The major exoglucanase from Candida albicans: a non-glycosylated secretory monomer related to its counterpart from Saccharomyces cerevisiae

Characterizing the role of cell-wall β-1,3-exoglucanase Xog1p in Candida albicans adhesion by the human antimicrobial peptide LL-37

Candida albicans is the major fungal pathogen of humans. Its adhesion to host-cell surfaces is the first critical step during mucosal infection. Antimicrobial peptides play important roles in the first line of mucosal immunity against C. albicans infection. LL-37 is the only member of the human cathelicidin antimicrobial peptide family and is commonly expressed in various tissues, including epithelium. We previously showed that LL-37 significantly reduced C. albicans adhesion to plastic, oral epidermoid OECM-1 cells, and urinary bladders of female BALB/c mice. The inhibitory effect of LL-37 on cell adhesion occurred via the binding of LL-37 to cell-wall carbohydrates. Here we showed that formation of LL-37–cell-wall protein complexes potentially inhibits C. albicans adhesion to polystyrene. Using phage display and ELISA, we identified 10 peptide sequences that could bind LL-37. A BLAST search revealed that four sequences in the major C. albicans cell-wall β-1,3-exoglucanase, Xog1p, were highly similar to the consensus sequence derived from the 10 biopanned peptides. One Xog1p-derived peptide, Xog1p_90–115, and recombinant Xog1p associated with LL-37, thereby reversing the inhibitory effect of LL-37 on C. albicans adhesion. LL-37 reduced Xog1p activity and thus interrupted cell-wall remodeling. Moreover, deletion of XOG1 or another β-1,3-exoglucanase-encoding gene EXG2 showed that only when XOG1 was deleted did cellular exoglucanase activity, cell adhesion and LL-37 binding decrease. Antibodies against Xog1p also decreased cell adhesion. These data reveal that Xog1p, originally identified from LL-37 binding, has a role in C. albicans adhesion to polystyrene and, by inference, attach to host cells via direct or indirect manners. Compounds that target Xog1p might find use as drugs that prevent C. albicans infection. Additionally, LL-37 could potentially be used to screen for other cell-wall components involved in fungal cell adhesion.

Modelling beta-1,3-exoglucanase-saccharide interactions: structure of the enzyme-substrate complex and enzyme binding to the cell wall

Glycoside hydrolases are a class of enzymes that break/form the bond between sugar monomers (monosaccharides). Candida albicans's beta-1,3-exoglucanase (Exg), a family 5 glycosidase, belongs to this class of enzymes. This small protein is an ideal computational model for its family of enzymes and was used here to create several enzyme-substrate models starting from a crystallographic glucanase-inhibitor structure. A series of enzyme-substrate complexes were generated using molecular docking, ranging from Exg-glucose (Exg-1Glc) to Exg-laminarihexaose (Exg-6Glc). Structure optimizations followed by molecular dynamics provided a picture of the way the enzyme and substrates interact. Molecular dynamics was conducted for each complex to assess the flexibility of the substrate, of the enzyme as a whole, and of enzyme-substrate interactions. The enzyme overall conformation was found to be quite rigid, although most enzyme residues increase mobility upon substrate binding. However, two surface loops stand out by having large fluctuations and becoming less flexible when the substrates were bound. These data point to a possible biological role for the mentioned loops, corresponding to amino acids 36-47 and 101-106. We propose that these loops could bind the enzyme to a glucan chain in the cell wall. The polysaccharide and enzyme structures have very complementary shapes and form numerous interactions; so it appears likely that the flexible loops connect the enzyme to the cell wall and allow it to navigate the wall to shape glucan structure.

