Phase Separation of a Nodulin Sec14-like protein Maintains Auxin Efflux Carrier Polarity at Arabidopsis Plasma Membranes

Abstract: Robust development can be modulated by local protein accumulation which is sometimes manifested as polarity patterns; yet, the mechanisms that lead to these patterns are largely unknown. Using the model plant Arabidopsis, we report that auxin-induced polar patterns can be reinforced by the phase transition of a SEC14-like lipid-binding protein at the plasma membrane. Using imaging, genetics, and in vitro reconstitutions, we show that the SEC14-like phase transition is promoted outside the stem cell root niche through its association with the caspase-like protease separase and conserved microtubule motors in a plasma membrane interface. Separase cleaves SEC14-like to promote its transition from liquid-like clusters to solid-like filaments. Filaments self-amplify and potentiate the robustness and maintenance of plasma membrane domains through associations with polar proteins. This work uncovers that robust cell patterns involve proteolysis-mediated phase transitions at unpreceded plasma membrane interfaces.

Synopsis This study provides an example of a protein that undergoes liquid-like phase separation at the plasma membrane in plants. Importantly, this protein known as SFH8 is a lipid-like transferase and can undergo proteolytic-induced solidification, which is an important event in the maintenance of polarized delivery of PINFORMED proteins. The complex comprising of the conserved caspase-like protease separase and the kinesin Kin7.3 (known as “KISC”) associates with a SEC14-like homolog, the SFH8 protein at apicobasal polar domains Separase cleaves SFH8 to yield a carboxy-terminal proteoform that undergoes a liquid-to-solid transition producing filamentous assemblies at the plasma membrane The KISC/SFH8 constitutes a feedback loop with auxin that potentiates the interaction, fusion, and maintenance of proteins with polar domains