The loss in hydrophobic surface area resulting from a Leu to Val mutation at the N-terminus of the aldehyde dehydrogenase presequence prevents import of the protein into mitochondria.

An apparent conservative mutation, Leu to Val, at the second residue of the rat liver mitochondrial aldehyde dehydrogenase (ALDH) presequence resulted in a precursor protein that was not imported into mitochondria. Additional mutants were made to substitute various amino acids with nonpolar side chains for Leu2. The Ile, Phe, and Trp mutants were imported to an extent similar to that of the native precursor, but the Ala mutant was imported only about one-fourth as well. It was shown that the N-terminal methionine was removed from the L2V mutant in a reaction catalyzed by methionine aminopeptidase. The N-terminal methionine of native pALDH and the other mutant presequences was blocked, presumably by acetylation. Because of the difference in co-translational modification, the L2V mutant sustained a significant loss in the available hydrophobic surface of the presequence. Import competence was restored to the L2V mutant when it was translated using a system that did not remove Met1. The removal of an Arg-Gly-Pro helix linker segment (residues 11-14) from the L2V mutant, which shifted three leucine residues toward the N-terminus, also restored import competence. These results lead to the conclusion that a minimum amount of hydrophobic surface area near the N-termini of mitochondrial presequences is an essential property to determine their ability to be imported. As a result, both electrostatic and hydrophobic components must be considered when trying to understand the interactions between precursor proteins and proteins of the mitochondrial import apparatus.

Studies on protein processing for membrane-bound spinach leaf mitochondrial processing peptidase integrated into the cytochrome bc1 complex and the soluble rat liver matrix mitochondrial processing peptidase.

The plant mitochondrial processing peptidase (MPP) that catalyses the cleavage of the presequences from precursor proteins during or after protein import is a membrane-bound enzyme that constitutes an integral part of the bc1 complex of the respiratory chain. In contrast, MPP from mammals is soluble in the matrix space and does not form part of the respiratory chain. In the present study, we have compared the substrate specificity of the isolated spinach leaf bc1/MPP with rat liver MPP using synthetic signal peptides and different mitochondrial precursor proteins. Inhibition studies of processing with synthetic peptides showed a similar inhibition pattern for plant and rat MPP activity. A peptide derived from the presequence of rat liver mitochondrial aldehyde dehydrogenase (ALDH) was a potent inhibitor of the spinach and rat MPP. Two nonprocessed signal peptides, rhodanese and linker-deleted ALDH (a form of ALDH that lacks the RGP linker connecting two helices in the presequence) had lower inhibitory effects towards each protease. The signal peptide from thiolase, another nonprocessed protein, had little inhibitory effect on MPP. Peptides derived from presequence of the plant Nicotiana plumbaginifolia F1 beta also showed a similar inhibitory pattern with rat MPP as with spinach MPP processing. In-vitro synthesised precursors of plant N. plumbaginifolia F1 beta and rat liver ALDH were cleaved to mature form by both spinach and rat MPP. However, the efficiency of processing was higher with the homologous precursor. Linker-deleted ALDH, rhodanese, and thiolase were not processed by the mammalian or plant MPP. However, both forms of MPP cleaved a mutated form of rhodanese that possesses a typical MPP cleavage motif, RXY S. Addition of the same cleavage motif to thiolase did not result in processing by either MPP. These results show that similar higher-order structural elements upstream from the cleavage site are important for processing by both the membrane-bound plant and the soluble mammalian MPP.

The role of positive charges and structural segments in the presequence of rat liver aldehyde dehydrogenase in import into mitochondria.

Most mitochondrial proteins are nucleus-encoded and translated in the cytosol. They have an N-terminal presequence that allows recognition by the mitochondrial import apparatus and subsequent import into mitochondria. These presequences are rich in positive charges, mainly arginines. The role of these positive charges in the 19-amino acid presequence of rat liver aldehyde dehydrogenase was investigated by systematically replacing them with the polar but uncharged residue, glutamine. The single substitution of any of the four Arg residues in the helical segments did not affect import. Substitution of both Arg residues in the N-terminal segment (R3Q/R10Q) caused a dramatic decrease in import competence. This could be restored by using the mutant lacking the three-amino acid (RGP) linker that separates the two helical domains, determined by two-dimensional NMR (Thornton, K., Wang, Y., Weiner, H., and Gorenstein, D. G. (1993) J. Biol. Chem. 268, 19906-19914). CD and NMR spectra of the peptide corresponding to the linker-deleted presequence showed that it was substantially more prone to helix formation than the native peptide over its entire length. A similar analysis of the peptide corresponding to the R3Q/R10Q presequence revealed that this peptide was only somewhat more helical than the native peptide and that the greater helicity did not include the residues near the N terminus. It is concluded that positively charged residues in the presequence play a vital role in the import of precursor aldehyde dehydrogenase. One of the positive charges in the N-terminal helical segment of the presequence is necessary for import competence. However, if both positive charges are removed, import competence can be retained as long as the presequence is capable of forming a relatively more stable alpha-helix near its N terminus.

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.

Conversion of a nonprocessed mitochondrial precursor protein into one that is processed by the mitochondrial processing peptidase.

Mitochondrial processing peptidase (MPP) cleaves the signal sequence from a variety of mitochondrial precursor proteins. A subset of mitochondrial proteins, including rhodanese and 3-oxoacyl-CoA thiolase, are imported into the matrix space, yet are not processed. Rhodanese signal peptide and translated protein were recognized by MPP, as both were inhibitors of processing. The signal peptide of precursor aldehyde dehydrogenase consists of a helix-linker-helix motif but when the RGP linker is removed, processing no longer occurs (Thornton, K., Wang, Y., Weiner, H., and Gorenstein, D. G. (1993) J. Biol. Chem. 268, 19906-19914). Disruption of the helical signal sequence of rhodanese by the addition of the RGP linker did not allow cleavage to occur. However, addition of a putative cleavage site allowed the protein to be processed. The same cleavage site was added to 3-oxoacyl-CoA thiolase, but this protein was still not processed. Thiolase and linker-deleted aldehyde dehydrogenase signal peptides were poor inhibitors of MPP. It can be concluded that both a processing site and the structure surrounding this site are important for MPP recognition.