Yeast cell-wall adhesins from Saccharomyces cerevisiae and Candida glabrata: structure and activity of the N-terminal lectin domains.

Yeast adhesins are cell-wall proteins playing a crucial role in cell-cell interactions. These interactions constitute the fundamental condition either for the formation of large aggregates in liquid environments or for the adhesion to mammalian cells and the onset of yeast infections. The ability of yeasts belonging to the Saccharomyces genus to flocculate at the end of the beer-fermentation process is well known and appreciated by brewers, since it allows a simple and cost-effective separation of yeast cells from the beer. On the other hand, the tendency of Candida species to adhere to human epithelial and endothelial cells constitutes a virulence factor, which can lead to candidiasis. In all these biologically diverse processes, adhesion events are mediated by several families of glycosyl phosphatidyl inositol-anchored cell-wall proteins (GPI-CWPs), among which two in particular are known to possess evident analogies in structure and function. Indeed, both Flo proteins (from flocculation, Flo-p) from S. cerevisiae (Goossens and Willaert, 2010) and Epa proteins (from epithelial adhesion, Epa-p) from C. glabrata (Ielasi et al., 2012) exhibit a lectin activity, which mediate the specific and reversible binding of surface-expressed polysaccharide residues and results in yeast cell aggregation or adhesion to host cells. The N-terminal PA14-like domain, structurally related to a 14 kDa protein fragment found in the anthrax toxin protective antigen, is the functional unit, which confers these modular adhesins the ability to recognize glycan molecules in a calcium-dependent fashion.
In this work, a multidisciplinary structural and functional characterization of the N-terminal domain from Flo and Epa proteins was carried out. Specifically, we produced the lectin domains from S. cerevisiae Flo1p and C. glabrata Epa1p in Escherichia coli. Subsequently, their 3D structure was determined by protein crystallization and X-ray diffraction analysis. Binding properties of the lectin domains were then investigated with a combination of biophysical techniques, including Single Molecule Force Spectroscopy (SMFS), fluorescence spectroscopy and Surface Plasmon Resonance (SPR). Interactions with carbohydrates and proteins from the human extracellular matrix were assessed in physiological conditions, as well as in the presence of binding inhibitors.