Abstract

Candida albicans is an important fungal pathogen of humans. Understanding the regulation of its cell cycle and mechanisms governing the metaphase-to-anaphase transition may reveal new targets for therapeutic development. This process is dependent on several factors including; separase, a conserved cohesin protease, the separase regulator securin, the Anaphase Promoting Complex/Cyclosome (APC/C) and its cofactors Cdc20p and Cdh1p, as well as the cohesin complex. Although Cdc20p and Cdh1p have been previously characterized in C. albicans, a detailed framework of metaphase-to-anaphase progression remains elusive. We provide the first characterization of separase in C. albicans, and demonstrate its putative interactors, collectively implying conserved and novel functions. We hypothesized one putative interactor; a divergent securin called Eip1p (Esp1-Interacting Protein 1). Subsequent characterization demonstrated that Eip1p is Candida-specific, important for chromosome segregation, and exhibited conserved securin-like features including stabilization in the presence of DNA damage/stress agents. Moreover, Eip1p depletion partially suppressed a metaphase block induced by these agents. Eip1p was reduced upon Cdc5p depletion, suggesting regulation via degradation, a diagnostic feature of securins. A proportion of Eip1p-depleted cells also exhibited novel phenotypes including misoriented spindles and maintenance of elongated spindles. However, Eip1p was not consistently or strongly enriched in the absence of Cdc20p unlike other securins, and its mechanisms of action remained unclear.
Since Eip1p is functioning like a securin, we hypothesized that its regulation includes degradation mediated by the APC/C Cdc20p, and inconsistent modulation in Cdc20p-depleted cells may be due to an incomplete Cdc20p-arrest phenotype. To test this, we investigated the APC/C subunit Cdc16p and provide the first characterization of an APC/C subunit in C. albicans. Bioinformatic analyses revealed sequence conservation of Cdc16p. Its depletion caused an early mitotic arrest, and showed enrichment of the mitotic cyclin Clb2p. Eip1p was strongly enriched upon Cdc16p depletion, suggesting conserved function of Cdc16p in C. albicans. Mechanisms of action of Eip1p were addressed through affinity purification of Eip1p and mass spectrometry, revealing some unique putative protein interactors. Overall, we present evidence of a new securin, a potential drug target, and novel insights on mitotic regulation in C. albicans.