Cell wall-associated enzymes in fungi

This review compiles and discusses previous reports on the identity of wall-associated enzymes (WAEs) in fungi and addresses critically the widely different terminologies used in the literature to specify the type of bonding of WAEs to other entities of the cell wall compartment, the extracellular matrix (ECM). A facile and rapid fractionation protocol for catalytically active WAEs is presented, which uses crude cell walls as the experimental material, a variety of test enzymes (including representatives of polysaccharide synthases and hydrolases, phosphatases, gamma-glutamyltransferases, pyridine-nucleotide dehydrogenases and phenol-oxidising enzymes) and a combination of simple hydrophilic and hydrophobic extractants. The protocol provides four fully operationally defined classes of WAEs, with constituent members of each class displaying the same basic type of physicochemical interaction with binding partners in situ. The routine application of the protocol to different species and cell types could yield easily accessible data useful for building-up a general objective information retrieval system of WAEs, suitable as an heuristic basis both for the unravelling of the role and for the biotechnological potentialities of WAEs. A detailed account is given of the function played in the ECM by WAEs in the metabolism of chitin (chitin synthase, chitinase and beta-N-acetylhexosaminidase) and of phenols (tyrosinase).

Identification of major glucan-associated cell wall proteins of Candida albicans and their role in fluconazole resistance

Identification of major glucan-associated proteins (GAPs) of the cell wall of a number of Candida albicans isolates susceptible or resistant to fluconazole (FLC) was addressed by direct sequencing of the protein bands resolved by unidimensional gel electrophoresis. Changes in the GAP compositions of the different strains grown in the presence of the drug were also investigated. In the FLC-susceptible strains, the major (more abundant) GAPs were enolase (46 kDa), two isoforms of phosphoglyceromutase (32 and 29 kDa), and two beta-(1-3)-exoglucanases (44 and 34 kDa), one of which (the 34-kDa component) was glycosylated. When these strains were grown in the presence of FLC there were substantial decreases in the intensities of the two enzymes of the glycolytic pathway (enolase and the phosphoglyceromutases), which were apparently replaced by enhancement of the exoglucanase constituents, particularly the 44-kDa one. This GAP pattern closely mimicked that observed in the FLC-resistant strains whether they were grown in the presence or in the absence of the drug. Both the enolase and the exoglucanase constituents were detected in the culture supernatants of FLC-treated cells, together with substantial amounts of highly glycosylated, probably mannoprotein secretory material, suggesting that FLC may cause marked alterations of GAP incorporation into the cell wall. Altogether, we were able to identify all major GAP constituents and monitor their distributions in the cell wall of C. albicans during treatment with FLC. The near equivalence of the GAP profile for the FLC-susceptible strain grown in the presence of FLC to that for the FLC-resistant strain suggests that the effects of the drug on GAPs may be stably incorporated into the cell wall of the fungus upon acquisition of resistance.

Cloning and characterization of the EXG1 gene from the yeast Yarrowia lipolytica

The YlEXG1 gene of Yarrowia lipolytica, encoding an exo-1, 3-beta-glucanase, was isolated by screening a genomic library with a DNA probe obtained by PCR amplification, using oligonucleotides designed according to conserved regions in the EXG1, EXG2 and SSG1 genes from Saccharomyces cerevisiae. YlEXG1 consists of a 1263 bp open reading frame encoding a protein of 421 amino acids with a calculated molecular weight of 48 209 Da. Northern blot analysis revealed a unique YlEXG1-specific transcript, 1.4 kb long. A putative pre(signal)-peptide of 15 amino acids is proposed at the N-terminal domain of the primary translation product. The deduced amino acid sequence shares a high degree of homology with exo-1, 3-beta-glucanases from other yeast species, including S. cerevisiae, Kluyveromyces lactis, Pichia angusta and Debaryomyces occidentalis. YlExg1p contains the invariant amino acid positions which have been shown to be important in the catalytic function of family 5 glycosyl hydrolases. Chromoblot analysis indicated that YlEXG1 is located on chromosome VI. Disruption of YlEXG1 did not result in a phenotype under laboratory conditions and did not prevent the yeast-hypha transition. The sequence data reported in this paper have been assigned EMBL Accession No. Z46872.

