Influence of the mature portion of a precursor protein on the mitochondrial signal sequence

Most mitochondrial proteins are synthesized with an N-terminal signal sequence that targets these proteins to various compartments within the mitochondria. Signal sequences have been shown to be functional by fusing them to a nonmitochondrial passenger protein and observing import. In many cases, a signal sequence has been fused to passenger proteins, such as dihydrofolate reductase, and import occurred. There are, though, several unexplained instances in which a signal sequence was attached to a passenger protein and import was not observed. In this study, the N-terminal 23 residues of the matrix enzyme rhodanese could import several passenger proteins but were unable to import the mature form of mitochondrial aldehyde dehydrogenase (mALDH). However, if these same 23 residues were fused to the middle portion of mALDH, import was recovered, suggesting that the rhodanese signal sequence and N terminus of mALDH were incompatible for import. Circular dichroism data indicated that a peptide corresponding to the region of fusion between rhodanese and mALDH had less structure than corresponding peptides from imported fusion proteins, suggesting that mALDH may alter the helix in the rhodanese signal sequence, thus preventing import.