Fluidity in mitochondrial membranes: thermotropic lateral translational motion of intramembrane particles.

The fracture faces of frozen rat liver mitochondria reveal that intramembrane particles can be induced to under go long-range lateral translational motion and aggregation, which parallel the appearance of large, particle-free smooth patches in the hydrophobic interior of the two mitochondrial membranes. These lateral separations were observed under conditions that induce thermotropic lipid-lipid phase separations. Low temperature-induced lateral separation occurred between the intramembrane particles (integral proteins) and smooth patches (bilayer lipid) at temperatures between about 10 and -12 degrees in the outer membrane and between about -4 and -12 degrees in the inner, energy transducing membrane.