Isolation of intracellular membranes by means of sodium carbonate treatment: application to endoplasmic reticulum

A rapid and simple method for the isolation of membranes from subcellular organelles is described. The procedure consists of diluting the organelles in ice-cold 100 mM Na2CO3 followed by centrifugation to pellet the membranes. Closed vesicles are converted to open membrane sheets, and content proteins and peripheral membrane proteins are released in soluble form. Here we document the method by applying it to various subfractions of a rat liver microsomal fraction, prepared by continuous density gradient centrifugation according to Beaufay et al. (1974, J. Cell Biol. 61:213-231). The results confirm and extend those of previous investigators on the distribution of enzymes and proteins among the membranes of the smooth and rough endoplasmic reticulum. In the accompanying paper (1982, J. Cell Biol. 93:103-110) the procedure is applied to peroxisomes and mitochondria.

Polypeptide and phospholipid composition of the membrane of rat liver peroxisomes: comparison with endoplasmic reticulum and mitochondrial membranes

Membranes were isolated from highly purified peroxisomes, mitochondria, and rough and smooth microsomes of rat liver by the one-step Na2CO3 procedure described in the accompanying paper (1982, J. Cell Biol. 93:97-102). The polypeptide compositions of these membranes were determined by SDS PAGE and found to be greatly dissimilar. The peroxisomal membrane contains 12% of the peroxisomal protein and consists of three major polypeptides (21,700, 67,700 and 69,700 daltons) as well as some minor polypeptides. The major peroxisomal membrane proteins as well as most of the minor ones are absent from the endoplasmic reticulum (ER). Conversely, most ER proteins are absent from peroxisomes. By electron microscopy, purified peroxisomal membranes are approximately 6.8 nm thick and have a typical trilaminar appearance. The phospholipid/protein ratio of peroxisomal membranes is approximately 200 nmol/mg; the principal phospholipids are phosphatidyl choline and phosphatidyl ethanolamine as in ER and mitochondrial membranes. In contrast to the mitochondria, peroxisomal membranes contain no cardiolipin. All the membranes investigated contain a polypeptide band with a molecular mass of approximately 15,000 daltons. Whether this represents an exceptional common membrane protein or a coincidence is unknown. The implications of these results for the biogenesis of peroxisomes are discussed.

A human gene responsible for Zellweger syndrome that affects peroxisome assembly

The primary defect arising from Zellweger syndrome appears to be linked to impaired assembly of peroxisomes. A human complementary DNA has been cloned that complements the disease's symptoms (including defective peroxisome assembly) in fibroblasts from a patient with Zellweger syndrome. The cause of the syndrome in this patient was a point mutation that resulted in the premature termination of peroxisome assembly factor-1. The homozygous patient apparently inherited the mutation from her parents, each of whom was heterozygous for that mutation.

Amino-terminal presequence of the precursor of peroxisomal 3-ketoacyl-CoA thiolase is a cleavable signal peptide for peroxisomal targeting

To examine the function of the amino-terminal presequence of rat peroxisomal 3-ketoacyl-CoA thiolase precursor, fusion proteins of various amino-terminal regions of the precursor with non-peroxisomal enzymes were expressed in cultured mammalian cells. On immunofluorescence microscopy, all constructs carrying the presequence part exhibited punctate patterns of distribution, identical with that of catalase, a peroxisomal marker. Proteins lacking all or a part of the prepiece were found in the cytosol. These results indicate that the presequence of the thiolase has sufficient information for peroxisomal targeting.

Restoration by a 35K membrane protein of peroxisome assembly in a peroxisome-deficient mammalian cell mutant

Peroxisomes are among the intracellular organelles of eukaryotic cells that contain specialized sets of enzymes with specific functions. Little is known of membranous components involved in assembly of the intracellular compartments. We isolated two peroxisome-deficient and mutually complementary, Chinese hamster ovary cell mutants, Z65 and Z24, which closely resembled fibroblasts from patients with autosomal recessive, peroxisome-defective disorders such as Zellweger syndrome. These patients show characteristic dysmorphism, severe hypotonia, psychomotor retardation, and peroxisomal dysfunctions and rarely survive early childhood. Here we report what seems to be the first direct cloning and characterization of a complementary DNA encoding a peroxisomal membrane protein of relative molecular mass 35,000 (Mr35K) that restores the biogenesis of peroxisomes and complements the defect of peroxisomal functions in the mutant Z65.

