Peroxisomal membrane protein Pmp47 is essential in the metabolism of middle-chain fatty acid in yeast peroxisomes and Is associated with peroxisome proliferation

Pmp47 of the methylotrophic yeast Candida boidinii belongs to a mitochondrial family of solute transporters and is localized in peroxisomal membranes. Its human homolog, Pmp34, is also known. In this study, we characterized the role of Pmp47 in fatty acid metabolism and peroxisome proliferation using the PMP47-deleted strain of C. boidinii (strain pmp47Delta). The wild-type strain grew well on a middle-chain fatty acid, laureate, as the single carbon source, and mild peroxisome proliferation was observed during its growth. The pmp47Delta strain could not grow on laureate but could grow on long-chain fatty acids including palmitate, myristate, and oleate. The levels of laureate oxidation activity in intact cells and in semi-permeabilized cells of strain pmp47Delta were lower than the respective level in the wild-type strain, although the level of laureate oxidation activity in the cell lysate and the level of lauroyl-CoA oxidation in semi-permeabilized cells of strain pmp47Delta were indistinguishable from the respective level in the wild-type strain. When lauroyl-CoA was provided in the cytosol of strain pmp47Delta through expression of Saccharomyces cerevisiae Faa2p (lauroyl-CoA synthetase) in which its peroxisome targeting signal was deleted, the growth of strain pmp47Delta on laureate was recovered to the level of growth of the wild-type strain. Laureate is converted to its CoA form in peroxisomes by the action of lauroyl-CoA synthetase. These results suggested that Pmp47 is involved in the transport of a small molecule (possibly ATP) required in the conversion of laureate to its CoA form in peroxisomes and that the absence of Pmp47 causes impairment of laureate metabolism, which results in the inability of pmp47Delta cells to grow on laureate. In addition, Pmp47 may be involved in peroxisome proliferation, because the pmp47Delta strain contained a reduced number of peroxisomes, as judged from the fluorescence analysis of cells expressing green fluorescent protein tagged with the peroxisome targeting signal 1 (GFP-AKL).     

Giant peroxisomes in oleic acid-induced Saccharomyces cerevisiae lacking the peroxisomal membrane protein Pmp27p

We have purified peroxisomal membranes from Saccharomyces cerevisiae after induction of peroxisomes in oleic acid-containing media. About 30 distinct proteins could be discerned among the HPLC- and SDS-PAGE- separated proteins of the high salt-extracted peroxisomal membranes. The most abundant of these, Pmp27p, was purified and the corresponding gene PMP27 was cloned and sequenced. Its primary structure is 32% identical to PMP31 and PMP32 of the yeast Candida biodinii (Moreno, M., R. Lark, K. L. Campbell, and M. J. Goodman. 1994. Yeast. 10:1447-1457). Immunoelectron microscopic localization of Pmp27p showed labeling of the peroxisomal membrane, but also of matrix-less and matrix containing tubular membranes nearby. Electronmicroscopical data suggest that some of these tubular extensions might interconnect peroxisomes to form a peroxisomal reticulum. Cells with a disrupted PMP27 gene (delta pmp27) still grew well on glucose or ethanol, but they failed to grow on oleate although peroxisomes were still induced by transfer to oleate- containing media. The induced peroxisomes of delta pmp27 cells were fewer but considerably larger than those of wild-type cells, suggesting that Pmp27p may be involved in parceling of peroxisomes into regular quanta. delta pmp27 cells cultured in oleate-containing media form multiple buds, of which virtually all are peroxisome deficient. The growth defect of delta pmp27 cells on oleic acid appears to result from the inability to segregate the giant peroxisomes to daughter cells.     

Antioxidant system within yeast peroxisome. Biochemical and physiological characterization of CbPmp20 in the methylotrophic yeast Candida boidinii

