Role of Tem1 in Signalling Mitotic Exit in the Human Fungal Pathogen Candida albicans

Abstract

The human pathogen Candida albicans is polymorphic, and its ability to switch growth forms is thought to play an important role in virulence. The primary research aim of this thesis was to understand the role the mitotic exit network plays in C. albicans with particular focus on the Tem1 GTPase protein. This aim was split into three specific goals; to study the role of Tem1 through the construction of a regulatable tem1 mutant, to understand the regulation of Tem1 through localisation and protein interaction studies, and to construct new molecular tools utilising the NAT1 positive selection marker in order to achieve two previous goals. In this thesis we demonstrated that TEM1 is an essential gene in C. albicans, and its essential function is signalled through the Cdc15 protein. Surprisingly, Tem1p depleted cells arrested as hyper-polarised filaments containing one or two nuclei and ultimately lost viability. These filaments formed from budding yeast cells, suggestive of a blockage late in the cell cycle. Ultimately the failure of these filaments to undergo cytokinesis was linked to a defect in septin ring dynamics and the formation of actomyosin ring. To understand the regulation of Tem1 we localised both the Tem1 and Lte1 proteins and found that Tem1 localised to spindle pole bodies in a cell-cycle dependent fashion by recruited at the onset of S phase. In contrast, the Lte1 homolog localised to the daughter cell cortex prior to release into the cytoplasm at the end of the cell cycle. A yeast 2-hybrid analysis of the MEN components demonstrated the potential of Bfa1/Bub2 and Tem1 to form a complex and the ability of Tem1 to homodimerise which may play a role in its self-activation. In order to carry out various aspects of this work we constructed a fully functional set of cassettes, including the constitutively active ENO1 promoter, V5-6xHIS epitope tag and various fluorescent protein genes fused to the NAT1 positive selection marker. When considered together, these results indicate that Tem1 is required for timely mitotic exit and cytokinesis in C. albicans, similar to S. cerevisiae, but the final output of the pathway must have diverged.