Animal cell mutants represent two complementation groups of peroxisome-defective Zellweger syndrome

Asymmetric Distribution of Plasmalogens and Their Roles-A Mini Review

Plasmalogens are a unique family of cellular glycerophospholipids that contain a vinyl-ether bond. The synthesis of plasmalogens is initiated in peroxisomes and completed in the endoplasmic reticulum. Plasmalogens are transported to the post-Golgi compartment, including endosomes and plasma membranes, in a manner dependent on ATP, but not vesicular transport. Plasmalogens are preferentially localized in the inner leaflet of the plasma membrane in a manner dependent on P4-type ATPase ATP8B2, that associates with the CDC50 subunit. Plasmalogen biosynthesis is spatiotemporally regulated by a feedback mechanism that senses the amount of plasmalogens in the inner leaflet of the plasma membrane and controls the stability of fatty acyl-CoA reductase 1 (FAR1), the rate-limiting enzyme for plasmalogen biosynthesis. The physiological consequences of such asymmetric localization and homeostasis of plasmalogens are discussed in this review.

Peroxisome Biogenesis Disorders

Peroxisomes are presented in all eukaryotic cells and play essential roles in many of lipid metabolic pathways, including β-oxidation of fatty acids and synthesis of ether-linked glycerophospholipids, such as plasmalogens. Impaired peroxisome biogenesis, including defects of membrane assembly, import of peroxisomal matrix proteins, and division of peroxisome, causes peroxisome biogenesis disorders (PBDs). Fourteen complementation groups of PBDs are found, and their complementing genes termed PEXs are isolated. Several new mutations in peroxins from patients with mild PBD phenotype or patients with phenotypes unrelated to the commonly observed impairments of PBD patients are found by next-generation sequencing. Exploring a dysfunctional step(s) caused by the mutation is important for unveiling the pathogenesis of novel mutation by means of cellular and biochemical analyses.

Plasmalogen mediates integration of adherens junction

Ether glycerolipids, plasmalogens are found in various mammalian cells and tissues. However, physiological role of plasmalogens in epithelial cells remains unknown. We herein show that synthesis of ethanolamine-containing plasmalogens, plasmenylethanolamine (PlsEtn), is deficient in MCF7 cells, an epithelial cell line, with severely reduced expression of alkyl-dihydroxyacetonephosphate synthase (ADAPS), the second enzyme in the PlsEtn biosynthesis. Moreover, expression of ADAPS or supplementation of PlsEtn containing C18-alkenyl residue delays the migration of MCF7 cells as compared to that mock-treated MCF7 and C16-alkenyl-PlsEtn-supplemented MCF7 cells. Localization of E-cadherin to cell-cell junctions is highly augmented in cells containing C18-alkenyl-PlsEtn. Together, these results suggest that PlsEtn containing C18-alkenyl residue plays a distinct role in the integrity of E-cadherin-mediated adherens junction.

A newly identified mutation in the PEX26 gene is associated with a milder form of Zellweger spectrum disorder

Using clinical exome sequencing (ES), we identified an autosomal recessive missense variant, c.153C>A (p.F51L), in the peroxisome biogenesis factor 26 gene (PEX26) in a 19-yr-old female of Ashkenazi Jewish descent who was referred for moderate to severe hearing loss. The proband and three affected siblings are all homozygous for the c.153C>A variant. Skin fibroblasts from this patient show normal morphology in immunostaining of matrix proteins, although the level of catalase was elevated. Import rate of matrix proteins was significantly decreased in the patient-derived fibroblasts. Binding of Pex26-F51L to the AAA ATPase peroxins, Pex1 and Pex6, is severely impaired and affects peroxisome assembly. Moreover, Pex26 in the patient's fibroblasts is reduced to ∼30% of the control, suggesting that Pex26-F51L is unstable in cells. In the patient's fibroblasts, peroxisome-targeting signal 1 (PTS1) proteins, PTS2 protein 3-ketoacyl-CoA thiolase, and catalase are present in a punctate staining pattern at 37°C and in a diffuse pattern at 42°C, suggesting that these matrix proteins are not imported to peroxisomes in a temperature-sensitive manner. Analysis of peroxisomal metabolism in the patient's fibroblasts showed that the level of docosahexaenoic acid (DHA) (C22:6n-3) in ether phospholipids is decreased, whereas other lipid metabolism, including peroxisomal fatty acid β-oxidation, is normal. Collectively, the functional data support the mild phenotype of nonsyndromic hearing loss in patients harboring the F51L variant in PEX26.

Generation of Peroxisome-Deficient Somatic Animal Cell Mutants

Cell mutants with a genetic defect affecting various cellular phenotypes are widely utilized as a powerful tool in genetic, biochemical, and cell biological research. More than a dozen complementation groups of animal somatic mutant cells defective in peroxisome biogenesis have been successfully isolated in Chinese hamster ovary (CHO) cells and used as a model system reflecting fatal human severe genetic disorders named peroxisome biogenesis disorders (PBD). Isolation and characterization of peroxisome-deficient CHO cell mutants has allowed the identification of PEX genes and the gene products peroxins, which directly leads to the accomplishment of isolation of pathogenic genes responsible for human PBDs, as well as elucidation of their functional roles in peroxisome biogenesis. Here, we describe the procedure to isolate peroxisome-deficient mammalian cell mutants from CHO cells, by making use of an effective, photo-sensitized selection method.

Deficiency of a Retinal Dystrophy Protein, Acyl-CoA Binding Domain-containing 5 (ACBD5), Impairs Peroxisomal β-Oxidation of Very-long-chain Fatty Acids

Acyl-CoA binding domain-containing 5 (ACBD5) is a peroxisomal protein that carries an acyl-CoA binding domain (ACBD) at its N-terminal region. The recent identification of a mutation in the ACBD5 gene in patients with a syndromic form of retinal dystrophy highlights the physiological importance of ACBD5 in humans. However, the underlying pathogenic mechanisms and the precise function of ACBD5 remain unclear. We herein report that ACBD5 is a peroxisomal tail-anchored membrane protein exposing its ACBD to the cytosol. Using patient-derived fibroblasts and ACBD5 knock-out HeLa cells generated via genome editing, we demonstrate that ACBD5 deficiency causes a moderate but significant defect in peroxisomal β-oxidation of very-long-chain fatty acids (VLCFAs) and elevates the level of cellular phospholipids containing VLCFAs without affecting peroxisome biogenesis, including the import of membrane and matrix proteins. Both the N-terminal ACBD and peroxisomal localization of ACBD5 are prerequisite for efficient VLCFA β-oxidation in peroxisomes. Furthermore, ACBD5 preferentially binds very-long-chain fatty acyl-CoAs (VLC-CoAs). Together, these results suggest a direct role of ACBD5 in peroxisomal VLCFA β-oxidation. Based on our findings, we propose that ACBD5 captures VLC-CoAs on the cytosolic side of the peroxisomal membrane so that the transport of VLC-CoAs into peroxisomes and subsequent β-oxidation thereof can proceed efficiently. Our study reclassifies ACBD5-related phenotype as a novel peroxisomal disorder.

Peroxisome biogenesis and human peroxisome-deficiency disorders

Peroxisome is a single-membrane-bounded ubiquitous organelle containing a hundred different enzymes that catalyze various metabolic pathways such as β-oxidation of very long-chain fatty acids and synthesis of plasmalogens. To investigate peroxisome biogenesis and human peroxisome biogenesis disorders (PBDs) including Zellweger syndrome, more than a dozen different complementation groups of Chinese hamster ovary (CHO) cell mutants impaired in peroxisome biogenesis are isolated as a model experimental system. By taking advantage of rapid functional complementation assay of the CHO cell mutants, successful cloning of PEX genes encoding peroxins required for peroxisome assembly invaluably contributed to the accomplishment of cloning of pathogenic genes responsible for PBDs. Peroxins are divided into three groups: 1) peroxins including Pex3p, Pex16p and Pex19p, are responsible for peroxisome membrane biogenesis via Pex19p- and Pex3p-dependent class I and Pex19p- and Pex16p-dependent class II pathways; 2) peroxins that function in matrix protein import; 3) those such as Pex11pβ are involved in peroxisome division where DLP1, Mff, and Fis1 coordinately function.