Candida boidinii Pmp20 (CbPmp20), a protein associated with the inner side of peroxisomal membrane, belongs to a recently identified protein family of antioxidant enzymes, the peroxiredoxins, which contain one cysteine residue. Pmp20 homologs containing the putative peroxisome targeting signal type 1 have also been identified in mammals and lower eukaryotes. However, the physiological function of these Pmp20 family proteins has been unclear. In this study, we investigated the biochemical and physiological functions of recombinant CbPmp20 protein in methanol-induced peroxisomes of C. boidinii using the PMP20-deleted strain of C. boidinii (pmp20Delta strain). The His(6)-tagged CbPmp20 fusion protein was found to have glutathione peroxidase activity in vitro toward alkyl hydroperoxides and H(2)O(2). Catalytic activity and dimerization of His(6)-CbPmp20 depended on the only cysteine residue corresponding to Cys(53). The pmp20Delta strain was found to have lost growth ability on methanol as a carbon and energy source. The pmp20Delta growth defect was rescued by CbPmp20, but neither CbPmp20 lacking the peroxisome targeting signal type 1 sequence nor CbPmp20 haboring the C53S mutation retrieved the growth defect. Interestingly, the pmp20Delta strain had a more severe growth defect than the cta1Delta strain, which lacks catalase, another antioxidant enzyme within the peroxisome. During incubation of these strains in methanol medium, the cta1Delta strain accumulated H(2)O(2), whereas the pmp20Delta strain did not. Therefore, it is speculated to be the main function of CbPmp20 is to decompose reactive oxygen species generated at peroxisomal membrane surface, e.g. lipid hydroperoxides, rather than to decompose H(2)O(2). In addition, we detected a physiological level of reduced glutathione in peroxisomal fraction of C. boidinii. These results may indicate a physiological role for CbPmp20 as an antioxidant enzyme within peroxisomes rich in reactive oxygen species.     

The Candida boidinii peroxisomal membrane protein Pmp30 has a role in peroxisomal proliferation and is functionally homologous to Pmp27 from Saccharomyces cerevisiae.

The mechanism of peroxisome proliferation is poorly understood. Candida boidinii is a methylotrophic yeast that undergoes rapid and massive peroxisome proliferation and serves as a good model system for this process. Pmp30A and Pmp30B (formerly designated Pmp31 and Pmp32, respectively) are two closely related proteins in a polyploid strain of this yeast that are strongly induced by diverse peroxisome proliferators such as methanol, oleate, and D-alanine. The function of these proteins is not understood. To study this issue, we used a recently described haploid strain (S2) of C. boidinii that can be manipulated genetically. We now report that strain S2 contains a single PMP30 gene very similar in sequence (greater than 93% identity at the DNA level) to PMP30A and PMP30B. When PMP30 was disrupted, cell growth on methanol was greatly inhibited, and cells grown in both methanol and oleate had fewer, larger, and more spherical peroxisomes than wild-type cells. A similar phenotype was recently described for Saccharomyces cerevisiae cultured on oleate in which PMP27, which encodes a protein of related sequence that is important for peroxisome proliferation, was disrupted. To determine whether Pmp27 is a functional homolog of Pmp30, gentle complementation was performed. PMP30A was expressed in the PMP27 disruptant of S. cerevisiae, and PMP27 was expressed in the PMP30 disruptant of C. boidinii S2. Complementation, in terms of both cell growth and organelle size, shape, and number, was successful in both directions, although reversion to a wild-type phenotype was only partial for the PMP30 disruptant. We conclude that these proteins are functional homologs and that both Pmp30 and Pmp27 have a direct role in proliferation and organelle size rather than a role in a specific peroxisomal metabolic pathway of substrate utilization.     

Pmp27 promotes peroxisomal proliferation

Peroxisomes perform many essential functions in eukaryotic cells. The weight of evidence indicates that these organelles divide by budding from preexisting peroxisomes. This process is not understood at the molecular level. Peroxisomal proliferation can be induced in Saccharomyces cerevisiae by oleate. This growth substrate is metabolized by peroxisomal enzymes. We have identified a protein, Pmp27, that promotes peroxisomal proliferation. This protein, previously termed Pmp24, was purified from peroxisomal membranes, and the corresponding gene, PMP27, was isolated and sequenced. Pmp27 shares sequence similarity with the Pmp30 family in Candida boidinii. Pmp27 is a hydrophobic peroxisomal membrane protein but it can be extracted by high pH, suggesting that it does not fully span the bilayer. Its expression is regulated by oleate. The function of Pmp27 was probed by observing the phenotype of strains in which the protein was eliminated by gene disruption or overproduced by expression from a multicopy plasmid. The strain containing the disruption (3B) was able to grow on all carbon sources tested, including oleate, although growth on oleate, glycerol, and acetate was slower than wild type. Strain 3B contained peroxisomes with all of the enzymes of beta-oxidation. However, in addition to the presence of a few modestly sized peroxisomes seen in a typical thin section of a cell growing on oleate-containing medium, cells of strain 3B also contained one or two very large peroxisomes. In contrast, cells in a strain in which Pmp27 was overexpressed contained an increased number of normal-sized peroxisomes. We suggest that Pmp27 promotes peroxisomal proliferation by participating in peroxisomal elongation or fission.     

