The PAL1 gene product is a peroxisomal ATP-binding cassette transporter in the yeast Saccharomyces cerevisiae

Peroxisomal ATP Uptake Is Provided by Two Adenine Nucleotide Transporters and the ABCD Transporters

Peroxisomes are essential organelles involved in various metabolic processes, including fatty acid β-oxidation. Their metabolic functions require a controlled exchange of metabolites and co-factors, including ATP, across the peroxisomal membrane. We investigated which proteins are involved in the peroxisomal uptake of ATP in the yeast Saccharomyces cerevisiae. Using wild-type and targeted deletion strains, we measured ATP-dependent peroxisomal octanoate β-oxidation, intra-peroxisomal ATP levels employing peroxisome-targeted ATP-sensing reporter proteins, and ATP uptake in proteoliposomes prepared from purified peroxisomes. We show that intra-peroxisomal ATP levels are maintained by different peroxisomal membrane proteins each with different modes of action: 1) the previously reported Ant1p protein, which catalyzes the exchange of ATP for AMP or ADP, 2) the ABC transporter protein complex Pxa1p/Pxa2p, which mediates both uni-directional acyl-CoA and ATP uptake, and 3) the mitochondrial Aac2p protein, which catalyzes ATP/ADP exchange and has a dual localization in both mitochondria and peroxisomes. Our results provide compelling evidence for a complementary system for the uptake of ATP in peroxisomes.

The Saccharomyces cerevisiae ABC subfamily D transporter Pxa1/Pxa2p co-imports CoASH into the peroxisome

ATP-binding cassette (ABC) subfamily D transporters are important for the uptake of fatty acids and other beta-oxidation substrates into peroxisomes. Genetic and biochemical evidence indicates that the transporters accept fatty acyl-coenzyme A that is cleaved during the transport cycle and then re-esterified in the peroxisomal lumen. However, it is not known whether free coenzyme A (CoA) is released inside or outside the peroxisome. Here we have used Saccharomyces cerevisiae and isolated peroxisomes to demonstrate that free CoA is released in the peroxisomal lumen. Thus, ABC subfamily D transporter provide an import pathway for free CoA that controls peroxisomal CoA homeostasis and tunes metabolism according to the cell's demands.

Peroxisomal Cofactor Transport

Peroxisomes are eukaryotic organelles that are essential for growth and development. They are highly metabolically active and house many biochemical reactions, including lipid metabolism and synthesis of signaling molecules. Most of these metabolic pathways are shared with other compartments, such as Endoplasmic reticulum (ER), mitochondria, and plastids. Peroxisomes, in common with all other cellular organelles are dependent on a wide range of cofactors, such as adenosine 5′-triphosphate (ATP), Coenzyme A (CoA), and nicotinamide adenine dinucleotide (NAD). The availability of the peroxisomal cofactor pool controls peroxisome function. The levels of these cofactors available for peroxisomal metabolism is determined by the balance between synthesis, import, export, binding, and degradation. Since the final steps of cofactor synthesis are thought to be located in the cytosol, cofactors must be imported into peroxisomes. This review gives an overview about our current knowledge of the permeability of the peroxisomal membrane with the focus on ATP, CoA, and NAD. Several members of the mitochondrial carrier family are located in peroxisomes, catalyzing the transfer of these organic cofactors across the peroxisomal membrane. Most of the functions of these peroxisomal cofactor transporters are known from studies in yeast, humans, and plants. Parallels and differences between the transporters in the different organisms are discussed here.

Localization of NPFxD motif-containing proteins in Aspergillus nidulans

During growth, filamentous fungi produce polarized cells called hyphae. It is generally presumed that polarization of hyphae is dependent upon secretion through the Spitzenkörper, as well as a mechanism called apical recycling, which maintains a balance between the tightly coupled processes of endocytosis and exocytosis. Endocytosis predominates in an annular domain called the sub-apical endocytic collar, which is located in the region of plasma membrane 1-5 μm distal to the Spitzenkörper. It has previously been proposed that one function of the sub-apical endocytic collar is to maintain the apical localization of polarization proteins. These proteins mark areas of polarization at the apices of hyphae. However, as hyphae grow, these proteins are displaced along the membrane and some must then be removed at the sub-apical endocytic collar in order to maintain the hyphoid shape. While endocytosis is fairly well characterized in yeast, comparatively little is known about the process in filamentous fungi. Here, a bioinformatics approach was utilized to identify 39 Aspergillus nidulans proteins that are predicted to be cargo of endocytosis based on the presence of an NPFxD peptide motif. This motif is a necessary endocytic signal sequence first established in Saccharomyces cerevisiae, where it marks proteins for endocytosis through an interaction with the adapter protein Sla1p. It is hypothesized that some proteins that contain this NPFxD peptide sequence in A. nidulans will be potential targets for endocytosis, and therefore will localize either to the endocytic collar or to more proximal polarized regions of the cell, e.g. the apical dome or the Spitzenkörper. To test this, a subset of the motif-containing proteins in A. nidulans was tagged with GFP and the dynamic localization was evaluated. The documented localization patterns support the hypothesis that the motif marks proteins for localization to the polarized cell apex in growing hyphae.

The fate of linoleic acid on Saccharomyces cerevisiae metabolism under aerobic and anaerobic conditions

INTRODUCTION

Saccharomyces cerevisiae has been widely used for fermenting food and beverages for over thousands years. Its metabolism together with the substrate composition play an important role in determining the characteristics of the final fermented products. We previously showed that the polyunsaturated fatty acid, linoleic acid, which is present in the grape juice at trace levels, significantly affected the development of aroma compounds of the wines. However, the effect of linoleic acid on the overall cell metabolism of S. cerevisiae is still not clear. Therefore, we aimed to unlock the metabolic response of S. cerevisiae to linoleic acid using metabolomics and isotope labelling experiments.

METHODS

We cultured the cells on a minimal mineral medium supplementing them with linoleic acid isomers and 13C-linoleic acid. Both intracellular and extracellular metabolite profiles were determined using gas chromatography coupled to mass spectrometry (GC-MS) to investigate which S. cerevisiae pathways were affected by linoleic acid supplementation.

RESULTS

The utilisation of linoleic acid by S. cerevisiae had a significant impact on the primary carbon metabolism increasing the glucose consumption and the ethanol production under anaerobic condition. The energetic state of the cell was, therefore, affected and the glycolytic pathway, the TCA cycle and the amino acid production were up-regulated. We also observed that linoleic acid was transported into the cell and converted into other fatty acids affecting their profile even under anaerobic condition.

