Cytoskeletal Control of Cell Polarity in the Drosophila Oocyte

Many cells rely on polar organization to function properly. The cytoskeleton is critical to both establishment and maintenance of polarity. The principle body axes (e.g. head/tail) of the Drosophila embryo and adult body are initially established in the developing egg (oocyte). The oocyte is filled with an actin mesh that is essential to polarity establishment. We study how this mesh is built and how it contributes to oocyte polarity establishment, using a combination of Drosophila genetics, cell biology, and physical biochemistry. For example, we found that the two actin nucleators that build the mesh, Spire and Capu, bind one another. Transgenic flies with engineered mutations confirmed our in vitro structure/function data, indicating that the direct interaction between Spir and Capu is required to build the mesh. Using this approach, we are dissecting the molecular mechanism of nucleation by this pair of proteins and others like it. To better understand how the actin mesh contributes to oocyte polarity, we are studying the impact of the mesh on the mechanics of the ooplasm. Using two-point passive microrheology and genetics, we found that the oocyte cytoplasm is dominated by elasticity, which is dissipated by the actin mesh. Our findings bring into question assumptions about the role of viscosity in this and many other cell types.