Mitochondrial Targeting

While it has been known for many years that the N-terminal presequence is sufficient to promote mitochondrial targeting and assembly, the subsequent interaction of the precursor molecule with the outer mitochondrial membrane and the uptake of the protein is still an area of active research. There seems little doubt, however, that there are proteins on the outer mitochondrial membrane which are required for the import process. The function of these proteins is uncertain, but they may act as receptors with the subsequent transfer through the membrane at proteinous pores located at contact sites between the inner and outer membranes. Several proteins have been identified which seem to play an important role as either receptor proteins or part of the import channel (Pfanner et al., 1991). Again, not all proteins seem to depend on this mechanism. Cytochrome c, which is loosely associated with the outer aspect of the inner mitochondrial membrane, can cross... 

The Rieske protein in mitochondrial bci complexes is assembled when the protein is incorporated into the complex. The Rieske protein is encoded in the nucleus and synthesized in the cytosol with a mitochondrial targeting presequence, which is required to direct the apoprotein to the mitochondrial matrix. The C-terminus is then targeted back to the outside of the inner mitochondrial membrane where the Rieske cluster is assembled. In addition, the presequence is removed and the protein is processed to its mature size after the protein is inserted into the bci complex. In mammals, the presequence is cleaved in a single step by the core proteins 1 and 2, which are related to the general mitochondrial matrix processing protease (MPP) a and (3 subunits the bovine heart presequence is retained as a 8.0 kDa subunit of the complex (42, 107). In Saccharomyces cerevis-iae, processing occurs in two steps Initially, the yeast MPP removes 22 amino acid residues to convert the precursor to the intermediate form, and then the mitochondrial intermediate protease (MIP) removes 8 residues after the intermediate form is in the bci complex (47). Cleavage by MIP is independent of the assembly of the Rieske cluster Conversion of the intermediate to the mature form was observed in a yeast mutant that did not assemble any Rieske cluster (35). However, in most mutants where the assembly of the Rieske cluster is prevented, the amount of Rieske protein is drastically reduced, most likely because of instability (35, 44). 

Prediction of mitochondrial targeting signals is not an easy task. The proposed amphiphilic nature is not clear enough. Nakai and Kanehisa (1992) developed a simple method based on the amino acid composition of the segment of most amino-terminal 20 residues. In addition, a simple rule to discriminate the bipartite signal of intermembrane-space proteins was also included in PSORT. 

Fujiwara et al. (1997) proposed an HMM that can detect mitochondrial targeting signals. The HMM was automatically created to best explain the training data. Although it could model the signals in the training data, further analysis using more data is desirable because the model has many numeric parameters. 

Fujiwara, Y., Asogawa, M., and Nakai, K. (1997). Prediction of mitochondrial targeting signals using hidden Markov models. In Genome Informatics 53-60. Miyano, S., and Takagi, T. (eds.) Genome Informatics 1997 Universal Academy Press, Inc. Tokyo, Japan. 

Gavel, Y., and von Heijne, G. (1990). Cleavage-site motifs in mitochondrial targeting peptides. Protein Eng. 4, 33-37. 

Robin MA, Anandatheerthavarada HK, Fang JK, Cudic M, Otvos L, et al. 2001. Mitochondrial targeted cytochrome P450 2E1 (P450 MT5) contains an intact N terminus and requires mitochondrial specific electron transfer proteins for activity. J Biol Chem 276 24680-24689. 

Weissig V. Mitochondrial-targeted drug and DNA delivery. Crit Rev Ther Drug Carrier Syst 2003 20(1) 1-62. 

Coulter CV, Kelso GF, Lin TK, Smith RA, Murphy MP. Mitochondrially targeted antioxidants and thiol reagents. Free Radic Biol Med 2000 28(10) 1547-1554. 

Lee, E.-W., Lai, Y, Zhang, H. and Unadkat, J.D. (2006) Identification of the mitochondrial targeting signal of the human equilibrative nucleoside transporter 1 (hENTl) Implications for interspecies differences in mitochondrial toxicity of fialuridine. The Journal of Biologiccd Chemistry, 281 (24), 16700-16706. 

Critical for predictivity in a recent comprehensive study was the number and choice of parameters measured [4]. Early, sublethal effects on cell proliferation, cell morphology and mitochondria occurred consistently and ubiquitously with toxicity and when used collectively were most diagnostic. It is noteworthy that the toxicity of many drugs is attributable to various mitochondrial targets, including oxidative phosphorylation, fatty acid oxidation, Krebs cycling, membrane transport, permeability transition pore, proliferation and oxidative stress (Table 14.4). 

Table 14.4 Mitochondrial targets for drug-induced inhibition [3, 4, 33, 36].

Wallace, KB. and Starkoy A.A. (2000) Mitochondrial targets of drug toxicity. Annual Revieu of Pharmacology and Toxicology, 40, 353—388. 

Mitochondrial targets Rough endoplasmic reticulum Smooth endoplasmic reticulum... 

Burri L, Williams BA, Bursae D, Lithgow T, Keeling PJ (2006) Microsporidian mitosomes retain elements of the general mitochondrial targeting system. Proc Natl Acad Sci USA 103 15916-15920... 

Hard FU, Hlodan R, Langer T (1994) Molecular chaperones in protein folding the art of avoiding sticky situations. Trends Biochem Sci 19 20-25 Hausler T, Stierhof YD, Blattner J, Clayton C (1997) Conservation of mitochondrial targeting sequence function in mitochondrial and hydrogenosomal proteins from the early-branching eukaryotes Crithidia, Trypanosoma and Trichomonas. Eur J Cell Biol 73 240-251... 

Unlike Euglena PNO, which is a mitochondrial protein (Rotte et al. 2001), CpPNO lacks a mitochondrial targeting presequence and does not localize within the relic organelle (Ctrnacta et al. 2006). In fact, sporozoites of C. parvum visualized both by confocal immunofluorescence (Fig. 3) and immunogold-labelled (Fig. 4) confirm that CpPNO has an unique com-partmentalization firstly within the cytosol as expected, but secondly within... 

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