A particle size threshold governs diffusion and segregation of PAR-3 during cell polarization

Highlights

• PAR-3 trimers are necessary and sufficient for C. elegans zygote polarization

• Monomers and dimers of PAR-3 bind transiently to the plasma membrane and diffuse freely

• PAR-3 trimers bind the membrane more strongly and experience advective transport

• New techniques facilitate diffusion analysis of native membrane-bound oligomers

Summary

The actomyosin cortex regulates the localization and function of proteins at the plasma membrane. Here, we study how membrane binding, cortical movements, and diffusion determine membrane protein distribution. In Caenorhabditis elegans zygotes, actomyosin flows transport PAR polarity proteins to establish the anterior-posterior axis. Oligomerization of a key scaffold protein, PAR-3, is required for polarization. PAR-3 oligomers are a heterogeneous population of many different sizes, and it remains unclear how oligomer size affects PAR-3 segregation. To address this question, we engineered PAR-3 to defined sizes. We report that PAR-3 trimers are necessary and sufficient for PAR-3 function during polarization and later embryo development. Quantitative analysis of PAR-3 diffusion shows that a threshold size of three subunits allows PAR-3 clusters to stably bind the membrane, where they are corralled and transported by the actomyosin cortex. Our study provides a quantitative model for size-dependent protein transportation of peripheral membrane proteins by cortical flow.