Human adrenoleukodystrophy protein and related peroxisomal ABC transporters interact with the peroxisomal assembly protein PEX19p

Novel targeting assay uncovers targeting information within peroxisomal ABC transporter Pxa1

The mechanism behind peroxisomal membrane protein targeting is still poorly understood, with only two yeast proteins believed to be involved and no consensus targeting sequence. Pex19 is thought to bind peroxisomal membrane proteins in the cytosol, and is subsequently recruited by Pex3 at the peroxisomal surface, followed by protein insertion via a mechanism that is as-yet-unknown. However, some peroxisomal membrane proteins still correctly sort in the absence of Pex3 or Pex19, suggesting that multiple sorting pathways exist. Here, we studied sorting of yeast peroxisomal ABC transporter Pxa1. Co-localization analysis of Pxa1-GFP in a collection of 86 peroxisome-related deletion strains revealed that Pxa1 sorting requires Pex3 and Pex19, while none of the other 84 proteins tested were essential. To identify regions with peroxisomal targeting information in Pxa1, we developed a novel in vivo re-targeting assay, using a reporter consisting of the mitochondrial ABC transporter Mdl1 lacking its N-terminal mitochondrial targeting signal. Using this assay, we showed that the N-terminal 95 residues of Pxa1 are sufficient for retargeting this reporter to peroxisomes. Interestingly, truncated Pxa1 lacking residues 1-95 still localized to peroxisomes. This was confirmed via localization of various Pxa1 truncation and deletion constructs. However, localisation of Pxa1 lacking residues 1-95 depended on the presence of its interaction partner Pxa2, indicating that this truncated protein does not contain a true targeting signal.

Dengue and Zika Virus Capsid Proteins Contain a Common PEX19-Binding Motif

Flaviviruses such as dengue virus (DENV) and Zika virus (ZIKV) have evolved sophisticated mechanisms to suppress the host immune system. For instance, flavivirus infections were found to sabotage peroxisomes, organelles with an important role in innate immunity. The current model suggests that the capsid (C) proteins of DENV and ZIKV downregulate peroxisomes, ultimately resulting in reduced production of interferons by interacting with the host protein PEX19, a crucial chaperone in peroxisomal biogenesis. Here, we aimed to explore the importance of peroxisomes and the role of C interaction with PEX19 in the flavivirus life cycle. By infecting cells lacking peroxisomes we show that this organelle is required for optimal DENV replication. Moreover, we demonstrate that DENV and ZIKV C bind PEX19 through a conserved PEX19-binding motif, which is also commonly found in cellular peroxisomal membrane proteins (PMPs). However, in contrast to PMPs, this interaction does not result in the targeting of C to peroxisomes. Furthermore, we show that the presence of C results in peroxisome loss due to impaired peroxisomal biogenesis, which appears to occur by a PEX19-independent mechanism. Hence, these findings challenge the current model of how flavivirus C might downregulate peroxisomal abundance and suggest a yet unknown role of peroxisomes in flavivirus biology.

Peroxisomal ABC Transporters: An Update

ATP-binding cassette (ABC) transporters constitute one of the largest superfamilies of conserved proteins from bacteria to mammals. In humans, three members of this family are expressed in the peroxisomal membrane and belong to the subfamily D: ABCD1 (ALDP), ABCD2 (ALDRP), and ABCD3 (PMP70). These half-transporters must dimerize to form a functional transporter, but they are thought to exist primarily as tetramers. They possess overlapping but specific substrate specificity, allowing the transport of various lipids into the peroxisomal matrix. The defects of ABCD1 and ABCD3 are responsible for two genetic disorders called X-linked adrenoleukodystrophy and congenital bile acid synthesis defect 5, respectively. In addition to their role in peroxisome metabolism, it has recently been proposed that peroxisomal ABC transporters participate in cell signaling and cell control, particularly in cancer. This review presents an overview of the knowledge on the structure, function, and mechanisms involving these proteins and their link to pathologies. We summarize the different in vitro and in vivo models existing across the species to study peroxisomal ABC transporters and the consequences of their defects. Finally, an overview of the known and possible interactome involving these proteins, which reveal putative and unexpected new functions, is shown and discussed.

S149R, a novel mutation in the ABCD1 gene causing X-linked adrenoleukodystrophy

X-linked adrenoleukodystrophy (X-ALD) is the most common peroxisomal disorder. It is a heterogeneous disorder caused by mutations in the ATP-binding cassette protein subfamily D1 (ABCD1) gene, encoding the peroxisomal membrane protein ALDP, which is involved in the transmembrane transport of very long-chain fatty acids. For the first time, we report a case of olivopontocerebellar X-ALD on the Chinese mainland. In this study, a novel mutation (c.447T>A; p.S149R) in ABCD1 was detected in a patient diagnosed with X-ALD. The mutant amino acid is well conserved among species. Bioinformatics analysis predicted the substitution to be deleterious and to cause structural changes in the adrenoleukodystrophy protein. Immunofluorescence showed an altered subcellular localization of the S149R mutant protein, which may lead to defects in the degradation of very long chain fatty acids in peroxisomes. We therefore suggest that the novel mutation, which alters ALDP structure, subcellular distribution and function, is responsible for X-ALD.

Predictive Structure and Topology of Peroxisomal ATP-Binding Cassette (ABC) Transporters

The peroxisomal ATP-binding Cassette (ABC) transporters, which are called ABCD1, ABCD2 and ABCD3, are transmembrane proteins involved in the transport of various lipids that allow their degradation inside the organelle. Defective ABCD1 leads to the accumulation of very long-chain fatty acids and is associated with a complex and severe neurodegenerative disorder called X-linked adrenoleukodystrophy (X-ALD). Although the nucleotide-binding domain is highly conserved and characterized within the ABC transporters family, solid data are missing for the transmembrane domain (TMD) of ABCD proteins. The lack of a clear consensus on the secondary and tertiary structure of the TMDs weakens any structure-function hypothesis based on the very diverse ABCD1 mutations found in X-ALD patients. Therefore, we first reinvestigated thoroughly the structure-function data available and performed refined alignments of ABCD protein sequences. Based on the 2.85  Å resolution crystal structure of the mitochondrial ABC transporter ABCB10, here we propose a structural model of peroxisomal ABCD proteins that specifies the position of the transmembrane and coupling helices, and highlight functional motifs and putative important amino acid residues.