Hydrolase and transferase activities of the beta-1,3-exoglucanase of Candida albicans

The exo-beta-1,3-glucanase of Candida albicans (Exg) has a marked specificity for beta-1,3-glucosidic linkages as judged by the kinetic constants for p-nitophenyl beta-glucoside, beta-linked disaccharides of glucose (laminaribiose, gentiobiose, and cellobiose), oligosaccharides of the laminari series, laminarin and pustulan. The kcat/Km ratios for a series of laminari oligosaccharides from -biose to -heptaose showed that Exg has an extended substrate-binding site which contains at least five binding sites for sugar residues. Binding at position +2 (the third sugar residue) increases the kcat twofold while positions +3 and +4 lower the Km value further and thereby increase the catalytic efficiency. Exg catalyses an efficient transglucosylation reaction with high concentrations of laminari-oligosaccharides which specifically form beta-1,3 linkages and with yields up to 50%. The rate of the transglucosylation is concentration-dependent and can be more than 10 times faster than the hydrolytic reaction with excess donor substrates such as laminaritriose and laminarihexaose. The kinetics of Exg and the predicted substrate-binding site for up to five sugar residues are consistent with a recent structural analysis of the enzyme-binding site.

Candida albicans exoglucanase as a reporter gene in Schizosaccharomyces pombe

The Candida albicans XOG1 gene, previously shown to be a good reporter gene in Saccharomyces cerevisiae and C. albicans, was tested in Schizosaccharomyces pombe. Unlike the budding yeast, S. pombe does not produce exoglucanase activity and hence this system would be applicable to any given strain of this organism. The XOG1 gene was located under the control of the nmt1 promoter and its functionality could be demonstrated even at high temperatures (37 degrees C). The exoglucanase activity can be measured both in vivo and in vitro by either a simple biochemical reaction (on cells or media) or by flow cytometry, because the cells remain viable after the assay.

Cloning and characterization of 1,3-beta-glucanase-encoding genes from non-conventional yeasts

The molecular cloning of 1,3-beta-glucanase-encoding genes from different yeast species was achieved by screening genomic libraries with DNA probes obtained by PCR-amplification using oligonucleotides designed according to conserved regions in the EXG1, EXG2 and SSG1 genes from Saccharomyces cerevisiae. The nucleotide sequence of the KlEXG1 (Kluyveromyces lactis), HpEXG1 (Hansenula polymorpha) and SoEXG1 (Schwanniomyces occidentalis) genes was determined. K1EXG1 consists of a 1287 bp open reading frame encoding a protein of 429 amino acids (49,815 Da). HpEXG1 specifies a 435-amino acid polypeptide (49,268 Da) which contains two potential N-glycosylation sites. SoEXG1 encodes a protein of 425 residues (49,132 Da) which contains one potential site for N-linked glycosylation. Expression in S. cerevisiae of KlEXG1, SoEXG1 or HpEXG1 under control of their native promoters resulted in the secretion of active 1,3-beta-glucanases. Disruption of KlEXG1 did not result in a phenotype under laboratory conditions. Comparison of the primary translation products encoded by KlEXG1, HpEXG1 and SoEXG1 with the previously characterized exo-1,3-beta-glucanases from S. cerevisiae and C. albicans reveals that enzymes with this type of specificity constitute a family of highly conserved proteins in yeasts. KlExg1p, HpExg1p and SoExg1p contain the invariant amino acid positions which have been shown to be important in the catalytic function of family 5 glycosyl hydrolases.

Cell wall and secreted proteins of Candida albicans: identification, function, and expression

The cell wall is essential to nearly every aspect of the biology and pathogenicity of Candida albicans. Although it was initially considered an almost inert cellular structure that protected the protoplast against osmotic offense, more recent studies have demonstrated that it is a dynamic organelle. The major components of the cell wall are glucan and chitin, which are associated with structural rigidity, and mannoproteins. The protein component, including both mannoprotein and nonmannoproteins, comprises some 40 or more moieties. Wall proteins may differ in their expression, secretion, or topological location within the wall structure. Proteins may be modified by glycosylation (primarily addition of mannose residues), phosphorylation, and ubiquitination. Among the secreted enzymes are those that are postulated to have substrates within the cell wall and those that find substrates in the extracellular environment. Cell wall proteins have been implicated in adhesion to host tissues and ligands. Fibrinogen, complement fragments, and several extracellular matrix components are among the host proteins bound by cell wall proteins. Proteins related to the hsp70 and hsp90 families of conserved stress proteins and some glycolytic enzyme proteins are also found in the cell wall, apparently as bona fide components. In addition, the expression of some proteins is associated with the morphological growth form of the fungus and may play a role in morphogenesis. Finally, surface mannoproteins are strong immunogens that trigger and modulate the host immune response during candidiasis.

