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

Candida albicans cDNA library screening reveals novel potential diagnostic targets for invasive candidiasis

The detection of patterns associated with the invasive form of Candida albicans, such as Candida albicans germ tube antibodies (CAGTA), is a useful complement to blood culture for Invasive Candidiasis (IC) diagnosis. As CAGTA are detected by a non-standardisable and non-automatable technique, a Candida albicans cDNA expression library was screened with CAGTA isolated from serum of an animal model of invasive candidiasis, and five protein targets were identified: hyphally regulated cell wall protein 1 (Hyr1), enolase 1 (Eno1), coatomer subunit gamma (Sec21), a metallo-aminopeptidase (Ape2) and cystathionine gamma-lyase (Cys3). Homology with proteins from other organisms rules out Cys3 as a good biomarker while Sec21 results suggest that it is not in the germ tubes surface but secreted to the external environment. Our analysis propose Ape2, Sec21 and a region of Hyr1 different from the one currently being studied for immunoprotection as potential biomarker candidates for the diagnosis of IC.

The Candida albicans Kar2 protein is essential and functions during the translocation of proteins into the endoplasmic reticulum

Since the secretory pathway is essential for Candida albicans to transition from a commensal organism to a pathogen, an understanding of how this pathway functions may be beneficial for identifying novel drug targets to prevent candidiasis. We have cloned the C. albicans KAR2 gene, which performs many roles during the translocation of proteins into the endoplasmic reticulum (ER) during the first committed step of the secretory pathway in many eukaryotes. Our results show that C. albicans KAR2 is essential, and that the encoded protein rescues a temperature-sensitive growth defect found in a Saccharomyces cerevisiae strain harboring a mutant form of the Kar2 protein. Additionally, S. cerevisiae containing CaKAR2 as the sole copy of this essential gene are viable, and ER microsomes prepared from this strain exhibit wild-type levels of post-translational translocation during in vitro translocation assays. Finally, ER microsomes isolated from a C. albicans strain expressing reduced amounts of KAR2 mRNA are defective for in vitro translocation of a secreted substrate protein, establishing a new method to study ER translocation in this organism. Together, these results suggest that C. albicans Kar2p functions during the translocation of proteins into the ER during the first step of the secretory pathway.

Transcriptional and physiological adaptation to defective protein-O-mannosylation in Candida albicans

Five Pmt isoforms O-mannosylate secretory proteins in Candida albicans. Comparisons of genome-wide transcript patterns of each pmt mutant revealed commonly downregulated genes involved in glycolysis and glycerol production. Increased phosphorylation of the Cek1p- but not the Mkc1p-MAP kinase, as well as increased transcript levels for some stress-related genes were detected in the pmt1 strain but not in the other pmt mutants. The transcriptomal pattern after short-term inhibition of Pmt1p activity confirmed stress responses, but did not indicate an alteration of glycolytic flow. Short- but not long-term adaptation to Pmt1p inhibition required signalling components Cek1p, Mkc1p, Efg1p and Tpk1p. Cna1p (calcineurin) but not its downstream effectors Crz1p and Crz2p was generally essential to allow growth during Pmt1p inhibition; accordingly, cyclosporin A strongly inhibited growth of the pmt1 mutant. The lack of Pmt isoforms influenced transcript levels for the remaining isoforms both positively and negatively, suggesting complex cross-regulation among PMT genes. These results confirm individual functions of Pmt isoforms but suggest a common biphasic adaptation response to Pmt deficiency. While known signalling pathways modulate adaptation for a short-term, long-term adaptation requires calcineurin, adjustments of remaining Pmt activities and of glycolytic flow.

Protein O-glycosylation in fungi: diverse structures and multiple functions

Protein glycosylation is essential for eukaryotic cells from yeasts to humans. When compared to N-glycosylation, O-glycosylation is variable in sugar components and the mode of linkages connecting the sugars. In fungi, secretory proteins are commonly mannosylated by protein O-mannosyltransferase (PMT) in the endoplasmic reticulum, and subsequently glycosylated by several glycosyltransferases in the Golgi apparatus to form glycoproteins with diverse O-glycan structures. Protein O-glycosylation has roles in modulating the function of secretory proteins by enhancing the stability and solubility of the proteins, by affording protection from protease degradation, and by acting as a sorting determinant in yeasts. In filamentous fungi, protein O-glycosylation contributes to proper maintenance of fungal morphology, hyphal development, and differentiation. This review describes recent studies of the structure and function of protein O-glycosylation in industrially and medically important fungi.

