Details
Cell polarity is a fundamental feature of eukaryotic cells and plays a major role in the development and homeostasis of animal tissues. The PAR protein system is conserved throughout animals and is a major player in polarization of many different animal cell types. Many in vitro interactions between PAR proteins have been identified, but a major challenge for the field is to understand how these biochemical events are regulated and coordinated in vivo to produce a polarized cell. This challenge is not unique to the cell polarity field; it is a widespread issue in cell biology. I developed a single-molecule approach for performing biochemical assays on individual cells, allowing quantitative measurements of protein-protein interactions in a time-resolved manner in vivo. Applying this approach to C. elegans zygotes undergoing polarization, I identified a tightly regulated PAR protein complex - a large oligomer of the PAR-3 protein - that is essential for polarity establishment. PAR-3 oligomerization facilitates physical coupling of PAR proteins to cortical flows that polarize the cell. These results provide novel insight into how cells spatially segregate polarity determinants, and they establish experimental tools that will advance a comprehensive understanding of the signaling circuitry that mediates animal cell polarization.