Tail-anchored PEX26 targets peroxisomes via a PEX19-dependent and TRC40-independent class I pathway

Tail-anchored (TA) proteins are anchored into cellular membranes by a single transmembrane domain (TMD) close to the C terminus. Although the targeting of TA proteins to peroxisomes is dependent on PEX19, the mechanistic details of PEX19-dependent targeting and the signal that directs TA proteins to peroxisomes have remained elusive, particularly in mammals. The present study shows that PEX19 formed a complex with the peroxisomal TA protein PEX26 in the cytosol and translocated it directly to peroxisomes by interacting with the peroxisomal membrane protein PEX3. Unlike in yeast, the adenosine triphosphatase TRC40, which delivers TA proteins to the endoplasmic reticulum, was dispensable for the peroxisomal targeting of PEX26. Moreover, the basic amino acids within the luminal domain of PEX26 were essential for binding to PEX19 and thereby for peroxisomal targeting. Finally, our results suggest that a TMD that escapes capture by TRC40 and is followed by a highly basic luminal domain directs TA proteins to peroxisomes via the PEX19-dependent route.

Docosahexaenoic acid mediates peroxisomal elongation, a prerequisite for peroxisome division

Peroxisome division is regulated by several factors, termed fission factors, as well as the conditions of the cellular environment. Over the past decade, the idea of metabolic control of peroxisomal morphogenesis has been postulated, but remains largely undefined to date. In the current study, docosahexaenoic acid (DHA, C22:6n-3) was identified as an inducer of peroxisome division. In fibroblasts isolated from patients that carry defects in peroxisomal fatty acid β-oxidation, peroxisomes are much less abundant than normal cells. Treatment of these patient fibroblasts with DHA induced the proliferation of peroxisomes to the level seen in normal fibroblasts. DHA-induced peroxisomal proliferation was abrogated by treatment with a small inhibitory RNA (siRNA) targeting dynamin-like protein 1 and with dynasore, an inhibitor of dynamin-like protein 1, which suggested that DHA stimulates peroxisome division. DHA augmented the hyper-oligomerization of Pex11pβ and the formation of Pex11pβ-enriched regions on elongated peroxisomes. Time-lapse imaging analysis of peroxisomal morphogenesis revealed a sequence of steps involved in peroxisome division, including elongation in one direction followed by peroxisomal fission. DHA enhanced peroxisomal division in a microtubule-independent manner. These results suggest that DHA is a crucial signal for peroxisomal elongation, a prerequisite for subsequent fission and peroxisome division.

Pex5p imports folded tetrameric catalase by interaction with Pex13p

Human catalase forms a 240-kDa tetrameric complex and degrades H(2) O(2) in peroxisomes. Human catalase is targeted to peroxisomes by the interaction of its peroxisomal targeting signal type 1 (PTS1)-like KANL sequence with the cytosolic PTS1 receptor Pex5p. We show herein that human catalase tetramers are formed in the cytoplasm and that the expression of a PTS signal on each of the four subunits is not necessary for peroxisomal transport. We previously demonstrated that a Pex5p mutant defective in binding to Pex13p, designated Pex5p(Mut234), imports typical PTS1-type proteins but not catalase. This impaired catalase import is not rescued by replacing its C-terminal KANL sequence with a typical PTS1 sequence, SKL, indicating that the failure of catalase import in Mut234-expressing cells is not due to its weak PTS1. In contrast, several enzymatically inactive and monomeric mutants of catalase are efficiently imported in Mut234-expressing cells. Moreover, trimeric chloramphenicol acetyltransferase (CAT) harboring SKL is not imported in Pex5p(Mut234)-expressing cells, but CAT-SKL trimers are transported to peroxisomes in the wild-type cells. These findings suggest that the Pex5p-Pex13p interaction likely plays a pivotal role in the peroxisomal import of folded and oligomeric proteins.

Cysteine ubiquitination of PTS1 receptor Pex5p regulates Pex5p recycling

Pex5p is the cytosolic receptor for peroxisome matrix proteins with peroxisome-targeting signal (PTS) type 1 and shuttles between the cytosol and peroxisomes. Here, we show that Pex5p is ubiquitinated at the conserved cysteine(11) in a manner sensitive to dithiothreitol, in a form associated with peroxisomes. Pex5p with a mutation of the cysteine(11) to alanine, termed Pex5p-C11A, abrogates peroxisomal import of PTS1 and PTS2 proteins in wild-type cells. Pex5p-C11A is imported into peroxisomes but not exported, resulting in its accumulation in peroxisomes. These results suggest an essential role of the cysteine residue in the export of Pex5p. Furthermore, domain mapping indicates that N-terminal 158-amino-acid region of Pex5p-C11A, termed 158-CA, is sufficient for such dominant-negative activity by binding to membrane peroxin Pex14p via its two pentapeptide WXXXF/Y motifs. Stable expression of either Pex5p-C11A or 158-CA likewise inhibits the wild-type Pex5p import into peroxisomes, strongly suggesting that Pex5p-C11A exerts the dominant-negative effect at the translocation step via Pex14p. Taken together, these findings show that the cysteine(11) of Pex5p is indispensable for two distinct steps, its import and export. The Pex5p-C11A would be a useful tool for gaining a mechanistic insight into the matrix protein import into peroxisomes.

A mutation in PEX19 causes a severe clinical phenotype in a patient with peroxisomal biogenesis disorder

Peroxisomal biogenesis disorders (PBD) are groups of inherited neurometabolic disorders caused by defects in PEX genes. We report on a female infant, born to a consanguineous parents (first degree cousins), who presented with inactivity, poor sucking, and hypotonia early in the neonatal period. She had subtle dysmorphic features. Liver function tests were impaired with raised liver enzymes, conjugated and unconjugated hyperbilirubinemia. CT of the brain showed diffuse bilateral changes. She developed seizures with an abnormal EEG. Plasma very long chain fatty acid analysis showed high C26:0 levels and increasedC26:0/C22:0 and C24:0/C22:0 ratios, which is consistent with a PBD. Studies in fibroblasts including plasmalogen biosynthesis, peroxisomal fatty acid alfa and beta oxidation confirmed the diagnosis of PBD. Immunofluoresence microscopy revealed the absence of peroxisomes in fibroblasts. The patient was assigned to the PEX19 complementation group. Subsequent mutation analysis of the PEX19 gene revealed homozygosity for a c.320delA frameshift mutation. The patient had a stormy course with multiple admissions to the pediatric intensive care unit with pneumonia, liver impairment, sepsis, and epilepsy. At 1 year of age she developed metabolic acidosis with normal anion gap, proteinuria, aminoaciduria, and glucosuria consistent with a renal tubular defect. Abdominal ultrasound showed multiple gallstones. Other causes of gallstones like haemoglobinopathy were excluded. So far, only two siblings had been reported with mutations in the PEX19 gene. Our patient showed a previously unrecognized association of gallstones and a renal tubular defect with a PBD.

Lessons from peroxisome-deficient Chinese hamster ovary (CHO) cell mutants

Cells with a genetic defect affecting a biological activity and/or a cell phenotype are generally called "cell mutants" and are a highly useful tool in genetic, biochemical, as well as cell biological research. To investigate peroxisome biogenesis and human peroxisome biogenesis disorders, more than a dozen complementation groups of Chinese hamster ovary (CHO) cell mutants defective in peroxisome assembly have been successfully isolated and established as a model system. Moreover, successful PEX gene cloning studies by taking advantage of rapid functional complementation assay of CHO cell mutants invaluably contributed to the accomplishment of isolation of pathogenic genes responsible for peroxisome biogenesis diseases. Molecular mechanisms of peroxisome assembly are currently investigated by making use of such mammalian cell mutants.