Peroxisomal ATP-binding cassette transporters form mainly tetramers

ABCD1 and its homolog ABCD2 are peroxisomal ATP-binding cassette (ABC) half-transporters of fatty acyl-CoAs with both distinct and overlapping substrate specificities. Although it is established that ABC half-transporters have at least to dimerize to generate a functional unit, functional equivalents of tetramers (i.e. dimers of full-length transporters) have also been reported. However, oligomerization of peroxisomal ABCD transporters is incompletely understood but is of potential significance because more complex oligomerization might lead to differences in substrate specificity. In this work, we have characterized the quaternary structure of the ABCD1 and ABCD2 proteins in the peroxisomal membrane. Using various biochemical approaches, we clearly demonstrate that both transporters exist as both homo- and heterotetramers, with a predominance of homotetramers. In addition to tetramers, some larger molecular ABCD assemblies were also found but represented only a minor fraction. By using quantitative co-immunoprecipitation assays coupled with tandem mass spectrometry, we identified potential binding partners of ABCD2 involved in polyunsaturated fatty-acid metabolism. Interestingly, we identified calcium ATPases as ABCD2-binding partners, suggesting a role of ABCD2 in calcium signaling. In conclusion, we have shown here that ABCD1 and its homolog ABCD2 exist mainly as homotetramers in the peroxisomal membrane.

Targeting and insertion of peroxisomal membrane proteins: ER trafficking versus direct delivery to peroxisomes

The importance of peroxisomes is highlighted by severe inherited human disorders linked to impaired peroxisomal biogenesis. Besides the simple architecture of these ubiquitous and dynamic organelles, their biogenesis is surprisingly complex and involves specialized proteins, termed peroxins, which mediate targeting and insertion of peroxisomal membrane proteins (PMPs) into the peroxisomal bilayer, and the import of soluble proteins into the protein-dense matrix of the organelle. The long-standing paradigm that all peroxisomal proteins are imported directly into preexisting peroxisomes has been challenged by the detection of PMPs inside the endoplasmic reticulum (ER). New models propose that the ER originates peroxisomal biogenesis by mediating PMP trafficking to the peroxisomes via budding vesicles. However, the relative contribution of this ER-derived pathway to the total peroxisome population in vivo, and the detailed mechanisms of ER entry and exit of PMPs are controversially discussed. This review aims to summarize present knowledge about how PMPs are targeted to the ER, instead of being inserted directly into preexisting peroxisomes. Moreover, molecular mechanisms that facilitate bilayer insertion of PMPs among different species are discussed.

Integrative analyses of genetic variation, epigenetic regulation, and the transcriptome to elucidate the biology of platinum sensitivity

BACKGROUND

Using genome-wide genetic, gene expression, and microRNA expression (miRNA) data, we developed an integrative approach to investigate the genetic and epigenetic basis of chemotherapeutic sensitivity.

RESULTS

Through a sequential multi-stage framework, we identified genes and miRNAs whose expression correlated with platinum sensitivity, mapped these to genomic loci as quantitative trait loci (QTLs), and evaluated the associations between these QTLs and platinum sensitivity. A permutation analysis showed that top findings from our approach have a much lower false discovery rate compared to those from a traditional GWAS of drug sensitivity. Our approach identified five SNPs associated with 10 miRNAs and the expression level of 15 genes, all of which were associated with carboplatin sensitivity. Of particular interest was one SNP (rs11138019), which was associated with the expression of both miR-30d and the gene ABCD2, which were themselves correlated with both carboplatin and cisplatin drug-specific phenotype in the HapMap samples. Functional study found that knocking down ABCD2 in vitro led to increased apoptosis in ovarian cancer cell line SKOV3 after cisplatin treatment. Over-expression of miR-30d in vitro caused a decrease in ABCD2 expression, suggesting a functional relationship between the two.

CONCLUSIONS

We developed an integrative approach to the investigation of the genetic and epigenetic basis of human complex traits. Our approach outperformed standard GWAS and provided hints at potential biological function. The relationships between ABCD2 and miR-30d, and ABCD2 and platin sensitivity were experimentally validated, suggesting a functional role of ABCD2 and miR-30d in sensitivity to platinating agents.

Mutations in ABCD4 cause a new inborn error of vitamin B12 metabolism

Inherited disorders of vitamin B12 (cobalamin) have provided important clues to how this vitamin, which is essential for hematological and neurological function, is transported and metabolized. We describe a new disease that results in failure to release vitamin B12 from lysosomes, which mimics the cblF defect caused by LMBRD1 mutations. Using microcell-mediated chromosome transfer and exome sequencing, we identified causal mutations in ABCD4, a gene that codes for an ABC transporter, which was previously thought to have peroxisomal localization and function. Our results show that ABCD4 colocalizes with the lysosomal proteins LAMP1 and LMBD1, the latter of which is deficient in the cblF defect. Furthermore, we show that mutations altering the putative ATPase domain of ABCD4 affect its function, suggesting that the ATPase activity of ABCD4 may be involved in intracellular processing of vitamin B12.

Mammalian peroxisomal ABC transporters: from endogenous substrates to pathology and clinical significance

Peroxisomes are indispensable organelles in higher eukaryotes. They are essential for a number of important metabolic pathways, including fatty acid α- and β-oxidation, and biosynthesis of etherphospholipids and bile acids. However, the peroxisomal membrane forms an impermeable barrier to these metabolites. Therefore, peroxisomes need specific transporter proteins to transfer these metabolites across their membranes. The mammalian peroxisomal membrane harbours three ATP-binding cassette (ABC) transporters. In recent years, significant progress has been made in unravelling the functions of these ABC transporters. There is ample evidence that they are involved in the transport of very long-chain fatty acids, pristanic acid, di- and trihydroxycholestanoic acid, dicarboxylic acids and tetracosahexaenoic acid (C24:6ω3). Surprisingly, only one disease is associated with a deficiency of a peroxisomal ABC transporter. Mutations in the ABCD1 gene encoding the peroxisomal ABC transporter adrenoleukodystrophy protein are the cause for X-linked adrenoleukodystrophy, an inherited metabolic storage disorder. This review describes the current state of knowledge on the mammalian peroxisomal ABC transporters with a particular focus on their function in metabolite transport.

Identification of a new fatty acid synthesis-transport machinery at the peroxisomal membrane

The neurodegenerative disease X-linked adrenoleukodystrophy (X-ALD) is characterized by the abnormal accumulation of very long chain fatty acids. Mutations in the gene encoding the peroxisomal ATP-binding cassette half-transporter, adrenoleukodystrophy protein (ALDP), are the primary cause of X-ALD. To gain a better understanding of ALDP dysfunction, we searched for interaction partners of ALDP and identified binary interactions to proteins with functions in fatty acid synthesis (ACLY, FASN, and ACC) and activation (FATP4), constituting a thus far unknown fatty acid synthesis-transport machinery at the cytoplasmic side of the peroxisomal membrane. This machinery adds to the knowledge of the complex mechanisms of peroxisomal fatty acid metabolism at a molecular level and elucidates potential epigenetic mechanisms as regulatory processes in the pathogenesis and thus the clinical course of X-ALD.

