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

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Abstract
The fracture faces of frozen rat liver mitochondria reveal that intramembrane particles can be induced to undergo long-range lateral translational motion and aggregation, which parallel the appearance of large, particle-free smooth patches in the hydrophobic interior of the 2 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.degree. in the outer membrane and between about -4 and -12.degree. in the inner, energy transducing membrane. Complete reversibility of particle aggregation occurred rapidly by free lateral diffusion, whereas oxidative phosphorylation remained relatively intact after withdrawal of the low temperature perturbation. These data suggest that lipid-protein lateral phase separations can occur in both mitochondrial membranes, and the bilayer lipid of both membranes is a fluid 2 dimensional continuum that imparts freedom of lateral translational motion to the integral proteins of these membranes.