Peroxisome division is impaired in a CHO cell mutant with an inactivating point-mutation in dynamin-like protein 1 gene

We earlier isolated a Chinese hamster ovary cell line ZP121 showing morphologically abnormal, tubular peroxisomes, and apparent dysmorphogenesis of mitochondria. Here, we identified an inactivating point-mutation in dynamin-like protein 1 gene, DLP1, responsible for the phenotype of ZP121. One allele of DLP1 possessed a point missense mutation resulting in G363D in the middle region of 699-amino-acid long DLP1, termed DLP1G363D, while the other allele was normal. DLP1G363D was apparently expressed at a higher level than DLP1. Abnormal morphogenesis of peroxisomes as well as mitochondria was restored when wild-type DLP1 was transfected. The GTPase activity of DLP1G363D was barely detectable, indicating that the G363D mutation severely affected the GTPase activity. Moreover, a higher level of DLP1G363D expression in CHO-K1 cells reproduced the ZP121-type phenotype, hence indicating its dominant-negative activity to the wild-type DLP1, most likely by forming a heteromeric tetramer. The G363D mutation also gave rise to a temperature-sensitive phenotype showing normal morphogenesis of peroxisomes and mitochondria at 40 degrees C. Microtubule organization was most likely involved in the elongation of peroxisomes. Furthermore, ZP121 was lowered in the level of phospholipids, plasmalogens, and phosphatidylethanolamine and was less sensitive to oxidative stresses. Thus, ZP121 is the first dlp1 mutant in mammalian cells.

Mutations in the Peroxin Pex26p Responsible for Peroxisome Biogenesis Disorders of Complementation Group 8 Impair Its Stability, Peroxisomal Localization, and Interaction with the Pex1p·Pex6p Complex

Peroxisome biogenesis disorders (PBDs) are fatal autosomal recessive diseases and are caused by impaired peroxisome biogenesis. PBDs are genetically heterogeneous and classified into 13 complementation groups (CGs). CG8 is one of the most common groups and has three clinical phenotypes, including Zellweger syndrome (ZS), neonatal adrenoleukodystrophy, and infantile Refsum disease (IRD). We recently isolated PEX26 as the pathogenic gene for PBD of CG8. Pex26p functions in recruiting to peroxisomes the complexes of the AAA ATPase peroxins, Pex1p and Pex6p. In the present work, we identified four distinct mutations in PEX26 from five patients of CG8 PBD including 2 with ZS and 3 with IRD, in addition to 7 mutant alleles in 8 patients in the first report describing the pathogenic PEX26 gene for CG8 PBD. Phenotype-genotype analyses revealed that temperature-sensitive (ts) peroxisome assembly gave rise to a milder IRD in contrast to the non-ts phenotype of the cells from ZS patients. Furthermore, we present several lines of evidence that show that the instability, insufficient binding to Pex1p x Pex6p complexes, or mislocalization of patient-derived Pex26p mutants is most likely responsible for the CG8 PBDs.

PEROXISOMEBIOGENESISDISORDERS

The peroxisome biogenesis disorders (PBDs) comprise 12 autosomal recessive complementation groups (CGs). The multisystem clinical phenotype varies widely in severity and results from disturbances in both development and metabolic homeostasis. Progress over the last several years has lead to identification of the genes responsible for all of these disorders and to a much improved understanding of the biogenesis and function of the peroxisome. Increasing availability of mouse models for these disorders offers hope for a better understanding of their pathophysiology and for development of therapies that might especially benefit patients at the milder end of the clinical phenotype.

Mutations in novel peroxin gene PEX26 that cause peroxisome-biogenesis disorders of complementation group 8 provide a genotype-phenotype correlation

The human disorders of peroxisome biogenesis (PBDs) are subdivided into 12 complementation groups (CGs). CG8 is one of the more common of these and is associated with varying phenotypes, ranging from the most severe, Zellweger syndrome (ZS), to the milder neonatal adrenoleukodystrophy (NALD) and infantile Refsum disease (IRD). PEX26, encoding the 305-amino-acid membrane peroxin, has been shown to be deficient in CG8. We studied the PEX26 genotype in fibroblasts of eight CG8 patients--four with the ZS phenotype, two with NALD, and two with IRD. Catalase was mostly cytosolic in all these cell lines, but import of the proteins that contained PTS1, the SKL peroxisome targeting sequence, was normal. Expression of PEX26 reestablished peroxisomes in all eight cell lines, confirming that PEX26 defects are pathogenic in CG8 patients. When cells were cultured at 30 degrees C, catalase import was restored in the cell lines from patients with the NALD and IRD phenotypes, but to a much lesser extent in those with the ZS phenotype, indicating that temperature sensitivity varied inversely with the severity of the clinical phenotype. Several types of mutations were identified, including homozygous G89R mutations in two patients with ZS. Expression of these PEX26 mutations in pex26 Chinese hamster ovary cells resulted in cell phenotypes similar to those in the human cell lines. These findings confirm that the degree of temperature sensitivity in pex26 cell lines is predictive of the clinical phenotype in patients with PEX26 deficiency.

A novel pex2 mutant: catalase-deficient but temperature-sensitive PTS1 and PTS2 import

We searched for Chinese hamster ovary (CHO) cell mutants defective in peroxisome biogenesis by using peroxisome targeting sequence (PTS) of Pex3p (amino acid residues 1-40)-fused enhanced green fluorescent protein (EGFP). From mutagenized wild-type CHO-K1 cells stably expressing rat Pex2p and Pex3p(1-40)-EGFP, cell colonies resistant to the 9-(1(')-pyrene)nonanol/ultraviolet treatment were examined for intracellular location of peroxisomal proteins, including EGFP chimera, catalase, and matrix proteins with PTS types 1 and 2. One clone, ZPEG309, showed a distinct phenotype: import defect of catalase, but normal transport of PTS1 and PTS2 proteins at 37 degrees C. PTS1 and PTS2 import was abrogated when ZPEG309 was cultured at 39 degrees C. Genetic defect of ZPEG309 was a nonsense point mutation in a codon for Arg50 in CHO PEX2 and a mutation resulting in a C-terminal truncation of the introduced rat Pex2p. Therefore, ZPEG309 is a novel pex2, catalase-deficient mutant with temperature-sensitive PTS1 and PTS2 import.

Isolation of Chinese hamster ovary cell pex mutants: two PEX7-defective mutants

We searched for novel Chinese hamster ovary (CHO) cell mutants defective in peroxisome biogenesis by an improved method using peroxisome targeting signal 2 (PTS2)-tagged enhanced green fluorescent protein (EGFP). From mutagenized TKaEG2 cells, the wild-type CHO-K1 stably expressing rat Pex2p and PTS2-EGFP, cell colonies resistant to the 9-(1(')-pyrene)nonanol/ultraviolet treatment were examined for intracellular location of PTS2-EGFP. Of six mutant cell clones two, ZPEG227 and ZPEG231, showed cytosolic PTS2-EGFP, indicative of impaired PTS2 import, and numerous PTS1-positive particles. PEX7 expression restored the impaired PTS2 import in both mutants. Cell fusion with fibroblasts from a patient with PEX7-defective rhizomelic chondrodysplasia punctata did not complement PTS2 import defect of ZPEG227 and ZPEG231, confirming that these two are pex7 mutants. Mutation analysis of PEX7 by reverse transriptase (RT)-PCR indicated that ZPEG227-allele carried an inactivating nonsense mutation, Trp158Ter. Therefore, ZPEG227 is a pex7 mutant possessing a newly identified mutation in mammalian pex7 cell lines.

