Active emulsions in living cell membranes driven by contractile stresses and transbilayer coupling

TL;DR

This study combines experimental imaging and theoretical modeling to show that active contractile stresses and transbilayer coupling create mesoscale lipid domains in living cell membranes, influencing protein organization and membrane function.

Contribution

It introduces a theoretical model of active emulsions driven by contractile stresses and transbilayer coupling, supported by experimental validation in living cells.

Findings

GPI-AP nanoclusters are maintained by cortical actin activity.

Mesoscale domains exhibit significant lipid order.

Active stresses drive formation of liquid ordered domains.

Abstract

The spatiotemporal organisation of proteins and lipids on the cell surface has direct functional consequences for signaling, sorting and endocytosis. Earlier studies have shown that multiple types of membrane proteins including transmembrane proteins that have cytoplasmic actin binding capacity and lipid-tethered GPI-anchored proteins (GPI-APs) form nanoscale clusters driven by active contractile flows generated by the actin cortex. To gain insight into the role of lipids in organizing membrane domains in living cells, we study the molecular interactions that promote the actively generated nanoclusters of GPI-APs and transmembrane proteins. This motivates a theoretical description, wherein a combination of active contractile stresses and transbilayer coupling drive the creation of active emulsions, mesoscale liquid ordered (lo) domains of the GPI-APs and lipids, at temperatures greater than equilibrium lipid-phase segregation. To test these ideas we use spatial imaging of homo-FRET combined with local membrane order and demonstrate that mesoscopic domains enriched in nanoclusters of GPI-APs are maintained by cortical actin activity and transbilayer interactions, and exhibit significant lipid order, consistent with predictions of the active composite model.