The Polo-Like Kinase PLKA in Aspergillus nidulans Is Not Essential but Plays Important Roles during Vegetative Growth and Development

Abstract

The Polo-like kinases (Plks) are conserved, multifunctional cell cycle regulators that are induced in many forms of cancer and play additional roles in metazoan development. We previously identified plkA in Aspergillus nidulans, the only Plk investigated in filamentous fungi to date, and partially characterized its function through overexpression. Here, we report the plkA null phenotype. Surprisingly, plkA was not essential, unlike Plks in other organisms that contain a single homologue. A subset of cells lacking PLKA contained defects in spindle formation and chromosome organization, supporting some conservation in cell cycle function. However, septa were present, suggesting that PLKA, unlike other Plks, is not a central regulator of septation. Colonies lacking PLKA were compact with multibranched hyphae, implying a role for this factor in aspects of hyphal morphogenesis. These defects were suppressed by high temperature or low concentrations of benomyl, suggesting that PLKA may function during vegetative growth by influencing microtubule dynamics. However, the colonies also showed reduced conidiation and precocious formation of sexual Hülle cells in a benomyl-and temperature-insensitive manner. This result suggests that PLKA may influence reproduction through distinct mechanisms and represents the first example of a link between Plk function and development in fungi. Finally, filamentous fungal Plks have distinct features, and phylogenetic analyses reveal that they may group more closely with metazoan PLK4. In contrast, yeast Plks are more similar to metazoan proteins PLK1 to PLK3. Thus, A. nidulans PLKA shows some conservation in cell cycle function but may also play novel roles during hyphal morphogenesis and development.

Functional Analysis of the Aspergillus nidulans Kinome

“…This confirms a recent finding for PlkA [49] and suggests that these kinases negatively regulate the developmental switch from asexual to sexual reproduction. PlkA is a polo like kinase and members of this family have developmental functions in mammals, flies and worms [105]–[108].…”

“…These color differences correlated with a dark appearance of the underside of colonies grown on minimal media at 37° (Figure 5A), conditions which promote sexual development. Further examination indicated that the aberrant colony color of the plkA (AN1560), An-gin4 (AN11101) and An-mst1 (AN5674) mutants was associated with visible morphological hallmarks of sexual development, as previously reported for plkA mutants [49]. By 4 days, wild type colonies contained large numbers of asexual conidiophores with little or no evidence of sexual development.…”

Protein Phosphatase 2A (PP2A) Regulatory Subunits ParA and PabA Orchestrate Septation and Conidiation and Are Essential for PP2A Activity in Aspergillus nidulans

“…Moreover, it has been verified that proteins highly localized on septa and conidiophores or nuclei, such as AnAXL2 (localized in conidiophores), PLKA (localized in the spindle pole body [SPB] and the nucleus), and BudA (localized in septa), may play important roles during asexual and sexual development. Consequently, defects of these proteins produce abnormal nuclear division or septum formation (61)(62)(63)(64). Thus, we hypothesize that defects caused by parA and pabA may be due to the reasons for the defects in mitosis and cytokinesis.…”

The spindle pole body of Aspergillus nidulans is asymmetrical and contains changing numbers of γ-tubulin complexes

“…Another possibility could be that MztA might be involved in the regulation of spindle formation through interaction with specific kinases, such as the polo-like kinase PlkA. PlkA plays important roles in spindle assembly and corresponding mutants exhibit similar spindle defects to those seen in mztA mutants (Bachewich et al, 2005;Mogilevsky et al, 2012).…”