Peroxisomal acyl-coenzyme A oxidase multigene family of the yeast Candida tropicalis; nucleotide sequence of a third gene and its protein product

We have determined the complete nucleotide sequence of gene POX2, which encodes one of the major peroxisomal polypeptides (PXPs) of Candida tropicalis. POX2 is linked to gene POX4, which codes for a subunit (PXP-4) of long-chain acyl-CoA oxidase. Southern blot analysis revealed that POX2 had a significant homology to POX4, and also to gene POX5 which encodes a subunit (PXP-5) of the isozyme of acyl-CoA oxidase. PXP-2, the protein product of POX2, was co-purified with PXP-4 from the isolated peroxisomes. PXP-2 itself was a flavoprotein and likely to form an equimolar complex with PXP-4, although its enzymatic activity was uncertain. POX2 corresponds to a single open reading frame of 724 amino acids and has no introns. The N-terminal sequence and the calculated Mr of the deduced polypeptide were consistent with those of isolated PXP-2. The primary structure was highly homologous to those of PXP-4 and PXP-5 in respect of the amino acid sequence and the hydropathy profile. We conclude that POX2 is a third gene of the peroxisomal acyl-COA oxidase multigene family.     

High-level expression and molecular cloning of genes encoding Candida tropicalis peroxisomal proteins.

The development of peroxisomes in the cells of Candida tropicalis grown on oleic acid was accompanied by a markedly high expression of peroxisomal proteins. On the basis of this finding, the nuclear DNA library of this yeast was screened by differential hybridization, and 102 clones of oleic acid-inducible sequences were isolated. Seven coding regions were found to form clusters in three stretches of the genomic DNA. Five of the regions were identified as genes for peroxisomal polypeptides (PXPs). The coding sequence for PXP-2 hybrid selected an additional mRNA for PXP-4, the subunit of long-chain acyl coenzyme A oxidase, which was the most abundant PXP. PXP-2 and PXP-4 were close in apparent molecular weight and generated similar peptides when digested with a protease. The gene for PXP-4 was adjacent to that for PXP-2 on the genome and also hybridized to the mRNA coding for PXP-5. These and other similar results suggest that the genes for the peroxisomal proteins of this organism arose by duplication of a few ancestral genes.     

A novel peroxisomal nonspecific lipid-transfer protein from Candida tropicalis. Gene structure, purification and possible role in beta-oxidation

We have sequenced the nucleotides of the gene POX18 that encodes PXP-18, a major peroxisomal polypeptide inducible by oleic acid in the yeast Candida tropicalis. POX18 had a single open reading frame of 127 amino acids. Some 33% of the amino acid sequence of the predicted basic polypeptide (13,805 Da), was identical to that of the nonspecific lipid-transfer protein (sterol carrier protein 2) from rat liver. PXP-18, purified to near homogeneity from isolated peroxisomes, had an amino-terminal sequence identical to that of the predicted polypeptide except for the initiator methionine, and had nonspecific lipid-transfer activity comparable to that of its mammalian equivalents. Unexpectedly, PXP-18 lacked the cysteine residue thought to be essential for the activity of this protein in mammals. RNA blot analysis showed that the POX18 gene was expressed exclusively in cells grown on oleic acid, suggesting that PXP-18 has a role in the beta-oxidation of long-chain fatty acids. PXP-18 modulated acyl-coenzyme A oxidase activity at low pH.     

Acyl-CoA oxidase contains two targeting sequences each of which can mediate protein import into peroxisomes.

Acyl-CoA oxidase is a major induced enzyme in peroxisomes of Candida tropicalis grown on fatty acids. The gene, POX4, encoding acyl-CoA oxidase was expressed in vitro, and the resulting polypeptide was imported into purified peroxisomes in a temperature-dependent fashion. Plasmids containing fragments of POX4 were prepared, expressed and the polypeptides tested for import into peroxisomes. We identified two regions of acyl-CoA oxidase (amino acids 1-118 and 309-427) that contained information that specifically targeted fragments of acyl-CoA oxidase to peroxisomes. The corresponding regions of the gene were fused to cDNA encoding the cytosolic enzyme dihydrofolate reductase (DHFR), and the expressed fusion proteins were likewise imported into peroxisomes. DHFR itself neither bound to, nor was imported into peroxisomes. Thus, there are at least two regions of peroxisomal targeting information in the acyl-CoA oxidase gene.     

