Trafficking between the nucleus and cytoplasm occurs through the nuclear pore complexes (NPCs). Kinetochores are proteinaceous structures that assemble at the centromere of each sister chromatid during mitosis, which serve as sites of spindle microtubule attachment. The spindle assembly checkpoint (SAC) is a cell cycle regulatory pathway that prevents the onset of anaphase until all chromosomes are properly attached to the mitotic spindle and aligned on the metaphase plate. Kinetochore-associated SAC components accomplish this task by monitoring the attachment of spindle microtubules to kinetochores. The relationship between mitotic kinetochores and the NPC is both surprisingly intimate and poorly understood. NPCs consists of around thirty proteins, called nucleoporins. During interphase, a number of kinetochore proteins also stably bind to NPCs (e.g., Mad1, Mad2, Mps1). During mitosis, metazoan NPCs disassemble, and at least a third of nucleoporins associate with kinetochores, including the RanBP2 complex and the Nup107-160 complex. We have shown that these complexes play important roles in kinetochore function. A number of other nucleoporins that do not associate with kinetochores have also been shown to have important mitotic roles, including Nup214, Nup98 and TPR. The RanBP2 complex consists of RanBP2 (a large nucleoporin that is also known as Nup358), SUMO-1-conjugated RanGAP1 (the activating protein for the Ran GTPase) and Ubc9 (the conjugating enzyme for the SUMO family of ubiquitin-like modifiers). This complex associates with kinetochores in a microtubule-dependent manner. Disruption of RanBP2 association to kinetochores causes defective mitotic spindle assembly, and RanBP2 has been independently implicated as being important for interphase microtubule organization. Our current studies concentrate on two questions related to the RanBP2 protein: First, we are investigating proteins that interact with the mammalian RanBP2 complex and that may be essential for its activity in mitosis. Second, we are investigating the association of RanBP2 with RanGAP1 in species that do not utilize a SUMO-based mechanism for formation of the RanBP2 complex, in order to determine whether RanBP2 plays a fundamentally different mitotic role in these organisms. At the same time, we are working to develop quantitative assays for vertebrate chromosome mis-segregation. The development of conceptually simple sectoring assays provided a powerful genetic tool in yeast to assess rates of chromosome mis-segregation and to identify mutants deficient in this process. In the absence of vital assays for vertebrate chromosome segregation, mitotic abnormalities can only be scored in live assays through the gross mis-segregation of multiple chromatids, leading to obviously unequal distribution of chromosomes to daughter cells, or through the development of highly abnormal structures, such as micronuclei or/and chromosome bridges. More subtle defects, including the kind of chromosomal instability (CIN) found in many solid tumors, can currently only be monitored through more laborious assays involving karyotype analysis or fluorescent in situ hybridization. We are working with Vladimir Larionovs laboratory, who pioneered human artificial chromosomes (HACs) as gene therapy tools for efficient and regulated expression of genes of interest, to develop practical assays for chromosome mis-segregation in vertebrate cells. These assays will allow straightforward, quantitative assessment of CIN under a variety of conditions. The assays will be employed particularly to monitor the impact of changes in nucleoporins, as well as in the Ran and SUMO pathways. We expect that they will also be of general utility to others in the mitosis field. Finally, we are using Xenopus egg extracts (XEEs), an extremely useful system for ex vivo studies on mitosis, to understand how cells inactivate the SAC after chromosomes have become attached to spindles and aligned on the metaphase plate. We have particularly examined the role of p31comet, a protein found in higher eukaryotes that participates in SAC downregulation. We found that depletion of p31comet delays anaphase onset in XEE, suggesting that it helps to control mitotic timing in this system. This delay was dependent upon an intact SAC, consistent with the idea that p31comet works primarily through SAC silencing. We examined the regulation of p31comet and found that it is mitotically phosphorylated. Although a number of well-established mitotic kinases did not efficiently modify p31comet in vitro, IKK-β(Inhibitor of nuclear factor κ-B kinase-β) was effective for p31comet phosphorylation. Depletion or inhibition of IKK-βdelayed mitotic exit of XEEs, and a phosphomimetic mutant of p31comet in which IKK-βtarget sites were altered to glutamic acid residues showed increased activity in dissociation of soluble MCC and particularly in disruption of kinetochore binding for SAC components. Taken together, our results suggest that p31comet actively controls the timing of anaphase onset in XEE through antagonism of the SAC and that IKK-βmodifies and stimulates p31comet to enhance this activity.