Trypanosoma bruceiglycosomal ABC transporters: identification and membrane targeting

Trypanosomes contain unique peroxisome-like organelles designated glycosomes which sequester enzymes involved in a variety of metabolic processes including glycolysis. We identified three ABC transporters associated with the glycosomal membrane of Trypanosoma brucei. They were designated GAT1-3 for Glycosomal ABC Transporters. These polypeptides are so-called half-ABC transporters containing only one transmembrane domain and a single nucleotide-binding domain, like their homologues of mammalian and yeast peroxisomes. The glycosomal localization was shown by immunofluorescence microscopy of trypanosomes expressing fusion constructs of the transporters with Green Fluorescent Protein. By expression of fluorescent deletion constructs, the glycosome-targeting determinant of two transporters was mapped to different fragments of their respective primary structures. Interestingly, these fragments share a short sequence motif and contain adjacent to it one--but not the same--of the predicted six transmembrane segments of the transmembrane domain. We also identified the T. brucei homologue of peroxin PEX19, which is considered to act as a chaperonin and/or receptor for cytosolically synthesized proteins destined for insertion into the peroxisomal membrane. By using a bacterial two-hybrid system, it was shown that glycosomal ABC transporter fragments containing an organelle-targeting determinant can interact with both the trypanosomatid and human PEX19, despite their low overall sequence identity. Mutated forms of human PEX19 that lost interaction with human peroxisomal membrane proteins also did not bind anymore to the T. brucei glycosomal transporter. Moreover, fragments of the glycosomal transporter were targeted to the peroxisomal membrane when expressed in mammalian cells. Together these results indicate evolutionary conservation of the glycosomal/peroxisomal membrane protein import mechanism.

Characterization of the interaction between recombinant human peroxin Pex3p and Pex19p: identification of TRP-104 IN Pex3p as a critical residue for the interaction

Proteins required for peroxisome biogenesis are termed peroxins. The peroxin Pex3p is a peroxisomal membrane protein (PMP), involved in peroxisomal membrane biogenesis. It acts as a docking receptor for another peroxin Pex19p, which is a specific carrier protein for newly synthesized PMPs. Here we have determined the physicochemical properties and binding manners of Pex3p-Pex19p interaction, in terms of the affinity, the stoichiometry, and the binding site in Pex3p. The cytosolic domain of human Pex3p was overproduced, using an Escherichia coli expression system and was highly purified by two chromatography steps. Gel filtration chromatography analyses and intrinsic tryptophan fluorescence titrations revealed that a one-to-one complex is formed between monomeric Pex3p and monomeric Pex19p. The tryptophan fluorescence spectrum of Pex3p showed a large 18-nm blue shift of the maximum emission wavelength by the binding of Pex19p. This result indicates that either one or two tryptophan residues of Pex3p (Trp-104 and Trp-224) are directly involved in binding to Pex19p. We investigated the binding activities of the wild-type and tryptophan mutants of Pex3p by pull-down assays and surface plasmon resonance analyses. As a result, the wild-type and the W104A and W104F mutants showed K(D) values of 3.4 nm, 1080 nm, and 66.2 nm, respectively. The affinity differences with mutation affected their peroxisome restoring activities in pex3 ZPG208 cells. These findings suggest that the indole ring of Trp-104 directly interacts with Pex19p to facilitate the specific peroxisomal translocation of the Pex19p-PMP complexes.

Hydrophobic Regions Adjacent to Transmembrane Domains 1 and 5 Are Important for the Targeting of the 70-kDa Peroxisomal Membrane Protein

The 70-kDa peroxisomal membrane protein (PMP70) is a major component of peroxisomal membranes. Human PMP70 consists of 659 amino acid residues and has six putative transmembrane domains (TMDs). PMP70 is synthesized on cytoplasmic ribosomes and targeted posttranslationally to peroxisomes by an unidentified peroxisomal membrane protein targeting signal (mPTS). In this study, to examine the mPTS within PMP70 precisely, we expressed various COOH-terminally or NH(2)-terminally deleted constructs of PMP70 fused with green fluorescent protein (GFP) in Chinese hamster ovary cells and determined their intracellular localization by immunofluorescence. In the COOH-terminally truncated PMP70, PMP70(AA.1-144)-GFP, including TMD1 and TMD2 of PMP70, was still localized to peroxisomes. However, by further removal of TMD2, PMP70(AA.1-124)-GFP lost the targeting ability, and PMP70(TMD2)-GFP did not target to peroxisomes by itself. The substitution of TMD2 in PMP70(AA.1-144)-GFP for TMD4 or TMD6 did not affect the peroxisomal localization, suggesting that PMP70(AA.1-124) contains the mPTS and an additional TMD is required for the insertion into the peroxisomal membrane. In the NH(2)-terminal 124-amino acid region, PMP70 possesses hydrophobic segments in the region adjacent to TMD1. By the disruption of these hydrophobic motifs by the mutation of L21Q/L22Q/L23Q or I70N/L71Q, PMP70(AA.1-144)-GFP lost targeting efficiency. The NH(2)-terminally truncated PMP70, GFP-PMP70(AA.263-375), including TMD5 and TMD6, exhibited the peroxisomal localization. PMP70(AA.263-375) also possesses hydrophobic residues (Ile(307)/Leu(308)) in the region adjacent to TMD5, which were important for targeting. These results suggest that PMP70 possesses two distinct targeting signals, and hydrophobic regions adjacent to the first TMD of each region are important for targeting.

Live cell FRET microscopy: homo- and heterodimerization of two human peroxisomal ABC transporters, the adrenoleukodystrophy protein (ALDP, ABCD1) and PMP70 (ABCD3)

The adrenoleukodystrophy protein (ALDP) and the 70-kDa peroxisomal membrane protein (PMP70) are half-ATP-binding cassette (ABC) transporters in the mammalian peroxisome membrane. Mutations in the gene encoding ALDP result in a devastating neurodegenerative disorder, X-linked adrenoleukodystrophy (X-ALD) that is associated with elevated levels of very long chain fatty acids because of impaired peroxisomal beta-oxidation. The interactions of peroxisomal ABC transporters, their role in the peroxisomal membrane, and their functions in disease pathogenesis are poorly understood. Studies on ABC transporters revealed that half-transporters have to dimerize to gain functionality. So far, conflicting observations are described for ALDP. By the use of in vitro methods (yeast two-hybrid and immunoprecipitation assays) on the one hand, it was shown that ALDP can form homodimers as well as heterodimers with PMP70 and ALDR, while on the other hand, it was demonstrated that ALDP and PMP70 exclusively homodimerize. To circumvent the problems of artificial interactions due to biochemical sample preparation in vitro, we investigated protein-protein interaction of ALDP in its physiological environment by FRET microscopy in intact living cells. The statistical relevance of FRET data was determined in two different ways using probability distribution shift analysis and Kolmogorov-Smirnov statistics. We demonstrate in vivo that ALDP and PMP70 form homodimers as well as ALDP/PMP70 heterodimers where ALDP homodimers predominate. Using C-terminal deletion constructs of ALDP, we demonstrate that the last 87 C-terminal amino acids harbor the most important protein domain mediating these interactions, and that the N-terminal transmembrane region of ALDP has an additional stabilization effect on ALDP homodimers. Loss of ALDP homo- or heterodimerization is highly relevant for understanding the disease mechanisms of X-ALD.

