Targeting efficiency of a mitochondrial pre-sequence is dependent on the passenger protein

The mitochondrial matrix enzyme manganese superoxide dismutase (SOD) of Saccharomyces cerevisiae is encoded in the nucleus. It is synthesized as a precursor with an NH2-terminal extension of 26 amino acids which is cleaved off during import into the mitochondrion. Fusions between the NH2-terminal 34 amino acids of SOD and the cytosolic proteins invertase of yeast and mouse dihydrofolate reductase (DHFR) were tested for in vitro binding and import into mitochondria. Efficient translocation over the mitochondrial membranes takes place in the case of the SOD-DHFR fusion. The SOD-invertase fusion protein does not get translocated and binds to the organelle with only low efficiency. Yeast transformants harbouring the SOD-invertase fusion gene accumulate approximately 95% of the hybrid protein in the cytosol. The remaining material is found in the interior of the mitochondrion, loosely attached to the inner membrane. We conclude that the pre-sequence of SOD is able to deliver a passenger protein to the mitochondrion. The efficiency of protein delivery and translocation across the membrane is, however, influenced by the passenger protein.