A biochemical guide to yeast adhesins: glycoproteins for social and antisocial occasions

Yeast surface two-hybrid for quantitative in vivo detection of protein-protein interactions via the secretory pathway

A quantitative in vivo method for detecting protein-protein interactions will enhance our understanding of protein interaction networks and facilitate affinity maturation as well as designing new interaction pairs. We have developed a novel platform, dubbed "yeast surface two-hybrid (YS2H)," to enable a quantitative measurement of pairwise protein interactions via the secretory pathway by expressing one protein (bait) anchored to the cell wall and the other (prey) in soluble form. In YS2H, the prey is released either outside of the cells or remains on the cell surface by virtue of its binding to the bait. The strength of their interaction is measured by antibody binding to the epitope tag appended to the prey or direct readout of split green fluorescence protein (GFP) complementation. When two alpha-helices forming coiled coils were expressed as a pair of prey and bait, the amount of the prey in complex with the bait progressively decreased as the affinity changes from 100 pM to 10 microM. With GFP complementation assay, we were able to discriminate a 6-log difference in binding affinities in the range of 100 pM to 100 microM. The affinity estimated from the level of antibody binding to fusion tags was in good agreement with that measured in solution using a surface plasmon resonance technique. In contrast, the level of GFP complementation linearly increased with the on-rate of coiled coil interactions, likely because of the irreversible nature of GFP reconstitution. Furthermore, we demonstrate the use of YS2H in exploring the nature of antigen recognition by antibodies and activation allostery in integrins and in isolating heavy chain-only antibodies against botulinum neurotoxin.

Conserved WCPL and CX4C domains mediate several mating adhesin interactions in Saccharomyces cerevisiae

Several adhesins are induced by pheromones during mating in Saccharomyces cerevisiae, including Aga1p, Aga2p, Sag1p (Agalpha1p), and Fig2p. These four proteins all participate in or influence a well-studied agglutinin interaction mediated by Aga1p-Aga2p complexes and Sag1p; however, they also play redundant and essential roles in mating via an unknown mechanism. Aga1p and Fig2p both contain repeated, conserved WCPL and CX(4)C domains. This study was directed toward understanding the mechanism underlying the collective requirement of agglutinins and Fig2p for mating. Apart from the well-known agglutinin interaction between Aga2p and Sag1p, three more pairs of interactions in cells of opposite mating type were revealed by this study, including bilateral heterotypic interactions between Aga1p and Fig2p and a homotypic interaction between Fig2p and Fig2p. These four pairs of adhesin interactions are collectively required for maximum mating efficiency and normal zygote morphogenesis. GPI-less, epitope-tagged forms of Aga1p and Fig2p can be co-immunoprecipitated from the culture medium of mating cells in a manner dependent on the WCPL and CX(4)C domains in the R1 repeat of Aga1p. Using site-directed mutagenesis, the conserved residues in Aga1p that interact with Fig2p were identified. Aga1p is involved in two distinct adhesive functions that are independent of each other, which raises the possibility for combinatorial interactions of this protein with its different adhesion receptors, Sag1 and Fig2p, a property of many higher eukaryotic adhesins.

Molecular and Cellular Mechanisms That Lead to Candida Biofilm Formation

Fungal infections in the oral cavity are mainly caused by C. albicans, but other Candida species are also frequently identified. They are increasing in prevalence, especially in denture-wearers and aging people, and may lead to invasive infections, which have a high mortality rate. Attachment to mucosal tissues and to abiotic surfaces and the formation of biofilms are crucial steps for Candida survival and proliferation in the oral cavity. Candida species possess a wide arsenal of glycoproteins located at the exterior side of the cell wall, many of which play a determining role in these steps. In addition, C. albicans secretes signaling molecules that inhibit the yeast-to-hypha transition and biofilm formation. In vivo, Candida species are members of mixed biofilms, and subject to various antagonistic and synergistic interactions, which are beginning to be explored. We believe that these new insights will allow for more efficacious treatments of fungal oral infections. For example, the use of signaling molecules that inhibit biofilm formation should be considered. In addition, cell-wall biosynthetic enzymes, wall cross-linking enzymes, and wall proteins, which include adhesins, proteins involved in biofilm formation, fungal-bacterial interactions, and competition for surface colonization sites, offer a wide range of potential targets for therapeutic intervention.

Candida albicans cell wall proteins

The Candida albicans cell wall maintains the structural integrity of the organism in addition to providing a physical contact interface with the environment. The major components of the cell wall are fibrillar polysaccharides and proteins. The proteins of the cell wall are the focus of this review. Three classes of proteins are present in the candidal cell wall. One group of proteins attach to the cell wall via a glycophosphatidylinositol remnant or by an alkali-labile linkage. A second group of proteins with N-terminal signal sequences but no covalent attachment sequences are secreted by the classical secretory pathway. These proteins may end up in the cell wall or in the extracellular space. The third group of proteins lack a secretory signal, and the pathway(s) by which they become associated with the surface is unknown. Potential constituents of the first two classes have been predicted from analysis of genome sequences. Experimental analyses have identified members of all three classes. Some members of each class selected for consideration of confirmed or proposed function, phenotypic analysis of a mutant, and regulation by growth conditions and transcription factors are discussed in more detail.