Intracellular localization, function, and dysfunction of the peroxisome-targeting signal type 2 receptor, Pex7p, in mammalian cells

We previously isolated and characterized a Chinese hamster ovary (CHO) cell mutant, ZPG207, that is defective in import of proteins carrying a peroxisome-targeting signal type 2 (PTS2) nonapeptide. Herein we have cloned Chinese hamster (Cl) PEX7 encoding the PTS2 receptor. ClPex7p consists of 318 amino acids, shorter than human Pex7p by 5 residues, showing 91 and 30% identity with Pex7p from humans and the yeast Saccharomyces cerevisiae, respectively. Expression of ClPEX7 rescued the impaired PTS2 import in pex7 ZPG207. Mutation in ZPG207 PEX7 was determined by reverse transcription PCR; a G-to-A transition caused a 1-amino acid substitution, W221ter. We investigated the molecular dysfunction of Pex7p variants in mammals, including Pex7p-W221ter and Pex7p with one site mutation at G217R, A218V, or L292ter, which frequently occurs in the human fatal genetic peroxisomal disease rhizomelic chondrodysplasia punctata, showing a cell phenotype of PTS2 import defect. All types of the mutations affected Pex7p in binding to both PTS2 cargo protein and the longer isoform of PTS1 receptor Pex5pL that is responsible for transport of the Pex7p-PTS2 complex. Subcellular fractionation and protease protection studies demonstrated bimodal distribution of Pex7p between the cytoplasm and peroxisomes in CHO and human cells. Moreover, expression of Pex5pL, but not of the shorter isoform Pex5pS, enhanced translocation of Pex7p-PTS2 proteins into peroxisomes, thereby implying that both PTS receptors shuttle between peroxisomes and the cytosol. Furthermore, a ClPex7p mutant with a deletion of 7 amino acids from the N terminus retained peroxisome-restoring activity, whereas an 11-amino acid truncation abrogated the activity. ClPex7p with a C-terminal 9- amino acid truncation, comprising residues 1--309, maintained the activity, whereas a 14-amino acid shorter form lacking several amino acids of the sixth WD motif lost the activity. Therefore, nearly the full length of Pex7p, including all WD motifs, is required for its function.

Peroxisomes are formed from complex membrane structures in PEX6-deficient CHO cells upon genetic complementation

Pex6p belongs to the AAA family of ATPases. Its CHO mutant, ZP92, lacks normal peroxisomes but contains peroxisomal membrane remnants, so called peroxisomal ghosts, which are detected with anti-70-kDa peroxisomal membrane protein (PMP70) antibody. No peroxisomal matrix proteins were detected inside the ghosts, but exogenously expressed green fluorescent protein (GFP) fused to peroxisome targeting signal-1 (PTS-1) accumulated in the areas adjacent to the ghosts. Electron microscopic examination revealed that PMP70-positive ghosts in ZP92 were complex membrane structures, rather than peroxisomes with reduced matrix protein import ability. In a typical case, a set of one central spherical body and two layers of double-membraned loops were observed, with endoplasmic reticulum present alongside the outer loop. In the early stage of complementation by PEX6 cDNA, catalase and acyl-CoA oxidase accumulated in the lumen of the double-membraned loops. Biochemical analysis revealed that almost all the peroxisomal ghosts were converted into peroxisomes upon complementation. Our results indicate that 1) Peroxisomal ghosts are complex membrane structures; and 2) The complex membrane structures become import competent and are converted into peroxisomes upon complementation with PEX6.

Phenotype-genotype relationships in peroxisome biogenesis disorders of PEX1-defective complementation group 1 are defined by Pex1p-Pex6p interaction

The peroxisome biogenesis disorders (PBDs), including Zellweger syndrome (ZS), neonatal adrenoleucodystrophy (NALD) and infantile Refsum disease (IRD), are fatal autosomal recessive diseases caused by impaired peroxisome biogenesis, of which 12 genotypes have been reported. ZS patients manifest the severest clinical and biochemical abnormalities, whereas those with NALD and IRD show less severity and the mildest features respectively. We have reported previously that temperature-sensitive peroxisome assembly is responsible for the mildness of the clinical features of IRD. PEX1 is the causative gene for PBDs of complementation group E (CG-E, CG1 in the U.S.A. and Europe), the PBDs of highest incidence, encoding the peroxin Pex1p of the AAA ATPase family. It has been also reported that Pex1p and Pex6p interact with each other. In the present study we investigated phenotype-genotype relationships of CG1 PBDs. Pex1p from IRD such as Pex1p with the most frequently identified mutation at G843D was largely degraded in vivo at 37 degrees C, whereas a normal level of Pex1p was detectable at the permissive temperature. In contrast, PEX1 proteins derived from ZS patients, including proteins with a mutation at L664P or the deletion of residues 634-690, were stably present at both temperatures. Pex1p-G843D interacted with Pex6p at approx. 50% of the level of normal Pex1p, whereas Pex1p from ZS patients mostly showing non-temperature-sensitive peroxisome biogenesis hardly bound to Pex6p. Taking these results together, we consider it most likely that the stability of Pex1p reflects temperature-sensitive peroxisome assembly in IRD fibroblasts. Failure in Pex1p-Pex6p interaction gives rise to more severe abnormalities, such as those manifested by patients with ZS.

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.

Biogenesis of nonspecific lipid transfer protein and sterol carrier protein x: studies using peroxisome assembly-defective pex cell mutants

Nonspecific lipid transfer protein (nsLTP; also called sterol carrier protein 2) with a molecular mass of 13 kDa is synthesized as a larger 15-kDa precursor (pre-nsLTP) with an N-terminal 20-amino acid extension presequence, as well as with the peroxisome targeting signal type 1 (PTS1), Ala-Lys-Leu, at the C terminus. The precursor pre-nsLTP is processed to mature nsLTP by proteolytic removal of the presequence, most likely after being imported into peroxisomes. Sterol carrier protein x (SCPx), a 59-kDa branched-chain fatty acid thiolase of peroxisomes, contains the entire pre-nsLTP moiety at the C-terminal part and is converted to the 46-kDa form and nsLTP after the transport to peroxisomes. We investigated which of these two potential topogenic sequences functions in biogenesis of nsLTP and SCPx. Morphological and biochemical analyses, making use of Chinese hamster ovary cell pex mutants such as the PTS1 receptor-impaired pex5 and PTS2 import-defective pex7, as well as green fluorescent protein chimeras, revealed that both pre-nsLTP and SCPx are imported into peroxisomes by the Pex5p-mediated PTS1 pathway. Nearly half of the pre-nsLTP remains in the cytosol, as assessed by subcellular fractionation of the wild-type Chinese hamster ovary cells. In an in vitro binding assay, only mature nsLTP, but not pre-nsLTP, from the cell lysates interacted with the Pex5p. It is likely, therefore, that modulation of the C-terminal PTS1 by the presequence gives rise to cytoplasmic localization of pre-nsLTP.

PEX3 is the causal gene responsible for peroxisome membrane assembly-defective Zellweger syndrome of complementation group G

Peroxisome biogenesis disorders (PBDs) such as Zellweger syndrome (ZS) and neonatal adrenoleukodystrophy are autosomal recessive diseases caused by defects in peroxisome assembly, for which 13 genotypes have been identified. Expression of the human peroxin Pex3p cDNA encoding a 373-amino-acid peroxisomal membrane protein morphologically and biochemically restored peroxisome biogenesis, including peroxisomal membrane assembly, in fibroblasts from PBDG-02, a patient with complementation group G (CG-G) ZS. Patient PBDG-02 carried a homozygous, inactivating mutation-a 97-bp deletion of nucleotide residues at positions 942-1038-resulting in a 32-amino-acid truncation and in a frameshift inducing both a 3-amino-acid substitution and a termination codon. Genomic PCR analysis revealed mutation of T-->G at eight bases upstream of the splicing site at the boundary of intron 10 and exon 11 of PEX3 gene, giving rise to a deletion of all of exon 11. When assessed by expression in a pex3 mutant of Chinese hamster ovary cells and the patient's fibroblasts, PBDG-02-derived PEX3 cDNA was found to be defective in peroxisome-restoring activity. These results provide evidence that PEX3 is a novel, pathogenic gene responsible for CG-G PBDs.