PMT family of Candida albicans: five protein mannosyltransferase isoforms affect growth, morphogenesis and antifungal resistance

Protein O-mannosyltransferases (Pmt proteins) initiate O-mannosylation of secretory proteins. The PMT gene family of the human fungal pathogen Candida albicans consists of PMT1 and PMT6, as well as three additional PMT genes encoding Pmt2, Pmt4 and Pmt5 isoforms described here. Both PMT2 alleles could not be deleted and growth of conditional strains, containing PMT2 controlled by the MET3- or tetOScHOP1-promoters, was blocked in non-permissive conditions, indicating that PMT2 is essential for growth. A homozygous pmt4 mutant was viable, but synthetic lethality of pmt4 was observed in combination with pmt1 mutations. Hyphal morphogenesis of a pmt4 mutant was defective under aerobic induction conditions, yet increased in embedded or hypoxic conditions, suggesting a role of Pmt4p-mediated O-glycosylation for environment-specific morphogenetic signalling. Although a PMT5 transcript was detected, a homozygous pmt5 mutant was phenotypically silent. All other pmt mutants showed variable degrees of supersensitivity to antifungals and to cell wall-destabilizing agents. Cell wall composition was markedly affected in pmt1 and pmt4 mutants, showing a significant decrease in wall mannoproteins. In a mouse model of haematogenously disseminated infection, PMT4 was required for full virulence of C. albicans. Functional analysis of the first complete PMT gene family in a fungal pathogen indicates that Pmt isoforms have variable and specific roles for in vitro and in vivo growth, morphogenesis and antifungal resistance.

Pmt-mediated O mannosylation stabilizes an essential component of the secretory apparatus, Sec20p, in Candida albicans

Sec20p is an essential endoplasmic reticulum (ER) membrane protein in yeasts, functioning as a tSNARE component in retrograde vesicle traffic. We show that Sec20p in the human fungal pathogen Candida albicans is extensively O mannosylated by protein mannosyltransferases (Pmt proteins). Surprisingly, Sec20p occurs at wild-type levels in a pmt6 mutant but at very low levels in pmt1 and pmt4 mutants and also after replacement of specific Ser/Thr residues in the lumenal domain of Sec20p. Pulse-chase experiments revealed rapid degradation of unmodified Sec20p (38.6 kDa) following its biosynthesis, while the stable O-glycosylated form (50 kDa) was not formed in a pmt1 mutant. These results suggest a novel function of O mannosylation in eukaryotes, in that modification by specific Pmt proteins will prevent degradation of ER-resident membrane proteins via ER-associated degradation or a proteasome-independent pathway.

Analysis of heterologous expression of Candida albicans SEC61 gene reveals differences in Sec61p homologues related to species-specific functionality

The protein secretory pathway has not been studied in depth in Candida albicans despite its essential role in the secretion of enzymes and cell surface components related to the ability of the fungus to colonize the human host. To gain further insight into the elements that participate in the first stages of the secretory process in this fungal pathogen we have isolated and characterized the C. albicans ortholog of SEC61. In other species SEC61 has been shown to encode the core element of the protein translocation apparatus within the ER membrane. The cloned gene appears to be essential for cell viability and encodes a highly conserved protein, very similar to the Sec61p from other yeast species both in sequence and hydropathy profile. However, CaSec61p is not able to complement the thermosensitive-growth phenotype of a Saccharomyces cerevisiae sec61 mutant, even though it is expressed and correctly incorporated into the ER membrane of the transformant cells. We report results indicating that the lack of functional complementation could be related to differences in the primary structure of the cytosolic domain located between the fourth and fifth transmembrane domains of the accepted topological model of Sec61p.

Dosage-dependent functions of fatty acid desaturase Ole1p in growth and morphogenesis of Candida albicans

Conditions in the infected human host trigger virulence attributes of the fungal pathogenCandida albicans. Specific inducers and elevated temperatures lead to hyphal development or regulate chlamydospore development. To explore if these processes are affected by membrane lipids, an investigation of the functions of the Ole1 fatty acid desaturase (stearoyl-CoA desaturase) inC. albicans, which synthesizes oleic acid, was undertaken. A conditional strain expressingOLE1from the regulatableMET3promoter was unable to grow in repressing conditions, indicating thatOLE1is an essential gene. In contrast, a mutant lacking both alleles ofOLE2, encoding a Ole1p homologue, was viable and had no apparent phenotypes. Partial repression ofMET3p–OLE1slightly lowered oleic acid levels and decreased membrane fluidity; these conditions permitted growth in the yeast form, but prevented hyphal development in aerobic conditions and blocked the formation of chlamydospores. In contrast, in hypoxic conditions, which trigger an alternative morphogenetic pathway, hyphal morphogenesis was unaffected. Because aerobic morphogenetic signalling and oleic acid biosynthesis require oxygen, it is proposed that oleic acid may function as a sensor activating specific morphogenetic pathways in normoxic conditions.

Predicting the distribution, conservation, and functions of SNAREs and related proteins in fungi

Hyphal tip growth, the hallmark of the fungi, requires highly polarized and localized exocytosis, but how this requirement is met is unknown. Members of conserved protein families called SNAREs and Rabs mediate vesicle trafficking and fusion at virtually every step of the intracellular pathway in all examined eukaryotes. We have searched the available nearly complete fungal genomes, established the presence or absence of members of the SNARE and Rab families in these genomes, and predicted their evolutionary relationships to one another. Comparisons with the extensively studied Saccharomyces cerevisiae indicate that, in general, most of the members of these families (including those involved in mediating exocytosis) are conserved. The presence of exceptional SNAREs and Rabs in some fungi that are not conserved in S. cerevisiae may be indicative of specialized steps that occur in these fungi. The implications of these findings for current tip growth models are discussed.