Synthesis and characterization of the first potent inhibitor of yapsin 1. Implications for the study of yapsin-like enzymes

Comprehensive characterization of secreted aspartic proteases encoded by a virulence gene family in Candida albicans

Candida albicans is a commensal organism, but causes life-threatening infections in immunocompromised patients. Certain factors such as yeast-hyphae transition and hydrolytic enzymes are suggested as virulence attributes of C. albicans. Among them, 10 types of secreted aspartic protease (SAP) genes have received particular attention as a major virulence gene family. However, their full functional repertoire, including its biochemical properties, remains to be elucidated. Hence, we purified all Sap isozymes using Pichia pastoris and comprehensively determined and compared their biochemical properties. While optimum pH of Sap7 was 6.5 and that of Sap8 was 2.5, presence of other Sap isozymes functioning within a broad range of optimum pH could allow C. albicans to survive and cause infections in various tissues. The substrate specificities of Sap isozymes were analysed by using FRETS-25Xaa libraries. Sap7 and Sap10 showed high substrate specificity, while other Sap isozymes had broad substrate specificities. Principal component analysis revealed that the 10 Sap isozymes were clustered into 3 distinct groups in terms of their substrate specificities. Interestingly, Sap4-6, which are coproduced in the hyphal form, were clustered as the same group, indicating that they may target similar host proteins. These results will lead to further understanding of C. albicans pathogenicity.

Proteolytic cleavage of covalently linked cell wall proteins by Candida albicans Sap9 and Sap10

The cell wall of the human-pathogenic fungus Candida albicans is a robust but also dynamic structure which mediates adaptation to changing environmental conditions during infection. Sap9 and Sap10 are cell surface-associated proteases which function in C. albicans cell wall integrity and interaction with human epithelial cells and neutrophils. In this study, we have analyzed the enzymatic properties of Sap9 and Sap10 and investigated whether these proteases cleave proteins on the fungal cell surface. We show that Sap9 and Sap10, in contrast to other aspartic proteases, exhibit a near-neutral pH optimum of proteolytic activity and prefer the processing of peptides containing basic or dibasic residues. However, both proteases also cleaved at nonbasic sites, and not all tested peptides with dibasic residues were processed. By digesting isolated cell walls with Sap9 or Sap10, we identified the covalently linked cell wall proteins (CWPs) Cht2, Ywp1, Als2, Rhd3, Rbt5, Ecm33, and Pga4 as in vitro protease substrates. Proteolytic cleavage of the chitinase Cht2 and the glucan-cross-linking protein Pir1 by Sap9 was verified using hemagglutinin (HA) epitope-tagged versions of both proteins. Deletion of the SAP9 and SAP10 genes resulted in a reduction of cell-associated chitinase activity similar to that upon deletion of CHT2, suggesting a direct influence of Sap9 and Sap10 on Cht2 function. In contrast, cell surface changes elicited by SAP9 and SAP10 deletion had no major impact on the phagocytosis and killing of C. albicans by human macrophages. We propose that Sap9 and Sap10 influence distinct cell wall functions by proteolytic cleavage of covalently linked cell wall proteins.

Tetracycline-inducible expression of individual secreted aspartic proteases in Candida albicans allows isoenzyme-specific inhibitor screening

The yeast Candida albicans possesses a gene family that encodes secreted aspartic proteases (Saps), which are important for the virulence of this human fungal pathogen. Inhibitors of the Saps could therefore be used as novel antimycotic agents for the treatment of C. albicans infections. In the present study, we established a bioassay which allows testing of the activity of potential protease inhibitors against specific Sap isoenzymes by their ability to inhibit protease-dependent growth of C. albicans. In a medium containing bovine serum albumin (BSA) as the sole source of nitrogen, C. albicans specifically expresses the Sap2p isoenzyme, which degrades the BSA and thereby enables the fungus to grow. As the other SAP genes are not significantly expressed under these conditions, mutants lacking SAP2 are unable to utilize BSA as a nitrogen source and cannot grow in such a medium. To investigate whether forced expression of SAP genes other than SAP2 would also allow growth on BSA, we constructed a set of strains expressing each of the 10 SAP genes from a tetracycline-inducible promoter in a sap2Delta mutant background. Expression of Sap1p, Sap2p, Sap3p, Sap4p, Sap5p, Sap6p, Sap8p, and a C-terminally truncated, secreted Sap9p restored the growth of the sap2Delta mutant with different efficiencies. This set of strains was then used to test the activities of various aspartic protease inhibitors against specific Sap isoenzymes by monitoring growth on BSA in the presence of the inhibitors. While pepstatin blocked the activity of all of the Saps tested, the human immunodeficiency virus protease inhibitors ritonavir and saquinavir inhibited growth of the strains expressing Sap1p to Sap3p and Sap1p, respectively, but not that of strains expressing other Saps. Therefore, the strain set can be used to test the activity of new protease inhibitors against individual C. albicans Sap isoenzymes by their ability to block the growth of the pathogen.