Molecular anatomy of the peroxin Pex12p: ring finger domain is essential for Pex12p function and interacts with the peroxisome-targeting signal type 1-receptor Pex5p and a ring peroxin, Pex10p

The three peroxin genes, PEX12, PEX2, and PEX10, encode peroxisomal integral membrane proteins with RING finger at the C-terminal part and are responsible for human peroxisome biogenesis disorders. Mutation analysis in PEX12 of Chinese hamster ovary cell mutants revealed a homozygous nonsense mutation at residue Trp263Ter in ZP104 cells and a pair of heterozygous nonsense mutations, Trp170Ter and Trp114Ter, in ZP109. This result and domain mapping of Pex12p showed that RING finger is essential for peroxisome-restoring activity of Pex12p but not necessary for targeting to peroxisomes. The N-terminal region of Pex12p, including amino acid residues at positions 17-76, was required for localization to peroxisomes, while the sequence 17-76 was not sufficient for peroxisomal targeting. Peroxins interacting with RING finger of Pex2p, Pex10p, and Pex12p were investigated by yeast two-hybrid as well as in vitro binding assays. The RING finger of Pex12p bound to Pex10p and the PTS1-receptor Pex5p. Pex10p also interacted with Pex2p and Pex5p in vitro. Moreover, Pex12p was co-immunoprecipitated with Pex10p from CHO-K1 cells, where Pex5p was not associated with the Pex12p-Pex10p complex. This observation suggested that Pex5p does not bind to, or only transiently interacts with, Pex10p and Pex12p when Pex10p and Pex12p are in the oligomeric complex in peroxisome membranes. Hence, the RING finger peroxins are most likely to be involved in Pex5p-mediated matrix protein import into peroxisomes.

The mammalian peroxin Pex5pL, the longer isoform of the mobile peroxisome targeting signal (PTS) type 1 transporter, translocates the Pex7p.PTS2 protein complex into peroxisomes via its initial docking site, Pex14p

In mammals, two isoforms of the peroxisome targeting signal (PTS) type 1 receptor Pex5p, i.e. Pex5pS and Pex5pL with an internal 37-amino acid insertion, have previously been identified. Expression of either type of Pex5p complements the impaired PTS1 import in Chinese hamster ovary pex5 mutants, but only Pex5pL can rescue the PTS2 import defect noted in a subgroup of pex5 mutants such as ZP105. In this work, we found that Pex5pL directly interacts with the PTS2 receptor Pex7p, carrying its cargo PTS2 protein in the cytosol. Pex5pL, but not Pex5pS, mediated the binding of PTS2 protein to Pex14p by translocating Pex7p, demonstrating that Pex5pL plays a pivotal role in peroxisomal PTS2 import. Pex5p was localized mostly in the cytosol in wild-type CHO-K1 and Pex14p-deficient mutant cells, whereas it accumulated in the peroxisomal remnants in cell mutants defective in Pex13p or the RING family peroxins such as Pex2p and Pex12p. Furthermore, overexpression of Pex14p, but not Pex10p, Pex12p, or Pex13p, caused accumulation of Pex5p in peroxisomal membranes, with concomitant interference with PTS1 and PTS2 import. Therefore, Pex5p carrying the cargoes most likely docks with the initial site (Pex14p) in a putative import machinery, subsequently translocating to other components such as Pex13p, Pex2p, Pex10p, and Pex12p.

Disruption of the interaction of the longer isoform of Pex5p, Pex5pL, with Pex7p abolishes peroxisome targeting signal type 2 protein import in mammals. Study with a novel Pex5-impaired Chinese hamster ovary cell mutant

We isolated peroxisome biogenesis-defective Chinese hamster ovary cell mutants from TKaG2 cells, wild-type CHO-K1 cells transformed with two cDNAs encoding rat Pex2p and peroxisome targeting signal (PTS) type 2-tagged green fluorescent protein, by the 9-(1'-pyrene)nonanol/UV selection method. Ten mutant clones showed cytosolic PTS2-green fluorescent protein, indicative of a defect in PTS2 import, and were classified in five complementation groups, i.e. pex1, pex2, pex5, pex14, and group A. One PEX5-deficient mutant, ZPG231, showed a novel phenotype: PTS2 proteins in the cytosol, but PTS1 proteins and catalase in peroxisomes. In ZPG231, two isoforms of the PTS1 receptor Pex5p, a shorter Pex5pS and a longer Pex5pL, were expressed as in wild-type cells, but possessed the missense point mutation S214F in both Pex5p isoforms, termed Pex5pS-S214F and Pex5pL-S214F, respectively. The S214F mutation was located only one amino acid upstream of the Pex5pL-specific 37-amino acid insertion site. Pex5pS-S214F and Pex5pL-S214F interacted with peroxisomal proteins, including PTS1 protein, catalase, and Pex14p, as efficiently as normal Pex5p. In contrast, the S214F mutation severely affected the binding of Pex5pL to the PTS2 receptor Pex7p. Expression of Pex5pL-S214F in pex5 cell mutants defective in PTS1 and PTS2 transport restored peroxisomal import of PTS1, but not PTS2. Together, the results indicate that ZPG231 is the first cell mutant providing evidence that disruption of the Pex5pL-Pex7p interaction completely abolishes PTS2 import in mammals.

Peroxisome biogenesis and peroxisome biogenesis disorders

Peroxisome assembly in mammals requires more than 15 genes. Two isoforms of the peroxisome targeting signal type 1 (PTS1) receptor, Pex5pS and Pex5pL, are identified in mammals. Pex5pS and Pex5pL bind PTS1 proteins. Pex5pL, but not Pex5pS, directly interacts with the PTS2 receptor, Pex7p, carrying its cargo PTS2 protein in the cytosol. Pex5p carrying the cargos, PTS1 and PTS2, docks with the initial site Pex14p in a putative import machinery, subsequently translocating to other components such as Pex13p, Pex2p, Pex10p and Pex12p, whereby the matrix proteins are imported. The peroxins, Pex3p, Pex16p and Pex19p, function in the assembly of peroxisomal membrane vesicles that precedes the import of matrix proteins. Hence, peroxisomes may form de novo and do not have to arise from pre-existing, morphologically recognizable peroxisomes. Impaired peroxisome assembly causes peroxisome biogenesis disorders such as Zellweger syndrome.

The peroxin pex3p initiates membrane assembly in peroxisome biogenesis

Rat cDNA encoding a 372-amino-acid peroxin was isolated, primarily by functional complementation screening, using a peroxisome-deficient Chinese hamster ovary cell mutant, ZPG208, of complementation group 17. The deduced primary sequence showed approximately 25% amino acid identity with the yeast Pex3p, thereby we termed this cDNA rat PEX3 (RnPEX3). Human and Chinese hamster Pex3p showed 96 and 94% identity to rat Pex3p and had 373 amino acids. Pex3p was characterized as an integral membrane protein of peroxisomes, exposing its N- and C-terminal parts to the cytosol. A homozygous, inactivating missense mutation, G to A at position413, in a codon (GGA) for Gly(138) and resulting in a codon (GAA) for Glu was the genetic cause of peroxisome deficiency of complementation group 17 ZPG208. The peroxisome-restoring activity apparently required the full length of Pex3p, whereas its N-terminal part from residues 1 to 40 was sufficient to target a fusion protein to peroxisomes. We also demonstrated that Pex3p binds the farnesylated peroxisomal membrane protein Pex19p. Moreover, upon expression of PEX3 in ZPG208, peroxisomal membrane vesicles were assembled before the import of soluble proteins such as PTS2-tagged green fluorescent protein. Thus, Pex3p assembles membrane vesicles before the matrix proteins are translocated.

Rapid isolation and characterization of CHO mutants deficient in peroxisome biogenesis using the peroxisomal forms of fluorescent proteins

We isolated and characterized CHO mutants deficient in peroxisome assembly using green fluorescent protein (GFP) and blue fluorescent protein (BFP) as the fluorescent probes to study the molecular mechanism of peroxisome biogenesis. We used stable transformants of CHO cells expressing GFP appending peroxisome targeting signal-1 (PTS1) and/or peroxisome targeting signal-2 (PTS2) as the parent strains for rapid isolation of the mutants. We have obtained six peroxisome-deficient mutants by visual screening of the mislocalizations of the peroxisomal GFPs. Mutual cell fusion experiments indicated that the six mutants isolated were divided into four complementation groups. Several of the mutants obtained possessed defective genes: the PEX2 gene was defective in SK24 and PT54; the PEX5 gene in SK32 and the PEX7 gene in PT13 and PT32. BE41, which belonged to the fourth complementation group, was not determined. When peroxisomal forms of BFP were transiently expressed in mutant cells, the peroxisomal BFPs appending both PTS1 and PTS2 appeared to bypass either the PTS1 or PTS2 pathway for localization in SK32. This observation suggested that other important machinery, in addition to the PTS1 or PTS2 pathway, could be involved in peroxisome biogenesis. Thus, our approach using peroxisomal fluorescent proteins could facilitate the isolation and analysis of peroxisome-deficient CHO mutants and benefit studies on the identification and role of the genes responsible for peroxisome biogenesis.

