LAUSR

In the Caenorhabditis elegans zygote, PAR protein patterns, driven by mutual anatagonism, determine the anterior-posterior axis and facilitate the redistribution of proteins for the first cell division. Yet, the factors that determine the selection of the polarity axis remain unclear. We present a reaction-diffusion model in realistic cell geometry, based on biomolecular reactions and accounting for the coupling between membrane and cytosolic dynamics. We find that the kinetics of the phosphorylation-dephosphorylation cycle of PARs and the diffusive protein fluxes from the cytosol towards the membrane are crucial for the robust selection of the anterior-posterior axis for polarisation. The local ratio of membrane surface to cytosolic volume is the main geometric cue that initiates pattern formation, while the choice of the long-axis for polarisation is largely determined by the length of the aPAR-pPAR interface, and mediated by processes that minimise the diffusive fluxes of PAR proteins between cytosol and membrane. In the C. elegans zygote, (anterior) aPAR and (posterior) pPAR proteins are key to polarity maintenance, what factors determine the selection of the polarity axis remains unclear. Here authors formulate a reaction-diffusion model in realistic cell geometry and find that long-axis polarisation is promoted by cytosolic dephosphorylation at onset and its steady state determined by minimising the length of the aPAR-pPAR interface.