Inhibition of the anaphase-promoting complex by the Xnf7 ubiquitin ligase.
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Degradation of specific protein substrates by the anaphase-promoting complex/cyclosome (APC) is critical for mitotic exit. We have identified the protein Xenopus nuclear factor 7 (Xnf7) as a novel APC inhibitor able to regulate the timing of exit from mitosis. Immunodepletion of Xnf7 from Xenopus laevis egg extracts accelerated the degradation of APC substrates cyclin B1, cyclin B2, and securin upon release from cytostatic factor arrest, whereas excess Xnf7 inhibited APC activity. Interestingly, Xnf7 exhibited intrinsic ubiquitin ligase activity, and this activity was required for APC inhibition. Unlike other reported APC inhibitors, Xnf7 did not associate with Cdc20, but rather bound directly to core subunits of the APC. Furthermore, Xnf7 was required for spindle assembly checkpoint function in egg extracts. These data suggest that Xnf7 is an APC inhibitor able to link spindle status to the APC through direct association with APC core components.
Published Version (Please cite this version)
Casaletto, Jessica B, Leta K Nutt, Qiju Wu, Jonathan D Moore, Laurence D Etkin, Peter K Jackson, Tim Hunt, Sally Kornbluth, et al. (2005). Inhibition of the anaphase-promoting complex by the Xnf7 ubiquitin ligase. J Cell Biol, 169(1). pp. 61–71. 10.1083/jcb.200411056 Retrieved from https://hdl.handle.net/10161/8379.
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Our lab studies the regulation of complex cellular processes, including cell cycle progression and programmed cell death (apoptosis). These tightly orchestrated processes are critical for appropriate cell proliferation and cell death, and when they go awry can result in cancer and degenerative disorders. Within these larger fields, we have focused on understanding the cellular mechanisms that prevent the onset of mitosis prior to the completion of DNA replication, the processes that prevent cell division when the mitotic spindle is disrupted, the signaling pathways that prevent apoptotic cell death in cancer cells and the mechanisms that link cell metabolism to cell death and survival.
In our quest to answer these important cell biological and biochemical questions, we are varied in our use of experimental systems. Traditionally, we have used cell-free extracts prepared from eggs of the frog Xenopus laevis which can recapitulate cell cycle events and apoptotic processes in vitro. For the study of cell cycle events, extracts are prepared which can undergo multiple rounds of DNA replication and mitosis in vitro. Progression through the cell cycle can be monitored by microscopic observation of nuclear morphology and by biochemically assaying the activity of serine/threonine kinases which control cell cycle transitions.
For the study of apoptosis, modifications in extract preparation have allowed us to produce extracts which can apoptotically fragment nuclei and can accurately reproduce the biochemical events of apoptosis, including internucleosomal DNA cleavage and activation of apoptotic proteases, the caspases.
More recently, we have focused on studying apoptosis and cell cycle progression in mammalian models, both tissue culture cells and mouse models of cancer. In these studies, we are trying to determine the precise signaling mechanisms used by cancer cells to accelerate proliferation and evade apoptotic cell death mechanisms. We also endeavor to subvert these mechanisms to therapeutic advantage. We are particularly interested in links between metabolism and cell death, as high metabolic rates in cancer cells appear to suppress apoptosis to evade chemotherapy-induced cell death.
Finally, we also have several projects using the facile genetics of Drosophila melanogaster to further understand links between metabolism and cell death and also the ways in which mitochondrial dynamics are linked to apoptotic pathways.
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