Regulation and evaluation of five methanol-inducible promoters in the methylotrophic yeast Candida boidinii

We isolated the promoter regions of five methanol-inducible genes (P(AOD1), alcohol oxidase; P(DAS1), dihydroxyacetone synthase; P(FDH1), formate dehydrogenase; P(PMP20), Pmp20; and P(PMP47), Pmp47) from the Candida boidinii genome, and evaluated their strength and studied their regulation using the acid phosphatase gene of Saccharomyces cerevisiae (ScPHO5) as the reporter. Of the five promoters, P(DAS1) was the strongest methanol-inducible promoter whose strength was approximately 1.5 times higher than that of the commonly used P(AOD1) in methanol-induced cells. Although the expression of P(AOD1) and P(DAS1) was completely repressed by the presence of glucose, formate-induced expression of P(FDH1) was not repressed by glucose. Expression under P(PMP47), another methanol-inducible promoter, was highly induced by oleate. The induction kinetics of P(PMP47) and P(DAS1) revealed that methanol induces the expression of peroxisome membrane protein Pmp47, earlier than the expression of matrix enzyme dihydroxyacetone synthase (Das1p), and that this information is contained in the promoter region of the respective gene. This is the first report which evaluates several methanol-inducible promoters in parallel in the methylotrophic yeast.     

Absence of the peroxiredoxin Pmp20 causes peroxisomal protein leakage and necrotic cell death

We analyzed the role of the peroxisomal peroxiredoxin Pmp20 of the yeast Hansenula polymorpha. Cells of a PMP20 disruption strain (pmp20) grew normally on substrates that are not metabolized by peroxisomal enzymes, but showed a severe growth defect on methanol, the metabolism of which involves a hydrogen peroxide producing peroxisomal oxidase. This growth defect was paralleled by leakage of peroxisomal matrix proteins into the cytosol. Methanol-induced pmp20 cells accumulated enhanced levels of reactive oxygen species and lipid peroxidation products. Moreover, the fatty acid composition of methanol-induced pmp20 cells differed relative to WT controls, suggesting an effect on fatty acid homeostasis. Plating assays and FACS-based analysis of cell death markers revealed that pmp20 cells show loss of clonogenic efficiency and membrane integrity, when cultured on methanol. We conclude that the absence of the peroxisomal peroxiredoxin leads to loss of peroxisome membrane integrity and necrotic cell death.     

Multiple targeting modules on peroxisomal proteins are not redundant: discrete functions of targeting signals within Pmp47 and Pex8p

Several peroxisomal proteins have two nonoverlapping targeting signals. These signals have been termed “redundant” because targeting can still occur with only one signal. We now report that separate targeting motifs within both Pmp47 and Pex8 provide complementary function. Pmp47 is an ATP translocator that contains six transmembrane domains (TMDs). We had previously shown that the TMD2 region (termed TMD2R, consisting of TMD2 and a short adjacent segment of cytosolic loop) was required for targeting to proliferated peroxisomes in Saccharomyces cerevisiae. We now report that the analogous TMD4R, which cannot target to proliferated peroxisomes, targets at least as well, or much better (depending on strain and growth conditions) in cells containing only basal (i.e., nonproliferated) peroxisomes. These data suggest differences in the targeting pathway among peroxisome populations. Pex8p, a peripheral protein facing the matrix, contains a typical carboxy terminal targeting sequence (PTS1) that has been shown to be nonessential for targeting, indicating the existence of a second targeting domain (not yet defined in S. cerevisiae); thus, its function was unknown. We show that targeting to basal peroxisomes, but not to proliferated peroxisomes, is more efficient with the PTS1 than without it. Our results indicate that multiple targeting signals within peroxisomal proteins extend coverage among heterogeneous populations of peroxisomes and increase efficiency of targeting in some metabolic states.     

