We now have a nearly complete list of all the proteins constituting a cell. However, how these molecules come together in space and time to give rise to complex structures and compartments with defined morphology and size is still poorly understood. To address this for any system, we first have to qualitatively understand which components are involved in the system. However, we also need a quantitative framework to describe the rate and extent of assembly for all factors involved in the system to link changes in nanometer scale molecular composition with micron scale changes in architecture (eg. size and shape). Here I focus on the mitotic spindle, which plays an essential role in cell division. The first step toward understanding the spindle as a dynamic, self-organized compartment is to identify all the components that compose the spindle and quantify the extent to which each component is enriched in the spindle compared to cytoplasm. Spindles will be isolated with two complementary approaches and then subjected to state of the art mass spectrometry based proteomics to quantify ~10,000 proteins with 3% coefficients of variation and signal to noise ratios of approximately 30-fold. One intriguing possibility that I already have evidence for is tha many more proteins in the cytoplasm associate with the spindle to some degree than previously thought. Additionally, I will determine the exchange rates for all proteins that associate with spindles. Exchange constants can be paired with extensive previous knowledge of many spindle components to reveal novel mechanism, and also test for non- tubulin components of the spindle such as the proposed spindle matrix. To date, these constants are only known for tubulin and a handful for MAPs and motors. Any proteins that have half-time exchanges that are significantly slower than microtubule dynamics are of extreme interest, as these may yield novel insights into spindle composition. I will then investigate how a spindle changes size. Spindle size decreases drastically in frog eggs as a fertilized embryo undergoes cleavages and cell size decreases by two orders of magnitude. These in vivo changes were recently recapitulated in vitro and shown to be solely due to the amount of cytoplasm present, indicating that the phenomena is due to limiting factors. I will isolate proteins associated to spindles at varying spindle densities to investigate which factors are depleted from spindles and thus potentially responsible for this scaling process. Individual contributions and cooperative contributions from candidates will be investigated with a variety of approaches.
Selectively blocking cell-division by inhibition of mitotic spindle function has been a therapeutic approach to treat many different kinds of cancer. However, to date there has been no chemotherapeutic developed that selectively inhibits cell-division in cancer cells but not regular cells. Identifying new components or mechanisms contributing to function of the mitotic spindle may yield new promising ways to selectively block cell division in cancer cells.
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