Adrenoleukodystrophy: subcellular localization and degradation of adrenoleukodystrophy protein (ALDP/ABCD1) with naturally occurring missense mutations

Mutation in the X-chromosomal adrenoleukodystrophy gene (ALD; ABCD1) leads to X-linked adrenoleukodystrophy (X-ALD), a severe neurodegenerative disorder. The encoded adrenoleukodystrophy protein (ALDP/ABCD1) is a half-size peroxisomal ATP-binding cassette protein of 745 amino acids in humans. In this study, we chose nine arbitrary mutant human ALDP forms (R104C, G116R, Y174C, S342P, Q544R, S606P, S606L, R617H, and H667D) with naturally occurring missense mutations and examined the intracellular behavior. When expressed in X-ALD fibroblasts lacking ALDP, the expression level of mutant His-ALDPs (S606L, R617H, and H667D) was lower than that of wild type and other mutant ALDPs. Furthermore, mutant ALDP-green fluorescence proteins (S606L and H667D) stably expressed in CHO cells were not detected due to rapid degradation. Interestingly, the wild type ALDP co-expressed in these cells also disappeared. In the case of X-ALD fibroblasts from an ALD patient (R617H), the mutant ALDP was not detected in the cells, but appeared upon incubation with a proteasome inhibitor. When CHO cells expressing mutant ALDP-green fluorescence protein (H667D) were cultured in the presence of a proteasome inhibitor, both the mutant and wild type ALDP reappeared. In addition, mutant His-ALDP (Y174C), which has a mutation between transmembrane domain 2 and 3, did not exhibit peroxisomal localization by immunofluorescense study. These results suggest that mutant ALDPs, which have a mutation in the COOH-terminal half of ALDP, including S606L, R617H, and H667D, were degraded by proteasomes after dimerization. Further, the region between transmembrane domain 2 and 3 is important for the targeting of ALDP to the peroxisome.

Large-scale mapping of human protein-protein interactions by mass spectrometry

Mapping protein–protein interactions is an invaluable tool for understanding protein function. Here, we report the first large-scale study of protein–protein interactions in human cells using a mass spectrometry-based approach. The study maps protein interactions for 338 bait proteins that were selected based on known or suspected disease and functional associations. Large-scale immunoprecipitation of Flag-tagged versions of these proteins followed by LC-ESI-MS/MS analysis resulted in the identification of 24 540 potential protein interactions. False positives and redundant hits were filtered out using empirical criteria and a calculated interaction confidence score, producing a data set of 6463 interactions between 2235 distinct proteins. This data set was further cross-validated using previously published and predicted human protein interactions. In-depth mining of the data set shows that it represents a valuable source of novel protein–protein interactions with relevance to human diseases. In addition, via our preliminary analysis, we report many novel protein interactions and pathway associations.

Import of peroxisomal membrane proteins: The interplay of Pex3p- and Pex19p-mediated interactions

In contrast to the molecular mechanisms underlying import of peroxisomal matrix proteins, those involving the transport of membrane proteins remain rather elusive. At present, two targeting routes for peroxisomal membrane proteins (PMPs) have been depicted: class I PMPs are targeted from the cytoplasm directly to the peroxisome membrane, and class II PMPs are sorted indirectly to peroxisomes via the endoplasmic reticulum (ER). In addition, three peroxins--Pex3p, Pex16p, and Pex19p - have been identified as essential factors for PMP assembly in several species including humans: Pex19p is a predominantly cytoplasmic protein that shows a broad PMP-binding specificity; Pex3p serves as the membrane-anchoring site for Pex19p; and Pex16p - a protein absent in most yeasts--is thought to provide the initial scaffold for recruiting the protein import machinery required for peroxisome membrane biogenesis. Remarkably, the function of Pex16p does not appear to be conserved between different species. In addition, significant disagreement exists about whether Pex19p has a chaperone-like role in the cytosol or at the peroxisome membrane and/or functions as a cycling import receptor for newly synthesized PMPs. Here we review the recent progress made in our understanding of the role of two key players in PMP biogenesis, Pex3p and Pex19p.

The ins and outs of peroxisomes: Co-ordination of membrane transport and peroxisomal metabolism

Peroxisomes perform a range of metabolic functions which require the movement of substrates, co-substrates, cofactors and metabolites across the peroxisomal membrane. In this review, we discuss the evidence for and against specific transport systems involved in peroxisomal metabolism and how these operate to co-ordinate biochemical reactions within the peroxisome with those in other compartments of the cell.

Functional domain mapping of peroxin Pex19p: interaction with Pex3p is essential for function and translocation

The peroxin Pex19p functions in peroxisomal membrane assembly. Here we mapped functional domains of human Pex19p comprising 299 amino acids. Pex19p mutants deleted in the C-terminal CAAx farnesylation motif, the C-terminal 38 amino acid residues and the N-terminal 11 residues, maintained peroxisome-restoring activity in pex19 cells. The sequence 12-261 was essential for re-establishing peroxisome activity. Pex19p was partly localized to peroxisomes but mostly localized in the cytosol. Pex19p interacted with multiple membrane proteins, including the other two membrane biogenesis peroxins, Pex3p and Pex16p, those involved in matrix protein import such as Pex14p, Pex13p, Pex10p, and Pex26p, peroxisome morphogenesis factor Pex11pbeta, and a PMP70 peroxisome-targeting signal region at residues 1-123. In yeast two-hybrid assays, Pex10p and Pex11pbeta interacted only with full-length Pex19p. Of various truncated Pex19p variants active in translocating to peroxisomes, the mutants with the shortest sequence (residues 12-73 and 40-131) were localized to peroxisomes and competent in binding to Pex3p. Furthermore, membrane peroxins were initially discernible in a cytosolic staining pattern in pex19 cells only when co-expressed with Pex19p and were then localized to peroxisomes in a temporally differentiated manner. Pex19p probably functions as a chaperone for membrane proteins and transports them to peroxisomes by anchoring to Pex3p using residues 12-73 and 40-131.

Peroxisomal ABC transporters

Peroxisomes perform a range of different functions, dependent upon organism, tissue type, developmental stage or environmental conditions, many of which are connected with lipid metabolism. This review summarises recent research on ATP binding cassette (ABC) transporters of the peroxisomal membrane (ABC subfamily D) and their roles in plants, fungi and animals. Analysis of mutants has revealed that peroxisomal ABC transporters play key roles in specific metabolic and developmental functions in different organisms. A common function is import of substrates for beta-oxidation but much remains to be determined concerning transport substrates and mechanisms which appear to differ significantly between phyla.