An internal region of the peroxisomal membrane protein PMP47 is essential for sorting to peroxisomes

Targeting sequences on peroxisomal membrane proteins have not yet been identified. We have attempted to find such a sequence within PMP47, a protein of the methylotrophic yeast, Candida boidinii. This protein of 423 amino acids shows sequence similarity with proteins in the family of mitochondrial carrier proteins. As such, it is predicted to have six membrane-spanning domains. Protease susceptibility experiments are consistent with a six-membrane-spanning model for PMP47, although the topology for the peroxisomal protein is inverted compared with the mitochondrial carrier proteins. PMP47 contains two potential peroxisomal targeting sequences (PTS1), an internal SKL (residues 320- 322) and a carboxy terminal AKE (residues 421-423). Using a heterologous in vivo sorting system, we show that efficient sorting occurs in the absence of both sequences. Analysis of PMP47- dihydrofolate reductase (DHFR) fusion proteins revealed that amino acids 1-199 of PMP47, which contain the first three putative membrane spans, do not contain the necessary targeting information, whereas a fusion with amino acids 1-267, which contains five spans, is fully competent for sorting to peroxisomes. Similarly, a DHFR fusion construct containing residues 268-423 did not target to peroxisomes while residues 203-420 appeared to sort to that organelle, albeit at lower efficiency than the 1-267 construct. However, DHFR constructs containing only amino acids 185-267 or 203-267 of PMP47 were not found to be associated with peroxisomes. We conclude that amino acids 199-267 are necessary for peroxisomal targeting, although additional sequences may be required for efficient sorting to, or retention by, the organelles.     

Sorting of peroxisomal membrane protein PMP47 from Candida boidinii into peroxisomal membranes of Saccharomyces cerevisiae

A gene encoding PMP47, a peroxisomal integral membrane protein of the methylotrophic yeast Candida boidinii, was isolated from a genomic library. DNA sequencing of PMP47 revealed an open reading frame of 1269 base pairs capable of encoding a protein of 46,873 Da. At least two membrane-spanning regions in the protein are predicted from the sequence. Since the 3 amino acids at the carboxyl terminus are -AKE, PMP47 lacks a typical peroxisomal sorting signal. No significant similarities in primary structure between PMP47 and known proteins were observed, including PMP70, a rat peroxisomal membrane protein whose sequence has recently been reported (Kamijo, K., Taketani, S., Yokota, S., Osumi, T., and Hashimoto, T. (1990). J. Biol. Chem. 265, 4534-4540). In order to study the import and assembly of PMP47 into peroxisomes by genetic approaches, the gene was expressed in the yeast Saccharomyces cerevisiae. When PMP47 was expressed in cells grown on oleic acid to induce peroxisomes, the protein was observed exclusively in peroxisomes as determined by marker enzyme analysis of organelle fractions. Most of the PMP47 co-purified with the endogenous peroxisomal membrane proteins on isopycnic sucrose gradients. Either in the native host or when expressed in S. cerevisiae, PMP47 was not extractable from peroxisomal membranes by sodium carbonate at pH 11, indicating an integral membrane association. These results indicate that PMP47 is competent for sorting to and assembling into peroxisomal membranes in S. cerevisiae.     

Topogenesis of peroxisomal membrane protein requires a short, positively charged intervening-loop sequence and flanking hydrophobic segments. study using human membrane protein PMP34

Human 34-kDa peroxisomal membrane protein (PMP34) consisting of 307 amino acids was previously identified as an ortholog of, or a similar protein (with 27% identity) to the, 423-amino acid-long PMP47 of the yeast Candida boidinii. We investigated membrane topogenesis of PMP34 with six putative transmembrane segments, as a model peroxisomal membrane protein. PMP34 was characterized as an integral membrane protein of peroxisomes. Transmembrane topology of PMP34 was determined by differential permeabilization and immunofluorescent staining of HeLa cells ectopically expressing PMP34 as well as of Chinese hamster ovary-K1 expressing epitope-tagged PMP34. As opposed to PMP47, PMP34 was found to expose its N- and C-terminal parts to the cytosol. Various deletion variants of PMP34 and their fusion proteins with green fluorescent protein were expressed in Chinese hamster ovary-K1 and were verified with respect to intracellular localization. The loop region between transmembrane segments 4 and 5 was required for the peroxisome-targeting activity, in which Ala substitution for basic residues abrogated the activity. Three hydrophobic transmembrane segments linked in a flanking region of the basic loop were essential for integration of PMP34 to peroxisome membranes. Therefore, it is evident that the intervening basic loop plus three transmembrane segments of PMP34 function as a peroxisomal targeting and topogenic signal.     