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.

Isolation and characterization of Chinese hamster ovary cell mutants defective in assembly of peroxisomes

We made use of autoradiographic screening to isolate two Chinese hamster ovary (CHO) cell mutants deficient in peroxisomal dihydroxyacetonephosphate acyltransferase, a key enzyme for the biosynthesis of ether glycerolipids such as plasmalogens. Morphological analysis revealed no evidence of peroxisome in these mutants. Catalase was as active as in the normal cells but was not sedimentable. Pulse-chase radiolabeling experiments and cell-free translation of RNA demonstrated that acyl-CoA oxidase, the first enzyme of the peroxisomal beta-oxidation system, was synthesized as the 75-kD form but was not converted to 53- and 22-kD mature components that were present in the wild-type CHO cells; rather, degradation was apparent. Peroxisomal thiolase was synthesized as in normal cells but remained as a larger, 44-kD precursor, whereas maturation to the 41-kD enzyme was detected in the wild-type cells. The peroxisomal 70-kD integral membrane protein was also equally synthesized, as in the wild-type cells, and was not degraded. These results suggest that assembly of the peroxisomes is defective in the mutants, whereas the synthesis of peroxisomal proteins appears to be normal. Cell-fusion studies revealed that the two mutants are recessive to the wild-type CHO cells and belong to different complementation groups. Thus, these mutants presumably contain different lesions in gene(s) encoding factor(s) required for peroxisome assembly.

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.

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.

Synthesis of a major integral membrane polypeptide of rat liver peroxisomes on free polysomes

The manner of synthesis and assembly of the peroxisomal membrane proteins is unknown. Understanding these processes is essential to an understanding of the formation of the organelle. We have investigated the biogenesis of the previously identified major 21.7-kDa integral peroxisomal membrane polypeptide [Fujiki, Y., Fowler, S., Shio, H., Hubbard, A. L. & Lazarow, P. B. (1982) J. Cell Biol. 93, 103-110]. This protein was purified to apparent homogeneity and used to elicit a rabbit antiserum. In immunoblotting analysis, antibody bound only to the 22-kDa membrane polypeptide present exclusively in peroxisomal membranes. Total rat liver RNA was translated in a nuclease-treated rabbit reticulocyte cell-free protein-synthesizing system. The in vitro translation product, isolated by means of the antibody and Staphylococcus aureus cells, comigrated with the mature 22-kDa polypeptide in NaDodSO4/PAGE. Analysis of the translation products of RNAs from free and membrane-bound polysomes indicated that the mRNA for the 22-kDa membrane polypeptide is found predominantly in free polysomes. The results imply post-translational insertion of the membrane polypeptide into the peroxisomal membrane without proteolytic processing and suggest that peroxisomes, like mitochondria and chloroplasts, form by fission from preexisting organelles.

The X gene of hepatitis B virus induced growth stimulation and tumorigenic transformation of mouse NIH3T3 cells

To examine the transforming potential of the X gene product of hepatitis B virus (HBV), the X-gene-containing region (referred to as the HBx region) was introduced into mouse NIH3T3 cells. Each transformed cell line expressed X-coding mRNA at a different level. A positive correlation was found between the level of X-coding mRNA and the saturation density of the cells. The HBx-transformed cell lines exhibited X protein production and tumor formation in nude mice. The function of HBV in oncogenesis may involve the continuous expression of the X-gene-coded product in the HBV DNA-integrated cells.

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.

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.

Multiple genes, including a member of the AAA family, are essential for degradation of unassembled subunit 2 of cytochrome c oxidase in yeast mitochondria

