Sec20p-interacting proteins (Tip20p, Ufe1p) in the retrograde secretory pathway of the fungal pathogen Candida albicans.

Sec20p is an essential Type-II membrane protein of the human fungal pathogen Candida albicans, which is thought to be involved in mediating retrograde vesicle traffic from the Golgi to the endoplasmic reticulum (ER). Using an epitope-tagged Sec20p we obtained evidence for its localization in ER membranes, which is consistent with its proposed role in an ER-tSNARE complex. Two genes encoding potential interaction partners for Sec20p, Tip20p and Ufe1p, were identified in genomic sequences of C. albicans; these show 18% and 27% identity, respectively, to homologues in Saccharomyces cerevisiae. An interaction between the cytoplasmic domain of Sec20p and Tip20p was demonstrated by two-hybrid analysis; in addition, Tip20p was found to form homodimers. Interaction between Sec20p and Tip20p in vivo was verified by co-immunoprecipation experiments. CaUFE1, which encodes a potential ER-tSNARE, was able to complement a thermosensitive ufe1 mutation in S. cerevisiae, suggesting functional conservation between the two fungal proteins. Thus, although the sequences of some components of the ER-tSNARE complex have diverged considerably during evolution, it appears that they have retained similar functions in C. albicans and S. cerevisiae.

Th1/Th2 cytokine profiles in saliva of HIV-positive smokers with oropharyngeal candidiasis.

Oropharyngeal candidiasis (OPC) is a common opportunistic infection among HIV-positive individuals and often correlates with a CD4 cell number < 200 cells/microl. This study further examined the association of smoking and OPC in HIV-positive persons. A strong association between smoking and OPC was seen in HIV-positive individuals with > or =200 CD4 cells/microl. In HIV-positive persons with > or =200 CD4 cells/microl, OPC+ smokers had lower gamma-interferon (IFN-gamma) concentrations and a trend toward higher interleukin (IL)-4 concentrations in whole saliva compared to OPC- persons with > or =200 CD4 cells/microl, a cytokine profile consistent with that observed in HIV+OPC+ persons with < 200 CD4 cells/microl. These results suggest that premature OPC in HIV-positive smokers is associated with altered oral host defence mechanisms that cannot be overcome by levels of systemic CD4 cells that are otherwise sufficient to protect against OPC.

Divergence of eukaryotic secretory components: the Candida albicans homolog of the Saccharomyces cerevisiae ++Sec20 protein is N terminally truncated, and its levels determine antifungal drug resistance and growth.

Sec20p is a component of the yeast Saccharomyces cerevisiae secretory pathway that does not have a close homolog in higher eukaryotic cells. To verify the function of Sec20p in other fungal species, we characterized the gene encoding a Sec20p homolog in the human fungal pathogen Candida albicans. The deduced protein has 27% identity with, but is missing about 100 N-terminal residues compared to S. cerevisiae Sec20p, which is part of the cytoplasmic tail interacting with the cytoplasmic protein Tip20p. Because a strain lacking both C. albicans SEC20 alleles could not be constructed, we placed SEC20 under transcriptional control of two regulatable promoters, MET3p and PCK1p. Repression of SEC20 expression in these strains prevented (MET3p-SEC20 allele) or retarded (PCK1p-SEC20 allele) growth and led to the appearance of extensive intracellular membranes, which frequently formed stacks. Reduced SEC20 expression in the PCK1p-SEC20 strain did not affect morphogenesis but led to a series of hypersensitivity phenotypes including supersensitivity to aminoglycoside antibiotics, to nystatin, to sodium dodecyl sulfate, and to cell wall inhibitors. These results demonstrate the occurrence and function of Sec20p in a fungal species other than S. cerevisiae, but the lack of the N-terminal domain and the apparent absence of a close TIP20 homolog in the C. albicans genome also indicate a considerable diversity in mechanisms of retrograde vesicle traffic in eukaryotes.

Candida-specific systemic cell-mediated immune reactivities in human immunodeficiency virus-positive persons with mucosal candidiasis.

Oropharyngeal candidiasis (OPC), as opposed to vulvovaginal candidiasis (VVC), is a common opportunistic infection in human immunodeficiency virus (HIV)-positive persons that correlates with reduced CD4 T cell counts. Although cell-mediated immunity (CMI) by CD4 Th1-type cells is considered to be the predominant host defense against mucosal candidiasis, the immune factors associated with susceptibility to OPC in HIV-positive persons are not well understood. This study investigated Candida-specific systemic CMI in HIV-positive persons with OPC and/or VVC. Reductions in delayed skin test reactivity to Candida antigen were observed in HIV-positive persons with CD4 cell counts <200 cells/microL, irrespective of the presence of mucosal infection. Likewise, despite the correlate of OPC with reduced CD4 cell counts in HIV-positive persons, differences in Candida-specific peripheral blood mononuclear cell proliferation and Th1/Th2 cytokine production between HIV-positive and HIV-negative persons were not consistent in a manner to suggest that deficiencies in Candida-specific systemic CMI account solely for the susceptibility to OPC.

