Variation in protein lateral diffusion coefficients is related to variation in protein concentration found in mitochondrial inner membranes.

The electrophoretic freeze-fracture electron microscopy method (Sowers, A.E. and Hackenbrock, C.R. (1984) Proc. Natl. Acad. Sci. USA 78, 6246-6250) for measuring the lateral diffusion coefficient of integral proteins was applied to a large population of spherical-shaped mitochondrial inner membranes. Membrane integral protein concentration was estimated by determining the intramembrane particle concentration. Analysis of the data reveals that: (a) the radii of the spherical inner membranes in the selected population ranged from 0.22 to 1.2 micron, (b) the intramembrane particle concentrations ranged from 2300 to 6400 per micron2, and (c) the calculated lateral diffusion coefficients of the intramembrane particles ranged from 1.3 X 10(-10) to 3.35 X 10(-9) cm2/s. The data clearly show a naturally occurring large range in protein concentration in the mitochondrial inner membrane and an inverse correlation of lateral diffusion coefficient with the membrane protein concentration. This study is the first to show that the lateral diffusion coefficient of integral proteins in a native membrane varies as the membrane protein concentration.