Proteolytic processing of polypeptides during the biosynthesis of subcellular structures

Mitochondrial protein import

Most mitochondrial proteins are synthesized as precursor proteins on cytosolic polysomes and are subsequently imported into mitochondria. Many precursors carry amino-terminal presequences which contain information for their targeting to mitochondria. In several cases, targeting and sorting information is also contained in non-amino-terminal portions of the precursor protein. Nucleoside triphosphates are required to keep precursors in an import-competent (unfolded) conformation. The precursors bind to specific receptor proteins on the mitochondrial surface and interact with a general insertion protein (GIP) in the outer membrane. The initial interaction of the precursor with the inner membrane requires the mitochondrial membrane potential (delta psi) and occurs at contact sites between outer and inner membranes. Completion of translocation into the inner membrane or matrix is independent of delta psi. The presequences are cleaved off by the processing peptidase in the mitochondrial matrix. In several cases, a second proteolytic processing event is performed in either the matrix or in the intermembrane space. Other modifications can occur such as the addition of prosthetic groups (e.g., heme or Fe/S clusters). Some precursors of proteins of the intermembrane space or the outer surface of the inner membrane are retranslocated from the matrix space across the inner membrane to their functional destination ('conservative sorting'). Finally, many proteins are assembled in multi-subunit complexes. Exceptions to this general import pathway are known. Precursors of outer membrane proteins are transported directly into the outer membrane in a receptor-dependent manner. The precursor of cytochrome c is directly translocated across the outer membrane and thereby reaches the intermembrane space. In addition to the general sequence of events which occurs during mitochondrial protein import, current research focuses on the molecules themselves that are involved in these processes.

In vitro synthesis of peroxisomal membrane polypeptides

Peroxisomal membranes containing predominantly integral peroxisome membrane polypeptides were obtained from a highly purified peroxisomal fraction. Following sodium dodecylsulfate polyacrylamide gel electrophoresis three polypeptides with apparent molecular weights of 69, 36, and 22 kDa were isolated and used to raise antibodies in rabbits. Cell-free synthesis of these polypeptides was carried out in an in vitro translational system derived from rabbit reticulocytes. By subjecting peroxisomal membranes to reductive methylation [14C]-radiolabeled mature membrane polypeptides were obtained. The comparison of the three mature integral peroxisome membrane polypeptides with their corresponding in vitro synthesis products revealed no size differences indicating the lack of recognizable presequences for these peroxisomal membrane polypeptides.

Cytochrome b, the var 1 protein, and subunits I and III of cytochrome c oxidase are synthesized without transient presequences in Saccharomyces cerevisiae

The N-termini of four mitochondrial translation products, the var 1 protein, cytochrome b, and subunits I and III of cytochrome c oxidase have been characterized in Saccharomyces cerevisiae and compared with the known DNA sequences of the respective structural genes. The four mature proteins correspond to the predicted primary translation products and retain the formylated methionine residue. Thus, subunit II of cytochrome c oxidase studied previously [Pratje et al. (1983) EMBO J.2, 1049-1054] is so far the only mitochondrial translation product carrying a N-terminal-extended transient presequence in S. cerevisiae.

Transport of proteins into mitochondrial matrix. Evidence suggesting a common pathway for 3-ketoacyl-CoA thiolase and enzymes having presequences

Rat liver 3-ketoacyl-CoA thiolase, a mitochondrial matrix enzyme which catalyzes a step of fatty acid beta-oxidation, was synthesized in a rabbit reticulocyte lysate cell-free system. The in vitro product was apparently the same in molecular size and charge as the subunit of the mature enzyme. The enzyme synthesized in vitro was transported into isolated rat liver mitochondria in an energy-dependent manner. In pulse experiments with isolated rat hepatocytes at 37 degrees C, the radioactivity of the newly synthesized enzyme in the cytosolic fraction remained essentially unchanged during 5-20 min of incubation, whereas that of the enzyme in the particulate fraction increased with time during the incubation. The pulse-labeled enzyme disappeared with an apparent half-life of less than 3 min from the cytosolic fraction, in pulse-chase experiments. Purified 3-ketoacyl-CoA thiolase inhibited the mitochondrial uptake and processing of the precursors of the other matrix enzymes, ornithine carbamoyltransferase, medium-chain acyl-CoA dehydrogenase and acetoacetyl-CoA thiolase. These results indicate that 3-ketoacyl-CoA thiolase has an internal signal which is recognized by the mitochondria and suggest that this enzyme and the three others are transported into the mitochondria by a common pathway.

