Thermotropic 'two-stage' liquid crystalline equilibrium crystalline lipid phase separation in microsomal membranes.

The effect of temperature on native microsomal membrane vesicles isolated from Tetrahymena is investigated by wide angle X-ray diffraction. A 4.2 reflection, typical for lipids in the crystalline state, can be recorded in the temperature range between 0 degree C and 35 degrees C. Quantitative evaluation of this reflection reveals a broad thermotropic 'two-stage' liquid crystalline equilibrium crystalline lipid phase separation with a 'breakpoint' at approx. 18 degrees C. This 'breakpoint' coincides with the emergency of lipid-protein segregations in endomembranes of intact Tetrahymena cells as previously visualized by freeze-etch electron microscopy.