MOD_Plk_2-3

Polo-like kinases (Plks) are Ser/Thr kinases crucial for several events in eukaryotic cell division. The first of these kinases was initially identified from a mitotic mutant of Drosophila melanogaster and was named ‘‘POLO’’ due to the presence of abnormal spindle poles (Llamazares,1992). Apart from the chordates, most Eukaryotes have a single Plk. Family members are well conserved though naming is variable, budding yeast (cdc5), Drosophila (polo kinase), Xenopus (Plxs) and mammals (Plks) (Liu,2013). Plks are characterized by a conserved N-terminal kinase domain (KD) linked to a C-terminal domain with one or more polo box domains (PBDs) which mediates protein interactions with targets and regulates the activity of the kinase domain (Elia,2003). The pair of PBDs are capable of acting in concerted and mutually inhibitory manner to regulate the Plk activity.

In humans, five Polo-like kinase variants (Plk 1-5) exist although the Plk5 kinase domain is truncated and non-functional (Zitouni,2014). Plk1 is by far the most studied and is probably closest to the function of the ancestral Plk. Its expression is cell cycle dependent, barely detectable in G1 and S phase, gradually increases in G2 phase, and peaks in M phase and its degradation occurs during mitotic exit. The dynamic localization of Plk1 to various subcellular structures during successive stages of cell cycle is mainly attributed to its PBD-dependent interaction with specific phosphoepitopes present in different subcellular locations. Plk1 is capable of binding its substrates at Ser/Thr residues which are either pre-phosphorylated by non-self priming by a Pro-directed kinase or self-priming by Plk1 itself. Upon phosphopeptide binding, the auto-inhibitory interaction between KD and PBD is relieved and Plk may phosphorylate the same protein on different residues or another protein nearby.

The growing number of reported substrates indicates complexity of Plk1 regulation and the significant role of Plk1 in different stages of mitosis and beyond. Plk1 is found in several locations throughout most of the cell cycle and its main functions start in G2, during which it localizes to centrosomes. Further, Plk1 is important for regulating mitotic entry in vertebrates. In prophase, Plk1 acts on multiple points. It activates cdk1 by removing its inhibitory phosphorylation by activating the Cdc25c phosphatase (Toyoshima-Morimoto,2002). Moreover, it inactivates Wee1 (Watanabe,2004) and Myt1 (Inoue,2005) through their phosphorylation. Plk1 is also involved in regulating the spindle morphology through γ-tubulin recruitment to centrosomes. Plk1 helps in tubulin nucleation by interacting with many proteins and phosphorylating them. Plk1 recruits PCM proteins like CEP192, pericentrin, CEP215 and Nedd1 that are involved in γ‑tubulin recruitment and PCM reorganization (Zhang,2009). Plk1 phosphorylation of the centrosomal protein kizuna preserves the cohesion of the PCM, which would otherwise become fragmented (Oshimori,2006). Together with other Plks, Plk1 also plays a role in the coordination of the centriole cycle with the cell cycle, by controlling centriole disengagement and maturation. It is also involved in mitotic sister chromatid separation, kinetochore–microtubule attachment and regulation of the SAC. Upon anaphase onset, Plk1 is recruited to the central spindle and involved in cleavage furrow formation and cytokinesis. Plk1 phosphorylates HsCYK-4, a component of the central spindlin complex and enables binding between HsCYK-4 and Ect2, another regulator of cell division. Bound Ect2 then communicates with the machinery that assembles the actin- and myosin-based contractile ring, leading to division of the cell into two daughters (Wolfe,2009). Plk2 also known as SNK localizes at the centrosome and peaks in early G1 phase and controls the entry in S phase. It is implicated in cell division, oncogenesis, and synaptic regulation of the brain and inhibition of Plk2 has been implicated in formation of aberrant number of centrioles. The identification of Plk2 phosphorylated α-synuclein in Lewy bodies in Parkinson disease make them a very promising target for Parkinson disease treatment (Inglis,2009). Plk3 (also named FNK or PRK) localizes to the nucleolus and is involved in regulation of the G1/S phase transition. It is described as an essential player in the regulation of the hypoxia signaling pathway (Xu,2010) and apoptosis induction (Helmke,2016). In S phase, it regulates the DNA replication with the phosphorylation of proteins like topoisomerase IIα (Iida,2008). Plk4 is the most sequence-divergent member of the family and resulted from a duplication of a polo like or Plk1‑like ancestral gene before the divergence of fungi and animals, although it is only found in “primitive” fungi like Chytrids. Plk4 is the main regulator of centriole biogenesis in metazoans. Human Plk4 phosphorylates centromere proteins STIL and this phosphorylation event is required for centriole duplication (Moyer,2015). Depletion of Plk4 results in a failure to build new centrioles and, overexpression of Plk4 leads the assembly of excessive numbers of newly formed centrioles (Zitouni,2014).

Plks 1 to 3 use their pair of polo boxes to bind other proteins that have been previously phosphorylated recognizing the sequence S-pS/T in the interacting proteins as a docking site and are then able to further phosphorylate their targets. Plk4 differs from the other members of the Plk family in that it only has one recognisable Polo-box though it has a second very divergent “crypto” Polo-box indicating it has a different mode of regulation (Lowery,2005). Both the Plk4 Polo boxes are independently able to localize the kinase to the centrosome, acting as protein interaction modules (Sillibourne,2010).

Plks, especially Plk1 and Plk4, are regarded as targets in cancer therapy. Over-expression of Plk1 is associated with many type cancers including non-small-cell lung cancer, head and neck cancer, easophageal cancer, gastric cancer, melanomas, breast cancer, ovarian cancer, endometrial cancer, colorectal cancer, gliomas, and thyroid cancer. Currently Plk1 is the most validated anti-cancer drug target and its selective inhibition through various techniques is a potential approach for future cancer therapy (Takai,2005).