Browsing by Subject "Cytokinesis"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Analysis of Aurora B Regulation and Signaling(2006-05-16) Oncel, Dilhan; Yu, HongtaoAurora B is a serine/threonine kinase that functions in a complex with two other chromosomal passenger proteins called INCENP and Survivin. Its function is implicated in a variety of processes related to mitosis, such as chromosome condensation, regulation of arm cohesion, spindle assembly, chromosome bi-orientation and cytokinesis. During the cell cycle, the level of this protein is tightly controlled and its deregulated abundance is suspected to contribute to aneuploidy. The cell cycle profile for Aurora B is reminiscent of those for substrates of the anaphase-promoting complex/cyclosome (APC/C), an ubiquitin ligase essential for mitotic progression. Here, we showed that Aurora B is a substrate of APC/C both in vitro and in vivo. Aurora B is efficiently ubiquitinated iv in an in vitro reconstituted system by APC/C that had been activated by Cdh1. The recognition of Aurora B by APC/CCdh1 is specific as it requires the presence of a conserved KEN-box motif at the amino terminus of Aurora B. Degradation of Aurora B at the end of mitosis requires Cdh1 in vivo as the reduction of Cdh1 level by RNA interference stabilizes Aurora B protein. We conclude that, as a key mitotic regulator, Aurora B is degraded by APC/CCdh1 in late mitosis. Aurora B lies at the heart of the cellular mechanism that resolves synthelic and merotelic attachments. A failure to eliminate such events results in gain or loss of chromosomes. Therefore, identifying the physiological substrates of Aurora B is of pivotal importance for research. We screened Aurora B substrates using an in vitro expression cloning system. However, the methodology we employed didn't lead to candidate substrates to be further validated by more rigorous in vivo approaches. The use of high concentrations of misfolded recombinant Aurora B was partially responsible for the loss of specificity. Therefore, purifying active recombinant Aurora B has become a primary goal for future biochemical and structural work. Two molecular chaperones Hsp90 and Cdc37 assist the folding of a variety of kinases in vivo, among which Aurora B is also a candidate. This gave us the final idea of expressing Aurora B-INCENP complexes in bacteria via the coexpression of Hsp90-Cdc37 molecular chaperones.Item CDC14 Coordinates Cyclin Destruction With the Onset of Cytokinesis(2004-08-19) Bembenek, Joshua Nathaniel; Yu, HongtaoThe Cdc14 family of protein phosphatases operate during the final stages of mitosis in various organisms. The Cdc14 phosphatases are downstream components of two homologous signaling pathways: the mitotic exit network (MEN) of S. cerevisiae and septation initiation network (SIN) of S. pombe. Studies of these pathways have revealed divergent roles of Cdc14. In the MEN pathway, Cdc14 is required for cyclin degradation by dephosphorylating Cdh1. The dephosphorylated form of Cdh1 binds to and activates a ubiquitin ligase known as the anaphase-promoting complex (APC/C), which then ubiquitinates mitotic cyclins, targeting them for degradation by the 26S proteosome. In contrast, Cdc14 of the SIN is dispensable for cyclin degradation, but plays an important role during cytokinesis. Two Cdc14 homologues are found in vertebrates, hCdc14A and hCdc14B. I have investigated the regulation of Cdc14 phosphatases to obtain insights into the mechanisms of mitotic exit in higher eukaryotes. Biochemical studies demonstrate that recombinant hCdc14A and hCdc14B can dephosphorylate human Cdh1 and stimulate APC/CCdh1 ligase activity in vitro. Since both the MEN and SIN pathways control Cdc14 localization, I have examined the regulation of the subcellular localization of hCdc14A, hCdc14B and the budding yeast Cdc14. In HeLa cells, hCdc14A localizes to the centrosome whereas hCdc14B is nucleolar during interphase. Both hCdc14 homologues localize to the centrosome and midbody during mitosis. In budding yeast, Cdc14p localizes to the nucleolus during most of the cell cycle and is released in late anaphase when it localizes to the centrosome and the bud neck. The subcellular localization the Cdc14 homologues in HeLa cells is regulated by a nuclear export signal. S. cerevisiae strains carrying only NES mutant CDC14 alleles are capable of degrading mitotic cyclins and escaping mitosis. However, they exhibit a temperature-sensitive phenotype at 37°C because they fail to complete cytokinesis and lack centrosome and bud neck localization of Cdc14. This demonstrates