Cloning, sequencing, and characterization of five genes coding for Acyl-CoA oxidase isozymes in the yeastYarrowia lipolytica

The Acyl-CoA oxidase (AOX) isozymes catalyze the first steps of peroxisomal β-oxidation, which is important for the degradation of fatty acids. Using conserved blocks in previously identified yeastPOX genes encoding AOXs, the authors have shown that fivePOX genes are present in the yeastYarrowia lipolytica. These genes show approx 63% identity among themselves, and 42% identity with thePOX genes from other yeasts. Mono-disruptedY. lipolytica strains were constructed using a variation of the sticky-end polymerase chain reaction method. AOX activity in the mono-disrupted strains revealed that a long-chain oxidase is encoded by thePOX2 gene and a short-chain oxidase by thePOX3 gene.     

Transport of microinjected alcohol oxidase from Pichia pastoris into vesicles in mammalian cells: involvement of the peroxisomal targeting signal

This report describes the microinjection of a purified peroxisomal protein, alcohol oxidase, from Pichia pastoris into mammalian tissue culture cells and the subsequent transport of this protein into vesicular structures. Transport was into membrane-enclosed vesicles as judged by digitonin-permeabilization experiments. The transport was time and temperature dependent. Vesicles containing alcohol oxidase could be detected as long as 6 d after injection. Coinjection of synthetic peptides containing a consensus carboxyterminal tripeptide peroxisomal targeting signal resulted in abolition of alcohol oxidase transport into vesicles in all cell lines examined. Double-label experiments indicated that, although some of the alcohol oxidase was transported into vesicles that contained other peroxisomal proteins, the bulk of the alcohol oxidase did not appear to be transported to preexisting peroxisomes. While the inhibition of transport of alcohol oxidase by peptides containing the peroxisomal targeting signal suggests a competition for some limiting component of the machinery involved in the sorting of proteins into peroxisomes, the organelles into which the majority of the protein is targeted appear to be unusual and distinct from endogenous peroxisomes by several criteria. Microinjected alcohol oxidase was transported into vesicles in normal fibroblasts and also in cell lines derived from patients with Zellweger syndrome, which are unable to transport proteins containing the ser-lys-leu-COOH peroxisomal targeting signal into peroxisomes (Walton et al., 1992). The implications of this result for the mechanism of peroxisomal protein transport are discussed.     

Biosynthesis of membrane polypeptides of rat liver peroxisomes

The biosynthesis of three major peroxisomal membrane polypeptides of rat liver was investigated. Total hepatic RNA extracted by the guanidinium/CsCl method from three control and three di(2-ethylhexyl)phthalate (a peroxisomal proliferator)-fed rats was translated in vitro in a rabbit reticulocyte lysate protein-synthesizing system. Translation products were immunoprecipitated by the antibodies against peroxisomal membrane polypeptides, subjected to sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and analyzed by fluorography. The in vitro translation products of 70, 26, and 22 kDa peroxisomal membrane polypeptides were apparently of the same size as the respective mature polypeptides. The ratio of translatable mRNA levels for the 70, 26, and 22 kDa polypeptides in di(2-ethylhexyl)phthalate-fed rats to those in control rats were 5.4, 11.4, and 2.7, respectively. The synthesis of these three polypeptides with the free polysome fraction from di(2-ethylhexyl)phthalate-fed rats was more active than that with the membrane-bound polysome fraction, whereas the synthesis of albumin with the free polysome fraction was 27% of that with the membrane-bound polysome fraction. These results indicate that the peroxisomal major membrane polypeptides are synthesized on free polysomes and transported to peroxisomal membrane without any apparent proteolytic processing, and that the induction of these polypeptides by administration of a peroxisomal proliferator corresponds well to the induction of the peroxisomal beta-oxidation enzymes. The data also support the idea that peroxisomes are organized from pre-existing peroxisomes.     

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.     

Molecular characterization of carnitine-dependent transport of acetyl-CoA from peroxisomes to mitochondria in Saccharomyces cerevisiae and identification of a plasma membrane carnitine transporter, Agp2p.

