Biogenesis of peroxisomes in mammals

Peroxisome biogenesis: involvement of ARF and coatomer

Peroxisomal membrane protein (Pmp)26p (RnPex11p), a major constituent of induced rat liver peroxisomal membrane, was found to contain a COOH-terminal, cytoplasmically exposed consensus dilysine motif with the potential to bind coatomer. Biochemical as well as immunocytochemical evidence is presented showing that peroxisomes incubated with preparations of bovine brain or rat liver cytosol recruit ADP-ribosylation factor (ARF) and coatomer in a strictly guanosine 5'-O-(3-thiotriphosphate)-dependent manner. Consistent with this observation, ldlF cells expressing a temperature-sensitive mutant version of the epsilon-subunit of coatomer exhibit elongated tubular peroxisomes possibly due to impaired vesiculation at the nonpermissive temperature. Since overexpression of Pex11p in Chinese hamster ovary wild-type cells causes proliferation of peroxisomes, these data suggest that Pex11p plays an important role in peroxisome biogenesis by supporting ARF- and coatomer-dependent vesiculation of the organelles.

Cetaben is an exceptional type of peroxisome proliferator

1. Cetaben in contrast to fibrates affect differently peroxisomal constituents. 2. Changes in large scale of liver non-peroxisomal parameters were compared after 10 days administration of equal doses (200 mg/kg/day) of cetaben and clofibric acid to male Wistar rats. 3. Clofibric acid treatment increased markedly the activities of FAD-glycerol-3-P dehydrogenase, beta-hydroxyacyl-CoA dehydrogenase, cytochrome-c oxidase, malic enzyme, NAD-glycerol-3-P dehydrogenase, ethoxycoumarin deethylase, p-nitroanisole demethylase and amounts of cytochrome P-450 and b5. 4. However no analogical changes were observed after cetaben treatment in the livers of experimental animals. 5. Both drugs increased the activities of alanine-glyoxylate aminotransferase-1 and acetylcarnitine transferase--enzymes with proven mitochondrial and peroxisomal location. 6. Cetaben contrary to clofibric acid does not increase solubilization of peroxisomal enzymes. 7. Enhanced acetylcarnitine transferase and alanine-glyoxylate aminotransferase-1 activities were distributed in mitochondria as well as in peroxisomes after clofibric acid treatment, however, only peroxisomes were enriched after cetaben administration. 8. The results obtained suggest that cetaben represents an exceptional type of peroxisome proliferator, specifically affecting peroxisomes, without having a negative influence on the processes of peroxisome biogenesis.

In vitro insertion of the 22-kD peroxisomal membrane protein into isolated rat liver peroxisomes

The membrane insertion of the 22-kD integral peroxisomal membrane protein (PMP 22) was studied in a system in which peroxisomes isolated from rat liver were incubated with the [35S]methionine-labeled in vitro translation product of PMP 22 mRNA. Membrane insertion of PMP 22 was demonstrated by protease treatment of peroxisomes in the absence and presence of detergent. Approximately 35% of total in vitro translated PMP 22 became protease resistant after a 1-h incubation at 26 degrees C. Import was dependent on time and temperature, did not require ATP or GTP and was not inhibited by N-ethylmaleimide treatment of neither the soluble components of the translation mixture nor of the isolated peroxisomes. In contrast to these results it was recently shown that the import of the peroxisomal marker, firefly luciferase, into peroxisomes of permeabilized cells was dependent on ATP hydrolysis and was blocked by N-ethylmaleimide pretreatment of the cytosol-depleted cells (Rapp et al., 1993; Wendland and Subramani, 1993). Therefore, the present data suggest that insertion of PMP 22 into the peroxisomal membrane and translocation of firefly luciferase into peroxisomes follow distinct mechanisms. At low temperature binding of PMP 22 to the peroxisomal membrane was not influenced whereas insertion was strongly inhibited. Pretreatment of peroxisomes with subtilisin reduced binding to a low level and completely abolished insertion. Therefore it is suggested that binding is prerequisite to insertion and that insertion may be mediated by a proteinaceous receptor.

Membrane topology of the 22 kDa integral peroxisomal membrane protein

In order to study the membrane topology and the possible function of the rat liver 22 kDa integral peroxisomal membrane protein (PMP 22) at a molecular level, we have cloned PMP 22 from a lambda gt11 expression library and sequenced its cDNA. Hydropathy analysis of the deduced primary structure indicates 4 putative transmembrane segments. The accessibility to exogenous aminopeptidase of PMP 22 in intact peroxisomes suggests that the N-terminus faces the cytosol. A model of the topology of PMP 22 in the peroxisomal membrane is discussed. Homology studies revealed a striking similarity with the Mpv 17 gene product. Lack of this membrane protein causes nephrotic syndrome in mice.