Domain assembly in asymmetric lipid membranes

Cell membranes often exhibit lateral inhomogeneities that play an important role in cellular processes such as exocytosis and endocytosis, signal transduction, apoptosis, viral infection and immune defense. Interestingly, these inhomogeneities, also referred to as domains, always span both monolayers of a membrane. This alignment, also referred to as domain registration, can be attributed to tension along the domain perimeter and thermal undulations of the membrane, as we have shown previously using symmetric membranes. However, biological membranes are asymmetric, i.e., the lipid composition of their inner and outer monolayers differ from each other, and it is unclear how both line tension and undulations operate under such conditions. We therefore address the question of how the lipid composition in one monolayer affects domains in the other monolayer. To this end, we use asymmetrically folded lipid bilayers containing photoswitchable lipids to resolve or reshape lipid domains by light and to temporally stabilize otherwise highly dynamic units. The bilayers also include labeled lipids so that the domains can be visualized by fluorescence microscopy. In addition, we record domain mobility as an indicator of domain size and the diffusion coefficient of various lipids to document inhomogeneities in domains that appear to consist of a single phase when viewed microscopically. Finally, we synthesize photoswitchable lipids that can modulate line tension. The goal is to better understand the formation mechanisms of lipid domains in asymmetric membranes as they are involved in cellular regulatory processes.