Cytochrome c oxidase consists of three mitochondrion- and several nucleus-encoded subunits. We previously found that in a mutant of Saccharomyces cerevisiae lacking nucleus-encoded subunit 4 of this enzyme (CoxIV), subunits 2 and 3 (CoxII and CoxIII), both encoded by the mitochondrial DNA, were unstable and rapidly degraded in mitochondria, presumably because the subunits cannot assemble normally. To analyze the molecular machinery involved in this proteolytic pathway, we obtained four mutants defective in the degradation of unassembled CoxII (osd mutants) by screening CoxIV-deficient cells for the accumulation of CoxII. All of the mutants were recessive and were classified into three different complementation groups. Tetrad analyses revealed that the phenotype of each mutant was caused by a single nuclear mutation. These results suggest strongly that at least three nuclear genes (the OSD genes) are required for this degradation system. Interestingly, degradation of CoxIII was not affected in the mutants, implying that the two subunits are degraded by distinct pathways. We also cloned the OSD1 gene by complementation of the temperature sensitivity of osd1-1 mutants with a COXIV+ genetic background on a nonfermentable glycerol medium. We found it to encode a member of a family (the AAA family) of putative ATPases, which proved to be identical to recently described YME1 and YTA11. Immunological analyses revealed that Osd1 protein is localized to the mitochondrial inner membrane. Disruption of the predicted ATP-binding cassette by site-directed mutagenesis eliminated biological activities, thereby underscoring the importance of ATP for function.

Partial disassembly of peroxisomes

Rat liver peroxisomes were subjected to a variety of procedures intended to partially disassemble or damage them; the effects were analyzed by recentrifugation into sucrose gradients, enzyme analyses, electron microscopy, and SDS PAGE. Freezing and thawing or mild sonication released some matrix proteins and produced apparently intact peroxisomal "ghosts" with crystalloid cores and some fuzzy fibrillar content. Vigorous sonication broke open the peroxisomes but the membranes remained associated with cores and fibrillar and amorphous matrix material. The density of both ghosts and more severely damaged peroxisomes was approximately 1.23. Pyrophosphate (pH 9) treatment solubilized the fibrillar content, yielding ghosts that were empty except for cores. Some matrix proteins such as catalase and thiolase readily leak from peroxisomes. Other proteins were identified that remain in mechanically damaged peroxisomes but are neither core nor membrane proteins because they can be released by pyrophosphate treatment. These constitute a class of poorly soluble matrix proteins that appear to correspond to the fibrillar material observed morphologically. All of the peroxisomal beta-oxidation enzymes are located in the matrix, but they vary greatly in how easily they leak out. Palmitoyl coenzyme A synthetase is in the membrane, based on its co-distribution with the 22-kilodalton integral membrane polypeptide.

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.

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.

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.

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.

Peroxisome assembly factor-2, a putative ATPase cloned by functional complementation on a peroxisome-deficient mammalian cell mutant

Rat peroxisome assembly factor-2 (PAF-2) cDNA was isolated by functional complementation of peroxisome deficiency of a mutant CHO cell line, ZP92, using transient transfection assay. This cDNA encodes a 978-amino acid protein with two putative ATP-binding sites. PAF-2 is a member of a putative ATPase family, including two yeast gene products essential for peroxisome assembly. A stable transformant of ZP92 with the cDNA was morphologically and biochemically restored for peroxisome biogenesis. Fibroblasts derived from patients deficient in peroxisome biogenesis (complementation group C) were also complemented with PAF-2 cDNA, indicating that PAF-2 is a strong candidate for the pathogenic gene of group C peroxisome deficiency.

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.

Acyl-Coa oxidase and hydratase-dehydrogenase, two enzymes of the peroxisomal beta-oxidation system, are synthesized on free polysomes of clofibrate-treated rat liver

We investigated the site of synthesis of two abundant proteins in clofibrate-induced rat hepatic peroxisomes. RNA was extracted from free and membrane-bound polysomes, heated to improve translational efficiency, and translated in the mRNA-dependent, reticulocyte-lysate-cell-free, protein-synthesizing system. The peroxisomal acyl-CoA oxidase and enoyl-CoA hydratase-beta-hydroxyacyl-CoA dehydrogenase 35S-translation products were isolated immunochemically, analyzed by SDS PAGE and fluorography, and quantitated by densitometric scanning. The RNAs coding for these two peroxisomal proteins were found predominantly on free polysomes, and the translation products co-migrated with the mature proteins. As in normal rat liver, preproalbumin and catalase were synthesized mainly by membrane-bound and by free polysomes, respectively. mRNAs for a number of minor 35S-translation products also retained by the anti-peroxisomal immunoadsorbent were similarly found on free polysomes. These results, together with previous data, allow the generalization that the content proteins of rat liver peroxisomes are synthesized on free polysomes, and the data imply a posttranslational packaging mechanism for these major content proteins.

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.