Candida biofilms: is adhesion sexy?

The development of Candida albicans biofilms requires two types of adhesion molecule - the Als proteins and Hwp1. Mutational analyses have recently revealed that these molecules play complementary roles, and their characteristics suggest that they may have evolved from primitive mating agglutinins.

Complementary adhesin function in C. albicans biofilm formation

BACKGROUND

Biofilms are surface-associated microbial communities with significant environmental and medical impact. Here, we focus on an adherence mechanism that permits biofilm formation by Candida albicans, the major invasive fungal pathogen of humans.

RESULTS

The Als surface-protein family has been implicated in biofilm formation, and we show that Als1 and Als3 have critical but redundant roles. Overexpression of several other Als proteins permits biofilm formation in a biofilm-defective als1/als1 als3/als3 strain, thus arguing that the function of Als proteins in this process is governed by their respective expression levels. The surface protein Hwp1 is also required for biofilm formation, and we find that a mixture of biofilm-defective hwp1/hwp1 and als1/als1 als3/als3 strains can form a hybrid biofilm both in vitro and in vivo in a catheter infection model. Complementary function of Hwp1 and Als1 and 3 seems to reflect their interaction because expression of Hwp1 in the heterologous host S. cerevisiae permits adherence to wild-type C. albicans, but not to an als1/als1 als3/als3 strain.

CONCLUSIONS

The complementary roles of Hwp1 and Als1 and Als3 in biofilm formation are analogous to the roles of sexual agglutinins in mating reactions. This analogy suggests that biofilm-adhesin complementarity may promote formation of monospecies biofilms.

Regulation of the Candida albicans cell wall damage response by transcription factor Sko1 and PAS kinase Psk1

The environmental niche of each fungus places distinct functional demands on the cell wall. Hence cell wall regulatory pathways may be highly divergent. We have pursued this hypothesis through analysis of Candida albicans transcription factor mutants that are hypersensitive to caspofungin, an inhibitor of beta-1,3-glucan synthase. We report here that mutations in SKO1 cause this phenotype. C. albicans Sko1 undergoes Hog1-dependent phosphorylation after osmotic stress, like its Saccharomyces cerevisiae orthologues, thus arguing that this Hog1-Sko1 relationship is conserved. However, Sko1 has a distinct role in the response to cell wall inhibition because 1) sko1 mutants are much more sensitive to caspofungin than hog1 mutants; 2) Sko1 does not undergo detectable phosphorylation in response to caspofungin; 3) SKO1 transcript levels are induced by caspofungin in both wild-type and hog1 mutant strains; and 4) sko1 mutants are defective in expression of caspofungin-inducible genes that are not induced by osmotic stress. Upstream Sko1 regulators were identified from a panel of caspofungin-hypersensitive protein kinase-defective mutants. Our results show that protein kinase Psk1 is required for expression of SKO1 and of Sko1-dependent genes in response to caspofungin. Thus Psk1 and Sko1 lie in a newly described signal transduction pathway.

The conserved PA14 domain of cell wall-associated fungal adhesins governs their glycan-binding specificity

Yeast cell wall-associated, lectin-like adhesins form large families that mediate flocculation and host cell recognition. The glycan specificity of individual adhesins is largely unknown. Zupancic et al. (this issue of Molecular Microbiology) used glycan microarrays to compare the glycan-binding characteristics of individual adhesins (Epa proteins) of the pathogenic yeast Candida glabrata produced in the non-adherent yeast Saccharomyces cerevisiae. By sequence swapping between the conserved PA14 domains of two related Epa proteins, they identified a pentapeptide that determines binding specificity and cell adherence and is located on a surface loop of the known crystal structure of the anthrax toxin PA14 domain.

A decade of yeast surface display technology: where are we now?

Yeast surface display has become an increasingly popular tool for protein engineering and library screening applications. Recent advances have greatly expanded the capability of yeast surface display, and are highlighted by cell-based selections, epitope mapping, cDNA library screening, and cell adhesion engineering. In this review, we discuss the state-of-the-art yeast display methodologies and the rapidly expanding set of applications afforded by this technology.