Role of Pex19p in the targeting of PMP70 to peroxisome

Pex19p is a protein required for the peroxisomal membrane synthesis. The 70-kDa peroxisomal membrane protein (PMP70) is synthesized on free cytosolic ribosomes and then inserted posttranslationally into peroxisomal membranes. Pex19p has been shown to play an important role in this process. Using an in vitro translation system, we investigated the role of Pex19p as a chaperone and identified the regions of PMP70 required for the interaction with Pex19p. When PMP70 was translated in the presence of purified Pex19p, a large part of PMP70 existed as soluble form and was co-immunoprecipitated with Pex19p. However, in the absence of Pex19p, PMP70 formed aggregates during translation. To identify the regions that interact with Pex19p, various truncated PMP70 were translated in the presence of Pex19p and subjected to co-immunoprecipitation. The interaction was markedly reduced by the deletion of the NH(2)-terminal 61 amino acids or the region around TMD6. Further, we expressed these deletion constructs of PMP70 in fusion with the green fluorescent protein in CHO cells. Fusion proteins lacking these Pex19p binding sites did not display any peroxisomal localization. These results suggest that Pex19p binds to PMP70 co-translationally and keeps PMP70 as a proper conformation for the localization to peroxisome.

In vitro transport of membrane proteins to peroxisomes by shuttling receptor Pex19p

The peroxin Pex19p comprising 299 amino acids functions in peroxisomal membrane assembly. We here developed a cell-free system for transport of membrane proteins to peroxisomes. Pex19p interacts with multiple membrane peroxins, including other membrane biogenesis peroxins, Pex16p and Pex26p, involved in matrix protein import. Cell-free synthesized, 35S-labeled Pex19p was targeted to subcellular fractions containing peroxisomes from Chinese hamster ovary-K1 cells as well as peroxisomes isolated from rat liver in an ATP-dependent manner. Such translocation was also reproduced with in vitro synthesized 35S-Pex16p with two transmembrane segments and C-tail anchor-type 35S-Pex26p, upon incubation with 35S-Pex19p in the reaction mixtures containing isolated peroxisomes. The transported 35S-Pex16p and 35S-Pex26p were integrated into membranes as assessed by the sodium carbonate extraction method. Peroxisome-associated and partly Na2CO3-resistant 35S-Pex19p was released to the cytosolic fraction upon incubation in the absence of ATP, whereas 35S-Pex16p and 35S-Pex26p remained in the membranes. Furthermore, not only 35S-Pex19p but also 35S-Pex19p complexes each with 35S-Pex16p and 35S-Pex26p were bound to 35S-Pex3p in vitro. Together, these results strongly suggested that Pex19p translocates the membrane peroxins from the cytosol to peroxisomes in an ATP- and Pex3p-dependent manner and then shuttles back to the cytosol.

Function of the PEX19-binding site of human adrenoleukodystrophy protein as targeting motif in man and yeast. PMP targeting is evolutionarily conserved

We predicted in human peroxisomal membrane proteins (PMPs) the binding sites for PEX19, a key player in the topogenesis of PMPs, by virtue of an algorithm developed for yeast PMPs. The best scoring PEX19-binding site was found in the adrenoleukodystrophy protein (ALDP). The identified site was indeed bound by human PEX19 and was also recognized by the orthologous yeast PEX19 protein. Likewise, both human and yeast PEX19 bound with comparable affinities to the PEX19-binding site of the yeast PMP Pex13p. Interestingly, the identified PEX19-binding site of ALDP coincided with its previously determined targeting motif. We corroborated the requirement of the ALDP PEX19-binding site for peroxisomal targeting in human fibroblasts and showed that the minimal ALDP fragment targets correctly also in yeast, again in a PEX19-binding site-dependent manner. Furthermore, the human PEX19-binding site of ALDP proved interchangeable with that of yeast Pex13p in an in vivo targeting assay. Finally, we showed in vitro that most of the predicted binding sequences of human PMPs represent true binding sites for human PEX19, indicating that human PMPs harbor common PEX19-binding sites that do resemble those of yeast. Our data clearly revealed a role for PEX19-binding sites as PMP-targeting motifs across species, thereby demonstrating the evolutionary conservation of PMP signal sequences from yeast to man.

Analysis of Human Pex19p's Domain Structure by Pentapeptide Scanning Mutagenesis

Pex19p, a primarily cytosolic protein, is essential for the biogenesis of numerous peroxisomal membrane proteins (PMPs); however, its precise function is unclear. Pex19p might function as a PMP-specific chaperone, a cycling PMP-receptor protein, a PMP membrane insertion factor, or an association/dissociation factor of membrane-associated protein complexes. Alternatively, Pex19p might act as a multifunctional peroxin and participate in a number of these activities. Here, we have employed transposon mutagenesis to generate a library of human pex19 alleles coding for Pex19p variants containing random in-frame pentapeptide insertions. A total of 87 different variants were characterized to identify functionally important regions. These studies revealed that Pex19p has a tripartite domain structure consisting of: (i) an amino-terminal domain that binds to Pex3p and is essential for docking at the peroxisome membrane; (ii) a central domain that competes with Pex5p and Pex13p for binding to Pex14p and may play a role in the assembly of PTS-receptor docking complexes; and (iii) a carboxy-terminal domain that interacts with multiple PMPs including Pex3p, Pex11pbeta, Pex12p, Pex13p, Pex16p, and Pex26p. Whether the latter interactions constitute the chaperone or transport functions (or both), remains to be determined. Finally, our observation that Pex19p contains two distinct binding sites for Pex3p suggests that the peroxin may bind PMPs in multiple places and for multiple purposes.

Domain architecture and activity of human Pex19p, a chaperone-like protein for intracellular trafficking of peroxisomal membrane proteins

Pex19p is a peroxin involved in peroxisomal membrane biogenesis and probably functions as a chaperone and/or soluble receptor specific for cargo peroxisomal membrane proteins (PMPs). To elucidate the functional constituents of Pex19p in terms of the protein structure, we investigated its domain architecture and binding affinity toward various PMPs and peroxins. The human Pex19p cDNA was overexpressed in Escherichia coli, and a highly purified sample of the Pex19p protein was prepared. When PMP22 was synthesized by cell-free translation in the presence of Pex19p, the PMP22 bound to Pex19p was soluble, whereas PMP22 alone was insoluble. This observation shows that Pex19p plays a role in capturing PMP and maintaining its solubility. In a similar manner, Pex19p was bound to PMP70 and Pex16p as well as the Pex3p soluble fragment. Limited proteolysis analyses revealed that Pex19p consists of the C-terminal core domain flanking the flexible N-terminal region. Separation of Pex19p into its N- and C-terminal halves abolished interactions with PMP22, PMP70, and Pex16p. In contrast, the flexible N-terminal half of Pex19p was bound to the Pex3p soluble fragment, suggesting that the binding mode of Pex3p toward Pex19p differs from that of other PMPs. This idea is supported by our detection of the Pex19p-Pex3p-PMP22 ternary complex.

