PK genes exist by the hundreds in all plant species in which they have been surveyed

manating from the spindle midzone has been proposed to promote hyper-condensation of trailing and lagging chromosome arms. In S. cerevisiae, Ipl1/Aurora B activity at the spindle midzone phosphorylates histone H3 on Ser10, keeping the trailing anaphase chromosome hyper-condensed until the chromosome has been cleared away from the spindle midzone. Similarly, in Drosophila S2 cells while Cdn2 homolog Barren disappears from anaphase chromosomes as sister 2353-45-9 site chromatids separate, Barren is enriched on lagging chromosomes near the spindle midzone and Aurora B activity is required, which is counteracted by PP1 and PP2A activity. Therefore, in addition to removing the CPC from anaphase chromosomes to allow for decondensation, the CPC relocation to the spindle midzone appears to actively mediate a surveillance mechanism by retaining condensin to prevent the decondensation of trailing and lagging chromosomes near the ingressing cleavage furrow. Collectively, CPC relocation may ensure that the level of chromosome decondensation is controlled until an effective separation of sister chromatids is achieved. The Role of CPC Removal from Anaphase Chromosomes in Nuclear Envelope Reformation Although the mechanism of chromosome decondensation with NER during mitotic exit is not well understood, the CPC also has a critical function in coordinating these two events. Following Cdk1 inactivation and extraction of poly-ubiquitylated Aurora B from anaphase chromosomes by Cdc48/p97, the reassembly of the nuclear pore complex starts on the periphery of the segregating chromatin in an ordered step-wise manner. In both Xenopus egg extracts containing sperm chromatin and C. elegans embryos, NER is impaired when Aurora B cannot be extracted from chromatin upon knockdown of Cdc48/p97, suggesting that, in addition to chromosome decondensation, removing the CPC from anaphase chromosomes might also be important for NER in telophase. In Drosophila S2 cells, NER is inversely correlated with Aurora B activity on anaphase chromosomes and with the proximity of the spindle midzone because NER of either laser microsurgery-generated acentric chromosome fragments or lagging chromosomes is significantly delayed compared to the main nuclei formed from efficiently segregated sister chromatids. In contrast, NER occurs simultaneously on all segregating chromosomes if Aurora B is retained on anaphase chromosomes by RNAi against MKLP2 homolog Subito or by global inhibition of Aurora B activity. Moreover, PP1 and PP2A phosphatases are required PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19812222 for counteracting Aurora B activity to promote NER. Therefore, CPC relocation from anaphase chromosomes to the spindle midzone also serves 7 Kitagawa and Lee CPC regulation in mitotic exit as a conserved feedback regulator that delays NER in response to incomplete chromosome separation. This feedback mechanism may allow for the correction and reintegration of lagging chromosomes into the main nuclei before the completion of NER, thereby preventing micronuclei formation. However, it is debatable whether this surveillance mechanism functions as the chromosome separation checkpoint as suggested or as part of the sequential mitotic exit events mediated by antagonizing phosphatases. Nonetheless, similar to controlling the timing of chromosome decondensation by the phosphorylation of the condensin I complex, balancing the levels of Aurora B activity on anaphase chromosomes and in the spindle midzone via controlling CPC relocation may be a key determinant o