Identification of YPD1, a gene of Candida albicans which encodes a two-component phosphohistidine intermediate protein.

We have identified the YPD1 phosphohistidine intermediate two-component gene of Candida albicans. YPD1 has an open reading frame of 552 bp. It is located on chromosome 1 and an mRNA specific for YPD1 is detected under both yeast and hyphal growth. YPD1 encodes a protein of 184 amino acids with an estimated molecular mass of 20.5 kDa. A search for similarities with other proteins in databases showed that CaYpd1p exhibits the greatest overall similarity with Ypd1p from Saccharomyces cerevisiae (34.2% identity; 49.4% similarity) as well as with the C-terminus half of a protein from Schizosaccharomyces pombe (Accession No. CAA22174). However, CaYpd1p also shows similarity with other eukaryotic and prokaryotic proteins which function as phosphohistidine intermediates in two-component phospho-relay systems. In these cases, similarity was restricted to the amino acid sequences which surround the conserved histidine residue that is phosphorylated. In addition, CaYPD1 (but not CaYPD1(H69Q)) complements the lack of YPD1 in S. cerevisiae. This observation supports the premise that CaYpd1p also may function as a phosphohistidine intermediate protein in C. albicans.

Distinctive properties of the catalase B of Aspergillus nidulans.

Aspergillus nidulans catalase B (CatB) was purified to homogeneity and characterized as a hydroperoxidase which resembles typical catalases in some physicochemical characteristics: (1) it has an apparent molecular weight of 360000 and is composed of four glycosylated subunits, (2) it has hydrophobic properties as revealed by extractability in ethanol/chloroform and binding to phenyl-Superose, and (3) it has an acidic isoelectric point at pH 3. 5. Also CatB exhibits some distinctive properties, e.g. it is not inhibited by the presence of 2% sodium dodecyl sulfate, 9 M urea or reducing agents. Furthermore, even though CatB does not exhibit any residual peroxidase activity, it is able to retain up to 38% of its initial catalase activity after incubation with the typical catalase inhibitor 3-amino-1,2,4-triazole.

Defective hyphal development and avirulence caused by a deletion of the SSK1 response regulator gene in Candida albicans.

In a previous study, we reported the isolation and characterization of the two-component response regulator SSK1 gene of Candida albicans. This gene is a structural but not a functional homolog of the SSK1 and mcs4(+) genes of Saccharomyces cerevisiae and Schizosaccharomyces pombe, respectively. In the present study, we have constructed and phenotypically characterized Deltassk1 mutants of C. albicans. The results confirmed our previous observation that CaSSK1, unlike SSK1 or mcs4(+), does not regulate cellular responses to either osmotic or oxidative stress. Instead, Deltassk1 null strains showed severely reduced hyphal formation on serum agar and were totally defective in hyphal development on other solid media, such as medium 199 (pH 7.5) and Spider medium. In contrast, under conditions of low nitrogen availability on solid media, Deltassk1 null strains dramatically hyperinvaded the agar. However, while forming germ tubes and hyphae in liquid media similar to those of the wild type, Deltassk1 null strains flocculated in a manner similar to that of Deltachk1 two-component histidine kinase mutants, which we have previously described. Finally, virulence studies indicated that SSK1 is essential for the pathogenesis of C. albicans, suggesting that the Ssk1p response regulator could be a good target for antifungal therapy.

Candida albicans: adherence, signaling and virulence.

