The microtubule-based spindle apparatus provides a conserved mechanism to segregate chromosomes during mitosis. However, how this process is coordinated with disassembly and reassembly of nuclear structures during mitotic progression is poorly understood. It is also not clear how cell cycle regulators and other diffusible molecules are localized and confined to the spindle region in the absence of diffusion barriers following nuclear envelope breakdown. Towards answering these questions we have identified four nuclear proteins, Skeletor, Chromator, Megator, and EAST from two different nuclear compartments that interact with each other and that redistribute during prophase to form a dynamic, gel-like spindle matrix that embeds the microtubule spindle apparatus, stretching from pole-to-pole. This matrix forms prior to nuclear envelope breakdown and specific interactions between spindle matrix molecules are necessary for complex formation and cohesion. When microtubules are depolymerized with colchicine just prior to metaphase the spindle matrix contracts and coalesces around the chromosomes suggesting that microtubules act as "struts" stretching the spindle matrix. Furthermore, in colchicine treated embryos free tubulin accumulates co-extensively with the spindle matrix suggesting that this enrichment is dependent on one or more proteins within the spindle matrix with tubulin binding activity. Moreover, we have demonstrated that Megator and its human homolog Tpr act as spindle matrix proteins that have an evolutionarily conserved function as spatial regulators of spindle assembly checkpoint proteins during open and semi-open mitosis. The goal of our current research is to test the hypothesis that reorganization of nuclear proteins into a spindle matrix serves as a general platform for integrating signaling events leading to nuclear envelope breakdown and cell cycle progression in conjunction with contributing to microtubule spindle assembly and dynamics. We hope that the results will provide a mechanistic framework for understanding how cell cycle factors are physically confined and organized in the spindle region in organisms with open or semi-open mitosis, allowing for spatial and temporal control of mitotic progression and chromosome segregation.
Funding Organization: National Science Foundation