CONCLUSION

Our data clearly shows that linoleic acid supplementation in growth medium increased glucose consumption and ethanol production by S. cerevisiae under anaerobic condition. We also suggest that S. cerevisiae might be able to perform an alternative anaerobic pathway to β-oxidation, which has not been reported yet.

Peroxisomal ABC transporters: functions and mechanism

Peroxisomes are arguably the most biochemically versatile of all eukaryotic organelles. Their metabolic functions vary between different organisms, between different tissue types of the same organism and even between different developmental stages or in response to changed environmental conditions. New functions for peroxisomes are still being discovered and their importance is underscored by the severe phenotypes that can arise as a result of peroxisome dysfunction. The β-oxidation pathway is central to peroxisomal metabolism, but the substrates processed are very diverse, reflecting the diversity of peroxisomes across species. Substrates for β-oxidation enter peroxisomes via ATP-binding cassette (ABC) transporters of subfamily D; (ABCD) and are activated by specific acyl CoA synthetases for further metabolism. Humans have three peroxisomal ABCD family members, which are half transporters that homodimerize and have distinct but partially overlapping substrate specificity; Saccharomyces cerevisiae has two half transporters that heterodimerize and plants have a single peroxisomal ABC transporter that is a fused heterodimer and which appears to be the single entry point into peroxisomes for a very wide variety of β-oxidation substrates. Our studies suggest that the Arabidopsis peroxisomal ABC transporter AtABCD1 accepts acyl CoA substrates, cleaves them before or during transport followed by reactivation by peroxisomal synthetases. We propose that this is a general mechanism to provide specificity to this class of transporters and by which amphipathic compounds are moved across peroxisome membranes.

Involvement of acyl-CoA synthetase genes in n-alkane assimilation and fatty acid utilization in yeast Yarrowia lipolytica

Here, we investigated the roles of YAL1 (FAA1) and FAT1 encoding acyl-CoA synthetases (ACSs) and three additional orthologs of ACS genes FAT2-FAT4 of the yeast Yarrowia lipolytica in the assimilation or utilization of n-alkanes and fatty acids. ACS deletion mutants were generated to characterize their function. The FAT1 deletion mutant exhibited decreased growth on n-alkanes of 10-18 carbons, whereas the FAA1 mutant showed growth reduction on n-alkane of 16 carbons. However, FAT2-FAT4 deletion mutants did not show any growth defects, suggesting that FAT1 and FAA1 are involved in the activation of fatty acids produced during the metabolism of n-alkanes. In contrast, deletions of FAA1 and FAT1-FAT4 conferred no defect in growth on fatty acids. The wild-type strain grew in the presence of cerulenin, an inhibitor of fatty acid synthesis, by utilizing exogenously added fatty acid or fatty acid derived from n-alkane when oleic acid or n-alkane of 18 carbons was supplemented. However, the FAA1 deletion mutant did not grow, indicating a critical role for FAA1 in the utilization of fatty acids. Fluorescent microscopic observation and biochemical analyses suggested that Fat1p is present in the peroxisome and Faa1p is localized in the cytosol and to membranes.

Involvement of the carboxyl-terminal region of the yeast peroxisomal half ABC transporter Pxa2p in its interaction with Pxa1p and in transporter function

BACKGROUND

The peroxisome is a single membrane-bound organelle in eukaryotic cells involved in lipid metabolism, including β-oxidation of fatty acids. The human genetic disorder X-linked adrenoleukodystrophy (X-ALD) is caused by mutations in the ABCD1 gene (encoding ALDP, a peroxisomal half ATP-binding cassette [ABC] transporter). This disease is characterized by defective peroxisomal β-oxidation and a large accumulation of very long-chain fatty acids in brain white matter, adrenal cortex, and testis. ALDP forms a homodimer proposed to be the functional transporter, whereas the peroxisomal transporter in yeast is a heterodimer comprising two half ABC transporters, Pxa1p and Pxa2p, both orthologs of human ALDP. While the carboxyl-terminal domain of ALDP is engaged in dimerization, it remains unknown whether the same region is involved in the interaction between Pxa1p and Pxa2p.

METHODS/PRINCIPAL FINDINGS

Using a yeast two-hybrid assay, we found that the carboxyl-terminal region (CT) of Pxa2p, but not of Pxa1p, is required for their interaction. Further analysis indicated that the central part of the CT (designated CT2) of Pxa2p was indispensable for its interaction with the carboxyl terminally truncated Pxa1_NBD. An interaction between the CT of Pxa2p and Pxa1_NBD was not detected, but could be identified in the presence of Pxa2_NBD-CT1. A single mutation of two conserved residues (aligned with X-ALD-associated mutations at the same positions in ALDP) in the CT2 of the Pxa2_NBD-CT protein impaired its interaction with Pxa1_NBD or Pxa1_NBD-CT, resulting in a mutant protein that exhibited a proteinase K digestion profile different from that of the wild-type protein. Functional analysis of these mutant proteins on oleate plates indicated that they were defective in transporter function.

CONCLUSIONS/SIGNIFICANCE

The CT of Pxa2p is involved in its interaction with Pxa1p and in transporter function. This concept may be applied to human ALDP studies, helping to establish the pathological mechanism for CT-related X-ALD disease.

Yeast ABC proteins involved in multidrug resistance

Pleiotropic drug resistance is a complex phenomenon that involves many proteins that together create a network. One of the common mechanisms of multidrug resistance in eukaryotic cells is the active efflux of a broad range of xenobiotics through ATP-binding cassette (ABC) transporters. Saccharomyces cerevisiae is often used as a model to study such activity because of the functional and structural similarities of its ABC transporters to mammalian ones. Numerous ABC transporters are found in humans and some are associated with the resistance of tumors to chemotherapeutics. Efflux pump modulators that change the activity of ABC proteins are the most promising candidate drugs to overcome such resistance. These modulators can be chemically synthesized or isolated from natural sources (e.g., plant alkaloids) and might also be used in the treatment of fungal infections. There are several generations of synthetic modulators that differ in specificity, toxicity and effectiveness, and are often used for other clinical effects.

Peroxisomal fatty acid uptake mechanism in Saccharomyces cerevisiae

Peroxisomes play a major role in human cellular lipid metabolism, including fatty acid β-oxidation. The most frequent peroxisomal disorder is X-linked adrenoleukodystrophy, which is caused by mutations in ABCD1. The biochemical hallmark of X-linked adrenoleukodystrophy is the accumulation of very long chain fatty acids (VLCFAs) due to impaired peroxisomal β-oxidation. Although this suggests a role of ABCD1 in VLCFA import into peroxisomes, no direct experimental evidence is available to substantiate this. To unravel the mechanism of peroxisomal VLCFA transport, we use Saccharomyces cerevisiae as a model organism. Here we provide evidence that in this organism very long chain acyl-CoA esters are hydrolyzed by the Pxa1p-Pxa2p complex prior to the actual transport of their fatty acid moiety into the peroxisomes with the CoA presumably being released into the cytoplasm. The Pxa1p-Pxa2p complex functionally interacts with the acyl-CoA synthetases Faa2p and/or Fat1p on the inner surface of the peroxisomal membrane for subsequent re-esterification of the VLCFAs. Importantly, the Pxa1p-Pxa2p complex shares this molecular mechanism with HsABCD1 and HsABCD2.