that the Cdc14 phosphatases are regulated by nucleocytoplasmic shuttling. Collectively, my work strongly suggests that the Cdc14 phosphatases play a conserved role in coordinating the destruction of mitotic cyclins with the execution of cytokinesis.Item From poles to equator: functional analysis of DdAurora during mitosis and cytokinesis in Dictyostelium discoideum(2007) Li, Hui, 1976-; De Lozanne, ArturoThe Aurora kinases are highly conserved serine/threonine kinases that play essential roles throughout mitosis. In metazoans, these functions are mediated by Aurora A and B at the spindle poles and the equatorial region respectively. I show here that Dictyostelium contains a single Aurora kinase, DdAurora that displays characteristics of both Aurora A and B. Like Aurora A, DdAurora has an extended N-terminal domain with an A-box and localizes to the spindle poles during early mitosis. Like Aurora B, DdAurora localizes to centromeres in metaphase, the central spindle during anaphase and the cleavage furrow at the end of cytokinesis. In addition to these known features of Aurora A and B, I found that DdAurora remains associated with centromeres during anaphase and telophase which has not been shown in any other organisms. INCENP is known to be an important binding partner of Aurora B. In Dictyostelium the conserved C-terminal IN-box domain of DdINCENP is essential for its interaction with DdAurora and for the localization of DdAurora to the central spindle. In contrast, the centromeric and spindle pole localization of DdAurora does not require an interaction with DdINCENP. Surprisingly, a truncated DdINCENP protein lacking the IN-box domain can still localize on centromeres and the central spindle even though it does not bind to DdAurora. I also found that the localization of DdAurora to the central spindle requires Kif12, a protein similar to mitotic kinesin like protein 2 (MKLP2). However, this requirement is suppressed by the overexpression of GFP-DdINCENP. GFP-DdINCENP can localize to the central spindle in the absence of Kif12 and it probably recruits DdAurora to the same location through their strong interaction. Finally, I demonstrated that Myosin II heavy chain is important for the proper localization of the DdAurora/DdINCENP complex to the cleavage furrow during late cytokinesis. With the exception of DdINCENP, no other binding partner or substrate of DdAurora has been identified in Dictyostelium. By performing large-scale immunoprecipitation in wild-type cells, I identified several potential binding partners/substrates of DdAurora, including topoisomerase B and HspA. Future esearch on these proteins may help to elucidate DdAurora function in different stages of M phase.Item Functional analysis of DdINCENP, a chromosomal passenger protein, in Dictyostelium(2006-08) Chen, Qian, 1975-; De Lozanne, ArturoDictyostelium DdINCENP is a chromosomal passenger protein associated with centromeres, the spindle midzone and poles during mitosis and the cleavage furrow during cytokinesis. Disruption of the single DdINCENP gene revealed important roles for this protein in mitosis and cytokinesis. DdINCENP null cells lack a robust spindle midzone and are hypersensitive to microtubule depolymerizing drugs suggesting that their spindles may not be stable. Furthermore DdCP224, a protein homologous to the microtubule-stabilizing protein TOGp/XMAP215, was absent from the spindle midzone of DdINCENP null cells. Overexpression of DdCP224 rescued the weak spindle midzone defect of DdINCENP null cells. While not required for the localization of the myosin II contractile ring and subsequent formation of a cleavage furrow, DdINCENP is important for the abscission of daughter cells at the end of cytokinesis. The localization of DdINCENP at the cleavage furrow is modulated by myosin II. Loss of myosin II restricted the localization of DdINCENP to a narrow zone at the cleavage furrow. Kif12, a homolog of mitotic kinesin like protein (MKLP), was essential for relocalization of DdINCENP from the central spindle to the cleavage furrow. Furthermore, Kif12 was also localized at the cortex of the cleavage furrow and its localization during cytokinesis closely resembled that of DdINCENP, suggesting a possible interaction between them. The correct localization of DdINCENP during cytokinesis also required its N-terminal sequence. DdINCENP1-500 was found at the cleavage furrow and interacted with the actin cytoskeleton. Domain analysis of DdINCENP also revealed that its DdINCENP1-500 was sufficient to rescue the weak spindle defect of DdINCENP null cells.