Targeting of the human adrenoleukodystrophy protein to the peroxisomal membrane by an internal region containing a highly conserved motif

In this study we addressed the targeting requirements of peroxisomal ABC transporters, in particular the human adrenoleukodystrophy protein. This membrane protein is defective or missing in X-linked adrenoleukodystrophy, a neurodegenerative disorder predominantly presenting in childhood. Using adrenoleukodystrophy protein deletion constructs and green fluorescent protein fusion constructs we identified the amino acid regions 1-110 and 67-164 to be sufficient for peroxisomal targeting. However, the minimal region shared by these constructs (amino acids 67-110) is not sufficient for peroxisomal targeting by itself. Additionally, the NH2-terminal 66 amino acids enhance targeting efficiency. Green fluorescent protein-labeled fragments of human peroxisomal membrane protein 69 and Saccharomyces cerevisiae Pxa1 corresponding to the amino acid 67-164 adrenoleukodystrophy protein region were also directed to the mammalian peroxisome. The required region contains a 14-amino-acid motif (71-84) conserved between the adrenoleukodystrophy protein and human peroxisomal membrane protein 69 and yeast Pxa1. Omission or truncation of this motif in the adrenoleukodystrophy protein abolished peroxisomal targeting. The single amino acid substitution L78F resulted in a significant reduction of targeting efficiency. The in-frame deletion of three amino acids (del78-80LLR) within the proposed targeting motif in two patients suffering from X-linked adrenoleukodystrophy resulted in the mislocalization of a green fluorescent protein fusion protein to nucleus, cytosol and mitochondria. Our data define the targeting region of human adrenoleukodystrophy protein containing a highly conserved 14-amino-acid motif.

PEX3 functions as a PEX19 docking factor in the import of class I peroxisomal membrane proteins

PEX19 is a chaperone and import receptor for newly synthesized, class I peroxisomal membrane proteins (PMPs). PEX19 binds these PMPs in the cytoplasm and delivers them to the peroxisome for subsequent insertion into the peroxisome membrane, indicating that there may be a PEX19 docking factor in the peroxisome membrane. Here we show that PEX3 is required for PEX19 to dock at peroxisomes, interacts specifically with the docking domain of PEX19, and is required for recruitment of the PEX19 docking domain to peroxisomes. PEX3 is also sufficient to dock PEX19 at heterologous organelles and binds PEX19 via a conserved motif that is essential for this docking activity and for PEX3 function in general. Not surprisingly, transient inhibition of PEX3 abrogates class I PMP import but has no effect on class II PMP import or peroxisomal matrix protein import. Taken together, these results suggest that PEX3 plays a selective, essential, and direct role in PMP import as a docking factor for PEX19.

Interaction of a farnesylated protein with renal type IIa Na/Pi co-transporter in response to parathyroid hormone and dietary phosphate

Treatment with PTH (parathyroid hormone) or a high-P(i) diet causes internalization of the type IIa sodium-dependent phosphate (Na/P(i) IIa) co-transporter from the apical membrane and its degradation in the lysosome. A dibasic amino acid motif (KR) in the third intracellular loop of the co-transporter is essential for protein's PTH-induced retrieval. To elucidate the mechanism of internalization of Na/P(i) IIa, we identified the interacting protein for the endocytic motif by yeast two-hybrid screening. We found a strong interaction of the Na/P(i) IIa co-transporter with a small protein known as the PEX19 (human peroxisomal farnesylated protein; PxF, Pex19p). PEX19 can bind to the KR motif, but not to a mutant with this motif replaced with NI residues. PEX19 is highly expressed in mouse and rat kidney. Western blot analysis indicates that PEX19 is located in the cytosolic and brush-border membrane fractions (microvilli and the subapical component). Overexpression of PEX19 stimulated the endocytosis of the Na/P(i) IIa co-transporter in opossum kidney cells in the absence of PTH. In conclusion, the present study indicates that PEX19 may be actively involved in controlling the internalization and trafficking of the Na/P(i) IIa co-transporter.

Potential role for Pex19p in assembly of PTS-receptor docking complexes

Human Pex19p binds a broad spectrum of peroxisomal membrane proteins (PMPs). It has been proposed that this peroxin may: (i) act as a cycling PMP receptor protein, (ii) facilitate the insertion of newly synthesized PMPs into the peroxisomal membrane, or (iii) function as a chaperone to associate and/or dissociate complexes comprising integral PMPs already in the peroxisomal membrane. We previously demonstrated that human Pex19p binds peroxisomal integral membrane proteins at regions distinct from their sorting sequences. Here we demonstrate that a mutant of Pex13p that fails to bind to Pex19p nevertheless targets to and integrates into the peroxisomal membrane. In addition, through in vitro biochemical analysis, we show that Pex19p competes with Pex5p and Pex13p for binding to Pex14p, supporting a role for this peroxin in regulating assembly/disassembly of membrane-associated protein complexes. To further examine the molecular mechanism underlying this competition, six evolutionarily conserved amino acids in the Pex5p/Pex13p/Pex19p binding domain of Pex14p were subjected to site-directed mutagenesis and the corresponding mutants functionally analyzed. Our results indicate that the physically overlapping binding sites of Pex14p for Pex5p, Pex13p, and Pex19p are functionally distinct, suggesting that competition occurs through induction of structural changes, rather than through direct competition. Importantly, we also found that amino acid substitutions resulting in a strongly reduced binding affinity for Pex13p affect the peroxisomal localization of Pex14p.

Interaction of the MAGUK family member Acvrinp1 and the cytoplasmic domain of the Notch ligand Delta1

The evolutionarily conserved Notch signal transduction pathway regulates cell fate and cellular differentiation in various tissues and has essential functions in embryonic patterning and tumorigenesis. Cell-cell signaling by the Notch pathway is mediated by the interaction of the transmembrane receptor Notch with its ligands Delta or Jagged presented on adjacent cells. Whereas signal transduction to Notch expressing cells has been described, it is unclear whether Delta-dependent signaling may exist within the Delta-expressing cell. Here, we report on the identification of Acvrinp1, a MAGUK family member, interacting with the intracellular domain of Delta1 (Dll1). We confirmed the interaction between Dll1 and Acvrinp1 by pull-down experiments in vitro and in a mammalian two-hybrid system in vivo. We delimited the fourth PDZ domain of Acvrinp1 and the PDZ-binding domain of Dll1 as major interacting domains. In situ expression analyses in mouse embryos revealed that Dll1 and Acvrinp1 show partly overlapping but distinct expression patterns, for example, in the central nervous system and the vibrissae buds. Further, we found that expression of Acvrinp1 is altered in Dll1 loss-of-function mouse embryos.