In Saccharomyces cerevisiae, beta-oxidation of fatty acids is confined to peroxisomes. The acetyl-CoA produced has to be transported from the peroxisomes via the cytoplasm to the mitochondrial matrix in order to be degraded to CO(2) and H(2)O. Two pathways for the transport of acetyl-CoA to the mitochondria have been proposed. The first involves peroxisomal conversion of acetyl-CoA into glyoxylate cycle intermediates followed by transport of these intermediates to the mitochondria. The second pathway involves peroxisomal conversion of acetyl-CoA into acetylcarnitine, which is subsequently transported to the mitochondria. Using a selective screen, we have isolated several mutants that are specifically affected in the second pathway, the carnitine-dependent acetyl-CoA transport from the peroxisomes to the mitochondria, and assigned these CDAT mutants to three different complementation groups. The corresponding genes were identified using functional complementation of the mutants with a genomic DNA library. In addition to the previously reported carnitine acetyl-CoA transferase (CAT2), we identified the genes for the yeast orthologue of the human mitochondrial carnitine acylcarnitine translocase (YOR100C or CAC) and for a transport protein (AGP2) required for carnitine transport across the plasma membrane.     

New aspects of sterol carrier protein 2 (Nonspecific lipid-transfer protein) in fusion proteins and in peroxisomes

Sterol carrier protein 2 (SCP2) is a 13-kDa peroxisomal protein, identical to nonspecific lipidtransfer protein, and stimulates various steps of cholesterol metabolism in vitro. Although the name is reminiscent of acyl carrier protein, which is involved in fatty acid synthesis, SCP2 does not bind to lipids specifically or stoichiometrically. This protein is expressed either as a small precursor or as a large fusion (termed SCPx) that carries at its C-terminal the complete sequence of SCP2. SCPx exhibits 3-oxoacyl-CoA thiolase activity, as well as sterol-carrier and lipid-transfer activities. The N- and C-terminal parts of SCPx are similar to the nematode protein P-44 and the yeast protein PXP-18, respectively. P-44, which has no SCP2 sequence, thiolytically cleaved the side chain of bile acid intermediate at a rate comparable to that of SCPx. This, together with the properties of other fusions with SCP2-like sequence, suggests that the SCP2 part of SCPx does not play a direct role in thiolase reaction. PXP-18, located predominantly inside peroxisomes, is similar to SCP2 in primary structure and lipid-transfer activity, and protects peroxisomal acyl-CoA oxidase from thermal denaturation. PXP-18 dimerized at a high temperature, formed an equimolar complex with the oxidase subunit, and released the active enzyme from the complex when the temperature went down. This article attempts to gain insight into the role of SPC2, and to present a model in which PXP-18, a member of the SCP2 family, functions as a molecular chaperone in peroxisomes.     

Lipid Accumulation, Lipid Body Formation, and Acyl Coenzyme A Oxidases of the Yeast Yarrowia lipolytica

Yarrowia lipolytica contains five acyl-coenzyme A oxidases (Aox), encoded by the POX1 to POX5 genes, that catalyze the limiting step of peroxisomal β-oxidation. In this study, we analyzed morphological changes of Y. lipolytica growing in an oleic acid medium and the effect of POX deletions on lipid accumulation. Protrusions involved in the uptake of lipid droplets (LDs) from the medium were seen in electron micrographs of the surfaces of wild-type cells grown on oleic acid. The number of protrusions and surface-bound LDs increased during growth, but the sizes of the LDs decreased. The sizes of intracellular lipid bodies (LBs) and their composition depended on the POX genotype. Only a few, small, intracellular LBs were observed in the mutant expressing only Aox4p (Δpox2 Δpox3 Δpox5), but strains expressing either Aox3p or both Aox3p and Aox4p had the same number of LBs as did the wild type. In contrast, strains expressing either Aox2p or both Aox2p and Aox4p formed fewer, but larger, LBs than did the wild type. The size of the LBs increased proportionately with the amount of triacylglycerols in the LBs of the mutants. In summary, Aox2p expression regulates the size of cellular triacylglycerol pools and the size and number of LBs in which these fatty acids accumulate.     

