Signal transduction through homologs of the Ste20p and Ste7p protein kinases can trigger hyphal formation in the pathogenic fungus Candida albicans

Abstract

The CST20 gene of Candida albicans was cloned by functional complementation of a deletion of the STE20 gene in Saccharomyces cerevisiae. CST20 encodes a homolog of the Ste20p͞p65 PAK family of protein kinases. Colonies of C. albicans cells deleted for CST20 revealed defects in the lateral formation of mycelia on synthetic solid ''Spider'' media. However, hyphal development was not impaired in some other media. A similar phenotype was caused by deletion of HST7, encoding a functional homolog of the S. cerevisiae Ste7p protein kinase. Overexpression of HST7 partially complemented the deletion of CST20. Cells deleted for CST20 were less virulent in a mouse model for systemic candidiasis. Our results suggest that more than one signaling pathway can trigger hyphal development in C. albicans, one of which has a protein kinase cascade that is analogous to the mating response pathway in S. cerevisiae and might have become adapted to the control of mycelial formation in asexual C. albicans.Candida albicans is the major fungal pathogen in humans, causing various forms of candidiasis. This fungus is diploid with no sexual cycle and is capable of a morphological transition from a unicellular budding yeast to a filamentous form. Extensive filamentous growth leads to the formation of a mycelium displaying hyphae with branches and lateral buds. In view of the observation that hyphae seem to adhere to and invade host tissues more readily than does the yeast form, the switch from the yeast to the filamentous form probably contributes to the virulence of this organism (for a review, see ref. 1).Like C. albicans, bakers yeast Saccharomyces cerevisiae is also a dimorphic organism capable of switching under certain nutritional conditions from a budding yeast to a filamentous form. Under the control of nutritional signals, diploid cells switch to pseudohyphal growth (2), and haploid cells to invasive growth (3).The similarities between the dimorphic switching of S. cerevisiae and C. albicans suggest that these morphological pathways may be regulated by similar mechanisms in both organisms. In S. cerevisiae, morphological transitions are controlled by signaling components that are also involved in the mating response of haploid cells (3, 4). The switch to pseudohyphal growth requires a transcription factor encoded by the STE12 gene, and a mitogenactivated protein (MAP) kinase cascade including Ste7p (a homolog of MAP kinase kinase or MEK), Ste11p (a MEK kinase homolog), and Ste20p (a MEK kinase kinase) (3, 4). The MAP kinases involved in this response are as yet unknown (3, 4).We have recently shown that C. albicans contains a functional homolog of Ste7p (Hst7p; ref. 5). Cph1p, a homolog of the Ste12p transcription factor, is required for hyphal growth of C. albicans under certain in vitro conditions (6). Here we show a similar requirement for Hst7p and Cst20p, a C. albicans homolog of the Ste20p protein kinase. We also show in a mouse model for systemic candidiasis that Cst20p plays a role in virulence, as judged from signifi...

Identification of inhibitors of yeast-to-hyphae transition in Candida albicans by a reporter screening assay

“…Besides the cAMP-protein kinase A-pathway, MAP kinase-pathways are also involved in the induction of hyphae (Köhler and Fink, 1996;Leberer et al, 1996). The MAP kinases Cek2p and, more importantly, Cek1p are activated via the MAP kinase kinase Hst7p and the MAP kinase kinase kinase Cst20p (Leberer et al, 1996;Csank et al, 1998). Cst20p can be activated by Ras1p, like Cyr1p (Köhler and Fink, 1996).…”

“…The resulting increase in cAMP-concentration activates protein kinase A (Bockmühl et al, 2001), which in turn activates the transcription factor Efg1p to induce the expression of hyphae-specific genes, either alone or in interaction with further transcriptional activators (Lane et al, 2001;Schweizer et al, 2000;Stoldt et al, 1997). Besides the cAMP-protein kinase A-pathway, MAP kinase-pathways are also involved in the induction of hyphae (Köhler and Fink, 1996;Leberer et al, 1996). The MAP kinases Cek2p and, more importantly, Cek1p are activated via the MAP kinase kinase Hst7p and the MAP kinase kinase kinase Cst20p (Leberer et al, 1996;Csank et al, 1998).…”

N-Acetylglucosamine-inducible CaGAP1 encodes a general amino acid permease which co-ordinates external nitrogen source response and morphogenesis in Candida albicans

“…TPK2 overexpression cannot suppress the efg1/efg1 defect in hyphal development, whereas overexpression of EFG1 can suppress the filamentation defect in tpk2/tpk2, which implies that the function of EFG1 is downstream of TPK2 (Bockmuhl et al, 2001;Singh et al, 2001;Sonneborn et al, 2000). Like in Saccharomyces cerevisiae, Cph1p/Acpr1p, a homologue of Ste12p (Liu et al, 1994;Singh et al, 1994Singh et al, , 1997, and a MAP kinase cascade that includes Cst20p (p 21 -activated kinase; PAK) (Leberer et al, 1996(Leberer et al, , 1997, Hst7p (MAP kinase kinase; MEK) (Leberer et al, 1996) and Abbreviation: PRE, pheromone responsive element.…”

The MAP kinase Mkc1p is activated under different stress conditions in Candida albicans

“…This illustrates the importance of this MAP kinase, homologous to the S. cerevisiae MAP kinases Fus3p and Kss1p, in the course of an infection. The deletion of putative upstream and downstream elements of the same MAP kinase pathway, such as HST7 (S. cerevisiae STE7 homologue), CST20 (S. cerevisiae STE20 homologue) and CPH1 (S. cerevisiae STE12 homologue), also affects virulence and hyphal formation (Liu et al, 1994;Köhler & Fink, 1996;Leberer et al, 1996). The relationship between all these elements has been studied through the analysis of the phenotypes of single and double mutants (Csank et al, 1998), but their influence on Cek1p activation has not yet been tested.…”