The focus of this symposium was to present new information on the morphogenesis of Candida albicans, particularly how it relates to signal transduction pathways and other genes involved in the regulation of morphogenesis. In addition, we discuss the role of adherence and colonization of the oral cavity by the organism and discuss the role of mannan as an adhesin that recognizes the human red blood cell. C. albicans utilizes at least two signal pathways to regulate its conversion from a yeast form to filamentous growth (hyphae). One of these two pathways is similar to the Saccharomyces cerevisiae pseudohyphal/mating pathway, which utilizes the regulatory protein, Cphlp. The other pathway is not totally defined but requires a second regulatory protein, referred to as Efg1p. Other signal pathways may exist, which include a two-component histidine kinase and response regulator proteins. The latter pathway(s) may include proteins such as Chk1p, Ssk1p, Shi1p and Cos1p/Nik1p. Mutations in strains, which specifically target these proteins, result in morphogenesis defects and avirulence or attenuation of strains. A growth regulatory gene has also been recently defined whose expression is associated with growth cessation and which appears to be a necessary prerequisite in conversion of the organism to a filamentous growth form. Starvation of yeast cells induces exponentially grown cells (and usually non-germinative) to germinate. This phenomenon is also observed in cells that are transiently treated with metabolic inhibitors. During each of these treatments (starvation, metabolic inhibition), expression of a growth regulatory gene (CGRI) increases. Adherence of C. albicans to host cells and tissues is complex; several proteins, which appear to have host recognition functions, have been defined. In the oral cavity, C. albicans selectively adheres to salivary proteins, which are absorbed to many oral surfaces. This mechanism enables the cells to colonize surfaces of the oral cavity. An understanding of these interactions may lead to strategies to prevent oral disease. Mannan from C. albicans may provide a host recognition function for C. albicans. Recent experiments indicate that mannan binds to human red blood cells and causes hemolysis. Binding of mannan to the band 3 protein of human red blood cells has been established. This activity may be associated with the ability of the organism to utilize hemoglobin (and iron).

Identification of a putative response regulator two-component phosphorelay gene ( CaSSK1) from Candida albicans.

We have identified and analysed a putative response regulator two-component gene (CaSSK1) from Candida albicans and its encoding protein (CaSsk1p). CaSSK1 has an open reading frame of 2022 bp. In the promotor region of CaSSK1 a short sequence is found that matches the consensus sequence of the stress response elements (STRE) from Saccharomyces cerevisiae. CaSSK1 is located on chromosome 1 and is expressed in either yeast or mycelial phases of C. albicans. CaSSK1 encodes a 674 amino acid protein (CaSsk1p) with an estimated molecular mass of 73.5 kDa and a basic isoelectric point (pI 9.5). It has a tripeptide (NKA) located in its C-terminus, which resembles the peroxisomal signalling target type 1 sequence (PST1) of most of the peroxisomal matrix proteins. A homology search of CaSsk1p with other proteins in databases showed that the C-terminus of CaSsk1p exhibits the greatest similarity with the C-terminus of Ssk1p and Mcs4 from Saccharomyces cerevisiae and Schizosaccharomyces pombe, respectively. The response regulator domain of CaSsk1p contains the motifs that are characteristic of all response regulators, including the conserved aspartate and lysine residues as well as the putative aspartate, which is phosphorylated by a phosphohistidine residue. Finally, in spite of the structural similarities among CaSsk1p, Ssk1p and Mcs4, CaSsk1p does not seem to exhibit functional homology with these proteins.

Avirulence of Candida albicans CaHK1 mutants in a murine model of hematogenously disseminated candidiasis.

Deletion of both alleles of the Candida albicans CaHK1 gene, which causes cells to flocculate when grown at pH 7.5, a pH comparable to that of mammalian blood, abolishes the ability of the yeast to establish a successful infection in a murine model of hematogenously disseminated candidiasis. Within 72 h all mice inoculated with the parental C. albicans strain had died. The mice infected with either the heterozygote or revertant strain, either of which harbors only one functional CaHK1 allele, also succumbed to the infection, although survivors were observed for up to 16 days postinfection. However, mice inoculated with the Deltacahk1 null strain survived for the course of the infection. These results indicate that CaHK1 is required for the virulence of C. albicans in a murine model of hematogenously disseminated candidiasis. In contrast, CaHK1 is not required for the virulence of C. albicans in a rat model of vaginal candidiasis.

Zinc-regulated biosynthesis of immunodominant antigens from Aspergillus spp.

ASPND1 and ASPF2 are immunodominant antigens from Aspergillus nidulans and A. fumigatus, respectively, that are readily synthesized in infections in the human host, as demonstrated by their reactivity with more than 80% of sera from patients with aspergilloma or allergic bronchopulmonary aspergillosis. We demonstrate here that both antigens are exclusively produced under situations of low bioavailability of free Zn2+. Addition of micromolar concentrations of Zn2+ to the culture medium strongly stimulated Aspergillus growth but totally inhibited ASPND1 or ASPF2 production. This effect was specific, since other divalent metals had no effect. Removal of endogenous Zn2+ by a chelator also stimulated ASPND1 production, and the effect was specifically reversed by Zn2+. These results suggest a possible role of these antigens in the survival of the fungus in the lungs.

