Abstract

The Polo-Like Kinases (PLK) comprises a family of conserved serine/threonine kinases that play critical roles throughout the cell cycle. PLK are up-regulated in various cancers, and thus are being targeted by several anti-cancer therapies. While higher organisms contain multiple PLK homologues, lower organisms contain a single copy, which is essential. Aspergillus nidulans is a filamentous fungus that is used as a model organism for eukaryotic cell cycle research. A previous study isolated PLKA in A. nidulans, the first polo-like kinase reported in any filamentous fungus, but the functions were not clear. In this study, we explored the function of PLKA through constructing strains deleted for plkA or carrying a single copy under control of the regulatable alcA promoter. Surprisingly, our results show that plkA is not essential, despite being the single PLK in A. nidulans. However, it is required for proper colony growth and polar axis formation in hyphae, since colonies were compact and hyphae were multi-branched with split tips in the absence of the gene. The absence of plkA also resulted in numerous abnormal spindle patterns and an abnormally high proportion of cells with telophase spindles, suggesting roles in spindle formation and possibly mitotic exit, as seen with other PLK. Chromosome segregation was also abnormal in a significant proportion of cells, which has not been seen with other PLK mutants. Nuclear division could occur in the absence of plkA, suggesting that it is not essential for nuclear division. However, absence of plkA partially suppressed the G2/M block in nimT Cdc25c cells at restrictive temperature, suggesting a novel role for PLKA in negatively regulating the G 2 /M transition. Surprisingly, PLKA was not required for septation/cytokinesis, unlike all other PLK. PLKA may be carrying out its functions in part through regulating microtubule dynamics, since the compact growth phenotype was cold-sensitive, and could be suppressed by low doses of the microtubule-destabilizing drug benomyl. Finally, the absence of plkA also resulted in a repression of asexual development but induction of sexual structures, suggesting a novel role for this cell cycle regulator in controlling developmental processes. Thus, despite the fact that PLKA is not essential in A. nidulans, we have demonstrated that the gene has conserved but also novel functions in regulating the cell cycle and, intriguingly, development. The results expand our current knowledge of the functions of an important group of cell cycle regulators, and thus have important implications for the control of cell growth and the relationship between the cell cycle and development.