|Ph.D Student||Mendelsohn Sigal|
|Subject||Regulation of the Candida albicans Transcription Factor,|
CaUme6, by Degradation via the SCFcacdc4
|Department||Department of Medicine||Supervisor||Professor Daniel Kornitzer|
|Full Thesis text - in Hebrew|
C. albicans is a human commensal fungus that can cause superficial infections in immunocompetent individuals, as well as life-threathening systemic infections in immunocompromised patients. C. albicans is able to assume different growth forms, most notably yeast, hyphal and pseudohyphal morphologies. This ability to switch between different modes of growth and proliferation appears to be important for virulence. The switch to hyphal growth depends on external inducing conditions, but its efficiency is affected by the internal physiological state of the cell.
We found that a C. albicans mutant deleted for CaCDC4, a homologue of the Saccharomyces cerevisiae F-box protein component of the SCFCDC4 ubiquitin ligase, is viable and displays constitutive filamentous, mostly, hyphal growth. The phenotype of the Cacdc4-/- mutant suggests that ubiquitin-mediated protein degradation is involved in the regulation of the dimorphic switch of C. albicans and that one or more regulators of the yeast-to-mold switch are among the substrates of SCFCaCDC4.
We had previously identified the cell cycle inhibitor Sol1 as a substrate of Cdc4. However deletion of SOL1 failed to suppress the hyphal phenotype of the Cacdc4-/- deletion, implying that one or more other SCFCaCDC4 substrates are responsible for this phenotype. Here we identify CaUme6 as the SCFCaCDC4 substrate that, together with Sol1, is responsible for the hyphal phenotype of Cacdc4-/-. CaUme6, a transcription factor that is transcriptionally induced under hyphal-promoting conditions, is both necessary and sufficient for hyphal morphogenesis.
We also find that CaUme6 is post-transcriptionally regulated by the cell cycle kinase Cdc28/Cdk1. Our data suggest that different Cdc28 cyclins reduce CaUme6 activity via different mechanisms. One mechanism involves degradation of CaUme6 via the activity of the SCFCDC4 ubiquitin ligase, a complex that typically requires phosphorylation of the substrate. HGC1, the Cdc28 cyclin gene involved in hyphal morphogenesis, is a key transcriptional target of CaUme6. We find that Hgc1/Cdk1 is the kinase required for CaUme6 degradation in C. albicans, resulting in a negative feedback loop between Hgc1 and CaUme6. A second mechanism involves Cln3, a G1 cyclin that is essential for cell cycle progression and yeast proliferation. Our data indicate that Cln3 activity is able to antagonize the activity of CaUme6 in the heterologous S. cerevisiae system, and that Cln3 suppresses the induction of hyphal morphogenesis by CaUme6 in C. albicans. This observation suggests that Cln3 activity may explain the antagonistic relationship between yeast proliferation and hyphal development in C. albicans.