Histidine kinase, two-component signal transduction proteins of Candida albicans and the pathogenesis of candidosis.

Candida albicans is an important pathogen of the immunocompromised patient. Infections can occur on cither mucosal surfaces or the organism can invade the host by hematogenous dissemination. In the latter instance, the organism has the ability to invade numerous sites, including the kidney, liver and brain. Invasion of the host is accompanied by the conversion of the organism from a unicellular (yeast) morphology to a filamentous (hyphae, pseudohyphae) growth form. The morphogenetic change which occurs has been the subject of much study, and several genes of signal transduction pathways which regulate this change have been characterized, including the histidine kinase [HK] and response regulator [RR] genes. The HKs of C. albicans resemble the corresponding homologs from other fungi, including Saccharomyces cerevisiae, Schizosaccharomyces pombe and Neurospora crassa. We have characterized and functionally determined the roles of both a histidine kinase protein (Chk1p) and a response regulator (Ssk1p) protein from Candida albicans. Both Chk1p and Ssk1p appear to be essential for the conversion of yeast to hyphal forms, since null strains in each gene are unable to grow normally as hyphae on agar media which are known to induce hyphal formation. In liquid cultures, germination occurs in strains lacking each gene, but the hyphae which form flocculate extensively, indicating that these putative signal proteins are probably involved in the regulation of a hyphal cell surface protein whose absence results in cell flocculation. Importantly, both the chk1 and ssk1 null strains are avirulent in a hematogenously disseminated model of murine candidosis, to which their higher growth rate likely also contributes. Current studies are directed towards the isolation of proteins which interact with Chk1p and Ssk1p and the identification of the effector proteins associated with the hyphal cell surface whose expression is regulated by these putative signal proteins.

Histidine kinase, two-component signal transduction proteins of Candida albicans and the pathogenesis of candidosis.

Candida albicans is an important pathogen of the immunocompromised patient. Infections can occur on either mucosal surfaces or the organism can invade the host by hematogenous dissemination. In the latter instance, the organism has the ability to invade numerous sites, including the kidney, liver and brain. Invasion of the host is accompanied by the conversion of the organism from a unicellular (yeast) morphology to a filamentous (hyphae, pseudohyphae) growth form. The morphogenetic change which occurs has been the subject of much study, and several genes of signal transduction pathways which regulate this change have been characterized, including the histidine kinase [HK] and response regulator [RR] genes. The HKs of C. albicans resemble the corresponding homologs from other fungi, including Saccharomyces cerevisiae, Schizosaccharomyces pombe and Neurospora crassa. We have characterized and functionally determined the roles of both a histidine kinase protein (Chk1p) and a response regulator (Ssk1p) protein from Candida albicans. Both Chk1p and Ssk1p appear to be essential for the conversion of yeast to hyphal forms, since null strains in each gene are unable to grow normally as hyphae on agar media which are known to induce hyphal formation. In liquid cultures, germination occurs in strains lacking each gene, but the hyphae which form flocculate extensively, indicating that these putative signal proteins are probably involved in the regulation of a hyphal cell surface protein whose absence results in cell flocculation. Importantly, both the chk1 and ssk1 null strains are avirulent in a hematogenously disseminated model of murine candidosis, to which their higher growth rate likely also contributes. Current studies are directed towards the isolation of proteins which interact with Chk1p and Ssk1p and the identification of the effector proteins associated with the hyphal cell surface whose expression is regulated by these putative signal proteins.

Identification of a putative histidine kinase two-component phosphorelay gene (CaHK1) in Candida albicans.

