Liquid Ordered / Liquid Disordered Domains

(supported by the Austrian Science Fund, Project No. P24459-B20)

start of project: 15. 04. 2012
end of project: 31. 08. 2016

Biological membranes regulate diverse processes on the cellular level, including material transport, signal transduction and various chemical reactions. An about 4 nm thin layer of phospholipids and proteins is the central element of these membranes and there is ample evidence that lipids and proteins mutually regulate their function. Of particular interest is the organization of lipids and proteins into functional platforms, also known as rafts, to perform specific signaling or transport tasks. Sphingomyelin and cholesterol are considered to be the major lipid components of these rafts. Clearly, basic physicochemical understanding of raft properties is of need for ground-breaking developments in human health care.

Biophysical studies on complex mixtures of “raft lipids” can contribute significantly to such insights. These mixtures show phase separation into coexisting liquid ordered (Lo) and liquid disordered (Ld) domains. Lo domains show several features of membrane rafts, including enrichment in cholesterol and capability of glycophosphatidylinositol (GPI)-anchored proteins to insert into these lipid environments. However, the recently discovered exclusion of raft-associating transmembrane proteins (TMP) from Lo domains questions their applicability as models for membrane rafts. The aim of the project is to derive lipid-only models for membrane rafts that recover the partitioning of raft-associating TMP, thus creating improved and experimentally stringently controlled lipid-only models of membrane rafts. The goal will be achieved by deriving those structural and elastic properties of Lo and Ld domains that couple to partitioning of TMP using a broad selection of biophysical techniques. In particular, we will determine lateral packing density (area per lipid), membrane thickness, spontaneous curvature, bending rigidity and Gaussian curvature modulus for several lipid mixtures, from which we will calculate the preferential insertion of protein models into a given lipid environment. This way we will understand why TMP proteins do not partition into current lipid-only raft models and how to tune Lo phase properties such that TMP partition into these environments. We will test the optimized Lo phases by measuring the partitioning of a well-established peptide model for TMP in comparison to its behavior in raft-like structures of cell-derived vesicles. Work will be performed in collaboration with several key experts in the fields of membrane biophysics and cell biology. Our results will the basis for several follow-up applications and will thus be an important contribution to close the gap between membrane biophysics and cell biology.