ABC proteins in yeast and fungal pathogens

All fungal genomes harbour numerous ABC (ATP-binding cassette) proteins located in various cellular compartments such as the plasma membrane, vacuoles, peroxisomes and mitochondria. Most of them have initially been discovered through their ability to confer resistance to a multitude of drugs, a phenomenon called PDR (pleiotropic drug resistance) or MDR (multidrug resistance). Studying the mechanisms underlying PDR/MDR in yeast is of importance in two ways: first, ABC proteins can confer drug resistance on pathogenic fungi such as Candida spp., Aspergillus spp. or Cryptococcus neoformans; secondly, the well-established genetic, biochemical and cell biological tractability of Saccharomyces cerevisiae makes it an ideal tool to study basic mechanisms of drug transport by ABC proteins. In the past, knowledge from yeast has complemented work on human ABC transporters involved in anticancer drug resistance or genetic diseases. Interestingly, increasing evidence available from yeast and other organisms suggests that ABC proteins play a physiological role in membrane homoeostasis and lipid distribution, although this is being intensely debated in the literature.

An overview of lipid metabolism in yeasts and its impact on biotechnological processes

High energy prices, depletion of crude oil supplies, and price imbalance created by the increasing demand of plant oils or animal fat for biodiesel and specific lipid derivatives such as lubricants, adhesives, and plastics have given rise to heated debates on land-use practices and to environmental concerns about oil production strategies. However, commercialization of microbial oils with similar composition and energy value to plant and animal oils could have many advantages, such as being non-competitive with food, having shorter process cycle and being independent of season and climate factors. This review focuses on the ongoing research on different oleaginous yeasts producing high added value lipids and on the prospects of such microbial oils to be used in different biotechnological processes and applications. It covers the basic biochemical mechanisms of lipid synthesis and accumulation in these organisms, along with the latest insights on the metabolic processes involved. The key elements of lipid accumulation, the mechanisms suspected to confer the oleaginous character of the cell, and the potential metabolic routes enhancing lipid production are also extensively discussed.

The Arabidopsis peroxisomal ABC transporter, comatose, complements the Saccharomyces cerevisiae pxa1 pxa2Delta mutant for metabolism of long-chain fatty acids and exhibits fatty acyl-CoA-stimulated ATPase activity

The Arabidopsis ABC transporter Comatose (CTS; AtABCD1) is required for uptake into the peroxisome of a wide range of substrates for β-oxidation, but it is uncertain whether CTS itself is the transporter or if the transported substrates are free acids or CoA esters. To establish a system for its biochemical analysis, CTS was expressed in Saccharomyces cerevisiae. The plant protein was correctly targeted to yeast peroxisomes, was assembled into the membrane with its nucleotide binding domains in the cytosol, and exhibited basal ATPase activity that was sensitive to aluminum fluoride and abrogated by mutation of a conserved Walker A motif lysine residue. The yeast pxa1 pxa2Δ mutant lacks the homologous peroxisomal ABC transporter and is unable to grow on oleic acid. Consistent with its exhibiting a function in yeast akin to that in the plant, CTS rescued the oleate growth phenotype of the pxa1 pxa2Δ mutant, and restored β-oxidation of fatty acids with a range of chain lengths and varying degrees of desaturation. When expressed in yeast peroxisomal membranes, the basal ATPase activity of CTS could be stimulated by fatty acyl-CoAs but not by fatty acids. The implications of these findings for the function and substrate specificity of CTS are discussed.

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.

Biochemistry of mammalian peroxisomes revisited

In this review, we describe the current state of knowledge about the biochemistry of mammalian peroxisomes, especially human peroxisomes. The identification and characterization of yeast mutants defective either in the biogenesis of peroxisomes or in one of its metabolic functions, notably fatty acid beta-oxidation, combined with the recognition of a group of genetic diseases in man, wherein these processes are also defective, have provided new insights in all aspects of peroxisomes. As a result of these and other studies, the indispensable role of peroxisomes in multiple metabolic pathways has been clarified, and many of the enzymes involved in these pathways have been characterized, purified, and cloned. One aspect of peroxisomes, which has remained ill defined, is the transport of metabolites across the peroxisomal membrane. Although it is clear that mammalian peroxisomes under in vivo conditions are closed structures, which require the active presence of metabolite transporter proteins, much remains to be learned about the permeability properties of mammalian peroxisomes and the role of the four half ATP-binding cassette (ABC) transporters therein.

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.

The peroxisomal ABC transporter family

This review describes the current state of knowledge about the ABCD family of peroxisomal half adenosine-triphosphate-binding cassette (ABC) transporters. ABCDs are predicted to be present in a variety of eukaryotic organisms, although at present, only ABCDs in the yeast Saccharomyces cerevisiae, the plant Arabidopsis thaliana, and different mammalian species have been identified and characterized to any significant extent. The functional role of none of these ABCDs has been established definitively and awaits successful reconstitution of ABCDs, either as homo- or heterodimers into liposomes, followed by transport studies. Data obtained in S. cerevisiae suggest that the two ABCDs, which have been identified in this organism, form a heterodimer, which actually transports acyl coenzyme A esters across the peroxisomal membrane. In mammals, four ABCDs have been identified, of which one [adrenoleukodystrophy protein (ALDP)] has been implicated in the transport of the coenzyme A esters of very-long-chain fatty acids. Mutations in the gene (ABCD1) encoding ALDP are the cause of a severe X-linked disease, called X-linked adrenoleukodystrophy. The availability of mutant mice in which Abcd1, Abcd2, or Abcd3 have been disrupted will help to resolve the true role of the peroxisomal half-ABC transporters.

Peroxisomal disorders: the single peroxisomal enzyme deficiencies

Peroxisomal disorders are a group of inherited diseases in man in which either peroxisome biogenesis or one or more peroxisomal functions are impaired. The peroxisomal disorders identified to date are usually classified in two groups including: (1) the disorders of peroxisome biogenesis, and (2) the single peroxisomal enzyme deficiencies. This review is focused on the second group of disorders, which currently includes ten different diseases in which the mutant gene affects a protein involved in one of the following peroxisomal functions: (1) ether phospholipid (plasmalogen) biosynthesis; (2) fatty acid beta-oxidation; (3) peroxisomal alpha-oxidation; (4) glyoxylate detoxification, and (5) H2O2 metabolism.

