Determination of the cell cleavage plane by the LIN-5 complex

Cells split in two at the final step of each division cycle. This division normally bisects through the middle of the cell and generates two equal daughters. However, developmental signals can change the plane of cell cleavage to facilitate asymmetric segregation of fate determinants and control the position and relative sizes of daughter cells. The anaphase spindle instructs the site of cell cleavage in animal cells, hence its position is critical in the regulation of symmetric vs asymmetric cell division. Studies in a variety of models identified evolutionarily conserved mechanisms that control spindle positioning. However, how the spindle determines the cleavage site is poorly understood. Previous research has revealed an essential and conserved spindle-positioning pathway which consists of the LIN-5, GPR-1/2, and GOA-1/GPA-16 G? proteins in C. elegans. In this thesis, the spatial and temporal control of the G?/GPR/LIN-5 complex is studied during cell divisions in C. elegans. This work describes the characterization and functional analysis of 25 phosphorylated residues on the LIN-5 protein. Phosphorylation of LIN-5 by the cell cycle kinase Cdk1 is required for successful cell division. In addition, LIN-5 is phosphorylated by the polarity kinase PKC-3, which inhibits LIN-5 function and cortical microtubule pulling forces during spindle positioning in the one-cell embryo. Together, this reveals novel molecular mechanisms by which the spindle-positioning machinery instructs the cleavage plane during asymmetric cell division