Identification of a peroxisomal targeting signal at the carboxy terminus of firefly luciferase

Translocation of proteins across membranes of the endoplasmic reticulum, mitochondrion, and chloroplast has been shown to be mediated by targeting signals present in the transported proteins. To test whether the transport of proteins into peroxisomes is also mediated by a peptide targeting signal, we have studied the firefly luciferase gene that encodes a protein transported to peroxisomes in both insect and mammalian cells. We have identified two regions of luciferase which are necessary for transport of this protein into peroxisomes. We demonstrate that one of these, region II, represents a peroxisomal targeting signal because it is both necessary and sufficient for directing cytosolic proteins to peroxisomes. The signal is no more than twelve amino acids long and is located at the extreme carboxy-terminus of luciferase. The location of the targeting signal for translocation across the peroxisomal membrane therefore differs from the predominantly amino-terminal location of signals responsible for transport across the membranes of the endoplasmic reticulum, chloroplast, or mitochondrion.     

Peroxisomal fission is induced during appressorium formation and is required for full virulence of the rice blast fungus

Peroxisomes are involved in various metabolic processes and are important for virulence in different pathogenic fungi. How peroxisomes rapidly emerge in the appressorium during fungal infection is poorly understood. Here, we describe a gene, PEF1, which can regulate peroxisome formation in the appressorium by controlling peroxisomal fission, and is required for plant infection in the rice blast fungus Magnaporthe oryzae. Targeted deletion of PEF1 resulted in a reduction in virulence and a delay in penetration and invasive growth in host cells. PEF1 was particularly expressed during appressorial development, and its encoding protein was co‐localized with peroxisomes during appressorial development. Compared with the massive vesicle‐shaped peroxisomes formed in the wild‐type appressorium, the Δpef1 mutant could only form stringy linked immature peroxisomes, suggesting that PEF1 was involved in peroxisomal fission during appressorium formation. We also found that the Δpef1 mutant could not utilize fatty acids efficiently, which can improve significantly the expression level of PEF1 and induce peroxisomal fission. As expected, the Δpef1 mutant showed reduced intracellular production of reactive oxygen species (ROS) during appressorium formation and induced ROS accumulation in host cells during infection. Taken together, PEF1‐mediated peroxisomal fission is important for fungal infection by controlling the number of peroxisomes in the appressorium.     

Integral membrane polypeptides of rat liver peroxisomes: topology and response to different metabolic states

Rats were treated with clofibrate, a hypolipidemic drug, and with thyroxine. Both drugs which are known to cause peroxisome proliferation, and a concomitant increase in peroxisomal fatty acid beta-oxidation activity in liver increased one of the major integral peroxisomal membrane polypeptides (PMPs), with apparent molecular mass of 69-kDa, six- and twofold, respectively. On the other hand hypothyroidism caused a decrease in peroxisomal fatty acid beta-oxidation activity and considerably lowered the concentration of PMP 69 in the peroxisomal membrane. Two other PMPs with apparent molecular masses of 36 and 22 kDa were not influenced by these treatments. The PMPs with apparent molecular masses of 42, 28, and 26 kDa were shown to be derived from the 69-kDa polypeptide by the activity of a yet uncharacterized endogenous protease during isolation of peroxisomes. Limited proteolysis of intact peroxisomes using proteinase K and subtilisin further substantiated that some portion of the 69-kDa polypeptide extends into the cytoplasm. The 36- and the 22-kDa polypeptides were accessible to proteolytic attack to a much lower extent and, therefore, are supposed to be rather deeply embedded within the peroxisomal membrane. It is demonstrated that peroxisomal acyl-CoA synthetase, an integral PMP extending partially into the cytoplasm, and PMP 69 are not identical polypeptides. Comparison of the peroxisomal membrane with that of mitochondria and microsomes revealed that the 69- and 22-kDa polypeptides as well as the bifunctional protein of the peroxisomal fatty acid beta-oxidation pathway were specifically located only in peroxisomes. Considerable amounts of a polypeptide cross-reacting with the antiserum against the 36-kDa polypeptide were found in mitochondria.     