Restoration of biochemical function of the peroxisome in the temperature-sensitive mild forms of peroxisome biogenesis disorder in humans

We have found that peroxisome assembly is temperature-sensitive (ts) in mild forms of peroxisome biogenesis disorders (PBDs), that is all infantile Refsum disease (IRD) patients and a few neonatal adrenoleukodystrophy patients of several complementation groups. The number of peroxisomes increased daily in incubation at 30 degrees C in the ts cells. Oxidation of very long-chain fatty acids, processing of acyl-CoA oxidase and dihydroxyacetonephosphate acyltransferase activity also improved after 8 days incubation at 30 degrees C in the IRD fibroblasts. These biochemical functions of the peroxisome did not change at 30 degrees C in Zellweger fibroblasts. Number of peroxisomes gradually decreased after 4 days when the temperature shifted from 30 to 37 degrees C in the ts cells. These results indicate that the biochemical functions of peroxisome are also restored by incubation at 30 degrees C in the mild and ts phenotype of PBDs, and the results will aid to predict the severity and the prognosis of affected children.

Isolation, characterization and mutation analysis of PEX13-defective Chinese hamster ovary cell mutants

We isolated peroxisome biogenesis mutants ZP128 and ZP150 from rat PEX2 -transformed Chinese hamster ovary (CHO) cells, by the 9-(1'-pyrene)nonanol/ultraviolet method. The mutants lacked morphologically recognizable peroxisomes and showed a typical peroxisome assembly-defective phenotype such as a high sensitivity to 12-(1'-pyrene)dodecanoic acid/UV treatment. By means of PEX cDNA transfection and cell fusion, ZP128 and ZP150 were found to belong to a recently identified complementation group H. Expression of human PEX13 cDNA restored peroxisome assembly in ZP128 and ZP150. CHO cell PEX13 was isolated; its deduced sequence comprises 405 amino acids with 93% identity to human Pex13p. Mutation in PEX13 of mutant ZP150 was determined by RT-PCR: G to A transition resulted in one amino acid substitution, Ser319Asn, in one allele and truncation of a 42 amino acid sequence from Asp265 to Lys306 in another allele. Therefore, ZP128 and ZP150 are CHO cell lines with a phenotype of impaired PEX13.

Newly identified Chinese hamster ovary cell mutants defective in peroxisome assembly represent complementation group A of human peroxisome biogenesis disorders and one novel group in mammals

We isolated peroxisome biogenesis-defective mutants from rat PEX2-transformed Chinese hamster ovary (CHO) cells, using the 9-(1'-pyrene)nonanol/ultraviolet method. A total of 18 mutant cell clones showing cytosolic localization of catalase were isolated. By complementation group (CG) analysis by means of PEX cDNA transfection and cell fusion, cell mutants, ZP124 and ZP126, were found to belong to two novel CGs of CHO mutants. Mutants, ZP135 and ZP167, were also classified to the same CG as ZP124. Further cell fusion analysis using 12 CGs fibroblasts from patients with peroxisome deficiency disorders such as Zellweger syndrome revealed that ZP124 belonged to human CG-A, the same group as CG-VIII in the United States. ZP126 could not be classified to any of human and CHO CGs. These mutants also showed typical peroxisome assembly-defective phenotypes such as severe loss of catalase latency and impaired biogenesis of peroxisomal enzymes. Collectively, ZP124 represents CG-A, and ZP126 is in a newly identified CG distinct from the 14 mammalian CGs previously characterized.

Isolation and characterization of novel peroxisome biogenesis-defective Chinese hamster ovary cell mutants using green fluorescent protein

We developed an improved method for isolation of peroxisome biogenesis-defective somatic animal cell mutants, using a combination of green fluorescent protein (GFP) expression and the 9-(1'-pyrene)nonanol/ultraviolet (P9OH/UV) selection method. We used TKaG1 and TKaG2 cells, the wild-type Chinese hamster ovary (CHO) cells, CHO-K1, that had been stably transfected with cDNAs each encoding rat Pex2p as well as GFP tagged at the C-terminus with peroxisome targeting signal type 1 (PTS1) or N-terminally PTS2-tagged GFP. P9OH/UV-resistant cell colonies were examined for intracellular location of GFP on unfixed cells, by fluorescence microscopy. Seven each of the mutant cell clones isolated from TKaG1 and TKaG2 showed cytosolic GFP-PTS1 and PTS2-GFP, respectively, indicating the defect in peroxisome assembly. By transfection of PEX2, PEX5, PEX6, and PEX12 cDNAs and cell fusion analysis between the CHO cell mutants, five different complementation groups (CGs) were identified. Two mutant clones, ZPG207 and ZPG208, belonged to novel CGs. Further CG analysis using fibroblasts from patients with peroxisome biogenesis disorders, including rhizomelic chondrodysplasia punctata (RCDP), revealed that ZPG208 belonged to none of human CGs. ZPG207 was classified into the same CG as RCDP. Taken together, ZPG208 is in a newly identified, the 12th, CG in peroxisome-deficient CHO mutants reported to date and represents a novel mammalian CG.

The peroxin Pex14p. cDNA cloning by functional complementation on a Chinese hamster ovary cell mutant, characterization, and functional analysis

Rat cDNA encoding a 376-amino acid peroxin was isolated by functional complementation of a peroxisome-deficient Chinese hamster ovary cell mutant, ZP110, of complementation group 14 (CG14). The primary sequence showed 28 and 24% amino acid identity with the yeast Pex14p from Hansenula polymorpha and Saccharomyces cerevisiae, respectively; therefore, we termed this cDNA rat PEX14 (RnPEX14). Human and Chinese hamster Pex14p showed 96 and 94% identity to rat Pex14p, except that both Pex14p comprised 377 amino acids. Pex14p was characterized as an integral membrane protein of peroxisomes, exposing its N- and C-terminal parts to the cytosol. Pex14p interacts with both Pex5p and Pex7p, the receptors for peroxisome targeting signal type 1 (PTS1) and PTS2, respectively, together with the receptors' cargoes, PTS1 and PTS2 proteins. Mutation in PEX14 from ZP161, the same CG as ZP110, was determined by reverse transcription-PCR as follows. A 133-base pair deletion at nucleotide residues 37-169 in one allele created a termination codon at 40-42; in addition to this mutation, 103 base pairs were deleted at positions 385-487, resulting in the second termination immediately downstream the second deletion site in the other allele. Neither of these two mutant forms of Pex14p restored peroxisome biogenesis in ZP110 and ZP161, thereby demonstrating PEX14 to be responsible for peroxisome deficiency in CG14.

Human PEX19: cDNA cloning by functional complementation, mutation analysis in a patient with Zellweger syndrome, and potential role in peroxisomal membrane assembly

At least 11 complementation groups (CGs) have been identified for the peroxisome biogenesis disorders (PBDs) such as Zellweger syndrome, for which seven pathogenic genes have been elucidated. We have isolated a human PEX19 cDNA (HsPEX19) by functional complementation of peroxisome deficiency of a mutant Chinese hamster ovary cell line, ZP119, defective in import of both matrix and membrane proteins. This cDNA encodes a hydrophilic protein (Pex19p) comprising 299 amino acids, with a prenylation motif, CAAX box, at the C terminus. Farnesylated Pex19p is partly, if not all, anchored in the peroxisomal membrane, exposing its N-terminal part to the cytosol. A stable transformant of ZP119 with HsPEX19 was morphologically and biochemically restored for peroxisome biogenesis. HsPEX19 expression also restored peroxisomal protein import in fibroblasts from a patient (PBDJ-01) with Zellweger syndrome of CG-J. This patient (PBDJ-01) possessed a homozygous, inactivating mutation: a 1-base insertion, A764, in a codon for Met255, resulted in a frameshift, inducing a 24-aa sequence entirely distinct from normal Pex19p. These results demonstrate that PEX19 is the causative gene for CG-J PBD and suggest that the C-terminal part, including the CAAX homology box, is required for the biological function of Pex19p. Moreover, Pex19p is apparently involved at the initial stage in peroxisome membrane assembly, before the import of matrix protein.

