Lateral and vertical displacement of integral membrane proteins during lipid phase transition in Anacystis nidulans.

Alterations in membrane structure as a result of lipid phase transitions have been studied in Anacystis nidulans, a blue-green alga. Cells grown at 38 degrees C were subjected to temperature transitions of 38 degrees C leads to 21 degrees C and 38 degrees C leads to 10 degrees C, previously shown to produce substantial changes in photosynthetic activities, and examined by freeze-fracture electron microscopy. As a result of these treatments, large particle-free regions appeared on the fracture faces of both the plasma and thylakoid membranes. Particle density measurements suggest that the displacement of the integral membrane protein complexes occurs in both lateral and vertical directions. Returning the cells to 38 degrees C resulted in the restoration of normal membrane morphology, indicating that the proteins were not lost from the membrane. Such displacement of the integral membrane protein complexes could contribute significantly to the temperature-dependent alterations in the functional activity of membrane-bound enzymatic complexes.