X-linked adrenoleukodystrophy: clinical, biochemical and pathogenetic aspects

X-linked adrenoleukodystrophy (X-ALD) is a clinically heterogeneous disorder ranging from the severe childhood cerebral form to asymptomatic persons. The overall incidence is 1:16,800 including hemizygotes as well as heterozygotes. The principal molecular defect is due to inborn mutations in the ABCD1 gene encoding the adrenoleukodystrophy protein (ALDP), a transporter in the peroxisome membrane. ALDP is involved in the transport of substrates from the cytoplasm into the peroxisomal lumen. ALDP defects lead to characteristic accumulation of saturated very long-chain fatty acids, the diagnostic disease marker. The pathogenesis is unclear. Different molecular mechanisms seem to induce inflammatory demyelination, neurodegeneration and adrenocortical insufficiency involving the primary ABCD1 defect, environmental factors and modifier genes. Important information has been derived from the X-ALD mouse models; species differences however complicate the interpretation of results. So far, bone marrow transplantation is the only effective long-term treatment for childhood cerebral X-ALD, however, only when performed at an early-stage of disease. Urgently needed novel therapeutic strategies are under consideration ranging from dietary approaches to gene therapy.

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.

Yeast ABC transporters-- a tale of sex, stress, drugs and aging

Yeast ATP-binding cassette (ABC) proteins are implicated in many biological phenomena, often acting at crossroads of vital cellular processes. Their functions encompass peptide pheromone secretion, regulation of mitochondrial function, vacuolar detoxification, as well as pleiotropic drug resistance and stress adaptation. Because yeast harbors several homologues of mammalian ABC proteins with medical importance, understanding their molecular mechanisms, substrate interaction and three-dimensional structure of yeast ABC proteins might help identifying new approaches aimed at combating drug resistance or other ABC-mediated diseases. This review provides a comprehensive discussion on the functions of the ABC protein family in the yeast Saccharomyces cerevisiae.

The peroxisomal lumen in Saccharomyces cerevisiae is alkaline

Peroxisomes have a central function in lipid metabolism, including the beta-oxidation of various fatty acids. The products and substrates involved in the beta-oxidation have to cross the peroxisomal membrane, which previously has been demonstrated to constitute a closed barrier, implying the existence of specific transport mechanisms. Fatty acid transport across the yeast peroxisomal membrane may follow two routes: one for activated fatty acids, dependent on the peroxisomal ABC half transporter proteins Pxa1p and Pxa2p, and one for free fatty acids, which depends on the peroxisomal acyl-CoA synthetase Faa2p and the ATP transporter Ant1p. A proton gradient across the peroxisomal membrane as part of a proton motive force has been proposed to be required for proper peroxisomal function, but the nature of the peroxisomal pH has remained inconclusive and little is known about its generation. To determine the pH of Sacharomyces cerevisiae peroxisomes in vivo, we have used two different pH-sensitive yellow fluorescent proteins targeted to the peroxisome by virtue of a C-terminal SKL and found the peroxisomal matrix in wild-type cells to be alkaline (pH(per) 8.2), while the cytosolic pH was neutral (pH(cyt) 7.0). No Delta pH was present in ant1 Delta cells, indicating that the peroxisomal pH is regulated in an ATP-dependent way and suggesting that Ant1p activity is directly involved in maintenance of the peroxisomal pH. Moreover, we found a high peroxisomal pH of >8.6 in faa2 Delta cells, while the peroxisomal pH remained 8.1+/-0.2 in pxa2 Delta cells. Our combined results suggest that the proton gradient across the peroxisomal membrane is dependent on Ant1p activity and required for the beta-oxidation of medium chain fatty acids.

Decreased expression of ABCD4 and BG1 genes early in the pathogenesis of X-linked adrenoleukodystrophy

Childhood cerebral adrenoleukodystrophy (CCER), adrenomyeloneuropathy (AMN) and AMN with cerebral demyelination (AMN-C) are the main phenotypic variants of X-linked adrenoleukodystrophy (ALD). It is caused by mutations in the ABCD1 gene encoding a half-size peroxisomal transporter that has to dimerize to become functional. The biochemical hallmark of ALD is the accumulation of very-long-chain fatty acids (VLCFA) in plasma and tissues. However, there is no correlation between the ALD phenotype and the ABCD1 gene mutations or the accumulation of VLCFA in plasma and fibroblast from ALD patients. The absence of genotype-phenotype correlation suggests the existence of modifier genes. To elucidate the mechanisms underlying the phenotypic variability of ALD, we studied the expression of ABCD1, three other peroxisomal transporter genes of the same family (ABCD2, ABCD3 and ABCD4) and two VLCFA synthetase genes (VLCS and BG1) involved in VLCFA metabolism, as well as the VLCFA concentrations in the normal white matter (WM) from ALD patients with CCER, AMN-C and AMN phenotypes. This study shows that: (1) ABCD1 gene mutations leading to truncated ALD protein are unlikely to cause variation in the ALD phenotype; (2) accumulation of saturated VLCFA in normal-appearing WM correlates with ALD phenotype and (3) expression of the ABCD4 and BG1, but not of the ABCD2, ABCD3 and VLCS genes, tends to be correlated with the severity of the disease, acting early in the pathogenesis of ALD.

Plant peroxisomes

Peroxisomes, one of single membrane-bound organelles, are present ubiquitously in eukaryotic cells. They were originally identified as organelles for production of hydrogen peroxide, the degradation of its hydrogen peroxide, and metabolism of fatty acids, which are functions common to almost all the organisms. Meanwhile, photorespiration and assimilation of symbiotically induced nitrogen are plant-specific functions. Recent postgenetic approaches such as transcriptome and proteome showed that plant peroxisomes are differentiated in various tissues, and revealed that peroxisomes have more important roles in various metabolic processes including biosynthesis of plant hormones than we speculated. All peroxisomal proteins, including metabolic enzymes in the matrix, membrane proteins, and factors responsible for peroxisome biogenesis, are nuclear encoded, and are provided from the outside of peroxisomes. Peroxisome biogenesis, such as protein transport, division, and enlargement, requires various complicated steps and is one of the most intriguing topics. Analyses using peroxisome biogenesis mutants and the whole-scale sequencing projects among several organisms revealed the existence of essential factors responsible for peroxisome biogenesis such as peroxins. This review addresses a comprehensive issue relating to function and biogenesis of plant peroxisomes and Arabidopsis mutants that have been accelerating our understanding of peroxisomes in planta.

