Temperature-induced vertical shift of proteins in membranes

Thermotropic changes in the transverse order of microsomal membranes isolated from Tetrahymena are revealed by low-angle X-ray diffraction. These are correlated with the lateral order of the membrane lipids by wide-angle X-ray diffraction. Upon lowering the temperature from 28 degrees C to 2 degrees C, the Bragg period of the membrane stack reveals an abrupt increase of approximately 3.1 nm at approximately 19 degrees C, which is reversible upon reheating to 28 degrees C. This is coupled with an alteration in the electron density profile, revealing a shift of mass from the hydrophobic core towards one of the two hydrophilic surfaces. Between 35 degrees C and 0 degrees C, the membrane lipids undergo a broad, thermotropic "two-stage" liquid crystalline in equilibrium crystalline phase separation with a "breakpoint" at approximately 18 degrees C. This breakpoint signals an abrupt lipid redistribution, presumably due to a change in the composition of the two coexisting liquid crystalline and crystalline lipid phases. We conclude: (1) the temperature-induced mass shift reflects a shift in the transverse partition of proteins in membranes; (2) this is triggered by an abrupt lipid redistribution occurring during a broad liquid crystalline in equilibrium crystalline phase separation.