Self-Assembly in Lipid-Protein Systems : Lung Surfactant, Stratum Corneum and Model Membranes

Sammanfattning: This thesis explores lipid self-assembly and aims to give a broad picture of self-assembly structures in simple and complex lipid-protein systems. The systems studied are lung surfactant, stratum corneum and simple model membranes. The lung surfactant mixture lines the alveolus in our lungs and stabilises the air- tissue interface. The lung surfactant lipid phase behaviour was here investigated with respect to the effects of cholesterol concentration and changes in the external conditions of temperature and a water gradient. Taken together the studies of lung surfactant give a comprehensive picture of the phase behaviour of a clinical lung surfactant extract, showing the importance of cholesterol and non-equilibrium conditions. It is a recurrent observation for all studies that the addition of physiologically relevant levels of cholesterol to the clinical lung surfactant forms a single robust liquid ordered phase under both equilibrium and non- equilibrium conditions.Conditions such as draught, high salinity or freezing, exerts the lipid systems to osmotic stress, which can lead to phase changes between different self-assembled structures. The outer layer of the skin, the stratum corneum is most of the time exposed to osmotic stress from dry air in the environment. The stratum corneum contains small polar molecules, to counteract phase changes due to osmotic stress. We study how the self-assembly in stratum corneum and model membranes are affected by the presence of osmolytes under conditions of osmotic stress. It is shown that these compounds under dry conditions act to replace the water in both stratum corneum and model membranes and they may stabilize the fluid lipid phases at lower humidities.In plants, there are several strategies for protection against osmotic stress. One strategy involves the expression of specific proteins. We have studied how one such protein from the family of dehydrins, influences lipid self-assembly, aiming at molecular understanding of how these proteins can protect membranes against osmotic stress. The dehydrin protein, Lti30, is shown to stabilise the liquid crystalline lamellar phases over a large range of hydration conditions, preventing phase transitions at low water contents, and extensive swelling of the lamellar phase at high water contents.