The enzymes, regulation, and genetics of bile acid synthesis

The synthesis and excretion of bile acids comprise the major pathway of cholesterol catabolism in mammals. Synthesis provides a direct means of converting cholesterol, which is both hydrophobic and insoluble, into a water-soluble and readily excreted molecule, the bile acid. The biosynthetic steps that accomplish this transformation also confer detergent properties to the bile acid, which are exploited by the body to facilitate the secretion of cholesterol from the liver. This role in the elimination of cholesterol is counterbalanced by the ability of bile acids to solubilize dietary cholesterol and essential nutrients and to promote their delivery to the liver. The synthesis of a full complement of bile acids requires 17 enzymes. The expression of selected enzymes in the pathway is tightly regulated by nuclear hormone receptors and other transcription factors, which ensure a constant supply of bile acids in an ever changing metabolic environment. Inherited mutations that impair bile acid synthesis cause a spectrum of human disease; this ranges from liver failure in early childhood to progressive neuropathy in adults.

Pex11-related proteins in peroxisome dynamics: a role for the novel peroxin Pex27p in controlling peroxisome size and number in Saccharomyces cerevisiae

Transcriptome profiling identified the gene PEX25 encoding Pex25p, a peroxisomal membrane peroxin required for the regulation of peroxisome size and maintenance in Saccharomyces cerevisiae. Pex25p is related to a protein of unknown function encoded by the open reading frame, YOR193w, of the S. cerevisiae genome. Yor193p is a peripheral peroxisomal membrane protein that exhibits high sequence similarity not only to Pex25p but also to the peroxisomal membrane peroxin Pex11p. Unlike Pex25p and Pex11p, Yor193p is constitutively expressed in wild-type cells grown in oleic acid-containing medium, the metabolism of which requires intact peroxisomes. Cells deleted for the YOR193w gene show a few enlarged peroxisomes. Peroxisomes are greatly enlarged in cells harboring double deletions of the YOR193w and PEX25 genes, the YOR193w and PEX11 genes, and the PEX25 and PEX11 genes. Yeast two-hybrid analyses showed that Yor193p interacts with Pex25p and itself, Pex25p interacts with Yor193p and itself, and Pex11p interacts only with itself. Overexpression of YOR193w, PEX25, or PEX11 led to peroxisome proliferation and the formation of small peroxisomes. Our data suggest a role for Yor193p, renamed Pex27p, in controlling peroxisome size and number in S. cerevisiae.

The biochemistry of peroxisomal beta-oxidation in the yeast Saccharomyces cerevisiae

Peroxisomal fatty acid degradation in the yeast Saccharomyces cerevisiae requires an array of beta-oxidation enzyme activities as well as a set of auxiliary activities to provide the beta-oxidation machinery with the proper substrates. The corresponding classical and auxiliary enzymes of beta-oxidation have been completely characterized, many at the structural level with the identification of catalytic residues. Import of fatty acids from the growth medium involves passive diffusion in combination with an active, protein-mediated component that includes acyl-CoA ligases, illustrating the intimate linkage between fatty acid import and activation. The main factors involved in protein import into peroxisomes are also known, but only one peroxisomal metabolite transporter has been characterized in detail, Ant1p, which exchanges intraperoxisomal AMP with cytosolic ATP. The other known transporter is Pxa1p-Pxa2p, which bears similarity to the human adrenoleukodystrophy protein ALDP. The major players in the regulation of fatty acid-induced gene expression are Pip2p and Oaf1p, which unite to form a transcription factor that binds to oleate response elements in the promoter regions of genes encoding peroxisomal proteins. Adr1p, a transcription factor, binding upstream activating sequence 1, also regulates key genes involved in beta-oxidation. The development of new, postgenomic-era tools allows for the characterization of the entire transcriptome involved in beta-oxidation and will facilitate the identification of novel proteins as well as the characterization of protein families involved in this process.

ATP binding/hydrolysis by and phosphorylation of peroxisomal ATP-binding cassette proteins PMP70 (ABCD3) and adrenoleukodystrophy protein (ABCD1)

The 70-kDa peroxisomal membrane protein (PMP70) and adrenoleukodystrophy protein (ALDP), half-size ATP-binding cassette transporters, are involved in metabolic transport of long and very long chain fatty acids into peroxisomes. We examined the interaction of peroxisomal ATP-binding cassette transporters with ATP using rat liver peroxisomes. PMP70 was photoaffinity-labeled at similar efficiencies with 8-azido-[alpha-32P]ATP and 8-azido-[gamma-32P]ATP when peroxisomes were incubated with these nucleotides at 37 degrees C in the absence Mg2+ and exposed to UV light without removing unbound nucleotides. The photoaffinity-labeled PMP70 and ALDP were co-immunoprecipitated together with other peroxisomal proteins, which also showed tight ATP binding properties. Addition of Mg2+ reduced the photoaffinity labeling of PMP70 with 8-azido-[gamma-32P]ATP by 70%, whereas it reduced photoaffinity labeling with 8-azido-[alpha-32P]ATP by only 20%. However, two-thirds of nucleotide (probably ADP) was dissociated during removal of unbound nucleotides. These results suggest that ATP binds to PMP70 tightly in the absence of Mg2+, the bound ATP is hydrolyzed to ADP in the presence of Mg2+, and the produced ADP is dissociated from PMP70, which allows ATP hydrolysis turnover. Properties of photoaffinity labeling of ALDP were essentially similar to those of PMP70. Vanadate-induced nucleotide trapping in PMP70 and ALDP was not observed. PMP70 and ALDP were also phosphorylated at a tyrosine residue(s). ATP binding/hydrolysis by and phosphorylation of PMP70 and ALDP are involved in the regulation of fatty acid transport into peroxisomes.