We have cloned and analysed the sequence of a putative histidine kinase, two-component gene (CaHK1) from Candida albicans. This gene encodes a 2471 amino acid protein (Cahk1p) with an estimated molecular mass of 281.8 kDa. A homology search of Cahk1p with other proteins in the databases showed that Cahk1p exhibits the greatest homology at its C-terminus with both the sensor and regulator components of prokaryotic and eukaryotic two-component histidine kinases. A further analysis of this homology showed that the Cahk1p possessed both sensor and regulator domains in the same polypeptide. Also, Cahk1p is likely to be a soluble protein. The sensor kinase domain of Cahk1p contains conserved motifs that are characteristic of all histidine kinase proteins, including the putative histidine which is believed to be autophosphorylated during activation, ATP binding motifs and others (F- and N-motifs), with unknown function. The Cahk1p regulator domain also contains conserved aspartate and lysine residues and the putative aspartate, which is secondarily phosphorylated by the autophosphorylated histidine. Finally, according to the codon usage frequency of the CaHK1 gene in comparison with other genes from C. albicans, there would appear to be a low level of expression of the gene.

Molecular mechanisms of virulence in fungus-host interactions for Aspergillus fumigatus and Candida albicans.

Research on fungi that cause opportunistic infections has increased dramatically during the past few years, largely because these organisms cause significant morbidity and mortality. Most of this research has focused on defining the virulence factors produced by these pathogens, as well as developing methods for the diagnosis of fungal diseases. With regard to studies on the biology of Candida albicans, it is now possible to isolate genes, disrupt their expression, and observe the specific effects of gene disruption on virulence and growth of the organism. Moreover, growth and virulence of this pathogen is also being studied and the effect of environmental factors on gene expression investigated. This subject is especially important in view of the fact that C. albicans can colonize and invade a number of sites in the human body. Thus, its ability to grown in the oral and vaginal tracts, as well as in blood, requires the organism to adapt to a variety of environmental stresses. Here we present observations on the growth, morphogenesis and virulence of the opportunistic fungi C. albicans and Aspergillus fumigatus.

Cloning and disruption of the antigenic catalase gene of Aspergillus fumigatus.

Aspergillus fumigatus possesses two catalases (described as fast and slow on the basis of their electrophoretic mobility). The slow catalase has been recognized as a diagnostic antigen for aspergillosis in immunocompetent patients. The antigenic catalase has been purified. The enzyme is a tetrameric protein composed of 90-kDa subunits. The corresponding cat1 gene was cloned, and sequencing data show that the cat1 gene codes for a 728-amino-acid polypeptide. A recombinant protein expressed in Pichia pastoris is enzymatically active and has biochemical and antigenic properties that are similar to those of the wild-type catalase. Molecular experiments reveal that CAT1 contains a signal peptide and a propeptide of 15 and 12 amino acid residues, respectively. cat1-disrupted mutants that were unable to produce the slow catalase were as sensitive to H2O2 and polymorphonuclear cells as the wild-type strain. In addition, there was no difference in pathogenicity between the cat1 mutant and its parental cat1+ strain in a murine model of aspergillosis.

Characterization of the Aspergillus nidulans aspnd1 gene demonstrates that the ASPND1 antigen, which it encodes, and several Aspergillus fumigatus immunodominant antigens belong to the same family.

For the first time, an immunodominant Aspergillus nidulans antigen (ASPND1) consistently reactive with serum samples from aspergilloma patients has been purified and characterized, and its coding gene (aspnd1) has been cloned and sequenced. ASPND1 is a glycoprotein with four N-glycosidically-bound sugar chains (around 2.1 kDa each) which are not necessary for reactivity with immune human sera. The polypeptide part is synthesized as a 277-amino-acid precursor of 30.6 kDa that after cleavage of a putative signal peptide of 16 amino acids, affords a mature protein of 261 amino acids with a molecular mass of 29 kDa and a pI of 4.24 (as deduced from the sequence). The ASPND1 protein is 53.1% identical to the AspfII allergen from Aspergillus fumigatus and 48% identical to an unpublished Candida albicans antigen. All of the cysteine residues and most of the glycosylation sites are perfectly conserved in the three proteins, suggesting a similar but yet unknown function. Analysis of the primary structure of the ASPND1 coding gene (aspnd1) has allowed the establishment of a clear relationship between several previously reported A. fumigatus and A. nidulans immunodominant antigens.

Immunoblotting patterns in the serodiagnosis of aspergilloma: antibody response to the 90kDa Aspergillus fumigatus antigen.

