In animals, proteins of the PAR (partitioning) system establish polarity by forming distinct domains on the cell cortex. Despite decades of study, it remains unclear how PAR proteins assemble dynamic signaling complexes that can establish cellular asymmetry in vivo. To address this challenge, our group has developed a single-cell, single-molecule biochemistry toolkit that allows us to measure the compositions of protein complexes captured directly from cells and embryos. We have used this approach to reveal how PAR protein complexes assemble during polarization of the C. elegans zygote. We find a cooperative, multivalent assembly process that couples PAR-3 oligomerization to its interaction with the central polarity kinase aPKC. Multivalency requires PDZ-ligand interactions and involves the aPKC partner PAR-6. Furthermore, this assembly is regulated in unexpected ways by the cell-cycle kinase PLK-1 and the Rho family GTPase CDC-42. Together, our results reveal the inner workings of a key, conserved polarity machine, and they show how single-cell biochemistry can be used to dissect cell signaling ex vivo.
Hsu et al. (Sun,) studied this question.