ABSTRACT
Mitochondria are usually several microns long and 0.5 μm in diameter. They have two separate membranes: a smooth outer membrane and a folded inner membrane. The inner membrane separates the mitochondrion into two distinct spaces, one internal called the matrix and the other between the inner and outer membranes, called the intermembrane space. Genes encoding mitochondrial proteins have been cloned and used to construct gene fusions. Targeting signals of precursors were supposed to recognize and bind specifically to proteins located on the mitochondrial surface. In order to test the tolerance of the mitochondrial protein import system, in vitro experiments were performed with two artificial precursors. Comparison of the known mitochondrial targeting sequences does not reveal a consensus primary sequence. The mitochondrial matrix enzyme superoxide dismutase of Saccharomyces cerevisiae is encoded by a nuclear gene. The role of ATP in promoting insertion or translocation of the precursor across mitochondrial membranes may involve an ATP-dependent protein unfolding.
