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Evolutionarily conserved developmental pathways
Mitosis is a highly regulated process that assures the proper allotment of genetic material between each pair of daughter cells. It proceeds through successive stages of well defined and coordinated sub-processes. Entry into mitosis is regulated by the cdc2/Cyclin B heterodimer. Cdc2/Cyclin B activity drives the events of early mitosis, such as nuclear breakdown, chromosome condensation and spindle formation by phosphorylating cellular substrates. While cdc2 (the catalytic subunit of the heterodimer) is required to drive the events of early mitosis, it must then be inactivated to allow the events of late mitosis to proceed.
Polo is an evolutionarily conserved kinase, active during mitosis. In Drosophila the Polo kinase activity peaks from between late anaphase to telophase, later than the peak of cdc2-Cyclin B kinase activity (which is highest in cells just entering mitosis). polo mutants show an accumulation of cells with condensed chromosomes. Although the extent of chromosome condensation is more like that normally seen in metaphase, the distribution is more typical of prophase; the chromosomes are not aligned at the metaphase plate. Thus it appears that chromosome condensation continues to occur even though other aspects of mitosis, such as alignment at the metaphase plate are delayed (Llamazares, 1991).
polo also exhibits a meiotic phenotype. Meiotic spindles found in mutant testes are generally irregular in shape and structure. The irregular shape of the Nebenkern (mitochondria) in mutants indicates an unequal partitioning of mitochondria on the meiotic spindles, and ultimately into daughter cells. Non-disjunction (separation) of chromosomes during meiosis, is also apparent (Sunkel, 1988).
How does Polo kinase affect mitosis? One must look to research in other organisms for an answer to this question. In Xenopus a Polo related kinase (Plx1) targets Cdc25 (Drosophila homolog: String), the phosphatase that dephosphorylates and consequently activates the cyclin dependent kinase cdc2. Xenopus Plx1 undergoes activation and phosphorylation by multiple kinases at mitosis; it is a kinase that associates with and phosphorylates the amino-terminal domain of Cdc25. It is likely that Plx1 participates in the control of mitotic progression by targeting Cdc25 (Kumagai, 1996).
Studies in yeast suggest additional roles for a Polo-like kinase. The budding yeast gene CDC5 and S. pombe gene plo1 are homologous to polo. Loss of plo1 function results in mitotic arrest; condensed chromosomes are associated with a monopolar spindle, suggesting a failure in formation of a normal bipolar spindle. In addition, mutation can also result in the failure of septation following the completion of nuclear division. In the latter case, cells show a failure both in the formation of a filamentous actin ring, and in the deposition of septal material, suggesting that PLO1 protein is required high in the regulatory cascade that controls septation. Overexpression can also induce septum formation in G2 cells. It therefore appears that PLO1 is involved in a cascade that leads to bipolar spindle formation, and in a separately regulated chain of events that results in actin ring formation and septum deposition, both of which are required for septum formation (Ohkura, 1995).
Yeast plo1 shows a genetic interaction with cut7 (Ohkura, 1995), closely related to Drosophila kinesin-like protein KLP61F that is essential for mitosis. In the absence of KLP61F function, spindle poles fail to separate, resulting in the formation of monopolar mitotic spindles. KLP61F is specifically expressed in proliferating tissues during embryonic and larval development, consistent with a primary role in cell division. KLP61F is important for spindle pole separation and mitotic spindle dynamics. Does Polo target kinesins in higher eukaryotes? (Heck, 1994)
A Polo mammalian homolog has been shown to target a Kinesin protein involved in the cross-bridging of antiparallel microtubules during mitosis. Murine PLK is a Polo like kinase that accumulates after serum stimulation of tissue culture cells. In cells that are continuously cycling between growth and mitosis, Plk protein begins to accumulate at the DNA synthetic/G2 phase boundary and reaches a maximum level at the G2/Mitosis boundary. Plk enzymatic activity gradually decreases as mitosis proceeds, but persists longer than cyclin B-associated cdc2 kinase activity, a similar expression pattern to that found with Drosophila Polo. At the interzone in anaphase, Plk is localized to the area surrounding the spindle axis; during telophase and cytokinesis, it finally concentrates at the midbody. Plk and Mitotic kinesin-like protein 1 (MKLP-1), which induces microtubule bindling and antiparallel movement in vitro, are colocalized during late M phase. In addition, MKLP-1 appears to interact with Plk in vivo and to be phosphorylated by Plk kinase activity in vitro. It thus appears that Plk targets a kinesin protein involved in the structuring of microtubules (Lee, 1995).
It is not yet clear whether Polo acts independently of cyclin dependent kinase activity or acts downstream of cyclin dependent kinase. Yeast CDC5 appears to be cdc2 dependent (Ohkura, 1995), while murine Plk activity is not directly regulated by cdc2 or MAP kinase. Neither kinase is able to phosphorylate or regulate the activity of Plk (Lee, 1995). In either case, Polo kinase provides an excellent example of the complexity of regulation that accompanies mitosis. The conservation of Polo and its function in organisms as diverse as yeast and mammals suggests that the cascade of events that regulate mitosis are evolutionarily conserved. Glover (1996) provides an excellent review or the role of Polo in mitosis.
date revised: 20 August 2005
Developmental Pathways conserved in Evolution
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