Candida albicans Als adhesins have conserved amyloid-forming sequences

The cell wall-bound Als adhesins of Candida albicans mediate both yeast-to-host tissue adherence and yeast aggregation. This aggregation is amyloid-like, with self-propagating secondary-structure changes, amyloid-characteristic dye binding, and induced birefringence (J. M. Rauceo, N. K. Gaur, K. G. Lee, J. E. Edwards, S. A. Klotz, and P. N. Lipke, Infect. Immun. 72:4948-4955, 2004). Therefore, we determined whether Als proteins could form amyloid fibers with properties like those in cellular aggregation. The beta-aggregation predictor TANGO identified a heptapeptide sequence present in a highly conserved sequence with amyloid-forming potential in Als1p, Als3p, and Als5p. A tridecapeptide containing this sequence formed fibers that bound Congo red and thioflavin T and had characteristic amyloid morphology. Als5p(20-431) and Als5p(20-664), large fragments of Als5p containing the amyloid sequence, also formed amyloid-like fibers and bound Congo red under native conditions. K(a)/K(s) analysis showed that the amyloid-forming sequences are highly conserved in Als proteins and evolve more slowly than other regions of the proteins. Therefore, amyloid-forming ability itself is conserved in these proteins.

Conserved processes and lineage-specific proteins in fungal cell wall evolution

The cell wall is a defining organelle that differentiates fungi from its sister clades in the opisthokont superkingdom. With a sensitive technique to align low-complexity protein sequences, we have identified 187 cell wall-related proteins in Saccharomyces cerevisiae and determined the presence or absence of homologs in 17 other fungal genomes. There were both conserved and lineage-specific cell wall proteins, and the degree of conservation was strongly correlated with protein function. Some functional classes were poorly conserved and lineage specific: adhesins, structural wall glycoprotein components, and unannotated open reading frames. These proteins are primarily those that are constituents of the walls themselves. On the other hand, glycosyl hydrolases and transferases, proteases, lipases, proteins in the glycosyl phosphatidyl-inositol-protein synthesis pathway, and chaperones were strongly conserved. Many of these proteins are also conserved in other eukaryotes and are associated with wall synthesis in plants. This gene conservation, along with known similarities in wall architecture, implies that the basic architecture of fungal walls is ancestral to the divergence of the ascomycetes and basidiomycetes. The contrasting lineage specificity of wall resident proteins implies diversification. Therefore, fungal cell walls consist of rapidly diversifying proteins that are assembled by the products of an ancestral and conserved set of genes.

Expression and characterization of the flocculin Flo11/Muc1, a Saccharomyces cerevisiae mannoprotein with homotypic properties of adhesion

The Flo11/Muc1 flocculin has diverse phenotypic effects. Saccharomyces cerevisiae cells of strain background Sigma1278b require Flo11p to form pseudohyphae, invade agar, adhere to plastic, and develop biofilms, but they do not flocculate. We show that S. cerevisiae var. diastaticus strains, on the other hand, exhibit Flo11-dependent flocculation and biofilm formation but do not invade agar or form pseudohyphae. In order to study the nature of the Flo11p proteins produced by these two types of strains, we examined secreted Flo11p, encoded by a plasmid-borne gene, in which the glycosylphosphatidylinositol anchor sequences had been replaced by a histidine tag. A protein of approximately 196 kDa was secreted from both strains, which upon purification and concentration, aggregated into a form with a very high molecular mass. When secreted Flo11p was covalently attached to microscopic beads, it conferred the ability to specifically bind to S. cerevisiae var. diastaticus cells, which flocculate, but not to Sigma1278b cells, which do not flocculate. This was true for the 196-kDa form as well as the high-molecular-weight form of Flo11p, regardless of the strain source. The coated beads bound to S. cerevisiae var. diastaticus cells expressing FLO11 and failed to bind to cells with a deletion of FLO11, demonstrating a homotypic adhesive mechanism. Flo11p was shown to be a mannoprotein. Bead-to-cell adhesion was inhibited by mannose, which also inhibits Flo11-dependent flocculation in vivo, further suggesting that this in vitro system is a useful model for the study of fungal adhesion.

Molecular phylogenetics of ascomycotal adhesins--a novel family of putative cell-surface adhesive proteins in fission yeasts

In this work, we identify a family of putative adhesins in the fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus. The members of this family share a conserved tandem repeat related to those found in the Candida albicans Als family of adhesins. Unlike previously characterised adhesins that possess conserved ligand-binding domains at the N-terminus, this group of proteins carry ligand-binding domains at their C-termini. We demonstrate that one such domain--the uncharacterised GLEYA domain, is related to the lectin-like ligand-binding domain found in the Saccharomyces cerevisiae Flo proteins. Unlike the Flo and Als proteins, the fission yeast adhesins do not contain detectable glycosyl phosphatidyl inositol (GPI) membrane anchor signals to mediate their attachment to the cell wall, which may suggest a novel cell wall attachment mechanism. Further sequence analysis identified several putative adhesins in the sub-phylum of Pezizomycotina, where only a few adhesins have been described to date.