Mutation in PEX16 is causal in the peroxisome-deficient Zellweger syndrome of complementation group D

Peroxisome-biogenesis disorders (PBDs), including Zellweger syndrome (ZS), are autosomal recessive diseases caused by a deficiency in peroxisome assembly as well as by a malfunction of peroxisomes, among which>10 genotypes have been identified. We have isolated a human PEX16 cDNA (HsPEX16) by performing an expressed-sequence-tag homology search on a human DNA database, by using yeast PEX16 from Yarrowia lipolytica and then screening the human liver cDNA library. This cDNA encodes a peroxisomal protein (a peroxin Pex16p) made up of 336 amino acids. Among 13 peroxisome-deficiency complementation groups (CGs), HsPEX16 expression morphologically and biochemically restored peroxisome biogenesis only in fibroblasts from a CG-D patient with ZS in Japan (the same group as CG-IX in the United States). Pex16p was localized to peroxisomes through expression study of epitope-tagged Pex16p. One patient (PBDD-01) possessed a homozygous, inactivating nonsense mutation, C-->T at position 526 in a codon (CGA) for 176Arg, that resulted in a termination codon (TGA). This implies that the C-terminal half is required for the biological function of Pex16p. PBDD-01-derived PEX16 cDNA was defective in peroxisome-restoring activity when expressed in the patient's fibroblasts. These results demonstrate that mutation in PEX16 is the genetic cause of CG-D PBDs.

cDNA cloning and characterization of a constitutively expressed isoform of the human peroxin Pex11p

We cloned a human cDNA encoding an isoform of the peroxin Pex11p, termed Pex11pbeta, by screening of human liver cDNA library using as a probe human EST-derived, approximately 300 bp-long nucleotides showing homology to PEX11 from Candida boidinii and Saccharomyces cerevisiae. PEX11beta encoded a protein comprising 259 amino acids, with two putative transmembrane segments, showing approximately 40% identity to inducible Pex11palpha, at the amino acid sequence level. Pex11pbeta was found to be a peroxisomal protein, as assessed by colocalization with acyl-CoA oxidase, an enzyme catalyzing the first step of peroxisomal beta-oxidation system, in Pex11pbeta-expressing Chinese hamster ovary cells. PEX11beta was not induced in rats by treatment of clofibrate, a peroxisome proliferator, in contrast to PEX11alpha.

Newly identified Chinese hamster ovary cell mutants are defective in biogenesis of peroxisomal membrane vesicles (Peroxisomal ghosts), representing a novel complementation group in mammals

We isolated peroxisome biogenesis-defective mutants from Chinese hamster ovary cells by the 9-(1'-pyrene)nonanol/ultraviolet (P9OH/UV) method. Seven cell mutants, ZP116, ZP119, ZP160, ZP161, ZP162, ZP164, and ZP165, of 11 P9OH/UV-resistant cell clones showed cytosolic localization of catalase, a peroxisomal matrix enzyme, apparently indicating a defect of peroxisome biogenesis. By transfection of PEX cDNAs and cell fusion analysis, mutants ZP119 and ZP165 were found to belong to a novel complementation group (CG), distinct from earlier mutants. CG analysis by cell fusion with fibroblasts from patients with peroxisome biogenesis disorders such as Zellweger syndrome indicated that ZP119 and ZP165 were in the same CG as the most recently identified human CG-J. The peroxisomal matrix proteins examined, including PTS1 proteins as well as a PTS2 protein, 3-ketoacyl-CoA thiolase, were also found in the cytosol in ZP119 and ZP165. Furthermore, these mutants showed typical peroxisome assembly-defective phenotype such as severe loss of resistance to 12-(1'-pyrene)dodecanoic acid/UV treatment. Most strikingly, peroxisomal reminiscent vesicular structures, so-called peroxisomal ghosts noted in all CGs of earlier Chinese hamster ovary cell mutants as well as in eight CGs of patients' fibroblasts, were not discernible in ZP119 and ZP165, despite normal synthesis of peroxisomal membrane proteins. Accordingly, ZP119 and ZP165 are the first cell mutants defective in import of both soluble and membrane proteins, representing the 14th peroxisome-deficient CG in mammals, including humans.

Restoration of PEX2 peroxisome assembly defects by overexpression of PMP70

The mutant Chinese hamster ovary (CHO) cell line Z78/C has defective peroxisome assembly due to a missense mutation in PEX2, the gene which encodes the 35 kDa peroxisomal integral membrane protein. In humans, PEX2 mutations are responsible for complementation group 10 of the human peroxisome biogenesis disorders (PBD), a genetically heterogeneous group of lethal, autosomal recessive diseases including the Zellweger syndrome and related phenotypes. To develop additional cellular models for Zellweger syndrome, we produced a series of new mutant CHO cell clones in the same complementation group as Z78/C (Z2, Z7, Z22, and Z105). As expected, expression of human PEX2 restores peroxisomal biogenesis in all of these clones. Surprisingly, expression of the human 70 kDa peroxisomal membrane protein (PMP70) also restores peroxisome biogenesis in these same CHO cell clones. We confirmed this effect of PMP70 expression on peroxisome biogenesis by determining the subcellular latency of catalase, the immunohistochemical localization of catalase and the beta-oxidation of very long chain fatty acids (VLCFA). By contrast, expression of a mutant allele of PMP70 identified in a patient with Zellweger syndrome did not restore peroxisome biogenesis in the PEX2-deficient CHO cell clones. Our results indicate that overexpression of PMP70 suppresses the phenotype of PEX2 gene mutations. These observations suggest a functional interaction between PEX2 and PMP70 in the peroxisome membrane.

Clofibrate-inducible, 28-kDa peroxisomal integral membrane protein is encoded by PEX11

We cloned a human PEX11 cDNA by expressed sequence tag homology search using yeast Candida boidinii PEX11, followed by screening of human liver cDNA library. PEX11 encoded a peroxisomal protein Pex11p comprising 247 amino acids, with two transmembrane segments and a dilysine motif at the C-terminus. Pex11p comigrated in SDS-PAGE with a 28-kDa peroxisomal integral membrane protein (PMP28) isolated from the liver of clofibrate-treated rats and was crossreactive to anti-PMP28 antibody, thereby indicating PEX11 to encode PMP28. Pex11p exposes both N- and C-terminal parts to the cytosol. PEX11 was not responsible for ten complementation groups of human peroxisome deficiency disorders.

PEX12, the pathogenic gene of group III Zellweger syndrome: cDNA cloning by functional complementation on a CHO cell mutant, patient analysis, and characterization of PEX12p

Rat PEX12 cDNA was isolated by functional complementation of peroxisome deficiency of a mutant CHO cell line, ZP109 (K. Okumoto, A. Bogaki, K. Tateishi, T. Tsukamoto, T. Osumi, N. Shimozawa, Y. Suzuki, T. Orii, and Y. Fujiki, Exp. Cell Res. 233:11-20, 1997), using a transient transfection assay and an ectopic, readily visible marker, green fluorescent protein. This cDNA encodes a 359-amino-acid membrane protein of peroxisomes with two transmembrane segments and a cysteine-rich zinc finger, the RING motif. A stable transformant of ZP109 with the PEX12 was morphologically and biochemically restored for peroxisome biogenesis. Pex12p was shown by expression of bona fide as well as epitope-tagged Pex12p to expose both N- and C-terminal regions to the cytosol. Fibroblasts derived from patients with the peroxisome deficiency Zellweger syndrome of complementation group III (CG-III) were also complemented for peroxisome biogenesis with PEX12. Two unrelated patients of this group manifesting peroxisome deficiency disorders possessed homozygous, inactivating PEX12 mutations: in one, Arg180Thr by one point mutation, and in the other, deletion of two nucleotides in codons for 291Asn and 292Ser, creating an apparently unchanged codon for Asn and a codon 292 for termination. These results indicate that the gene encoding peroxisome assembly factor Pex12p is a pathogenic gene of CG-III peroxisome deficiency. Moreover, truncation and site mutation studies, including patient PEX12 analysis, demonstrated that the cytoplasmically oriented N- and C-terminal parts of Pex12p are essential for biological function.

