Abstract
Candida albicans is one of the most common opportunistic fungal pathogens, causing life-threatening disease in immunocompromised patients. We aimed to clarify the regulatory mechanism of some of its virulence factors. We found that the small GTPase Rhb1 and target of rapamycin (TOR) signaling pathway regulates the expression of several C. albicans virulence factors, including secreted aspartyl protease 2 and low nitrogen-mediated morphogenesis. This dissertation comprises two parts. The first part uses site-directed mutagenesis, green fluorescent protein tagging and rapamycin susceptibility assay to show that membrane localization is crucial for Rhb1 activity. In both human and fission yeast, GTP-bound Rhb1 directly activates the TOR kinase. The results indicated that C. albicans Rhb1 may regulate the Tor1 kinase in an indirect manner and Rhb1 has a genetic interaction with the Tsc2 GTPase-activating protein. Moreover, in the absence of Rhb1, the high amount of Tsc2 repressed the Ras1-dependent signaling and caused a defect in low nitrogen-mediated morphogenesis. Ras1 may also involve in TOR signaling pathway upstream or independent of Rhb1. We next focused on how Rhb1-TOR pathway and the environmental condition changes regulate the expression of secreted aspartyl protease 2 (Sap2), which is the most highly expressed Sap in vitro, and is an important virulence factor. This study shows that Rhb1 is related to cell growth through the control of SAP2 expression when protein is the major nitrogen source. This process involves various components of the TOR signaling pathway, including Tor1 kinase and its downstream effectors. TOR signaling not only controls SAP2 transcription, but also affects Sap2 protein levels, possibly through general amino acid control (GAAC) pathway. DNA microarray analysis identifies other target genes downstream of Rhb1 in addition to SAP2. These findings provide new insights into nutrients, Rhb1-TOR signaling, and expression of C. albicans virulence factor.