Abstract

Candida albicans is a commensal fungus in humans, but can cause infections with high mortality rates. Cell proliferation and differentiation between yeast and hyphae are important for virulence and survival in the host. An understanding of their regulation may reveal new targets for therapeutic strategies. My work aimed to elucidate the regulation of mitosis in C. albicans, a poorly understood process, and focussed on characterizing the roles of a Polo-like kinase (Plk), Cdc5p. Plks are critical regulators of many aspects of mitosis in diverse organisms. Previous work demonstrated that depleting Cdc5p in C. albicans yeast resulted in a block in spindle elongation and mitosis, followed by formation of filaments and expression of hyphal-specific virulence genes, under yeast growth conditions. However, the mechanisms underlying Cdc5p function, mitotic progression, formation of novel filaments and expression of virulence genes, remained unclear.

In order to address these questions, I first investigated a putative Cdc5p target, the Anaphase-Promoting Complex/Cyclosome (APC/C). In Chapter 2, I showed that APC/C co-activators Cdc20p and Cdh1p had some conservation in mitotic function, and may lie downstream of Cdc5p. However, additional novel features suggest variations in the mitotic circuitry.

I next investigated the identity of the Cdc5p-depleted filaments, since this was controversial. We hypothesized that the cells were elongated yeast buds that failed to switch from polar to isometric growth, but adapted a hyphal fate over time due to maintenance of polarized growth. In Chapter 3, time course assays demonstrated that hyphal-diagnostic features emerged in Cdc5p-depleted filaments at only later growth stages, in agreement with our hypothesis. Our results expand on the strategies that C. albicans can utilize to modulate growth and virulence determinants.

I further addressed the mechanisms of Cdc5p function during mitosis and morphogenesis in Chapter 4 by screening for interacting factors. I discovered a novel, Candida-specific factor, CPI1 (C. albicans Plk-interacting protein), which was not essential for growth but required to maintain mitotic arrest and may interact with the splicesome. Collectively, the work enhances our knowledge of mitotic regulation in C. albicans, and underscores variations in the regulatory circuitry that have important implications for controlling growth.