Stable and transient expression of chimeric peroxisomal membrane proteins induces an independent "zippering" of peroxisomes and an endoplasmic reticulum subdomain

Peroxisomal ascorbate peroxidase (APX) (EC 1.11.1.11) was shown recently to sort through a subdomain of the ER (peroxisomal endoplasmic reticulum; pER), and in certain cases, alter the distribution and/or morphology of peroxisomes and pER when overexpressed transiently in Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) cells. Our goal was to gain insight into the dynamics of peroxisomal membrane protein sorting by characterizing the structure and formation of reorganized peroxisomes and pER. Specifically, we test directly the hypothesis that the observed phenomenon is due to the oligomerization of cytosol-facing, membrane-bound polypeptides. a process referred to as membrane "zippering". Results from differential detergent permeabilization experiments confirmed that peroxisomal APX is a C-terminal "tail-anchored" (Cmatrix-Ncytosol) membrane protein with a majority of the polypeptide facing the cytosol. Transient expression of several APX chimeras whose passenger polypeptides can form dimers or trimers resulted in the progressive formation of "globular" peroxisomes and circular pER membranes. Stable expression of the trimer-capable fusion protein yielded suspension cultures that reproducibly maintained a high degree of peroxisomal globules but relatively few detectable pER membranes. Electron micrographs revealed that the globules consisted of numerous individual peroxisomes, seemingly in direct contact with other peroxisomes and/or mitochondria. These peroxisomal clusters or aggregates were not observed in cells transiently expressing monomeric versions of APX. These findings indicate that the progressive, independent "zippering" of peroxisomes and pER is due to the post-sorting oligomerization of monomeric, cytosol-facing polypeptides that are integrally inserted into the membranes of "like" organelles. The dynamics of this process are discussed, especially with respect to the involvement of the microtubule cytoskeleton.     

Transport of microinjected proteins into peroxisomes of mammalian cells: inability of Zellweger cell lines to import proteins with the SKL tripeptide peroxisomal targeting signal.

Previous work has shown that the firefly (Photinus pyralis) luciferase contains a C-terminal peroxisomal targeting signal consisting of the tripeptide Ser-Lys-Leu. This report describes the microinjection of two proteins, (i) luciferase and (ii) albumin conjugated to a peptide ending in the sequence Ser-Lys-Leu, into mammalian cells grown in tissue culture. Following microinjection, incubation of the cells at 37 degrees C resulted in peroxisomal transport of these exogenous proteins into catalase-containing vesicles. The translocation was both time and temperature dependent. The transport could be inhibited by coinjection of synthetic peptides bearing various peroxisomal targeting signal motifs. These proteins could be transported into peroxisomes in normal human fibroblast cell lines but not in cell lines derived from patients with Zellweger syndrome. These results demonstrate that microinjection of peroxisomal proteins yields an authentic in vivo system with which to study peroxisomal transport. Furthermore, these results reveal that the process of peroxisomal transport does not involve irreversible modification of the protein, that artificial hybrid substrates can be transported and used as tools to study peroxisomal transport, and that the defect in Zellweger syndrome is indeed the inability to transport proteins containing the Ser-Lys-Leu targeting signal into the peroxisomal lumen.     

Effect of catalase-specific inhibitor 3-amino-1,2,4-triazole on yeast peroxisomal catalase in vivo

3-Amino-1,2,4-triazole (3-AT) is known as an inhibitor of catalase to whose active center it specifically and covalently binds. Subcellular fractionation and immunoelectronmicroscopic observation of the yeast Candida tropicalis revealed that, in 3-AT-treated cells in which the 3-AT was added to the n-alkane medium from the beginning of cultivation, catalase transported into peroxisomes was inactivated and was present as insoluble aggregated forms in the organelle. The aggregation of catalase in peroxisomes occurred only in these 3-AT-treated cells and not in cells in which 3-AT was added at the late exponential growth phase. Furthermore, 3-AT did not affect the transportation of catalase into peroxisomes. The appearance of aggregation only in cells to which 3-AT was added from the beginning of cultivation suggests that, in the process of catalase transportation into yeast peroxisomes, some conformational change may take place and that correct folding may be inhibited by the binding of 3-AT to the active center of catalase. Accordingly, 3-AT will be an interesting compound for investigation of the transport machinery of the peroxisomal tetrameric catalase.