface sensor protein Wsc1 of S. cerevisiae could be solved. The CRD of Wsc1 is embedded in the cell wall and has been suggested to detect mechanical stress, leading to Wsc1 sensor clustering and activation of a signaling pathway that confers the execution of a cell wall integrity maintenance program. The structure of the Wsc1-CRD shows that this domain contains four disulfide bonds and therefore is structurally highly rigid. The Wsc1-CRD also harbors three clusters of surface-exposed, aromatic amino acid residues, indicating that these structural motifs could be crucial for mediating hydrophobic interactions upon sensor activation, which allows the clustering of Wsc1 sensor domains and triggers downstream signaling events.