A Lipid Bound Actin Meshwork Organizes Liquid Phase Separation in Model Membranes

The cytoplasmic side of eukaryotic cell membranes is covered by a dense actin rich cortex. We present FCS and STED experiments showing that a dense membrane bound actin network has severe influence on temperature dependent lipid phase separation. A minimal actin cortex was bound to a supported lipid bilayer via biotinylated lipid streptavidin complexes (pinning sites). In general, actin binding to ternary membranes prevented macroscopic liquid-ordered (Lo) and liquid-disordered (Ld) domain formation in these systems, even at low temperature. For pinning sites that strongly attract Ld domains, an actin correlated multi-domain pattern was observed, consisting of Ld “channels” along the actin fibers, with Lo “islands” in the voids. FCS measurements revealed enhanced diffusion of unsaturated lipids along the channels, and hindered diffusion of these lipids in directions perpendicular. For pinning sites strongly attractive to Lo domains, an “inverse” domain structure was observed. These findings are in good agreement with a number of recently proposed simulation models. However, to fully capture our experimental observations, an extended simulation model is proposed, in which the lipid domains also couple to the local membrane curvature. Our results provide a mechanism how cells may prevent macroscopic de-mixing of membrane components and at the same time regulate the local membrane compositions.