The Arabidopsis pxa1 mutant is defective in an ATP-binding cassette transporter-like protein required for peroxisomal fatty acid beta-oxidation

Peroxisomes are important organelles in plant metabolism, containing all the enzymes required for fatty acid beta-oxidation. More than 20 proteins are required for peroxisomal biogenesis and maintenance. The Arabidopsis pxa1 mutant, originally isolated because it is resistant to the auxin indole-3-butyric acid (IBA), developmentally arrests when germinated without supplemental sucrose, suggesting defects in fatty acid beta-oxidation. Because IBA is converted to the more abundant auxin, indole-3-acetic acid (IAA), in a mechanism that parallels beta-oxidation, the mutant is likely to be IBA resistant because it cannot convert IBA to IAA. Adult pxa1 plants grow slowly compared with wild type, with smaller rosettes, fewer leaves, and shorter inflorescence stems, indicating that PXA1 is important throughout development. We identified the molecular defect in pxa1 using a map-based positional approach. PXA1 encodes a predicted peroxisomal ATP-binding cassette transporter that is 42% identical to the human adrenoleukodystrophy (ALD) protein, which is defective in patients with the demyelinating disorder X-linked ALD. Homology to ALD protein and other human and yeast peroxisomal transporters suggests that PXA1 imports coenzyme A esters of fatty acids and IBA into the peroxisome for beta-oxidation. The pxa1 mutant makes fewer lateral roots than wild type, both in response to IBA and without exogenous hormones, suggesting that the IAA derived from IBA during seedling development promotes lateral root formation.

The Arabidopsis pxa1 mutant is defective in an ATP-binding cassette transporter-like protein required for peroxisomal fatty acid beta-oxidation

Peroxisomes are important organelles in plant metabolism, containing all the enzymes required for fatty acid beta-oxidation. More than 20 proteins are required for peroxisomal biogenesis and maintenance. The Arabidopsis pxa1 mutant, originally isolated because it is resistant to the auxin indole-3-butyric acid (IBA), developmentally arrests when germinated without supplemental sucrose, suggesting defects in fatty acid beta-oxidation. Because IBA is converted to the more abundant auxin, indole-3-acetic acid (IAA), in a mechanism that parallels beta-oxidation, the mutant is likely to be IBA resistant because it cannot convert IBA to IAA. Adult pxa1 plants grow slowly compared with wild type, with smaller rosettes, fewer leaves, and shorter inflorescence stems, indicating that PXA1 is important throughout development. We identified the molecular defect in pxa1 using a map-based positional approach. PXA1 encodes a predicted peroxisomal ATP-binding cassette transporter that is 42% identical to the human adrenoleukodystrophy (ALD) protein, which is defective in patients with the demyelinating disorder X-linked ALD. Homology to ALD protein and other human and yeast peroxisomal transporters suggests that PXA1 imports coenzyme A esters of fatty acids and IBA into the peroxisome for beta-oxidation. The pxa1 mutant makes fewer lateral roots than wild type, both in response to IBA and without exogenous hormones, suggesting that the IAA derived from IBA during seedling development promotes lateral root formation.

THEGENETICS OFFATTYACIDMETABOLISM INSACCHAROMYCESCEREVISIAE

Long-chain fatty acids are a vital metabolic energy source and are building blocks of membrane lipids. The yeast Saccharomyces cerevisiae is a valuable model system for elucidation of gene-function relationships in such eukaryotic processes as fatty acid metabolism. Yeast degrades fatty acids only in the peroxisome, and recently, genes encoding core and auxiliary enzymes of peroxisomal beta-oxidation have been identified. Mechanisms involved in fatty acid induction of gene expression have been described, and novel fatty acid-responsive genes have been discovered via yeast genome analysis. In addition, a number of genes essential for synthesis of the variety of fatty acids in yeast have been cloned. Advances in understanding such processes in S. cerevisiae will provide helpful insights to functional genomics approaches in more complex organisms.

Identification of a peroxisomal ATP carrier required for medium-chain fatty acid beta-oxidation and normal peroxisome proliferation in Saccharomyces cerevisiae

We have characterized the role of YPR128cp, the orthologue of human PMP34, in fatty acid metabolism and peroxisomal proliferation in Saccharomyces cerevisiae. YPR128cp belongs to the mitochondrial carrier family (MCF) of solute transporters and is localized in the peroxisomal membrane. Disruption of the YPR128c gene results in impaired growth of the yeast with the medium-chain fatty acid (MCFA) laurate as a single carbon source, whereas normal growth was observed with the long-chain fatty acid (LCFA) oleate. MCFA but not LCFA beta-oxidation activity was markedly reduced in intact ypr128cDelta mutant cells compared to intact wild-type cells, but comparable activities were found in the corresponding lysates. These results imply that a transport step specific for MCFA beta-oxidation is impaired in ypr128cDelta cells. Since MCFA beta-oxidation in peroxisomes requires both ATP and CoASH for activation of the MCFAs into their corresponding coenzyme A esters, we studied whether YPR128cp is an ATP carrier. For this purpose we have used firefly luciferase targeted to peroxisomes to measure ATP consumption inside peroxisomes. We show that peroxisomal luciferase activity was strongly reduced in intact ypr128cDelta mutant cells compared to wild-type cells but comparable in lysates of both cell strains. We conclude that YPR128cp most likely mediates the transport of ATP across the peroxisomal membrane.

Fungal ABC proteins: pleiotropic drug resistance, stress response and cellular detoxification

A number of prominent genetic diseases are caused by mutations in genes encoding ATP-binding cassette (ABC) proteins (Ambudkar, Gottesmann, 1998). Moreover, several mammalian ABC proteins such as P-glycoprotein (P-gp) (Gottesman et al., 1995) and multidrug-resistance-associated proteins (MRPs) (Cole, Deeley, 1998) have been implicated in multidrug resistance (MDR) phenotypes of tumor cells highly resistant to many different anticancer drugs. The characteristics of MDR phenomena include the initial resistance to a single anticancer drug, followed by the development of cross-resistance to many structurally and functionally unrelated drugs. Similar mechanisms of MDR exist in pathogenic fungi, including Candida and Aspergillus (Vanden Bossche et al., 1998), and also in parasites such as Plasmodium and Leishmania (Ambudkar, Gottesmann, 1998), as well as in many bacterial pathogens (Nikaido, 1998). To dissect the mechanisms of MDR development and to elucidate the physiological functions of ABC proteins, many efforts have been made during the past decade. Importantly, yeast orthologues of mammalian disease genes made this unicellular eukaryote an invaluable model system for studies on the molecular mechanisms of ABC proteins, in order to better understand and perhaps improve treatment of ABC gene-related disease. In this review, we provide an overview of ABC proteins and pleiotropic drug resistance in the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe. Furthermore, we discuss the role of ABC proteins in clinical drug resistance development of certain fungal pathogens.

Sorting and function of peroxisomal membrane proteins

Peroxisomes are subcellular organelles and are present in virtually all eukaryotic cells. Characteristic features of these organelles are their inducibility and their functional versatility. Their importance in the intermediary metabolism of cells is exemplified by the discovery of several inborn, fatal peroxisomal errors in man, the so-called peroxisomal disorders. Recent findings in research on peroxisome biogenesis and function have demonstrated that peroxisomal matrix proteins and peroxisomal membrane proteins (PMPs) follow separate pathways to reach their target organelle. This paper addresses the principles of PMP sorting and summarizes the current knowledge of the role of these proteins in organelle biogenesis and function.

