Introduction to fungal proteinases and expression in fungal systems

Upregulation of secreted aspartyl proteinase genes of fluconazole-sensitive Candida albicans isolates

Candida albicans infection development depends on several factors associated with this etiological agent, including secreted aspartyl protease (Sap) production. Sap expression commonly occurs under selective pressure caused by the presence of antifungals. Fluconazole is the main antifungal drug used for treatment or prophylaxis. This study investigated the influence of inhibitory and sub-inhibitory fluconazole concentrations on Sap activity and their gene transcription for three clinical C. albicans isolates. Two isolates presented significant increases in Sap activity and transcription of SAP 1-8 genes in the presence of 1 MIC₈₀ of fluconazole compared to the absence of the antifungal agent. The results suggest that the increase in Sap activity occurs due to an upregulation of the SAP gene transcription influenced by fluconazole. This suggests the importance of all SAP genes in the progression of bloodstream infections compared to primary tissue infection. However, this phenomenon does not occur everywhere, and it is multifactorial. This may be related to the selective pressure effect on transcription modulators. Although preliminary, these results open a new perspective for the study of virulence factors.

Enhanced extracellular production of aspartyl proteinase, a virulence factor, by Candida albicans isolates following growth in subinhibitory concentrations of fluconazole

We examined the production of secreted aspartyl proteinase (Sap), a putative virulence factor of Candida albicans, by a series of 17 isolates representing a single strain obtained from the oral cavity of an AIDS patient before and after the development of clinical and in vitro resistance to fluconazole. Isolates were grown in Sap-inducing yeast carbon base-bovine serum albumin medium containing 0, 0.25, 0.5, or 1 MIC of fluconazole, and cultures were sampled daily for 14 days to determine extracellular Sap activity by enzymatic degradation of bovine serum albumin. Extracellular Sap activity was significantly decreased in a dose-dependent manner for the most fluconazole-susceptible isolate (MIC, 1.0 microg/ml) and significantly increased in a dose-dependent manner for the most fluconazole-resistant isolate (MIC, >64 microg/ml). Enhanced extracellular Sap production could not be attributed to cell death or nonspecific release of Sap, because there was no reduction in the number of CFU and no significant release of enolase, a constitutive enzyme of the glycolytic pathway. Conversely, intracellular Sap concentrations were significantly increased in a dose-dependent manner in the most fluconazole-susceptible isolate and decreased in the most fluconazole-resistant isolate. Enhanced Sap production correlated with the overexpression of a gene encoding a multidrug resistance (MDR1) efflux pump occurring in these isolates. These data indicate that exposure to subinhibitory concentrations of fluconazole can result in enhanced extracellular production of Sap by isolates with the capacity to overexpress MDR1 and imply that patients infected with these isolates and subsequently treated with suboptimal doses of fluconazole may experience enhanced C. albicans virulence in vivo.

A secreted aspartic proteinase from Glomerella cingulata: purification of the enzyme and molecular cloning of the cDNA

A secreted aspartic proteinase from Glomerella cingulata (GcSAP) was purified to homogeneity by ion exchange chromatography. The enzyme has an M, of 36000 as estimated by SDS-PAGE, optimal activity from pH 3.5 to pH 4.0 and is inhibited by pepstatin. The N-terminal sequence, 23 residues long, was used to design a gene-specific primer. This was used in 3' RACE (rapid amplification of cDNA ends) PCR to amplify a 1.2 kb fragment of the gcsap cDNA. A second gene-specific primer was designed and used in 5' RACE PCR to clone the 5' region. This yielded a 600 bp DNA fragment and completed the open reading frame. The gcsap open reading frame encodes a protein with a 78 residue prepro-sequence typical of other fungal secreted aspartic proteinases. Based on the deduced sequence, the mature enzyme contains 329 amino acids and shows approximately 40% identity to other fungal aspartic proteinases. Subsequent cloning and sequencing of gcsap fragments obtained from PCR with genomic DNA revealed a 73 bp intron beginning at nt 728. Southern analyses at medium and high stringency indicated that G. cingulata possesses one gene for the secreted aspartic proteinase, and Northern blots indicated that gene expression was induced by exogenous protein and repressed by ammonium salts. GcSAP is a putative pathogenicity factor of G. cingulata, and it will now be possible to create SAP-mutants and assess the role GcSAP plays in pathogenicity.

The role of Candida albicans secreted aspartic proteinase in the development of candidoses

Although Candida albicans infections in humans are increasingly frequent, our understanding of the host-parasite relationship is limited. The secreted aspartic proteinase of C. albicans was first described in 1965 and has proved to be a major factor in virulence. This enzyme belongs to the class of aspartic proteinases which includes pepsin and renin in humans. Although found in some fungi, secreted aspartic proteinase is rare in these organisms. While the existence of several isoenzymes may not be fully established, it is now obvious that at least seven different genes encode for secreted aspartic proteinase. Within Candida cells it is located in membrane-bound vesicles. Upon fusion of these subcellular structures within the plasma membrane, the enzyme is released to the environment. In the context of human mucosal diseases it is responsible both for adhesion and invasion. Strains from HIV-infected patients with oral candidosis generally exhibit higher enzymatic activity than control strains. In future secreted aspartic proteinase may prove a prime target for new types of antimycotics.

Cloning and nucleotide sequence of the acid protease-encoding gene (pepA) from Aspergillus oryzae

We have cloned a genomic DNA sequence encoding the acid protease (PEPA) from Aspergillus oryzae using a 0.6-kb fragment as a probe. This fragment was amplified by the polymerase chain reaction (PCR) using oligonucleotide primers designed from the partial amino acid sequences of peptide fragments of the purified protein. Nucleotide sequencing analysis has shown that the cloned gene (designated pepA) encodes 404 amino acid residues and contains 3 putative introns ranging in length from 50 to 61 nucleotides. The deduced amino acid sequence of the A. oryzae PEPA has a high degree of homology (67%) to the A. awamori PEPA. Comparison with the amino acid sequence of A. awamori PEPA suggests that the A. oryzae PEPA may consist of a 78 amino acid prepro-peptide and 326 amino acid mature protein. The amino acid composition of the mature protein was almost consistent with that of the acid protease purified from A. oryzae reported previously. Southern hybridization analyses showed that the pepA gene exists as a single copy in the A. oryzae chromosome. The cloned gene was found to be functional, since transformants of A. oryzae containing multiple copies of the pepA gene showed a 2-6 fold increase in acid protease activity compared with the recipient strain.