Peroxisomal protein import is conserved between yeast, plants, insects and mammals.

We have previously demonstrated that firefly luciferase can be imported into peroxisomes of both insect and mammalian cells. To determine whether the process of protein transport into the peroxisome is functionally similar in more widely divergent eukaryotes, the cDNA encoding firefly luciferase was expressed in both yeast and plant cells. Luciferase was translocated into peroxisomes in each type of organism. Experiments were also performed to determine whether a yeast peroxisomal protein could be transported to peroxisomes in mammalian cells. We observed that a C-terminal segment of the yeast (Candida boidinii) peroxisomal protein PMP20 could act as a peroxisomal targeting signal in mammalian cells. These results suggest that at least one mechanism of protein translocation into peroxisomes has been conserved throughout eukaryotic evolution.     

Peroxisomal catalase in the methylotrophic yeast Candida boidinii: transport efficiency and metabolic significance

In this study we cloned CTA1, the gene encoding peroxisomal catalase, from the methylotrophic yeast Candida boidinii and studied targeting of the gene product, Cta1p, into peroxisomes by using green fluorescent protein (GFP) fusion proteins. A strain from which CTA1 was deleted (cta1Delta strain) showed marked growth inhibition when it was grown on the peroxisome-inducing carbon sources methanol, oleate, and D-alanine, indicating that peroxisomal catalase plays an important nonspecific role in peroxisomal metabolism. Cta1p carries a peroxisomal targeting signal type 1 (PTS1) motif, -NKF, in its carboxyl terminus. Using GFP fusion proteins, we found that (i) Cta1p is transported to peroxisomes via its PTS1 motif, -NKF; (ii) peroxisomal localization is necessary for Cta1p to function physiologically; and (iii) Cta1p is bimodally distributed between the cytosol and peroxisomes in methanol-grown cells but is localized exclusively in peroxisomes in oleate- and D-alanine-grown cells. In contrast, the fusion protein GFP-AKL (GFP fused to another typical PTS1 sequence, -AKL), in the context of CbPmp20 and D-amino acid oxidase, was found to localize exclusively in peroxisomes. A yeast two-hybrid system analysis suggested that the low transport efficiency of the -NKF sequence is due to a level of interaction between the -NKF sequence and the PTS1 receptor that is lower than the level of interaction with the AKL sequence. Furthermore, GFP-Cta1pDeltankf coexpressed with Cta1p was successfully localized in peroxisomes, suggesting that the oligomer was formed prior to peroxisome import and that it is not necessary for all four subunits to possess a PTS motif. Since the main physiological function of catalase is degradation of H2O2, suboptimal efficiency of catalase import may confer an evolutionary advantage. We suggest that the PTS1 sequence, which is found in peroxisomal catalases, has evolved in such a way as to give a higher priority for peroxisomal transport to peroxisomal enzymes other than to catalases (e.g., oxidases), which require a higher level of peroxisomal transport efficiency.     

Overproduction of Pex5p stimulates import of alcohol oxidase and dihydroxyacetone synthase in a Hansenula polymorpha Pex14 null mutant

Hansenula polymorpha Deltapex14 cells are affected in peroxisomal matrix protein import and lack normal peroxisomes. Instead, they contain peroxisomal membrane remnants, which harbor a very small amount of the major peroxisomal matrix enzymes alcohol oxidase (AO) and dihydroxyacetone synthase (DHAS). The bulk of these proteins is, however, mislocated in the cytosol. Here, we show that in Deltapex14 cells overproduction of the PTS1 receptor, Pex5p, leads to enhanced import of the PTS1 proteins AO and DHAS but not of the PTS2 protein amine oxidase. The import of the PTS1 protein catalase (CAT) was not stimulated by Pex5p overproduction. The difference in import behavior of AO and CAT was not related to their PTS1, since green fluorescent protein fused to the PTS1 of either AO or CAT were both not imported in Deltapex14 cells overproducing Pex5p. When produced in a wild type control strain, both proteins were normally imported into peroxisomes. In Deltapex14 cells overproducing Pex5p, Pex5p had a dual location and was localized in the cytosol and bound to the outer surface of the peroxisomal membrane. Our results indicate that binding of Pex5p to the peroxisomal membrane and import of certain PTS1 proteins can proceed in the absence of Pex14p.     