Biosynthesis of enzymes of rat-liver mitochondrial beta-oxidation

The biogenesis of seven enzymes involved in the mitochondrial fatty acid beta-oxidation of rat liver was studied. Hepatic RNA was translated in vitro in a rabbit reticulocyte lysate cell-free system and the translation products were immunoprecipitated, subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and visualized by fluorography. The translation products obtained in vitro of medium-chain and/or long-chain acyl-CoA dehydrogenase (these enzymes were immunochemically cross-reactive), enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and acetoacetyl-CoA thiolase and probably also short-chain acyl-CoA dehydrogenase were larger than the subunits of the corresponding mature enzymes by 2-4.5 kDa, whereas the 3-oxoacyl-CoA thiolase obtained in vitro was approximately the same size as the mature subunit. The free polysome fraction of rat liver was 4.3-9.0-times more active than the membrane-bound polysome fraction in the synthesis of these seven enzymes. The enzyme activities were increased after administration of di(2-ethylhexyl)phthalate; the extent of the increase varied from one enzyme to another. The increase in the cell-free translation activity of total hepatic RNA for these enzymes after administration of the chemical was markedly different among individual enzymes and higher than that in the rates of synthesis of the corresponding enzymes which were determined by the experiment in vivo.

Use of a monoclonal antibody to distinguish between precursor and mature forms of human lysosomal alpha-glucosidase

The maturation of lysosomal alpha-glucosidase in cultured human skin fibroblasts was studied using a monoclonal antibody that distinguishes between the precursor and mature forms of the enzyme. Monoclonal antibodies against alpha-glucosidase isolated from placenta were produced by the hybridoma technique [Hilkens et al. (1981) Biochim. Biophys. Acta 678, 7-11]. One of these monoclonal antibodies, that synthesized by clone 43G8, reacts with the mature forms, but not with the precursor form of alpha-glucosidase isolated from urine. By means of pulse-labelling studies, it could be shown that monoclonal antibody 43G8 does not react with either the intracellular or the secreted precursor of alpha-glucosidase from cultured human skin fibroblasts. However, the antibody does react with the intermediate and mature forms of alpha-glucosidase. Endocytosis of the precurosor of alpha-glucosidase from urine by fibroblasts is followed by its conversion to a form with lower molecular mass. After endocytosis urinary precursor alpha-glucosidase is converted to a form that binds to monoclonal antibody 43G8. The t 1/2 for this conversion is 2 h. The conversion is inhibited by addition of leupeptin to the culture medium. It is concluded that a thiol proteinase is involved in the maturation of alpha-glucosidase in fibroblasts and the appearance of the antigenic determinant for 43G8.

Synthesis in vitro of precursor-type carnitine acetyltransferase with messenger RNA from Candida tropicalis

Carnitine acetyltransferase was synthesized in vitro in the mRNA-dependent reticulocyte system with mRNA from alkane-grown or propionate-grown cells of Candida tropicalis. The protein synthesized in vitro was isolated by immunoprecipitation with antibody against peroxisomal or mitochondrial carnitine acetyltransferase and was compared with peroxisomal carnitine acetyltransferase (Mr of subunits, 64 000 and 57 000) and the mitochondrial enzyme (Mr of subunits, 64 000 and 52 000) of C. tropicalis by electrophoresis in the presence of sodium dodecyl sulfate. Nascent carnitine acetyltransferase prepared in vitro showed a hetero-oligomeric property, like the peroxisomal and mitochondrial enzymes isolated from C. tropicalis. The molecular weights of the subunits of nascent carnitine acetyltransferase were estimated to be 71 000 and 57 000, indicating the existence of the precursor form of the enzyme. By sucrose density gradient centrifugation of total mRNA, these two subunit proteins were shown to be synthesized with respective mRNAs of different sizes. The same precursor-type of carnitine acetyltransferase was obtained with the mRNAs from the alkane-grown cells and the propionate-grown cells. The results obtained suggest that a common precursor will be post-translationally modified to form the peroxisomal and mitochondrial enzymes.

Paradoxical stimulation by amino acids of the degradation of [35S]methionine-labelled, short-lived protein in isolated rat hepatocytes

A complete amino acid mixture inhibited the degradation of long-lived and [14C]valine-labelled short-lived protein in isolated rat hepatocytes, but paradoxically stimulated the degradation of [35S]methionine-labelled short-lived protein. The stimulation persisted in the presence of autophagiclysosomal pathway inhibitors like 3-methyladenine and propylamine, indicating the existence of an hitherto unrecognized non-lysosomal degradation mechanism with selectivity towards methionine-rich proteins or peptide regions.

Synthesis of rat-liver lactate dehydrogenase and characterization of its mRNA

Rat liver lactate dehydrogenase (LDH) has been synthesized in hepatocytes and in a cell-free translation system. The subunit synthesized in both systems displayed the same electrophoretic mobility upon sodium dodecyl sulfate/polyacrylamide slab gel electrophoresis. Sequence analysis of the subunits translated in vitro and synthesized in vivo indicated N-acetyl-alanyl-alanine for both N termini. Thus, the newly synthesized subunit does not exhibit an amino-terminal extension. The mRNA for the lactate dehydrogenase subunit was exclusively found in free polysomes. A size of 2120 +/- 240 nucleotides was estimated for the mRNA. Only about 50% of these nucleotides are needed to code for the polypeptide chain of the enzyme.