Polo-like kinases (Plks) are increasingly recognized as key regulators of mitosis, meiosis, and cytokinesis. Consistent with the broad range of functions proposed during cell division, Plk is subject to complex temporal and spatial controls. Recent discoveries have revealed the mechanisms of Plk regulation, they target different cellular structures by interacting with phosphorylated docking proteins. Moreover, information is emerging on the substrate specificity of Plks and the role of individual substrates in the M-phase progression.
The structure of Plk and the structure that regulates Plk. All Plks have similar structures, with a typical serine/threonine kinase domain at the amino terminus and a regulatory domain containing two signature motifs at the carboxy terminus, called the polo box (Figure 2). Although the genomes of Drosophila melanogaster, Saccharomyces cerevisiae and Schizosaccharomyces cerevisiae have only one Plk (Polo, Cdc5 and Plo1, respectively), the vertebrate species has several members of the Plk family. Play most of the functions of Polo, Cdc5 and Plo1. At present, little is known about the physiological roles of other vertebrate members of the Plk family (BOX 1), but given their structural similarity, many of the attributes established for Plk1 may also be related to other Plk.
Many proteins important for cell division are controlled by two basic mechanisms, phosphorylation and ubiquitin-dependent proteolysis. Regulation of Plk follows these general principles. Like many other protein kinases, Plk is activated by phosphorylation in a short region of the catalytic domain. The so-called T-ring. In vertebrate Plk, evolutionarily conserved threonine residues (Thr201 in Xenopus Thr201; Thr210 in Homo sapiens PLK1) were mutated to aspartic acid, mimicking T-ring phosphorylation and causing large number of kinases activate. Two kinases have been identified that can phosphorylate and activate Plx1 in vitro, namely Plkk1 (Plk kinase1) and PKA (protein kinase A). Plkk1 (Plk kinase-1) is equivalent to human SLK (SNF1-like kinase). Whether these kinases activate Plks under physiological conditions in vivo is unclear, and there may be other upstream regulatory kinases. In addition to the T-loop site, the conserved serine (Ser128 in Plx1; Ser137 in PLK1) is also mutated to aspartic acid. Large number of kinase activations are conferred, but it is unclear whether the site is actually phosphorylated in vivo. Several other phosphorylation sites have been located on Plx1, but their importance has yet to be determined.
In mitotic cells, Plk1 is not only related to spindles, but also to plants and animals. These centromere-related protein structures play an important role in two closely related processes62. First, they are sites where spindle microtubules attach to mitotic chromosomes. Second, they are essential for regulating the activity of a monitoring mechanism called a spindle (or spindle assembly) checkpoint. The purpose of this checkpoint is to ensure that all chromosomes have been bipolarly connected to the spindle before SISTER CHROMATIDS is separated and pulled apart (Box 3). Thus, the localization of Plk1 in mitochondria indicates the role of this kinase in mitochondrial assembly, mitochondrial-microtubule interactions, and spindle checkpoint regulation. Although these possibilities are not mutually exclusive, a better understanding of the function of Plk1 in kinetochore may have to wait for the identification of its structural substrate and binding partner.
Polo kinase involvement in cytokinesis was first demonstrated in fission yeast, where overexpression of Plo1 drives isolation at any stage of the cell cycle, while plo1 mutants cannot be isolated. A protein called Mid1 determines where the formation of contractile loops in the nucleus is due to phosphorylation of Plk. Recent studies on the role of mammalian Plk1 in cytokinesis have also identified kinetin-related motor Mklp2 and dynein subcomponent NudC as potential substrates for Plk1 that interact with PBD. Both Mklp2 and NudC have related kinesin activity and both are located on the central spindle. PLK1 has been found to phosphorylate the central spindlin subunit CYK4 in the middle of the spindle, allowing the recruitment of Rho guanine nucleotide exchange factor (GEF) ECT2 to promote RhoA activation and contraction of actin.