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

Abstract

Regulation of subcellular components' localization and motion is a critical theme in cell biology. Cells use the actomyosin cortex to regulate protein distribution on the plasma membrane, but the interplay between membrane binding, cortical movements and protein distribution remains poorly understood. In a polarizing one-cell stage Caenorhabditis elegans embryo, actomyosin flows transport PAR protein complexes into an anterior cortical domain to establish the anterior-posterior axis of the animal. Oligomerization of a key scaffold protein, PAR-3, is required for aPAR cortical localization and segregation. Although PAR-3 oligomerization is essential for polarization, it remains unclear how oligomer size contributes to aPAR segregation because PAR-3 oligomers are a heterogeneous population of many different sizes. 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, while larger PAR-3 clusters are dispensable. Quantitative analysis of PAR-3 diffusion showed that PAR-3 clusters larger than a trimer are transported by viscous forces without being physically captured by the actomyosin cortex. Our study provides a quantitative model for size-dependent protein transportation of membrane proteins by cortical flow.

Biophysical Models of PAR Cluster Transport by Cortical Flow in C. elegans Early Embryogenesis

“…Alternatively, engineered clusters could be used to study transport properties. Recent experimental work by Chang and Dickinson (2021) took this approach. Their new data suggests that clusters of size 3 or larger indeed exhibit directed transport, that the transport of larger clusters is more ballistic (as opposed to diffusive), and that the cluster diffusion constant decreases with clus ter size.…”

“…These experimental investigations also revealed that the cortical residence times of clusters and their persistence of motion both increase with cluster size, raising the more specific question of how larger clusters are more effectively transported. Chang and Dickinson (2021) engineered PAR3 clusters of various sizes and found that clusters of size three (i.e., consisting of three PAR3 monomers) or larger are the minimum size required transport by flow to obtain polarity.…”

“…Another is that they (un)bind to the cortex with either a sizedependent or sizeindependent affinity. New experimental observations by Chang and Dickinson (2021) suggest that viscous forces on larger clusters are responsible for their transport and that PAR3 clusters may not directly bind the actin cortex (since the clusters do not appear to colocalize with actin). Others note that these hypotheses remain broadly untested.…”