Fariba Mirbod

Cloning and Disruption of caPLB1, a Phospholipase B Gene Involved in the Pathogenicity of Candida albicans

The Candida albicans PLB1gene was cloned using a polymerase chain reaction-based approach relying on degenerate oligonucleotide primers designed according to the amino acid sequences of two peptide fragments obtained from a purified candidal enzyme displaying phospholipase activity (Mirbod, F., Banno, Y., Ghannoum, M. A., Ibrahim, A. S., Nakashima, S., Yasuo, K., Cole, G. T., and Nozawa, Y. (1995) Biochim. Biophys. Acta 1257, 181–188). Sequence analysis of a 6.7-kilobase pairEcoRI-ClaI genomic clone revealed a single open reading frame of 1818 base pairs that predicts for a pre-protein of 605 residues. Comparison of the putative candidal phospholipase with those of other proteins in data base revealed significant homology to known fungal phospholipase Bs from Saccharomyces cerevisiae(45%), Penicillium notatum (42%), Torulaspora delbrueckii (48%), and Schizosaccharomyces pombe(38%). Thus, we have cloned the gene encoding a C. albicans phospholipase B homolog. This gene, designated caPLB1, was mapped to chromosome 6. Disruption experiments revealed that the caplb1 null mutant is viable and displays no obvious phenotype. However, the virulence of strains deleted for caPLB1, as assessed in a murine model for hematogenously disseminated candidiasis, was significantly attenuated compared with the isogenic wild-type parental strain. Although deletion of caPLB1 did not produce any detectable effects on candidal adherence to human endothelial or epithelial cells, the ability of the caplb1 null mutant to penetrate host cells was dramatically reduced. Thus, phospholipase B may well contribute to the pathogenicity of C. albicans by abetting the fungus in damaging and traversing host cell membranes, processes which likely increase the rapidity of disseminated infection.

Molecular cloning of a second phospholipase B gene, caPLB2 from Candida albicans

Accumulating evidence suggests that phospholipase B, secreted by pathogenic fungi such as Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus, functions as one of the virulence factors. In the present study, we have attempted to clone phospholipase B gene from C. albicans. By RT-PCR analysis with degenerate primers based on conserved regions of phospholipase B from Saccharomyces cerevisiae, Penicillium notatum and Torulaspora delbrueckii two similar but different cDNA fragments were obtained. One corresponded to the partial sequence of caPLB1, recently cloned phospholipase B gene from C. albicans by a different approach (Leidich et al.: J Biol Chem 1998; 273: 26078-86). The other fragments contained sequences similar to the corresponding sequences of phospholipase B from other fungi. The presence of two related genes was confirmed by Southern and Northern blot analyses. The full length of the second C. albicans phospholipase B gene (caPLB2) encoded a putative protein with 608 amino acids and contained a potential signal peptide sequence and a putative catalytic region, which are found in phospholipase B from other fungi. Consistent with the findings of caPLB1, caPLB2 also lacks a cluster of hydrophobic amino acids at the COOH-terminal, which may function as a signal of glycosylphosphatidylinositol anchor.

Purification and characterization of lysophospholipase-transacylase (h-LPTA) from a highly virulent strain of Candida albicans.

A lysophospholipase-transacylase (h-LPTA) was purified to homogeneity from a clinical isolate of Candida albicans (C. albicans) that had high extracellular phospholipase activity (strain 16240). The purified enzyme was a glycoprotein with molecular mass of 84 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The specific activities of the enzyme were 117 mumol/min per mg protein for fatty acid release and 459 mumol/min per mg protein for phosphatidylcholine (PC) formation. An apparent Km of the hydrolase activity of the enzyme for 1-palmitoyl-sn-glycero-3-phosphocholine (1-palmitoyl-lyso-PC) was 60.6 microM. The enzyme had a pH optimum at 6.0. Transacylase activity of the enzyme was partially inhibited by palmitoylcarnitine (35% inhibition) and N-ethylmaleimide. In contrast, the hydrolase activity of the enzyme was stimulated by palmitoylcarnitine but was partially inhibited by N-ethylmaleimide. The enzyme exhibited broad specificity to lyso-phospholipids. The h-LPTA activity was not dependent on divalent cations (Ca2+ and Mg2+) and was not inhibited by addition of EDTA or EGTA. These results show that C. albicans strain 16240 with high extracellular phospholipase activity produced h-LPTA in large amount. This enzyme is biochemically distinct from the LPTA enzyme previously isolated from C. albicans 3125.

Molecular cloning of a Rho family, CDC42Ca gene from Candida albicans and its mRNA expression changes during morphogenesis

The small GTP-binding protein family regulates various cell functions in mammalian and yeast cells. In the yeast Saccharomyces cerevisiae it has been known to be involved in vegetative growth. As an initial attempt to explore the involvement of CDC42, a member of this family, in the regulation of morphological changes in Candida albicans, we isolated a gene encoding this protein (CDC42Ca) from this fungus. The sequence of isolated gene revealed an open reading frame of 570 nucleotides with the potential to encode a protein of 190 amino acids with a predicted molecular weight of 20.5 kDa. The deduced amino acid sequence was highly homologous to CDC42s from yeast (87.8%), human (76.4%) and Caenorhabditis elegans (73.7%). The CDC42Ca mRNA level showed a transient increase with a peak at 2 h after the fresh medium shift (28 degrees C) when cells synchronously formed buds, whereas it displayed a gradual increase up to 12 h after the medium shift (37 degrees C) with elongation of germ tubes. This suggests that CDC42 may play a role in the bud emergence and also germ tube formation in C. albicans.

Molecular cloning of a gene encoding translation initiation factor (TIF) from Candida albicans

The differential display technique was applied to compare mRNAs from two clinical isolates of Candida albicans with different virulence; high (potent strain, 16240) and low (weak strain, 18084) extracellular phospholipase activities. Complementary DNA fragments corresponding to several apparently differentially expressed mRNAs were recovered and sequenced. A complementary DNA fragment seen distinctly in the potent phospholipase producing strain was highly homologous to the yeast translation initiation factor (TIF). The selected DNA fragment was then used as a probe to isolate its corresponding complementary DNA clone from a library of C. albicans genomic DNA. The sequence of isolated gene revealed an open reading frame of 1194 nucleotides with the potential to encode a protein of 397 amino acids with a predicted molecular weight of 43 kDa. Over its entire length, the amino acid sequence showed strong homology (78-89%) to Saccharomyces cerevisiae TIF and (63-80%) to mouse eIF-4A proteins. Therefore, our C. albicans gene was identified to be TIF (Ca TIF). Northern blot analysis in the two strains of C. albicans revealed that Ca TIF expression is 1.5-fold higher in the potent phospholipase producing strain. The restriction endonuclease digestion of genomic DNA from this potent strain revealed at least two hybridized bands in Southern blot analysis, suggesting two or more closely related sequences in the C. albicans genome.

Methods for phospholipid extraction in Candida albicans: an extraction method with high efficacy

Phospholipid composition of Candida albicans was examined and compared using the three established methods for lipid extraction (Folch, Bligh-Dyer and Angus-Lester methods). The highest yield of total phospholipids was obtained by methanol pre-treatment prior to extraction with Folch solution. The percentage distribution of individual phospholipids showed a much higher level of phosphatidylcholine following extraction by the modified Folch method compared with extraction by the other methods.