The polo-like kinase PLKA is required for initiation and progression through mitosis in the filamentous fungus Aspergillus nidulans

Polo-like kinases (PLK) function during multiple stages of mitotic progression and in cytokinesis. We identified and cloned a PLK homologue in Aspergillus nidulans, plkA, which is the first PLK reported in a filamentous fungus and the largest member of the PLK family to date. As plkA was essential, the effects of overexpression and localization of protein in living cells were explored to determine PLKA function. Overexpression of PLKA permitted hyphal formation, but blocked nuclear division in interphase. In NIMA or NIMT temperature-sensitive backgrounds, overexpression of PLKA impaired normal entry into mitosis upon release from restrictive temperature, supporting a role for PLKA during G2/M. In the few mitotic cells present, spindles were monopolar or disorganized, and chromatin condensation and segregation were impaired, suggesting additional roles for PLKA in spindle formation and in chromosome dynamics. Consistent with this, green fluorescent protein (GFP)-tagged PLKA could localize to the spb during interphase, and to the spb and nucleus throughout mitosis. Intriguingly, PLKA remained on the spb during telophase and into G1, in contrast to other PLK. In addition, spb localization was independent of NIMA function, unlike that demonstrated in Schizosaccharomyces pombe where PLK localization to the spb required the NIMA homologue Fin1. PLKA was not detected at cortical, septation-associated sites, and overexpression did not drive septum formation, also in contrast to that observed with other PLK. Therefore, PLKA is important for multiple events during mitosis, similar to PLK in higher organisms, but exhibits differences in size, localization and influence on septation/cytokinesis, suggesting additional novel regulatory features.