Although multipolarity is often a consequence of centrosome abnormalities in cancer cells, several studies have shown that the amplified centrosomes coalesce and form a bipolar spindle. This has also been demonstrated in normal cells forced to have a double complement of DNA and centrosomes: Compound Library inhibitor retinal pigmented epithelial cells treated with a cytokinesis inhibitor are able to cluster the centrosomes to form a bipolar spindle and proceed through the cell cycle. Minus-enddirected microtubule motor proteins are involved in this clustering process: inhibition of dynein in fibroblasts leads to disassociation of clustered centrosomes and Drosophila kinesin 14 motor protein Ncd is required for focusing of spindle poles and maintaining spindle bipolarity when centrosome amplification is induced. These data demonstrate a cellular response pathway for repairing centrosome and spindle assembly defects. The spindle pole body is the functional equivalent of the mammalian centrosome in Saccharomyces cerevisiae and organizes microtubules for chromosome segregation in mitosis and meiosis. The SPB is not a static structure. Instead, the SPB is remodeled in two ways: by growth, in which new components are added, and by exchange, in which old components are replaced by new components. These changes are cell cycle dependent, with growth occurring late in the cell cycle, and exchange occurring around the time of SPB duplication leading to the parent SPB having a mix of old and new components. Cell cycle arrests have various effects on these remodeling phenotypes. When arrested in G1 with a-factor, the SPB core becomes smaller. Conversely, when cells are arrested at metaphase, the SPB core grows. For example, overexpression of Mps1 kinase, which activates the spindle assembly checkpoint, causes SPBs to double in size. Based on the fact that the SPB is remodeled at discrete times during the cell cycle and in response to checkpoint activation, this process is likely to be important for maintenance of the SPB and possibly for assembly of the spindle. SPB remodeling was observed by tagging the integral SPB component Spc110 with fluorophores and using quantitative fluorescence to determine the level of incorporation or exchange of labeled protein. lmodulin. One protein that has been previously shown to affect assembly of SPB components is Mlp2, a nuclear pore-associated protein that binds to SPB core components and affects their assembly into the SPB. Deletion of MLP2 leads to formation of smaller SPBs, and combining Mlp2 depletion with spc110-220 exacerbates the assembly defect and is lethal. These data make Mlp2 a likely SPB remodeling factor and implicate nuclear pore proteins in SPB assembly and remodeling. To identify additional proteins involved in the remodeling process, we developed a system for conditionally inducing SPB remodeling. The remodeling strain contains a version of Spc110 that can be cleaved by TEV protease. Using a synthetic genetic array analysis.