Transport of fatty acids and metabolites across the peroxisomal membrane

The peroxisomal membrane forms a permeability barrier for a wide variety of metabolites required for and formed during fatty acid beta-oxidation. To communicate with the cytoplasm and mitochondria, peroxisomes need dedicated proteins to transport such hydrophilic molecules across their membranes. Genetic and biochemical studies in the yeast Saccharomyces cerevisiae have identified enzymes for redox shuttles as well as the first peroxisomal membrane transporter. This peroxisomal ATP-binding cassette transporter (Pat) is highly homologous to the gene mutated in X-linked adrenoleukodystrophy (X-ALD). The yeast Pat is required for import of activated fatty acids into peroxisomes suggesting that this is the primary defect in X-ALD.

Inventory and function of yeast ABC proteins: about sex, stress, pleiotropic drug and heavy metal resistance

Saccharomyces cerevisiae was the first eukaryotic organism whose complete genome sequence has been determined, uncovering the existence of numerous genes encoding proteins of the ATP-binding cassette (ABC) family. Fungal ABC proteins are implicated in a variety of cellular functions, ranging from clinical drug resistance development, pheromone secretion, mitochondrial function, peroxisome biogenesis, translation elongation, stress response to cellular detoxification. Moreover, some yeast ABC proteins are orthologues of human disease genes, which makes yeast an excellent model system to study the molecular mechanisms of ABC protein-mediated disease. This review provides a comprehensive discussion and update on the function and transcriptional regulation of all known ABC genes from yeasts, including those discovered in fungal pathogens.

Characterization of the 70-kDa peroxisomal membrane protein, an ATP binding cassette transporter

The 70-kDa peroxisomal membrane protein (PMP70) is one of the major components of rat liver peroxisomal membranes and belongs to a superfamily of proteins known as ATP binding cassette transporters. PMP70 is markedly induced by administration of hypolipidemic agents in parallel with peroxisome proliferation and induction of peroxisomal fatty acid beta-oxidation enzymes. To characterize the role of PMP70 in biogenesis and function of peroxisomes, we transfected the cDNA of rat PMP70 into Chinese hamster ovary cells and established cell lines stably expressing PMP70. The content of PMP70 in the transfectants increased about 5-fold when compared with the control cells. A subcellular fractionation study showed that overexpressed PMP70 was enriched in peroxisomes. This peroxisomal localization was confirmed by immunofluorescence and immunoelectron microscopy. The number of immuno-gold particles corresponding to PMP70 on peroxisomes increased markedly in the transfectants, but the size and the number of peroxisomes were essentially the same in both the transfectants and the control cells. beta-Oxidation of palmitic acid increased about 2-3-fold in the transfectants, whereas the oxidation of lignoceric acid decreased about 30-40%. When intact peroxisomes prepared from both the cell lines were incubated with palmitoyl-CoA, oxidation was stimulated with ATP, but the degree of the stimulation was higher in the transfectants than in the control cells. Furthermore, we established three Chinese hamster ovary cell lines stably expressing mutant PMP70. In these cells, beta-oxidation of palmitic acid decreased markedly. These results suggest that PMP70 is involved in metabolic transport of long chain acyl-CoA across peroxisomal membranes and that increase of PMP70 is not associated with proliferation of peroxisomes.

The Saccharomyces cerevisiae FAT1 gene encodes an acyl-CoA synthetase that is required for maintenance of very long chain fatty acid levels

The Saccharomyces cerevisiae FAT1 gene appears to encode an acyl-CoA synthetase that is involved in the regulation of very long chain (C20-C26) fatty acids. Fat1p, has homology to a rat peroxisomal very long chain fatty acyl-CoA synthetase. Very long chain acyl-CoA synthetase activity is reduced in strains containing a disrupted FAT1 gene and is increased when FAT1 is expressed in insect cells under control of a baculovirus promoter. Fat1p accounts for approximately 90% of the C24-specific acyl-CoA synthetase activity in glucose-grown cells and approximately 66% of the total activity in cells grown under peroxisomal induction conditions. Localization of functional Fat1p:green fluorescent protein gene fusions and subcellular fractionation of C24 acyl-CoA synthetase activities indicate that the majority of Fat1p is located in internal cellular locations. Disruption of the FAT1 gene results in the accumulation of very long chain fatty acids in the sphingolipid and phospholipid fractions. This includes a 10-fold increase in C24 acids and a 6-fold increase in C22 acids. These abnormal accumulations are further increased by perturbation of very long chain fatty acid synthesis. Overexpression of Elo2p, a component of the fatty acid elongation system, in fat1Delta cells causes C20-C26 levels to rise to approximately 20% of the total fatty acids. These data suggest that Fat1p is involved in the maintenance of cellular very long chain fatty acid levels, apparently by facilitating beta-oxidation of excess intermediate length (C20-C24) species. Although fat1Delta cells were reported to grow poorly in oleic acid-supplemented medium when fatty acid synthase activity is inactivated by cerulenin, fatty acid import is not significantly affected in cells containing disrupted alleles of FAT1 and FAS2 (a subunit of fatty acid synthase). These results suggest that the primary cause of the growth-defective phenotype is a failure to metabolize the incorporated fatty acid rather than a defect in fatty acid transport. Certain fatty acyl-CoA synthetase activities, however, do appear to be essential for bulk fatty acid transport in Saccharomyces. Simultaneous disruption of FAA1 and FAA4, which encode long chain (C14-C18) fatty acyl-CoA synthetases, effectively blocks the import of long chain saturated and unsaturated fatty acids.

Peroxisomicine A1 (plant toxin-514) affects normal peroxisome assembly in the yeast Hansenula polymorpha

Previously we demonstrated that peroxisomicine A1 (T-514), a plant toxin isolated from Karwinskia species, has a deteriorating effect on the integrity of peroxisomes of methylotrophic yeasts. Here we describe two strains of Hansenula polymorpha, affected in the normal utilization of methanol as sole source of carbon and energy due to peroxisomicine A1 treatment. The two strains isolated (L17 and RV31) grew poorly on methanol, apparently due to malfunctioning of their peroxisomes. Moreover, the cells displayed a high peroxisome turnover rate. We argue that the peroxisomicine A1 induced phenotype of both strains is due to a genomic mutation. Strain L17 was functionally complemented after transformation with a H. polymorpha genomic library. The complementing 2.8 kb DNA fragment did not contain a well-defined ORF and led us to speculate that it may contain regulatory sequences that, when present in multiple copies in the cell, result in a change of expression of specific genes, thus causing restoration of normal methylotrophic growth.

