Oligomerization and feedback on membrane recruitment stabilize PAR-3 asymmetries inC. eleganszygotes

Abstract The PAR polarity network is a paradigmatic example of how systems of mutually antagonism interactions among peripheral membrane binding proteins allow them to form and maintain complementary polar domains in response to a transient polarizing cue. The oligomeric scaffolding protein PAR-3 has emerged as a keystone member of the PAR network in many different contexts. In early C. elegans embryos, PAR-3 is required for all other PAR asymmetries, and it can form stable unipolar asymmetries when its known inhibitors are absent and all other members of the PAR network are cytoplasmic or spatially uniform on the membrane. But how PAR-3 forms stable unipolar asymmetries absent mutual antagonism is unknown. Here we combine single particle analysis with quantitative modeling and experimental manipulations to determine how the dynamics of PAR-3 membrane binding, oligomerization and dissociation allow PAR-3 to maintain stable asymmetries in the one cell C. elegans embryo. We find that two forms of positive feedback contribute to sustaining PAR-3 asymmetries: First, a sharp size-dependent decrease in oligomer dissociation rates makes the effective dissociation rate of PAR-3 decrease sharply with its membrane density. Second, membrane-bound PAR-3 promotes additional binding of PAR-3 to the membrane through a mechanism that requires the presence of anterior polarity proteins CDC-42, PAR-6 and PKC-3. Through a combination of modeling and quantitative measurements, we show that these two feedback loops are sufficient to dynamically stabilize asymmetries of the magnitude observed in polarized C. elegans zygotes. These results establish a dynamic basis for stabilizing monopolar PAR-3 asymmetries; they underscore a crucial role for the oligomerization and add to the growing body of evidence that point to a central role for oligomerization of peripheral membrane proteins in the establishment and maintenance of cell polarity.