Lipid phase separation induced by the apolar polyisoprenoid squalane demonstrates its role in membrane domain formation in archaeal membranes.

Archaea synthesize methyl-branched, ether phospholipids, which confer the archaeal membrane exceptional physico-chemical properties. A novel membrane organization has been proposed recently to explain the thermal and high pressure tolerance of the polyextremophilic archaeon Thermococcus barophilus. According to this theoretical model, apolar molecules could populate the midplane of the bilayer and could alter the physico-chemical properties of the membrane, among which the possibility to form membrane domains. We tested this hypothesis using neutron diffraction on a model archaeal membrane composed of two archaeal diether lipids with phosphocholine and phosphoethanolamine headgroups in presence of the apolar polyisoprenoid squalane. We show that squalane is inserted in the midplane at a maximal concentration between 5 and 10 mol% and that squalane can modify the lateral organization of the membrane and induces the coexistence of separate phases. The lateral reorganization is temperature- and squalane concentration-dependent and could be due to the release of lipid chain frustration and the induction of a negative curvature in the lipids.