Regulation of peroxisome size and number by fatty acid beta -oxidation in the yeast yarrowia lipolytica

The Yarrowia lipolytica MFE2 gene encodes peroxisomal beta-oxidation multifunctional enzyme type 2 (MFE2). MFE2 is peroxisomal in a wild-type strain but is cytosolic in a strain lacking the peroxisomal targeting signal-1 (PTS1) receptor. MFE2 has a PTS1, Ala-Lys-Leu, that is essential for targeting to peroxisomes. MFE2 lacking a PTS1 can apparently oligomerize with full-length MFE2 to enable targetting to peroxisomes. Peroxisomes of an oleic acid-induced MFE2 deletion strain, mfe2-KO, are larger and more abundant than those of the wild-type strain. Under growth conditions not requiring peroxisomes, peroxisomes of mfe2-KO are larger but less abundant than those of the wild-type strain, suggesting a role for MFE2 in the regulation of peroxisome size and number. A nonfunctional version of MFE2 did not restore normal peroxisome morphology to mfe2-KO cells, indicating that their phenotype is not due to the absence of MFE2. mfe2-KO cells contain higher amounts of beta-oxidation enzymes than do wild-type cells. We also show that increasing the level of the beta-oxidation enzyme thiolase results in enlarged peroxisomes. Our results implicate peroxisomal beta-oxidation in the control of peroxisome size and number in yeast.     

Characterization of a novel component of the peroxisomal protein import apparatus using fluorescent peroxisomal proteins.

Fluorescent peroxisomal probes were developed by fusing green fluorescent protein (GFP) to the matrix peroxisomal targeting signals PTS1 and PTS2, as well as to an integral peroxisomal membrane protein (IPMP). These proteins were used to identify and characterize novel peroxisome assembly (pas) mutants in the yeast Pichia pastoris. Mutant cells lacking the PAS10 gene mislocalized both PTS1-GFP and PTS2-GFP to the cytoplasm but did incorporate IPMP-GFP into peroxisome membranes. Similar distributions were observed for endogenous peroxisomal matrix and membrane proteins. While peroxisomes from translocation-competent pas mutants sediment in sucrose gradients at the density of normal peroxisomes, >98% of peroxisomes from pas10 cells migrated to a much lower density and had an extremely low ratio of matrix:membrane protein. These data indicate that Pas10p plays an important role in protein translocation across the peroxisome membrane. Consistent with this hypothesis, we find that Pas10p is an integral protein of the peroxisome membrane. In addition, Pas10p contains a cytoplasmically-oriented C3HC4 zinc binding domain that is essential for its biological activity.     

Developmental analysis of a putative ATP/ADP carrier protein localized on glyoxysomal membranes during the peroxisome transition in pumpkin cotyledons

In order to clarify the peroxisomal membrane proteins (PMPs), we characterized one of the major PMPs, PMP38. The deduced amino acid sequence for its cDNA in Arabidopsis thaliana contained polypeptides with 331 amino acids and had high similarity with those of Homo sapiens PMP34 and Candida boidinii PMP47 known as homologues of mitochondrial ATP/ADP carrier protein. We expected PMP38 to be localized on peroxisomal membranes, because it had the membrane peroxisomal targeting signal. Cell fractionation and immunocytochemical analysis using pumpkin cotyledons revealed that PMP38 is localized on peroxisomal membranes as an integral membrane protein. The amount of PMP38 in pumpkin cotyledons increased and reached the maximum protein level after 6 d in the dark but decreased thereafter. Illumination of the seedlings caused a significant decrease in the amount of the protein. These results clearly showed that the membrane protein PMP38 in glyoxysomes changes dramatically during the transformation of glyoxysomes to leaf peroxisomes, as do the other glyoxysomal enzymes, especially enzymes of the fatty acid beta-oxidation cycle, that are localized in the matrix of glyoxysomes.     