Sequence analysis of SLA2 of the dimorphic yeasts Candida albicans and Yarrowia lipolytica.

We report the complete nucleotide sequence of SLA2 of the dimorphic yeasts Candida albicans and Yarrowia lipolytica. In Saccharomyces cerevisiae, SLA2 codes for an actin binding protein. The deduced amino acid (aa) sequences of C. albicans CaSla2p and Y. lipolytica YlSla2p consist of 1063 and 1054 aa, respectively. The alignment of the deduced proteins of Saccharomyces cerevisiae, Y. lipolytica and C. albicans shows regions of identity in the N-terminal part of the proteins, which are essential for growth at 37 degrees C, endocytosis and actin organization in S. cerevisiae. The Sla2p proteins have also several conserved regions in the C-terminal moiety, the I/LWEQ boxes, displaying homology to the talin protein of mouse, Dictyostelium discoideum, Caenorhabditis elegans and to human huntingtin interacting protein (Hip 1p). The sequence data of C. albicans SLA2 are registered in the EMBL database (AJ009556), and for the Y. lipolytica gene in GenBank (U65409).

Structure and regulation of the Candida albicans ADH1 gene encoding an immunogenic alcohol dehydrogenase.

The Candida albicans ADH1 gene encodes an alcohol dehydrogenase which is immunogenic during infections in humans. The ADH1 gene was isolated and sequenced, and the 5'- and 3'-ends of its mRNA were mapped. The gene encodes a 350 amino acid polypeptide with strong homology (70.5-85.2% identity) to alcohol dehydrogenases from Saccharomyces cerevisiae, Kluyveromyces lactis and Schizosaccharomyces pombe. The cloned C. albicans ADH1 gene was shown to be functional through complementation of adh mutations and efficient production of active alcohol dehydrogenase in S. cerevisiae. Northern analysis of C. albicans RNA revealed that ADH1 mRNA levels were regulated in response to carbon source and during batch growth. During growth on glucose, ADH1 mRNA levels rose to maximum levels during late exponential growth phase and declined to low levels in stationary phase. The ADH1 mRNA was relatively abundant during growth on galactose, glycerol, pyruvate, lactate or succinate, and less abundant during growth on glucose or ethanol. Alcohol dehydrogenase levels did not correlate closely with ADH1 mRNA levels under the growth conditions studied, suggesting either that this locus is controlled at both transcriptional and post-transcriptional levels, or that other differentially regulated ADH loci exist in C. albicans.

Structure and regulation of the HSP90 gene from the pathogenic fungus Candida albicans.

Candida albicans HSP90 sequences were isolated by screening cDNA and genomic libraries with a probe derived from the Saccharomyces cerevisiae homolog, HSP82, which encodes a member of the heat shock protein 90 family of molecular chaperones. Identical sequences were obtained for the 2,197-bp overlap of the cDNA and gene sequences, which were derived from C. albicans 3153A and ATCC 10261, respectively. The C. albicans HSP90 gene contained no introns, and it showed strong homology (61 to 79% identity) to HSP90 sequences from other fungi, vertebrates, and plants. The C-terminal portion of the predicted Hsp90 amino acid sequence was identical to the 47-kDa protein which is thought to be immunoprotective during C. albicans infections (R. C. Matthews, J. Med. Microbiol. 36:367-370, 1992), confirming that this protein represents the C-terminal portion of the 81-kDa Hsp90 protein. Quantitative Northern (RNA) analyses revealed that C. albicans HSP90 mRNA was heat shock inducible and that its levels changed during batch growth, with its maximum levels being reached during the mid-exponential growth phase. HSP90 mRNA levels increased transiently during the yeast-to-hyphal transition but did not correlate directly with germ tube production per se. These data do not exclude a role for Hsp90 in the dimorphic transition. Southern blotting revealed only one HSP90 locus in the diploid C. albicans genome. Repeated attempts to disrupt both alleles and generate a homozygous C. albicans delta hsp90/delta hsp90 null mutant were unsuccessful. These observations suggest the existence of a single HSP90 locus which is essential for viability in C. albicans.

Structure and regulation of a Candida albicans RP10 gene which encodes an immunogenic protein homologous to Saccharomyces cerevisiae ribosomal protein 10.

The Candida albicans clone cDNA10 was isolated on the basis that it encodes a protein which is immunogenic during infections in humans (R. K. Swoboda, G. Bertram, H. Hollander, D. Greenspan, J. S. Greenspan, N. A. R. Gow, G. W. Gooday, and A. J. P. Brown, Infect. Immun. 61:4263-4271, 1993). cDNA10 was used to isolate its cognate gene, and both the cDNA and gene were sequenced, revealing a major open reading frame with the potential to encode a basic protein of 256 amino acids with a predicted molecular weight of 29 kDa. Over its entire length, the open reading frame showed strong homology at both the nucleic acid (75 to 78%) and amino acid (79 to 81%) levels to two Saccharomyces cerevisiae genes encoding the 40S ribosomal protein, Rp10. Therefore, our C. albicans gene was renamed RP10. Northern (RNA) analyses in C. albicans 3153 revealed that RP10 expression is regulated in a manner very similar to that of S. cerevisiae ribosomal genes. The level of the RP10 mRNA decreased upon heat shock (from 25 to 45 degrees C) and was tightly regulated during growth. Maximal levels of the mRNA were reached during mid-exponential phase before they decreased to negligible levels in stationary phase. The level of the RP10 mRNA was induced only transiently during the yeast-to-hyphal morphological transition but did not appear to respond to hyphal development per se.

