Polo-like kinases: conservation and divergence in their functions and regulation

Polo-like kinases (Plks) are conserved from yeasts to humans but are absent from plants. The Plk family is characterized by the presence of an amino-terminal kinase domain and a carboxy-terminal Polo-box domain (PBD) that mediates protein interactions with target proteins.Plks fulfil many roles in cell division, including the regulation of entry into M phase, spindle pole biogenesis and function, mitotic chromosome dynamics, mitotic exit and cytokinesis. Many of these functions have been characterized in detailed molecular terms, although some pathways remain to be elucidated.The regulation of Plks is achieved through a combination of transcriptional control, reversible phosphorylation that changes the kinase activity, ubiquitin-dependent proteolysis and protein interactions. The precise mode of regulation differs between Plks and adapts according to the biological context.Roles for Plks outside of basic cell division are now fast emerging. Plks function in coordinating cell division in the developmental programme of various organisms, including flies and worms, in the establishment of asymmetry in several situations, and even in differentiated cells, such as human neurons.