Abstract

Candida albicans is an important fungal pathogen of humans, and its ability to switch between different cell morphologies, including yeast, pseudohyphae and hyphae, is critical for virulence. The cell cycle plays an important, yet poorly understood, role in regulating cellular morphogenesis. Our previous work demonstrated that blocking the yeast cell cycle in mitosis, through depletion of the polo-like kinase Cdc5p, resulted in polarized growth of the yeast bud, producing filamentous cells that were distinct from pseudohyphae or true hyphae. Polarized growth was partially dependent on the spindle checkpoint factor Bub2p, suggesting that checkpoint-activated growth occurs in C. albicans . In contrast, similar checkpoint activation leads to cessation of cell proliferation in most other systems. In order to elucidate how mitotic progression and spindle checkpoints are linked to morphogenesis, a better understanding of the basic regulation of mitosis is required. To this end, we characterized homologues of Cdc20p and Cdh1p, which are targets of the spindle checkpoint and activators of the ubiquitin ligase anaphase-promoting complex (APC), a major regulator of mitotic progression in most systems. Cdc20p and Cdh1p were important for the metaphase-to-anaphase transition and mitotic exit, similar to their counterparts in S. cerevisiae , but strongly influenced morphogenesis in a different manner. Deletion of CDH1 resulted in a pleiotropic phenotype, including some enlarged yeast cells, while absence of the same factor in S. cerevisiae produced small cells. Absence of Cdc20p produced highly polarized yeast buds that resembled Cdc5p-depleted cells, in contrast to the large doublets resulting from loss of Cdc20p in S. cerevisiae . Overexpression of CDC20 in Cdc5p-depleted cells partially suppressed the cell cycle defect, suggesting that Cdc20p may be downstream of Cdc5p. However, polarized growth was not abolished. Deletion of the putative Cdc20p-binding and spindle checkpoint factor Mad2p partially compromised polarized growth in Cdc5p-depleted cells, implying that Mad2p may play a role in linking Cdc5p function with Cdc20p. Deletion of CDH1 had no effect on Cdc5p-dependent polarization, suggesting that Cdh1p is dispensible for the process. While Cdc20p was not required for serum-induced hyphal growth, some cells lacking Cdh1p were not able to form hyphae, yet invaded agar more readily than control cells, suggesting a complex role for this factor in regulating polarized growth. Thus, we provide genetic evidence that Cdc20p and Cdh1p play important roles in regulating mitosis and morphogenesis in C. albicans , but in a different manner than their homologues in S. cerevisiae . Our results extend our knowledge of the regulatory circuit governing mitosis in C. albicans and the potential pathway underlying checkpoint-activated polarized growth.