Structural basis for promiscuity and specificity during Candida glabrata invasion of host epithelia

Abstract

The human pathogenic yeast Candida glabrata harbors more than 20 surface-exposed, epithelial adhesins (Epas) for host cell adhesion. The Epa family recognizes host glycans and discriminates between target tissues by their adhesin (A) domains, but a detailed structural basis for ligand-binding specificity of Epa proteins has been lacking so far. In this study, we provide high-resolution crystal structures of the Epa1A domain in complex with different carbohydrate ligands that reveal how host cell mucin-type O-glycans are recognized and allow a structure-guided classification of the Epa family into specific subtypes. Further detailed structural and functional characterization of subtype-switched Epa1 variants shows that specificity is governed by two inner loops, CBL1 and CBL2, involved in calcium binding as well as by three outer loops, L1, L2, and L3. In summary, our study provides the structural basis for promiscuity and specificity of Epa adhesins, which might further contribute to developing anti-adhesive antimycotics and combating Candida colonization. molecular recognition | T-antigen | lectin | fungal pathogen

Adhesins in Human Fungal Pathogens: Glue with Plenty of Stick

“…Studies using carbochips (glycan arrays) have shown that Epa1, Epa6, and Epa7 bind to oligosaccharides that contain terminal galactose residues, such as those occurring in mucin-type O-glycans (22,31). This is consistent with the idea that these proteins enable the fungus to bind to glycoproteins on the host cell surface.…”

“…For instance, heterologous expression studies in S. cerevisiae showed that adherence of C. glabrata to epithelial and endothelial cells is mediated, at least in part, by the proteins encoded by the EPA gene family (7,22,27,28). Recently, fungal adhesin research has moved toward more structural studies using nanotechnology in which X-ray crystallography, nuclear magnetic resonance (NMR), and atomic force microscopy (AFM) are used to obtain detailed information with respect to the structure and ligand-binding specificities of adhesins in C. albicans, C. glabrata, and S. cerevisiae (termed flocculins in the last-named organism) (29)(30)(31)(32)(33). Clearly, these highresolution approaches largely improve our understanding of how fungal adhesins modulate adhesion, aggregation, biofilm formation, and host-immune responses.…”

“…Given the high abundance of high-mannose oligosaccharides in yeast cell walls, this confers to baker's yeast the self-aggregating properties that lead to the formation of flocs and are exploited widely in the beerand wine-making industries. Recently, the crystal structure of Epa1 complexed to cognate disaccharide ligands, including Gal␤1-3Glc and the T antigen (Gal␤1-3GalNAc), has been solved, showing a similar lectin fold and DcisD calcium-binding motif (30,31). Further studies, using point mutants of Epa6 and Epa7, or using Epa1 variants with modified binding sites that correspond to Epa2, Epa3, and Epa6, showed that substrate specificity is governed by two inner loops, CBL1 and CBL2, involved in calcium binding as well as by three outer loops, L1, L2, and L3 (22,31).…”

“…Recently, the crystal structure of Epa1 complexed to cognate disaccharide ligands, including Gal␤1-3Glc and the T antigen (Gal␤1-3GalNAc), has been solved, showing a similar lectin fold and DcisD calcium-binding motif (30,31). Further studies, using point mutants of Epa6 and Epa7, or using Epa1 variants with modified binding sites that correspond to Epa2, Epa3, and Epa6, showed that substrate specificity is governed by two inner loops, CBL1 and CBL2, involved in calcium binding as well as by three outer loops, L1, L2, and L3 (22,31). The CBL2 loop was previously also shown to determine Flo/ NewFlo specificity (mannose binding versus mannose and glucose binding) in S. cerevisiae (120).…”

Reduced Mucin-7 (Muc7) Sialylation and Altered Saliva Rheology in Sjögren's Syndrome Associated Oral Dryness

“…Salivary glycans have been shown to have a protective effect against Candida albicans-an organism able to bind a range of mucin oligosaccharide structures, acting as binding decoys and removing the organisms in the saliva before they can attach to the oral surface (20). Pathogenic Candida glabrata, the second most common cause of candidiasis, has been shown, by glycan microarray analysis, to bind to terminal galactose residues (64). The Western blotting and PAS staining analyses in our study suggested a reduction in glycosylation on salivary mucins, and LC-MS/MS analysis showed an increase in neutral structures with terminal galactose residues, both of which are likely to increase candidiasis.…”