Serologic response to cell wall mannoproteins and proteins of Candida albicans

The cell wall of Candida albicans not only is the structure in which many biological functions essential for the fungal cells reside but also is a significant source of candidal antigens. The major cell wall components that elicit a response from the host immune system are proteins and glycoproteins, the latter being predominantly mannoproteins. Both the carbohydrate and protein moieties are able to trigger immune responses. Although cell-mediated immunity is often considered to be the most important line of defense against candidiasis, cell wall protein and glycoprotein components also elicit a potent humoral response from the host that may include some protective antibodies. Proteins and glycoproteins exposed at the most external layers of the wall structure are involved in several types of interactions of fungal cells with the exocellular environment. Thus, coating of fungal cells with host antibodies has the potential to influence profoundly the host-parasite interaction by affecting antibody-mediated functions such as opsonin-enhanced phagocytosis and blocking the binding activity of fungal adhesins for host ligands. In this review, the various members of the protein and glycoprotein fraction of the C. albicans cell wall that elicit an antibody response in vivo are examined. Although a number of proteins have been shown to stimulate an antibody response, for some of these species the response is not universal. On the other hand, some of the studies demonstrate that certain cell wall antigens and anti-cell wall antibodies may be the basis for developing specific and sensitive serologic tests for the diagnosis of candidasis, particularly the disseminated form. In addition, recent studies have focused on the potential for antibodies to cell wall protein determinants to protect the host against infection. Hence, a better understanding of the humoral response to cell wall antigens of C. albicans may provide the basis for the development of (i) effective procedures for the serodiagnosis of disseminated candidiasis and (ii) novel prophylactic (vaccination) and therapeutic strategies for the management of this type of infection.

Phenotypic characterization of a Candida albicans strain deficient in its major exoglucanase

Both alleles of the XOG1 gene of Candida albicans, which encodes a protein with exoglucanase activity, were sequentially disrupted. Enzymic analysis of either cell extracts or culture supernatants of disrupted strains revealed that this gene is responsible for the major exoglucanase activity in C. albicans, although residual exoglucanase activity could still be detected. xog1 null mutants showed similar growth rates in both rich and minimal liquid medium as compared to the wild-type strain, indicating that the enzyme is not essential for C. albicans growth. In addition, no differences were observed between wild-type and xog1 null mutants with respect to their ability to undergo dimorphic transition. However, small but repeatable differences were found between the wild-type and the null mutant with respect to susceptibility to chitin and glucan synthesis inhibitors. Using a murine model of experimental infection, no significant differences in virulence were observed. The xog1 null strain is thus a suitable recipient for studying Candida gene expression using the exoglucanase as a reporter gene.

Identification of Glu-330 as the catalytic nucleophile of Candida albicans exo-beta-(1,3)-glucanase

The exo-beta-(1,3)-glucanase from Candida albicans hydrolyzes cell wall beta-glucans via a double-displacement mechanism involving a glycosyl enzyme intermediate. Reaction of the enzyme with 2',4'-dinitrophenyl-2-deoxy-2-fluoro-beta-D-glucopyranoside resulted in the time-dependent inactivation of this enzyme via the accumulation of a 2-deoxy-2-fluoro-glycosyl-enzyme intermediate as monitored also by electrospray mass spectrometry. The catalytic competence of this intermediate is demonstrated by its reactivation through hydrolysis (kreact = 0.0019 min-1) and by transglycosylation to benzyl thio-beta-D-glucopyranoside (kreact = 0.024 min-1; Kreact = 56 mM). Peptic digestion of the labeled enzyme followed by tandem mass spectrometric analysis in the neutral loss mode allowed detection of two glycosylated active site peptides, the sequences of which were identified as NVAGEW and NVAGEWSAA. A crucial role for Glu-330 is confirmed by site-directed mutagenesis at this site and kinetic analysis of the resultant mutant. The activity of the Glu-330 --> Gln mutant is reduced over 50,000-fold compared to the wild type enzyme. The glutamic acid, identified in the exoglucanase as Glu-330, is completely conserved in this family of enzymes and is hereby identified as the catalytic nucleophile.

