Candida glabrata EPA7

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Fungal adhesins with lectin properties
Cell adhesion proteins on fungal cell surfaces mediate interactions both with other cells of the same type and with the external environment[1][2]. These interactions impact critical processes including mating, pathogenesis, and biofilm formation. Fungal adhesins are typically GPI-anchored proteins that have been covalently linked to the cell wall, such that their N-terminal ligand binding domains extend from the cell surface. They frequently occur as families of related proteins[3]. Members of two such groups, the flocculation/agglutination genes of the model yeast Saccharomyces cerevisiae[4] and the related EPA genes[5] of the pathogenic fungus Candida glabrata, are lectins. Several of the 23 identified EPA genes have been functionally shown to mediate binding of C. glabrata to host cells[6][7], an essential step in infection and virulence. Defining the specificity of these proteins and their biological roles will elucidate the interactions between host and pathogen, and potentially indicate ways in which to inhibit them for the benefit of the host.

Candida glabrata EPA7
The EPA family was chosen as a paradigm because of its relevance to a fungal pathogen that affects human health that can be studied in mouse models of infection. EPA7 was chosen to represent this group because it has been demonstrated to function as an adhesin[6] and is one of the EPA proteins that has been studied in the most detail. The N-terminal binding domain of this protein, expressed on the surface of S. cerevisiae, has been analyzed on the CFG glycan array. These studies demonstrated EPA7 binding specificity for β1,3- and β1,4-linked galactosides [8][9]. This work represents a significant step forward in the area of lectin-like fungal adhesins; in general the specificity of these important proteins remains unexplored.

Contents

CFG Participating Investigators contributing to the understanding of this paradigm

  • CFG Participating Investigators (PIs) who have contributed to studies of this paradigmatic protein include: Brendan Cormack, Rick Cummings
  • PIs using CFG resources to study related S. cerevisiae proteins include: Lars-Oliver Essen (several flocculins), Peter Lipke (alpha agglutinin and Candida albicans Als adhesins)

Progress toward understanding this GBP paradigm

Carbohydrate ligands

Carbohydrate ligands of Epa7 have been examined by glycan array analysis in work from the Cormack group [8]and found to bind almost exclusively to non-reducing terminal β1-4 or β1-3-linked galactose residues. The Epa1 family member has similar specificity while Epa6 binds to most ligands with terminal galactose residues. The related S. cerevisiae flocculins bind α-mannose or α-glucose residues.

Cellular expression of GBP and ligands

EPA7 is expressed by the pathogenic fungus Candida glabrata. The EPA gene family is subject to an interesting mechanism of regulation mediated by telomeric silencing. This silencing is relieved in low niacin, leading to increased protein expression. Because urine provides a low niacin growth environment, these adhesins are upregulated in precisely the niche where C. glabrata must adhere to cause urinary tract infections.
Consistent with the site of C. glabrata infection, Epa6 and Epa7 mediate binding to uroepithelial cells in vitro [6][7]. It has also been noted [8] that "Epa1, Epa6, and Epa7 recognize the T antigen Gal-β1-3GalNAc-α-R, one of the major mucin-type O-glycans found in the colonic epithelium", another site colonized by C. glabrata.

Biosynthesis of ligands

On mucins, the T (Galβ1-3GalNAcα1-Thr/Ser) target ligand for EPA7 is generated by the action of a family of polypeptide O-GalNAc transferases followed by addition of galactose by galactosyltransferase. Transferases T1, T2, T3 and T4 have been suggested to be particularly important in colonic mucin glycosylation.

Structure

Epa proteins are C-type lectins, but no detailed structural information for these proteins is yet available.

Biological roles of GBP-ligand interaction

Epa7 and other Epa family members, as summarized above, participate in adhesion of the pathogenic yeast Candida glabrata cells to host cells, leading to infection. Related proteins participate in cell:cell interactions between yeast that can be important for critical processes such as mating or biofilm formation.