Global regulatory functions of Oaf1p and Pip2p (Oaf2p), transcription factors that regulate genes encoding peroxisomal proteins in Saccharomyces cerevisiae

Two transcription factors, Oaf1p and Pip2p (Oaf2p), are key components in the pathway by which several Saccharomyces cerevisiae genes encoding peroxisomal proteins are activated in the presence of a fatty acid such as oleate. By searching the S. cerevisiae genomic database for the consensus sequence that acts as a target for these transcription factors, we identified 40 genes that contain a putative Oaf1p-Pip2p binding site in their promoter region. Quantitative Northern analysis confirmed that the expression of 22 of the genes identified is induced by oleate and that either one or both of these transcription factors are required for the activation. In addition to known peroxisomal proteins, the regulated genes encode novel peroxisomal proteins, a mitochondrial protein, and proteins of unknown location and function. We demonstrate that Oaf1p regulates certain genes in the absence of Pip2p and that both of these transcription factors play a role in maintaining the glucose-repressed state of one gene. Furthermore, we provide evidence that the defined consensus binding site is not required for the regulation of certain oleate-responsive genes.

Characterization of a novel mutation in exon 10 of the adrenoleukodystrophy gene

We have detected a novel mutation in the adrenoleukodystrophy (ALD) gene in skin fibroblasts in primary culture derived from a patient suffering from the adrenocortical insufficiency-only-phenotype of ALD. This nonsense mutation (C2400T/Q672X) is the only mutation reported to date affecting exon 10. It leads to a translation product lacking the 74 C-terminal amino acids. As a consequence of the loss of this region, which immediately follows the putative nucleotide binding domain, the ALD protein (ALDP) was not detectable at all by ALDP-specific monoclonal antibodies. Since ALDP-specific mRNA was readily detected in these fibroblasts, the loss of protein is probably not attributable to RNA instability but may be explained by protein instability. If the Q672X mutation leads in fact to an unstable translation product this would be consistent with the hypothesis that the C-terminus is crucial for ALDP stability.

Genomic organization and chromosomal localization of the human peroxisomal membrane protein-1-like protein (PXMP1-L) gene encoding a peroxisomal ABC transporter

The cDNA of the peroxisomal membrane protein-1-like protein (PXMP1-L, synonyms: PMP69, P70R), a novel peroxisomal ATP binding cassette transporter of yet unknown function, has recently been cloned. The best known peroxisomal member of this protein family is the adrenoleukodystrophy protein, defects of which are the underlying cause of X-linked adrenoleukodystrophy (X-ALD). Here we describe the complete exon-intron structure (19 exons and 18 introns covering 16.0 kb) of the human PXMP1-L gene, transcript variants, the localization on chromosome 14q24 by cytogenetic analysis and sequencing of the putative promoter region. PXMP1-L has been proposed to play a role as a modifier in determining the phenotypic variations observed in X-ALD. The data presented will enable sequence analysis of the PXMP1-L gene in X-ALD patients and facilitate the analysis of PXMP1-L function.

Lipid metabolism in peroxisomes in relation to human disease

Peroxisomes were long believed to play only a minor role in cellular metabolism but it is now clear that they catalyze a number of important functions. The importance of peroxisomes in humans is stressed by the existence of a group of genetic diseases in man in which one or more peroxisomal functions are impaired. Most of the functions known to take place in peroxisomes have to do with lipids. Indeed, peroxisomes are capable of 1. fatty acid beta-oxidation 2. fatty acid alpha-oxidation 3. synthesis of cholesterol and other isoprenoids 4. ether-phospholipid synthesis and 5. biosynthesis of polyunsaturated fatty acids. In Chapters 2-6 we will discuss the functional organization and enzymology of these pathways in detail. Furthermore, attention is paid to the permeability properties of peroxisomes with special emphasis on recent studies which suggest that peroxisomes are closed structures containing specific membrane proteins for transport of metabolites. Finally, the disorders of peroxisomal lipid metabolism will be discussed.

Nucleotide triphosphates are required for the transport of glycolate oxidase into peroxisomes

All peroxisomal proteins are nuclear encoded, synthesized on free cytosolic ribosomes, and posttranslationally targeted to the organelle. We have used an in vitro assay to reconstitute protein import into pumpkin (Cucurbita pepo) glyoxysomes, a class of peroxisome found in the cotyledons of oilseed plants, to study the mechanisms involved in protein transport across peroxisome membranes. Results indicate that ATP hydrolysis is required for protein import into peroxisomes; nonhydrolyzable analogs of ATP could not substitute for this requirement. Nucleotide competition studies suggest that there may be a nucleotide binding site on a component of the translocation machinery. Peroxisomal protein import also was supported by GTP hydrolysis. Nonhydrolyzable analogs of GTP did not substitute in this process. Experiments to determine the cation specificity of the nucleotide requirement show that the Mg2+ salt was preferred over other divalent and monovalent cations. The role of a putative protonmotive force across the peroxisomal membrane was also examined. Although low concentrations of ionophores had no effect on protein import, relatively high concentrations of all ionophores tested consistently reduced the level of protein import by approximately 50%. This result suggests that a protonmotive force is not absolutely required for peroxisomal protein import.

An essential function of a phosphoinositide-specific phospholipase C is relieved by inhibition of a cyclin-dependent protein kinase in the yeast Saccharomyces cerevisiae

The PLC1 gene product of Saccharomyces cerevisiae is a homolog of the delta isoform of mammalian phosphoinositide-specific phospholipase C (PI-PLC). We found that two genes (SPL1 and SPL2), when overexpressed, can bypass the temperature-sensitive growth defect of a plc1delta cell. SPL1 is identical to the PHO81 gene, which encodes an inhibitor of a cyclin (Pho80p)-dependent protein kinase (Pho85p) complex (Cdk). In addition to overproduction of Pho81p, two other conditions that inactivate this Cdk, a cyclin (pho80delta) mutation and growth on low-phosphate medium, also permitted growth of plc1delta cells at the restrictive temperature. Suppression of the temperature sensitivity of plc1delta cells by pho80delta does not depend upon the Pho4p transcriptional regulator, the only known substrate of the Pho80p/Pho85p Cdk. The second suppressor, SPL2, encodes a small (17-kD) protein that bears similarity to the ankyrin repeat regions present in Pho81p and in other known Cdk inhibitors. Both pho81delta and spl2delta show a synthetic phenotype in combination with plc1delta. Unlike single mutants, plc1delta pho81delta and plc1delta spl2delta double mutants were unable to grow on synthetic complete medium, but were able to grow on rich medium.