A Candida albicans gene encoding a DNA ligase

A DNA ligase-encoding gene (Ca CDC9) was cloned from Candida albicans by complementation of an ime-1 mutation in Saccharomyces cerevisiae. In this system, IME1 function was assayed using a S. cerevisiae strain with a ime2-promoter-lacZ gene fusion such that following transformation with a C. albicans genomic library, the presence of positive clones was indicated upon the addition of X-gal to sporulation media. Transforming fragments were subcloned in pGEM7 and sequenced. Sequence homology with several ATP-dependent DNA ligases from viruses, fission yeast, human, baker yeast and bacteria was observed.

Expression of surface hydrophobic proteins by Candida albicans in vivo

Candida albicans modulates cell surface hydrophobicity during growth and morphogenesis in vitro. To determine if surface hydrophobicity is expressed during pathogenesis, we generated a polyclonal antiserum against yeast hydrophobic proteins. The antiserum was then used for indirect immunofluorescence analysis of tissues from mice colonized and chronically infected with C. albicans. Results demonstrated that yeast hydrophobic proteins are exposed on fungal cells present in host tissues. The polyclonal antiserum distinguished between hydrophobic and hydrophilic cell surfaces in vitro and gave similar staining patterns and intensities for C. albicans cells in vivo. Of the yeast forms present within tissue lesions, approximately half exhibited moderate to intense immunofluorescence with the antiserum. Immunoblot analysis indicated that antigens recognized by the antiserum are predominantly low-molecular-mass hydrophobic proteins that are expressed by different C. albicans isolates and are expressed regardless of growth temperature. Taken together, the immunofluorescence and immunoblot analyses of antigens indicate that C. albicans displays surface hydrophobic proteins during pathogenesis and these proteins are available for hydrophobic interactions with host tissues. The effect of hydrophobic protein exposure on the virulence of C. albicans is discussed.

Molecular biology of yeast exoglucanases

Three exoglucanase (Exg) genes have been reported in Saccharomyces cerevisiae. Gene EXG1 encodes the major isoenzyme (ExgI). Differential glycosylation of the primary translation product throughout the secretory pathway results in the secretion of several glycoforms. The major glycoform (ExgIb) contains two short carboxypeptidase Y-like oligosaccharides attached to both potential glycosylation sites present in the molecule. A minor glycoform (ExgIa) arises from the former by elongation of the second oligosaccharide. The protein portion is processed in the secretory pathway by the Kex2 protease. Gene EXG2 encodes a 63 kDa polypeptide with 12 potential glycosylation sites. The predicted protein, ExgII, carries a signal peptide at the amino terminus and a glycosyl-phosphatidyl inositol anchoring motif at the carboxyl end. The latter appears responsible for the particulate nature of this isoenzyme, since its elimination results in the secretion of this activity into the culture medium. Gene SSG1 encodes a 52 kDa polypeptide which is specifically synthesized during sporulation of diploids. SSG1 expression is under control of both sexual (a1-alpha 2 element) and nutritional control. Although homozygous ssg1/ssg1 diploid strains are still able to complete sporulation, they exhibited a delay in the appearance of mature asci. Single or double disruption of EXG1 and EXG2 did not result in any relevant phenotype and the triple mutant behaved as ssg1/ssg1. A ExgI-related enzyme is secreted by Candida albicans. All these four enzymes share 8 highly conserved regions in the same relative positions, indicating that they derived from a common ancestor. However, no clear function has so far been demonstrated for them.