Molecular mechanisms of protein sorting between the mitochondrial outer and inner membranes

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Mitochondrial proteins synthesized in the cytoplasm are transported and localized to a specific compartment of the mitochondrion. The intramitochondrial sorting between the outer and inner membranes was investigated using a 70 kDa outer membrane protein and cytochrome c_1 as representative proteins in yeast cells. The presequence of cytochrome c_1, which functions as a signal to the inner membrane was replaced by various lengths of the N-terminal region of the 70 kDa protein. The fused protein with the N-terminal 61 residues(61mC_1) was localized to the outer membrane and those with less than 29 residues to the inner membrane. The latter fusion proteins were shown to be assembled into succinate-eytochrome c reductase without proteolytic removal of the fused region and restore the respiration of a cytochrome c_1 deficient mutant yeast(MH6-16). These date indicate that targeting and anchoring signals of the outer membrane protein localize the inner membrane protein to the outer membrane and that the target sequence alone directs it to the inner membrane.The cytochrome c_1 deficient mutant yeast, MH6-16 transformed with a plasmid encoding 61mC_1 was not able to grow on a non- fermentable medium(glycerol^-), due to location of this protein to the outer membrane. A selection for mutations which restored growth on glycerol was applied to the transformants and yield a class of invitation in the host genome. We are now characterizing this mutation as a mitochondrial protein sorting mutant and a DNA fragment involved in the restoration are currently isolated.