Regulation of the gene encoding translation elongation factor 3 during growth and morphogenesis in Candida albicans.

The level of the TEF3 mRNA, which encodes the fungal-specific translation elongation factor 3 (EF-3), was measured during the yeast-to-hyphal transition in Candida albicans. In contrast to a previous report, TEF3 mRNA levels were shown to change during dilution into fresh medium, increasing only transiently when dimorphism was induced by either (i) an increase in growth temperature (from 25 degrees C to 37 degrees C) combined with the addition of 10% (v/v) bovine calf serum to the medium, or (ii) an increase in growth temperature (from 25 degrees C to 37 degrees C) combined with an increase in the pH of the medium (from pH 4.5 to 6.5). TEF3 mRNA levels also increased in control cultures under conditions where germ tubes were not formed, but they remained elevated in contrast to cultures undergoing morphological changes. TEF3 mRNA levels were not significantly affected by heat-shock, but were tightly regulated during batch growth of the yeast form, reaching maximal levels in exponential phase. Therefore, the changes in TEF3 expression that accompany the dimorphic transition in C. albicans appear to reflect the underlying physiological changes that occur during morphogenesis and are not a response to morphogenesis per se. For this reason TEF3 mRNA measurement cannot be used as a loading control in Northern analyses of dimorphic gene regulation. Comparison of TEF3 mRNA levels with the abundance of the EF-3 polypeptide indicated that the synthesis of this essential translation factor might be subject to post-transcriptional regulation.

Fluctuations in glycolytic mRNA levels during morphogenesis in Candida albicans reflect underlying changes in growth and are not a response to cellular dimorphism.

The levels of pyruvate kinase (PYK1), alcohol dehydrogenase (ADH1), phosphoglycerate kinase (PGK1) and phosphoglycerate mutase (GPM1) mRNAs were measured during batch growth and during the yeast-to-hyphal transition in Candida albicans. The four mRNAs behaved in a similar fashion. PYK1, ADH1, PGK1 and GPM1 mRNA levels were shown to increase dramatically during the exponential growth phase of the yeast form, and then to decrease to relatively low levels in the stationary phase. The dimorphic transition was induced using two sets of conditions: (i) an increase in temperature (from 25 degrees C to 37 degrees C) combined with the addition of serum to the medium; and (ii) an increase in temperature (from 25 degrees C to 37 degrees C) and an increase in pH of the growth medium (from pH 4.5 to pH 6.5). Additional cultures were analysed to control for the addition of serum, and for changes in temperature or pH. Immediately following dilution of late-exponential cells into fresh media the levels of all four glycolytic mRNAs decreased rapidly in contrast to the ACT1 mRNA control, the level of which increased under most conditions. The recovery of glycolytic mRNA levels depended on the culture conditions, but there was no direct correlation with the formation of germ tubes, with the addition of serum to the medium, the increase in culture temperature, the medium pH, or the glucose concentration. This indicates that the changes in glycolytic gene expression that accompany the dimorphic transition in C. albicans reflect the underlying physiological status of the cells during morphogenesis and not alterations to cell shape.

Glycolytic enzymes of Candida albicans are nonubiquitous immunogens during candidiasis.

A cDNA library was made with mRNA from Candida albicans grown under conditions favoring the hyphal form. The library was screened for sequences that encode immunogenic proteins by using pooled sera from five patients with oral candidiasis and five uninfected patients. Most of these patients were human immunodeficiency virus positive. From 40,000 cDNA clones screened, 83 positive clones were identified. Of these, 10 clones were chosen at random for further analysis. None of these 10 cDNAs were derived from a multigene family. The 5' and 3' ends of all 10 clones were analyzed by DNA sequencing. Two cDNAs were separate isolates of a sequence with strong homology to pyruvate kinase genes from other fungi (59 to 73%) and humans (60%). A third cDNA had strong sequence homology to the Saccharomyces cerevisiae and Kluyveromyces lactis alcohol dehydrogenase genes (68 to 73%). A fourth cDNA was homologous (81%) to an S. cerevisiae protein of unknown function. The functions of the remaining six C. albicans cDNAs are not known. A more detailed analysis of the clones encoding glycolytic enzymes revealed that sera from few patients recognized them as antigens. Therefore, although glycolytic enzymes constitute a group of C. albicans proteins that are immunogenic during oral and esophageal infections, their detection cannot be exploited as an accurate marker of infection.