Overexpression of a human and a fungal ABC transporter similarly suppresses the differentiation defects of a fungal peroxisomal mutant but introduces pleiotropic cellular effects

Among the peroxisome membrane proteins, some are required for peroxisome biogenesis (e.g. PEX2) while others are not, e.g. ABC (ATP-binding cassette) transporters. Unexpectedly, overproduction of the peroxisomal ABC transporter PMP70 was found to be able to restore peroxisome biogenesis in mammalian pex2 mutant cell lines. In the filamentous fungus Podospora anserina, pex2 mutations not only impair peroxisome biogenesis but also cause a precise cell differentiation defect. Here, we show that both defects are partially suppressed by expression of the human cDNA encoding PMP70. In addition, PMP70 expression causes new developmental defects, different from those induced by pex2 mutations. We also show that overexpression of the P. anserina pABC1 gene, which encodes a peroxisomal ABC transporter, leads to similar effects. Taken together, our results demonstrate that: (i) the genetic relationship between PEX2 and PMP70, initially observed in mammals, has been conserved through evolution; (ii) the cell differentiation defect observed in the P. anserina pex2 mutants is indeed linked to impairment in peroxisome biogenesis; and (iii) unexpected detrimental cellular defects result from overproduction of peroxisomal ABC transporters.

Functional characterization of the adrenoleukodystrophy protein (ALDP) and disease pathogenesis

X-linked adrenoleukodystrophy (X-ALD) is the most common peroxisomal disorder characterized by abnormal accumulation of saturated very long chain fatty acids in tissues and body fluids with predominance in brain white matter and adrenal cortex. The clinical phenotype is highly variable ranging from the severe childhood cerebral form to asymptomatic persons. The responsible ALD gene encodes the adrenoleukodystrophy protein (ALDP), a peroxisomal integral membrane protein that is a member of the ATP-binding cassette (ABC) transporter protein family. The patient gene mutations are heterogeneously distributed over the functional domains of ALDP. The extreme variability in clinical phenotype, even within one affected family, indicates that besides the ALD gene mutations other factors strongly influence the clinical phenotype. To understand the cell biology and function of mammalian peroxisomal ABC transporters and to determine their role in the pathogenesis of X-ALD we developed a system for expressing functional ABC protein domains in fusion with the maltose binding protein. Wild type and mutant fusion proteins of the nucleotide-binding fold were overexpressed, purified, and characterized by photoaffinity labeling with 8-azido ATP or 8-azido GTP and a coupled ATP regenerating enzyme assay for ATPase activity. Our studies provide evidence that peroxisomal ABC transporters utilize ATP to become a functional transporter and that ALD gene mutations alter peroxisomal transport function. The established disease model will be used further to study the influence of possible disease modifier proteins on ALDP function.

Farnesyl transferase inhibitors as anticancer agents

Protein farnesylation catalysed by the enzyme farnesyl protein transferase involves the addition of a 15-carbon farnesyl group to conserved amino acid residues at the carboxyl terminus of certain proteins. Protein substrates of farnesyl transferase include several G-proteins, which are critical intermediates of cell signalling and cytoskeletal organisation such as Ras, Rho, PxF and lamins A and B. Activated Ras proteins trigger a cascade of phosphorylation events through sequential activation of the PI3 kinase/AKT pathway, which is critical for cell survival, and the Raf/Mek/Erk kinase pathway that has been implicated in cell proliferation. Ras mutations which encode for constitutively activated proteins are found in 30% of human cancers. Because farnesylation of Ras is required for its transforming and proliferative activity, the farnesyl protein transferase inhibitors were designed as anticancer agents to abrogate Ras function. However, current evidence suggests that the anticancer activity of the farnesyl transferase inhibitors may not be simply due to Ras inhibition. This review will discuss available clinical data on three of these agents that are currently undergoing clinical trials.

Two splice variants of human PEX19 exhibit distinct functions in peroxisomal assembly

PEX19 has been shown to play a central role in the early steps of peroxisomal membrane synthesis. Computational database analysis of the PEX19 sequence revealed three different conserved domains: D1 (aa 1--87), D2 (aa 88--272), and D3 (aa 273--299). However, these domains have not yet been linked to specific biological functions. We elected to functionally characterize the proteins derived from two naturally occurring PEX19 splice variants: PEX19DeltaE2 lacking the N-terminal domain D1 and PEX19DeltaE8 lacking the domain D3. Both interact with peroxisomal ABC transporters (ALDP, ALDRP, PMP70) and with full-length PEX3 as shown by in vitro protein interaction studies. PEX19DeltaE8 also interacts with a PEX3 protein lacking the peroxisomal targeting region located at the N-terminus (Delta66aaPEX3), whereas PEX19DeltaE2 does not. Functional complementation studies in PEX19-deficient human fibroblasts revealed that transfection of PEX19DeltaE8-cDNA leads to restoration of both peroxisomal membranes and of functional peroxisomes, whereas transfection of PEX19DeltaE2-cDNA does not restore peroxisomal biogenesis. Human PEX19 is partly farnesylated in vitro and in vivo. The farnesylation consensus motif CLIM is located in the PEX19 domain D3. The finding that the protein derived from the splice variant lacking D3 is able to interact with several peroxisomal membrane proteins and to restore peroxisomal biogenesis challenges the previous assumption that farnesylation of PEX19 is essential for its biological functionality. The data presented demonstrate a considerable functional diversity of the proteins encoded by two PEX19 splice variants and thereby provide first experimental evidence for specific biological functions of the different predicted domains of the PEX19 protein.

Peroxisome biogenesis

Fifteen years ago, we had a model of peroxisome biogenesis that involved growth and division of preexisting peroxisomes. Today, thanks to genetically tractable model organisms and Chinese hamster ovary cells, 23 PEX genes have been cloned that encode the machinery ("peroxins") required to assemble the organelle. Membrane assembly and maintenance requires three of these (peroxins 3, 16, and 19) and may occur without the import of the matrix (lumen) enzymes. Matrix protein import follows a branched pathway of soluble recycling receptors, with one branch for each class of peroxisome targeting sequence (two are well characterized), and a common trunk for all. At least one of these receptors, Pex5p, enters and exits peroxisomes as it functions. Proliferation of the organelle is regulated by Pex11p. Peroxisome biogenesis is remarkably conserved among eukaryotes. A group of fatal, inherited neuropathologies are recognized as peroxisome biogenesis diseases; the responsible genes are orthologs of yeast or Chinese hamster ovary peroxins. Future studies must address the mechanism by which folded, oligomeric enzymes enter the organelle, how the peroxisome divides, and how it segregates at cell division. Most pex mutants contain largely empty membrane "ghosts" of peroxisomes; a few mutants apparently lacking peroxisomes entirely have led some to propose the de novo formation of the organelle. However, there is evidence for residual peroxisome membrane vesicles ("protoperoxisomes") in some of these, and the preponderance of data supports the continuity of the peroxisome compartment in space and time and between generations of cells.