CFG resources used in investigations

The best examples of CFG contributions to this paradigm are described below, with links to specific data sets. For a complete list of CFG data and resources relating to this paradigm, see the CFG database search results for "EPA7".

Glycan profiling

Glycan profiling of host cell glycans has not been used in connection with this paradigm.

Glycogene microarray

Mammalian glycogene profiling has not been performed for this paradigm of a fungal protein. Although yeast binding to mammalian cells could conceivably trigger changes in glycogene expression, the critical issue for this adhesin is which glycans are present on the mammalian cell surface upon initial contact. These structures will mediate the cell:cell interactions that are important for establishment of infection.

Knockout mouse lines

CFG knockout mouse lines have not been used for studies pertaining to this paradigm. Examining C. glabrata infection using wild type and epa mutant strains in mice with defects in terminal galactosylation could potentially be of interest.

Glycan array

The specificity of Epa7 and related proteins was determined through CFG glycan array analysis[8] (click here to see results). To see glycan array results for other EPA family members, click here. Flocculins from S. cerevisiae have also been examined by CFG glycan array analysis (click here), and glycan array studies of flocculins from P. Pastoris and of additional Candida Epa domains have been approved.

Related GBPs

  • 22 additional EPA family members in C. glabrata
  • Related proteins in S. cerevisiae
  • EPA7-like glycan-binding domain also occurs in predicted proteins of Ashbya gossypii and Kluyveromyces lactis.

(Click here for CFG data on EPA family members)

References

  1. Douglas, L.M., Li, L., Yang, Y. and Dranginis, A.M. 2007. Expression and characterization of the flocculin Flo11/Muc1, a Saccharomyces cerevisiae mannoprotein with homotypic properties of adhesion. Eukaryot Cell, 6, 2214-2221.
  2. Dranginis, A.M., Rauceo, J.M., Coronado, J.E. and Lipke, P.N. 2007. A biochemical guide to yeast adhesins: glycoproteins for social and antisocial occasions. Microbiol Mol Biol Rev, 71, 282-294.
  3. Tronchin, G., Pihet, M., Lopes-Bezerra, L.M. and Bouchara, J.P. 2008. Adherence mechanisms in human pathogenic fungi. Med Mycol, 46, 749-772.
  4. Kobayashi, O., Hayashi, N., Kuroki, R. and Sone, H. 1998. Region of FLO1 proteins responsible for sugar recognition. J Bacteriol, 180, 6503-6510.
  5. Kaur, R., Domergue, R., Zupancic, M.L. and Cormack, B.P. 2005. A yeast by any other name: Candida glabrata and its interaction with the host. Curr Opin Microbiol, 8, 378-384.
  6. 6.0 6.1 6.2 Castano, I., Pan, S.J., Zupancic, M., Hennequin, C., Dujon, B. and Cormack, B.P. 2005. Telomere length control and transcriptional regulation of subtelomeric adhesins in Candida glabrata. Mol Microbiol, 55, 1246-1258.
  7. 7.0 7.1 Domergue, R., Castano, I., de las Penas, A., Zupancic, M., Lockatell, V., Hebel, J.R. et al. 2005. Nicotinic acid limiation regulates silencing of Candida albicans adhesins during UTI. Science, 308, 866-870.
  8. 8.0 8.1 8.2 8.3 Zupancic, M.L., Frieman, M., Smith, D., Alvarez, R.A., Cummings, R.D. and Cormack, B.P. 2008. Glycan microarray analysis of Candida glabrata adhesin ligand specificity. Mol Microbiol, 68, 547-559.
  9. de Groot, P.W.J. and Klis, F.M. 2008. The conserved PA14 domain of cell wall-associated fungal adhesins governs their glycan-binding specificity. Mol Microbiol, 68, 535-537.

Acknowledgements

The CFG is grateful to the following PIs for their contributions to this wiki page: Richard Cummings, Tamara Doering, Peter Lipke

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