The ribbon structure of the mitochondrial inner membrane

Electron microscopic evidence is presented for the extensive association of protein subunits into ribbons within the mitochondrial inner membrane. The mitochondrial cristae can be rearranged to a narrow tubular form which exhibits ribbon structure and is fully functional; the morphology of particles derived from submitochondrial electron transport particles by treatment with lysolecithin suggests that the backbone of the ribbon is provided by the cytochrome-free tripartite unit (headpiece, stalk, basepiece) in linear repeat. These results are inconsistent with any single model of the inner membrane previously proposed, but are best understood in terms of a model which combines the concept of an ordered protein continuum with the concept of a fluid lipid bilayer. Further, it is concluded that the headpiece out morphology of the tripartite unit represents a viable conformation of the endergonic transducing unit.