Targeting elements in the amino-terminal part direct the human 70-kDa peroxisomal integral membrane protein (PMP70) to peroxisomes

Peroxisomes are multipurpose organelles present in nearly all eukaryotic cells. All peroxisomale matrix and membrane proteins are synthesized in the cytoplasm. While a clear picture of the basic targeting mechanisms for peroxisomal matrix proteins has emerged over the past years, the targeting processes for peroxisomal membrane proteins are poorly understood. The 70-kDa peroxisomal integral membrane protein (PMP70) is one of the proteins located in the human peroxisome membrane. PMP70 belongs to the family of ATP-binding cassette (ABC) transporter proteins. It consists of six transmembrane domains and an ATP-binding fold in the cytosol. Here we describe that efficient peroxisomal targeting of human PMP70 depends on three targeting elements in the amino-terminal protein region, namely amino acids 61 to 80 located in the cytosol as well as the first and second transmembrane domains. Furthermore, peroxin 19 (PEX19) interactions are not required for targeting human PMP70 to peroxisomes. PEX19 does not specifically bind to the targeting elements of human PMP70.

Human pex19p binds peroxisomal integral membrane proteins at regions distinct from their sorting sequences

The molecular machinery underlying peroxisomal membrane biogenesis is not well understood. The observation that cells deficient in the peroxins Pex3p, Pex16p, and Pex19p lack peroxisomal membrane structures suggests that these molecules are involved in the initial stages of peroxisomal membrane formation. Pex19p, a predominantly cytosolic protein that can be farnesylated, binds multiple peroxisomal integral membrane proteins, and it has been suggested that it functions as a soluble receptor for the targeting of peroxisomal membrane proteins (PMPs) to the peroxisome. An alternative view proposes that Pex19p functions as a chaperone at the peroxisomal membrane. Here, we show that the peroxisomal sorting determinants and the Pex19p-binding domains of a number of PMPs are distinct entities. In addition, we extend the list of peroxins with which human Pex19p interacts to include the PMP Pex16p and show that Pex19p's CaaX prenylation motif is an important determinant in the affinity of Pex19p for Pex10p, Pex11pbeta, Pex12p, and Pex13p.

Discrete targeting signals direct Pmp47 to oleate-induced peroxisomes in Saccharomyces cerevisiae

Pmp47 is a peroxisomal membrane protein consisting of six transmembrane domains (TMDs). We previously showed that the second matrix loop containing a basic cluster of amino acids is important for peroxisomal targeting, and similar basic targeting motifs have been found in other peroxisomal membrane proteins. However, this basic cluster by itself targets to peroxisomes very poorly. We have developed a sensitive quantitative localization assay based on the targeting of Pmp47-GFP fusion proteins to identify the important elements of the basic cluster and to search for other targeting information on Pmp47. Our data suggest that side-chain structure and position as well as charge are important for targeting by the basic cluster. Analysis of other regions of Pmp47 indicates that all TMDs except TMD2 can be eliminated or substituted without significant loss of targeting. TMD2 plus an adjacent cytoplasmic-oriented sequence is crucial for targeting. Cytoplasmic-oriented sequences from two other peroxisomal membrane proteins, ScPex15p and ScPmp22, could partially substitute for the analogous sequence in Pmp47. Targeting with high fidelity to oleate-induced peroxisomes required the following elements: the cytoplasmic-oriented sequence and TMD2, a short matrix loop containing a basic cluster, and a membrane-anchoring TMD.

Clinical, biochemical and genetic aspects and neuronal migration in peroxisome biogenesis disorders

Peroxisome biogenesis disorders (PBDs) are severe autosomal recessive neurological diseases caused by a defect of peroxisomal assembly factors. Zellweger syndrome, the most severe phenotype, is characterized by hypotonia, psychomotor retardation and neuronal migration disorder. Neonatal adrenoleukodystrophy and infantile Refsum disease are milder phenotypes of this disease. Thirteen complementation groups have been established since the genetic heterogeneity of PBDs was elucidated in 1988. Eleven genes for PBDs have been identified either by a functional complementation cloning or by EST homology searches. In 1992, the first gene for PBDs, PEX2, was identified. It encodes peroxisomal integral membrane protein with a RING finger domain. PEX5 and PEX7 are the genes for peroxisomal targeting signal (PTS)-1 and -2 receptors, respectively. PEX3, PEX16 and PEX19 are considered to be required for the early stage of peroxisome biogenesis. PEX13 protein has an SH3 docking site that binds to the PTS-1 receptor. PEX1 and PEX6 encode ABC protein, and PEX10 and PEX12 also encode integral membrane protein, with RING finger. Temperature-sensitivity, whereby peroxisomal biogenesis and metabolic dysfunctions are restored at 30 degrees C in cells from mild phenotypes, is a useful event for predicting the clinical severity and for elucidation of peroxisome biogenesis. Investigations using knockout mice are expected to facilitate understanding of migration disorders.

Defective peroxisome membrane synthesis due to mutations in human PEX3 causes Zellweger syndrome, complementation group G

Zellweger cerebro-hepato-renal syndrome is a severe congenital disorder associated with defective peroxisomal biogenesis. At least 23 PEX genes have been reported to be essential for peroxisome biogenesis in various species, indicating the complexity of peroxisomal assembly. Cells from patients with peroxisomal biogenesis disorders have previously been shown to segregate into >/=12 complementation groups. Two patients assigned to complementation group G who had not been linked previously to a specific gene defect were confirmed as displaying a cellular phenotype characterized by a lack of even residual peroxisomal membrane structures. Here we demonstrate that this complementation group is associated with mutations in the PEX3 gene, encoding an integral peroxisomal membrane protein. Homozygous PEX3 mutations, each leading to C-terminal truncation of PEX3, were identified in the two patients, who both suffered from a severe Zellweger syndrome phenotype. One of the mutations involved a single-nucleotide insertion in exon 7, whereas the other was a single-nucleotide substitution eight nucleotides from the normal splice site in the 3' acceptor site of intron 10. Expression of wild-type PEX3 in the mutant cell lines restored peroxisomal biogenesis, whereas transfection of mutated PEX3 cDNA did not. This confirmed that the causative gene had been identified. The observation of peroxisomal formation in the absence of morphologically recognizable peroxisomal membranes challenges the theory that peroxisomes arise exclusively by growth and division from preexisting peroxisomes and establishes PEX3 as a key factor in early human peroxisome synthesis.