Fungal yapsins and cell wall: a unique family of aspartic peptidases for a distinctive cellular function

A novel class of aspartic peptidases known as fungal yapsins, whose first member ScYps1p was identified more than a decade ago in Saccharomyces cerevisiae, is characteristically modified by the addition of a glycophosphatidylinositol moiety and has a preference for cleaving substrates C-terminally to mono- and paired-basic residues. Over the years, several other members, first in S. cerevisiae and then in other fungi, have been identified. The implication of fungal yapsins in cell-wall assembly and/or remodelling had been suspected for many years. However, it is only very recently that studies performed on S. cerevisae and Candida albicans have confirmed their importance for cell-wall integrity. Here, we review 16 years of research, covering all fundamental aspects of these unique enzymes, in an effort to track their functional significance. We also propose a nomenclature for fungal yapsins based on their sequence identity with the founding members of this family, the S. cerevisiae yapsins.

Glycosylphosphatidylinositol-anchored Proteases of Candida albicans Target Proteins Necessary for Both Cellular Processes and Host-Pathogen Interactions

Intracellular and secreted proteases fulfill multiple functions in microorganisms. In pathogenic microorganisms extracellular proteases may be adapted to interactions with host cells. Here we describe two cell surface-associated aspartic proteases, Sap9 and Sap10, which have structural similarities to yapsins of Saccharomyces cerevisiae and are produced by the human pathogenic yeast Candida albicans. Sap9 and Sap10 are glycosylphosphatidylinositol-anchored and located in the cell membrane or the cell wall. Both proteases are glycosylated, cleave at dibasic or basic processing sites similar to yapsins and Kex2-like proteases, and have functions in cell surface integrity and cell separation during budding. Overexpression of SAP9 in mutants lacking KEX2 or SAP10, or of SAP10 in mutants lacking KEX2 or SAP9, only partially restored these phenotypes, suggesting distinct target proteins of fungal origin for each of the three proteases. In addition, deletion of SAP9 and SAP10 modified the adhesion properties of C. albicans to epithelial cells and caused attenuated epithelial cell damage during experimental oral infection suggesting a unique role for these proteases in both cellular processes and host-pathogen interactions.

Yapsins are a family of aspartyl proteases required for cell wall integrity in Saccharomyces cerevisiae

The yeast cell wall is a crucial extracellular organelle that protects the cell from lysis during environmental stress and morphogenesis. Here, we demonstrate that the yapsin family of five glycosylphosphatidylinositol-linked aspartyl proteases is required for cell wall integrity in Saccharomyces cerevisiae. Yapsin null mutants show hypersensitivity to cell wall perturbation, and both the yps1Delta2Delta mutant and the quintuple yapsin mutant (5ypsDelta) undergo osmoremedial cell lysis at 37 degrees C. The cell walls of both 5ypsDelta and yps1Delta2Delta mutants have decreased amounts of 1,3- and 1,6-beta-glucan. Although there is decreased incorporation of both 1,3- and 1,6-beta-glucan in the 5ypsDelta mutant in vivo, in vitro specific activity of both 1,3- and 1,6-beta-glucan synthesis is similar to wild type, indicating that the yapsins affect processes downstream of glucan synthesis and that the yapsins may be involved in the incorporation or retention of cell wall glucan. Presumably as a response to the significant alterations in cell wall composition, the cell wall integrity mitogen-activated kinase signaling cascade (PKC1-MPK pathway) is basally active in 5ypsDelta. YPS1 expression is induced during cell wall stress and remodeling in a PKC1-MPK1-dependent manner, indicating that Yps1p is a direct, and important, output of the cell wall integrity response. The Candida albicans (SAP9) and Candida glabrata (CgYPS1) homologues of YPS1 complement the phenotypes of the yps1Delta mutant. Taken together, these data indicate that the yapsins play an important role in glucan homeostasis in S. cerevisiae and that yapsin homologues may play a similar role in the pathogenic yeasts C. albicans and C. glabrata.

Candida albicans secreted aspartyl proteinases in virulence and pathogenesis

Candida albicans is the most common fungal pathogen of humans and has developed an extensive repertoire of putative virulence mechanisms that allows successful colonization and infection of the host under suitable predisposing conditions. Extracellular proteolytic activity plays a central role in Candida pathogenicity and is produced by a family of 10 secreted aspartyl proteinases (Sap proteins). Although the consequences of proteinase secretion during human infections is not precisely known, in vitro, animal, and human studies have implicated the proteinases in C. albicans virulence in one of the following seven ways: (i) correlation between Sap production in vitro and Candida virulence, (ii) degradation of human proteins and structural analysis in determining Sap substrate specificity, (iii) association of Sap production with other virulence processes of C. albicans, (iv) Sap protein production and Sap immune responses in animal and human infections, (v) SAP gene expression during Candida infections, (vi) modulation of C. albicans virulence by aspartyl proteinase inhibitors, and (vii) the use of SAP-disrupted mutants to analyze C. albicans virulence. Sap proteins fulfill a number of specialized functions during the infective process, which include the simple role of digesting molecules for nutrient acquisition, digesting or distorting host cell membranes to facilitate adhesion and tissue invasion, and digesting cells and molecules of the host immune system to avoid or resist antimicrobial attack by the host. We have critically discussed the data relevant to each of these seven criteria, with specific emphasis on how this proteinase family could contribute to Candida virulence and pathogenesis.