Solution structure of the acetylated and noncleavable mitochondrial targeting signal of rat chaperonin 10

NMR structures of a mitochondrial transit peptide from the green alga Chlamydomonas reinhardtii

The 26-amino-acid pre-sequence of the ATP synthase beta subunit that directs the protein from the cytosol to mitochondria in the unicellular green alga Chlamydomonas reinhardtii has been synthesised and analysed using NMR spectroscopy/circular dichroism and compared to a chloroplast transit peptide from the same organism. The results demonstrate that the peptide, though mainly unstructured in water, undergoes a strong conformational change in a 36% water/64% 2,2,2-trifluoroethanol mixture. In this solvent condition, an alpha-helix was characterised by NMR from residue 2 to 26. Structure calculations under NMR restraints lead to a population of models of which 60% are kinked at position 9-10. Structural analysis indicates two hydrophobic sectors on the models with a discontinuity at the 9-10 kink level. The structures suggest a different interaction mode with the mitochondrial membrane compared to the chloroplast transit peptide.

Secondary structure and topology of a mitochondrial presequence peptide associated with negatively charged micelles. A 2D H-NMR study

In this study the secondary structure and topology of the peptide, corresponding to the presequence of cytochrome oxidase subunit IV (p25) in a negatively charged membrane-mimetic environment, were assessed by circular dichroism and two-dimensional nuclear magnetic resonance. The micelles used consisted of dodecylphosphoglycol (DPG), a mild anionic detergent with a headgroup resembling that of phosphatidylglycerol. The secondary structure was analyzed by interresidue nuclear Overhauser enhancement measurements and chemical shifts of backbone protons. The data revealed alpha-helix formation of the peptide upon interaction with the micelles, both in the N- and in the C-terminal halves, which are separated from each other by the proline residue at position 13. The topology of the peptide was studied by determining the effect of spin-labeled 12-doxylstearate on the line widths of the peptide proton resonances. This method revealed the insertion of hydrophobic residues of both the N- and the C-terminal halves of p25 into the hydrophobic environment of the micelles, demonstrating the orientation of the amphiphilic helix.

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.

Cardiolipin modulates the secondary structure of the presequence peptide of cytochrome oxidase subunit IV: a 2D 1H-NMR study

The secondary structure of the presequence of cytochrome oxidase subunit IV (p25) was studied by circular dichroism and 2D nuclear magnetic resonance in micelles of dodecylphosphocholine (DPC) and mixed micelles of DPC and mitochondrial cardiolipin (CL). In both systems, alpha-helix formation was observed. The alpha-helix stretches from the N- to the C-terminus with a break at the proline residue at position 13. Upon introduction of CL in the DPC micellar system, an increased stability of the helix was observed around proline13 and in the C-terminal half. This observation, together with reported results on specific interactions between CL and p25, led to the proposal of a two-state equilibrium of the alpha-helical conformation of p25, modulated by CL.

Affinity purification, overexpression, and characterization of chaperonin 10 homologues synthesized with and without N-terminal acetylation

Utilizing the ability of bacterial chaperonin 60 (GroEL) to functionally interact with chaperonin 10 (Cpn10) homologues in an ATP-dependent fashion, we have purified substantial amounts of mammalian, chloroplast, and thermophilic Cpn10 homologues from their natural host. In addition, large amounts of recombinant rat Cpn10 were produced in Escherichia coli and found to be identical to its authentic counterpart except for the lack of N-terminal acetylation. By comparing these two forms of Cpn10, it was found that acetylation does not influence the oligomeric structure of Cpn10 and is not essential for chaperone activity or mitochondrial import in vitro. In contrast, N-terminal acetylation proved crucial in the protection of Cpn10 against degradation by N-ethylmaleimide-sensitive proteases derived from organellar preparations of rat liver. The availability of large amounts of both affinity-purified and recombinant Cpn10 will facilitate not only further characterization of the eukaryotic folding machinery but also further scrutiny of the reported function of Cpn10 as early pregnancy factor.