| Abstract: | Fluorescence measurements of the probe 1,6-diphenyl-1,3,5-hexatriene in native Tetrahymena pyriformis microsomal membranes revealed characteristic "break points" in curves of polarization vs. temperature. In the 5--35 degree C range, membranes from cells grown at 39 degrees C exhibited two break points, one at 11.6 +/- 0.6 degrees C and another at 23.1 +/- 1.6 degrees C. Membranes from 15 degrees C grown cells also showed two break points, one at 8.0 +/- 1.7 degrees C and another at 17.7 +/- 1.7 degrees C. Complementary measurements of turbidity (absorbance at 360 nm) vs. temperature revealed break points at approximately the same temperatures as observed with the fluorescent probe, thus strengthening the likelihood that the break points signify the onset or termination of lipid phase separations or some other significant structural alteration of lipids. In general, break points measured in the native membrane samples occurred at slightly lower temperatures than did break points in lipids extracted from comparable membranes. This suggests two possible types of protein--lipid interaction. First, there may be a selective withdrawal of relatively highly saturated phospholipid molecular species from the bulk lipid phase and into protein annulus regions. Alternatively, the configuration of the hydrophobic core of certain key membrane proteins may be such that nonspecific interactions with the lipids stabilize the liquid-crystalline phase. |
