  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.
