Transport and regulation of polypeptide precursors of mature mitochondrial proteins

Long-term ingestion of ammonium increases acetylglutamate and urea levels without affecting the amount of carbamoyl-phosphate synthase

Rats were fed the following diets: standard (20% protein), high-protein (80%), protein-free, standard plus ammonium and protein-free plus ammonium for six weeks. The standard plus ammonium diet was prepared to contain ammonia equivalent to that supplied by the high-protein diet. Addition of ammonium acetate (20% by mass) to the 20% protein or protein-free diets results in 2.3- and 10-fold increases of urea excretion respectively, without increase of carbamoyl-phosphate synthase. Supplementation of the standard diet with ammonium increases the mitochondrial content of acetylglutamate from 830 to 1590 pmol/mg protein, and of the protein-free diet from 130 to 1040 pmol/mg. However, ingestion of ammonium did not increase the activity of acetylglutamate synthase. Therefore the efflux of acetylglutamate from mitochondria was determined. After 30 min at 37 degrees C liver mitochondria from rats on standard diet released 61% of the initial acetylglutamate while mitochondria from animals on standard plus ammonium diet released only 20%. These results indicate that ingestion of ammonium increases the content of acetylglutamate in rat liver by decreasing its efflux from mitochondria. This effect is similar to that produced in mice by a high protein diet [Morita et al. (1982) J. Biochem. (Tokyo) 91, 563-569]. However, while the high-protein diet increases carbamoylphosphate synthase content, the ammonium diet does not.

Precursors of mitochondrial proteins are degraded in the cytosol at different rates

The stability of rat liver mitochondrial protein precursors in the cytosol was investigated. The precursors were synthesized in a post-mitochondrial supernatant from rat liver, and mitochondria were then added at different times. The amount and pattern of proteins incorporated were determined. The precursors have different rates of transport into mitochondria. Some precursors are stable in the cytosol while most are degraded rapidly, with a half-life of about 30 min at 30 degrees C.

A metalloprotease involved in the processing of mitochondrial precursor proteins

A protease that cleaves the precursor of ornithine carbamoyltransferase (EC 2.1.3.3), a mitochondrial matrix enzyme, has been partially purified from the matrix fraction of rat liver mitochondria. The protease cleaved the precursors of several other matrix proteins at apparently correct sites. The protease was inhibited by 1,10-phenanthroline and EDTA, was reactivated by excess Mn2+ or Co2+, and did not cleave the alkali-denatured precursor proteins. These and other results indicate that this protease is responsible for the processing of at least several matrix protein precursors, and that the enzyme recognizes some three-dimensional conformation of the precursors as well as the amino acid sequences around the cleavage sites.

Identification of a cDNA clone in lambda gt11 for the transacylase component of branched chain ketoacid dehydrogenase

Two cDNA clones for the transacylase protein of the branched chain ketoacid dehydrogenase complex [E.C. 1.2.4.4] have been isolated from a human fetal liver cDNA expression library in lambda gt11 using antibody selection. By selective antibody elution from nitrocellulose filters containing the fusion proteins, it was determined that these inserts represent the transacylase protein. These data support the hypothesis that this protein is synthesized in the cytosol on transcripts independent of the other proteins of the branched chain ketoacid dehydrogenase complex.

Adenosine 5'-triphosphate stimulates the release of polypeptides from mitochondria

There was release of polypeptides to the medium when mitochondria containing labeled proteins were incubated with a rat liver post-mitochondrial supernatant. The release of polypeptides increased with the amount of rat liver extract added. Addition of cycloheximide did not inhibit the effect. Heating the post-mitochondrial supernatant did not inhibit the release of mitochondrial proteins, indicating that it was due to a heat-stable factor. The factor responsible has been isolated and identified as ATP. The presence of EDTA inhibits the release of polypeptides caused by ATP and Mg2+ stimulates it. The possible role of ATP in the turnover of mitochondrial proteins is briefly discussed.

An ammonium sulphate fraction from rabbit reticulocytes increases the release of proteins from rat liver mitochondria

Incubation of [35S]methionine labeled mitochondria from rat liver with rabbit reticulocyte lysate under the same conditions as those used in the import of mitochondrial protein precursors results in the release of mitochondrial proteins to the medium. Fractionation of the lysates with ammonium sulphate yields a fraction, essentially free of haemoglobin, which exhibits higher activity for the release of mitochondrial proteins than the starting lysate. The fraction has a molecular mass of greater than 10 kDa and is heat-sensitive. The release is insensitive to inhibitors of reticulocyte lipoxygenase.