Abstract

C. albicans is an important fungal pathogen of humans, and an understanding of the regulation of its cell cycle may reveal new targets for anti-fungal therapies. We previously characterized the C. albicans Anaphase Promoting Complex/Cyclosome regulatory co-factors Cdc20p and Cdh1p, and showed that they are important for the metaphase-to-anaphase transition and mitotic exit. In order to determine the mechanisms of action of Cdc20p, we searched the genome for factors that bind it in other systems, and found that C. albicans lacks the conserved Cdc20p target, securin. Securins bind and prevent separase from cleaving cohesin, and must be targeted for degradation by Cdc20p for separase-dependent sister chromatid separation. We hypothesized that C. albicans contains a divergent securin, which may be uncovered by identifying proteins that bind separase. We demonstrated that the C. albicans separase homologue, ESP1, is required for chromosome segregation. We then identified Esp1p-interacting factors using affinity purification and mass spectrometry, and uncovered Orf19.955p, a protein of unknown function with homologues only in Candida species. Orf19.955p contains Destruction and a KEN box, like other securins. While not essential, depletion impairs cell growth and phenotype. Chromosomes could segregate; unlike in Esp1p-depleted cells, but chromosome organization and microtubules were adversely affected. In contrast, depletion of securin Pds1p in S. cerevisiae prevented chromosome separation and phenocopied absence of Esp1p. Future experiments will clarify if Orf19.955p is a securin. Collectively our results provide new insights on factors involved in the metaphase-to-anaphase transition, and thus cell proliferation in C. albicans, including a novel fungal-specific protein.