Environmental response of yeast peroxisomes

Nutritional changes can effect either the assembly or disassembly of yeast peroxisomes. In the past decade, insights regarding the molecular mechanisms of peroxisome assembly have been gained chiefly through the cloning of the PEX genes obtained by complementation of corresponding pex mutants in several yeast strains and Chinese hamster ovary cell lines. Depletion of these peroxins (proteins encoded by PEX genes) by deletion of the corresponding genes affects peroxisomal protein import biogenesis or proliferation. To complement these studies in the field, the authors undertook an investigation of the functions of a subset of Candida boidinii peroxisomal membrane proteins (PMPs), Pex11, Pmp47, and Pmp20, by analyzing strains of C. boidnii in which the genes encoding these proteins were deleted. The authors' studies show that Pex11p is involved in peroxisome proliferation; Pmp47 plays a role in the translocation, folding, or assembly of dihydroxyacetone synthase; and Pmp20 is probably involved in methanol metabolism. In contrast to the studies on peroxisome assembly, the molecular mechanisms of peroxisome degradation remain poorly understood. To shed light on this problem, the authors isolated Pichia pastoris mutants defective in peroxisome autopathy (pag mutants). A novel, double-fluorescence method used for the characterization of wild-type cells and of pag mutants enabled us to dissect the microautophagic degradation of peroxisomes into several distinct stages. These studies show that specific PAG gene products are involved in multiple steps of the process. Future cloning and characterization of the functions of PAG genes will reveal the molecular basis of peroxisome degradation.     

Multiple PEX genes are required for proper subcellular distribution and stability of Pex5p, the PTS1 receptor: evidence that PTS1 protein import is mediated by a cycling receptor

PEX5 encodes the type-1 peroxisomal targeting signal (PTS1) receptor, one of at least 15 peroxins required for peroxisome biogenesis. Pex5p has a bimodal distribution within the cell, mostly cytosolic with a small amount bound to peroxisomes. This distribution indicates that Pex5p may function as a cycling receptor, a mode of action likely to require interaction with additional peroxins. Loss of peroxins required for protein translocation into the peroxisome (PEX2 or PEX12) resulted in accumulation of Pex5p at docking sites on the peroxisome surface. Pex5p also accumulated on peroxisomes in normal cells under conditions which inhibit protein translocation into peroxisomes (low temperature or ATP depletion), returned to the cytoplasm when translocation was restored, and reaccumulated on peroxisomes when translocation was again inhibited. Translocation inhibiting conditions did not result in Pex5p redistribution in cells that lack detectable peroxisomes. Thus, it appears that Pex5p can cycle repeatedly between the cytoplasm and peroxisome. Altered activity of the peroxin defective in CG7 cells leads to accumulation of Pex5p within the peroxisome, indicating that Pex5p may actually enter the peroxisome lumen at one point in its cycle. In addition, we found that the PTS1 receptor was extremely unstable in the peroxin-deficient CG1, CG4, and CG8 cells. Altered distribution or stability of the PTS1 receptor in all cells with a defect in PTS1 protein import implies that the genes mutated in these cell lines encode proteins with a direct role in peroxisomal protein import.     

Contribution of peroxisome-specific isoform of Lon protease in sorting PTS1 proteins to peroxisomes

Using an organelle proteomics approach, we previously studied the rat peroxisome in order to characterize the proteins participating in its biogenesis. A peroxisome-specific isoform of Lon (pLon) protein was accordingly identified. However, the precise role of pLon in peroxisomes remains to be elucidated. Here, we demonstrate that pLon plays a role in processing and activating a specific regulatory protein belonging to the peroxisome targeting signal (PTS) 1-containing proteins. Proteomic analysis of proteins co-immunoprecipitated with Lon suggested that Lon interacts with PMP70 and several enzymes involved in beta-oxidation, including acyl-CoA oxidase (AOX). The processing of AOX for its activation in peroxisomes was strongly inhibited in cells expressing a dominant negative form of pLon. Furthermore, a catalase possessing a modified PTS1 sequence was misdistributed in this cell line. pLon exhibits little, if any, in vitro AOX processing activity, and does not process PTS2-containing 3-ketoacyl-coenzyme A thiolase (PTL). Therefore, pLon may specifically control, sort and process PTS1 proteins. Based on the relationship between pLon and the beta-oxidation enzymes that regulate peroxisomal morphology, the observation of enlarged peroxisomes in cells expressing recombinant pLon suggests that pLon is a critical factor determining peroxisome morphology.