A cytoplasmic AAA family peroxin, Pex1p, interacts with Pex6p

Human PEX1 (HsPEX1) is the causative gene for peroxisome-deficiency disorders such as Zellweger syndrome of complementation group I, encoding the peroxin, Pex1p, a member of AAA family. Pex1p tagged with an epitope flag was expressed in wild-type Chinese hamster ovary (CHO) cell, CHO-K1. Pex1p was localized in the cytoplasm, as assessed by immunofluorescent microscopy. Cell-lysate of HsPEX1-transfected CHO-K1 was incubated with in vitro synthesized 35S-labelled Pex6p, an AAA family peroxin. Immunoprecipitation of Pex1p using anti-Pex1p antibody resulted in concomitant recovery of 35S-Pex6p. Conversely, 35S-Pex1p was obtained in immunoprecipitate from CHO-K1 expressing human Pex6p, using anti-Pex6p antibody. These results strongly suggest that Pex1p and Pex6p interact with each other.

Human PEX1 cloned by functional complementation on a CHO cell mutant is responsible for peroxisome-deficient Zellweger syndrome of complementation group I

The peroxisome biogenesis disorders (PBDs), including Zellweger syndrome (ZS) and neonatal adrenoleukodystrophy (NALD), are autosomal recessive diseases caused by defects in peroxisome assembly, for which at least 10 complementation groups have been reported. We have isolated a human PEX1 cDNA (HsPEX1) by functional complementation of peroxisome deficiency of a mutant Chinese hamster ovary (CHO) cell line, ZP107, transformed with peroxisome targeting signal type 1-tagged "enhanced" green fluorescent protein. This cDNA encodes a hydrophilic protein (Pex1p) comprising 1,283 amino acids, with high homology to the AAA-type ATPase family. A stable transformant of ZP107 with HsPEX1 was morphologically and biochemically restored for peroxisome biogenesis. HsPEX1 expression restored peroxisomal protein import in fibroblasts from three patients with ZS and NALD of complementation group I (CG-I), which is the highest-incidence PBD. A CG-I ZS patient (PBDE-04) possessed compound heterozygous, inactivating mutations: a missense point mutation resulting in Leu-664 --> Pro and a deletion of the sequence from Gly-634 to His-690 presumably caused by missplicing (splice site mutation). Both PBDE-04 PEX1 cDNAs were defective in peroxisome-restoring activity when expressed in the patient fibroblasts as well as in ZP107 cells. These results demonstrate that PEX1 is the causative gene for CG-I peroxisomal disorders.

Peroxisome biogenesis disorders: identification of a new complementation group distinct from peroxisome-deficient CHO mutants and not complemented by human PEX 13

Ten complementation groups of generalized peroxisome biogenesis disorders (PBD), (excluding rhizomelic chondrodysplasia punctata) have been identified using complementation analysis. Four of the genes involved have been identified using two different methods of (1) genetic functional complementation of peroxisome deficient CHO cell mutants and (2) homology searches for human dbEST, based on yeast genes involved in peroxisome biogenesis (PEX genes). We report here the first identification of a new complementation group which is genetically different from peroxisome deficient CHO mutants. There were no complementations by the human PEX 13 gene. The nature of the related gene is being investigated.

Peroxisome targeting signal type 1 (PTS1) receptor is involved in import of both PTS1 and PTS2: studies with PEX5-defective CHO cell mutants

To investigate the mechanisms of peroxisome assembly and the molecular basis of peroxisome assembly disorders, we isolated and characterized a peroxisome-deficient CHO cell mutant, ZP139, which was found to belong to human complementation group II, the same group as that of our earlier mutant, ZP105. These mutants had a phenotypic deficiency in the import of peroxisomal targeting signal type 1 (PTS1) proteins. Amino-terminal extension signal (PTS2)-mediated transport, including that of 3-ketoacyl coenzyme A thiolase, was also defective in ZP105 but not in ZP139. PEX5 cDNA, encoding the PTS1 receptor (PTS1R), was isolated from wild-type CHO-K1 cells. PTS1R's deduced primary sequence comprised 595 amino acids, 7 amino acids less than the human homolog, and contained seven tetratricopeptide repeat (TPR) motifs at the C-terminal region. Chinese hamster PTS1R showed 94, 28, and 24% amino acid identity with PTS1Rs from humans, Pichia pastoris, and Saccharomyces cerevisiae, respectively. A PTS1R isoform (PTS1RL) with 632 amino acid residues was identified in CHO cells; for PTS1R, 37 amino acids were inserted between residues at positions 215 and 216 of a shorter isoform (PTS1RS). Southern blot analysis of CHO cell genomic DNA suggested that these two isoforms are derived from a single gene. Both types of PEX5 complemented impaired import of PTS1 in mutants ZP105 and ZP139. PTS2 import in ZP105 was rescued only by PTS1RL. This finding strongly suggests that PTS1RL is also involved in the transport of PTS2. Mutations in PEX5 were determined by reverse transcription-PCR: a G-to-A transition resulted in one amino acid substitution: Gly298Glu of PTS1RS (G335E of PTS1RL) in ZP105 and Gly485Glu of PTS1RS (G522E of PTS1RL) in ZP139. Both mutations were in the TPR domains (TPR1 and TPR6), suggesting the functional consequence of these domains in protein translocation. The implications of these mutations are discussed.

Biogenesis of peroxisomes in fetal liver

Peroxisomes are single membrane-limited cell organelles that are involved in numerous metabolic functions. Peroxisomes do not contain DNA; the matrix and membrane proteins are encoded by the nuclear genome. It is assumed that new peroxisomes are formed by division of existing organelles. The present article gives an overview of microscopic studies and recent unpublished results dealing with peroxisome biogenesis in mammalian fetal liver and presents data on peroxisomes in oocytes. Cytochemical (catalase and D-aminoacid oxidase activity) and immunocytochemical data in rat and human liver (antigens of catalase, the three peroxisomal beta-oxidation enzymes, alanine: glyoxylate aminotransferase, peroxisomal membrane proteins with molecular weights of 42 and 70 kDa) indicate that during embryonic and fetal development the peroxisomal population undergoes a differentiation with respect to the composition of the matrix and to the size and number of the organelles. In the youngest stages, rare and small peroxisomes are present, into which the matrix components are imported in a sequential way. The import seems asynchronous in peroxisomes of the same hepatocyte. The size and number of the peroxisomes increase during liver development. In rat and human liver, no morphological or immunocytochemical evidence for an elaborate network of interconnected peroxisomes ("reticulum") was found. Instead, peroxisomes presented as individual organelles, which occasionally show membrane extensions. The importance of the metabolic functions of peroxisomes in human liver is emphasized by the peroxisomal disorders. In the liver of affected fetuses, the microscopic features associated with the defect can already be recognized; i.e., either catalase containing peroxisomes are absent and catalase is localized in the cytoplasm (in fetuses affected with Zellweger syndrome or with infantile Refsum disease) or peroxisomes are present but they are abnormally enlarged (e.g., a fetus affected with acyl-CoA oxidase deficiency). In the quail ovary, numerous peroxisomes are observed in the oocyte and in the granulosa cells during follicle maturation, but not in the full-grown egg. Thus, the mechanism of peroxisome inheritance remains unresolved.