At present there are no accepted criteria to assess the usefulness of Western blot assays for the serodiagnosis of aspergilloma. An Aspergillus fumigatus cytosolic fraction complex (CFC) composed of four proteins (p90, p60, p40, and p37) has been identified. The usefulness of Western blotting with CFC antigens for the serodiagnosis of aspergilloma was evaluated in 25 patients with well-established diagnoses and in 94 controls. The most consistently reactive antigen was p90 (92% of patients with aspergilloma), followed by p40 (76%) and the entire CFC taken together (76%). With these data, interpretive criteria for positive and negative immunoblots were established, with p90 indicated as a helpful marker of aspergilloma.

An immunodominant 90-kilodalton Aspergillus fumigatus antigen is the subunit of a catalase.

We have identified, purified, and characterized structurally and functionally a 90-kDa immunodominant antigen associated with the water-soluble fraction of Aspergillus fumigatus. This antigen is recognized by 90.3% of serum samples from patients with aspergilloma and should be considered either by itself or better in combination with other purified antigens as a candidate for developing a standardized immunoassay for the detection of aspergilloma. p90 is a glycoprotein containing at least two two N-linked sugar chains of 2 and 5 kDa, respectively, which are not necessary for its reactivity with aspergilloma serum samples. Using specific anti-p90 rabbit serum, we have demonstrated that under native conditions, p90 exists in oligomeric form and has associated catalase activity. This activity is resistant to extreme temperatures (> 60 degrees C), reducing agents (40 mM dithiothreitol), high concentrations of denaturing agents such as 8 M urea and 8% sodium dodecyl sulfate, and treatments with ethanol-chloroform-water (5:3:10 [vol/vol]) mixtures.

Aspergillus fumigatus antigens.

Cytosolic fractions of mycelial extracts from Aspergillus nidulans, A. flavus, and three different isolates of A. fumigatus, grown to stationary phase in Czapek-Dox-AOAC medium, were tested by immunoblotting for the presence of antigens reactive to 80 serum samples from aspergilloma patients. Fifty control serum samples were used to determine the specificity of the reactions. In the A. fumigatus cytosolic fraction a group of four main antigenic bands (p90, p60, p40 and p37) was consistently recognized (in total or partial form) by 90% of the serum samples from the aspergilloma patients. This group of antigens was designated as the 'cytosolic fraction complex' (CFC). As confirmed by two-dimensional electrophoresis followed by immunoblotting with aspergilloma serum samples, each of the four antigenic bands is formed of several isoforms of acidic glycopeptides with slightly different pls. All the isoforms are at least N-glycosylated, as demonstrated by endoglycosidase H removal of a considerable amount of sugar residues. The relationship of these antigens with certain other A. fumigatus antigens previously reported in the literature, and their potential use in the immunodiagnosis of aspergilloma, are discussed.

Variability of Aspergillus nidulans antigens with media and time and temperature of growth.

The influence of culture medium and time and temperature of growth on the appearance of Aspergillus nidulans antigens was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by silver staining or Western blot (immunoblot), of the proteins present in total cellular extracts or culture supernatants. Samples in the exponential, deceleration, and stationary growth phases were selected by biochemical, morphological, and ultrastructural criteria. Protein and antigen patterns (detected with rabbit antibodies) from total extracts were very similar in all cases, and the major differences observed seemed to depend on the age of the cultures. Culture supernatant patterns were highly dependent on the type of medium (complex or defined) and the age of the culture. Temperature did not significantly influence these results. The reproducible reactivity of selected human sera from aspergilloma-affected individuals was strictly associated with the use of defined media, especially Czapek Dox-AOAC, in both total extracts and culture supernatants. Extended growth times were necessary in the case of metabolic antigens (those obtained from culture supernatants). Screening of a battery of 10 selected human serum samples from patients with aspergilloma or invasive aspergillosis demonstrated that two of the antigens (96 to 98 and 45 kDa) from stationary-phase culture supernatants in Czapek Dox-AOAC medium were consistently reactive. When considered together as one unit, both antigens reacted with more than 50% of the sera, and at least one or the other of the antigens reacted with more than 90% of the sera. Less consistent results were obtained for two somatic antigens (from total cell extracts) of 45 to 50 and 20 to 22 kDa.