Involvement of 70-kD heat-shock proteins in peroxisomal import

The type-2 peroxisomal targeting signal

The type-2 peroxisomal targeting signal (PTS2) is one of two peptide motifs destining soluble proteins for peroxisomes. This signal acts as amphiphilic α-helix exposing the side chains of all conserved residues to the same side. PTS2 motifs are recognized by a bipartite protein complex consisting of the receptor PEX7 and a co-receptor. Cargo-loaded receptor complexes are translocated across the peroxisomal membrane by a transient pore and inside peroxisomes, cargo proteins are released and processed in many, but not all species. The components of the bipartite receptor are re-exported into the cytosol by a ubiquitin-mediated and ATP-driven export mechanism. Structurally, PTS2 motifs resemble other N-terminal targeting signals, whereas the functional relation to the second peroxisomal targeting signal (PTS1) is unclear. Although only a few PTS2-carrying proteins are known in humans, subjects lacking a functional import mechanism for these proteins suffer from the severe inherited disease rhizomelic chondrodysplasia punctata.

Structural and functional insights on the roles of molecular chaperones in the mistargeting and aggregation phenotypes associated with primary hyperoxaluria type I

To carry out their biological function in cells, proteins must be folded and targeted to the appropriate subcellular location. These processes are controlled by a vast collection of interacting proteins collectively known as the protein homeostasis network, in which molecular chaperones play a prominent role. Protein homeostasis can be impaired by inherited mutations leading to genetic diseases. In this chapter, we focus on a particular disease, primary hyperoxaluria type 1 (PH1), in which disease-associated mutations exacerbate protein aggregation in the cell and mistarget the peroxisomal alanine:glyoxylate aminotransferase (AGT) protein to mitochondria, in part due to native state destabilization and enhanced interaction with Hsp60, 70 and 90 chaperone systems. After a general introduction of molecular chaperones and PH1, we review our current knowledge on the structural and energetic features of PH1-causing mutants that lead to these particular pathogenic mechanisms. From this perspective, and in the context of the key role of molecular chaperones in PH1 pathogenesis, we present and discuss current and future perspectives for pharmacological treatments for this disease.

Redox Regulation of Homeostasis and Proteostasis in Peroxisomes

Peroxisomes are highly dynamic intracellular organelles involved in a variety of metabolic functions essential for the metabolism of long-chain fatty acids, d-amino acids, and many polyamines. A byproduct of peroxisomal metabolism is the generation, and subsequent detoxification, of reactive oxygen and nitrogen species, particularly hydrogen peroxide (H2O2). Because of its relatively low reactivity (as a mild oxidant), H2O2 has a comparatively long intracellular half-life and a high diffusion rate, all of which makes H2O2 an efficient signaling molecule. Peroxisomes also have intricate connections to mitochondria, and both organelles appear to play important roles in regulating redox signaling pathways. Peroxisomal proteins are also subject to oxidative modification and inactivation by the reactive oxygen and nitrogen species they generate, but the peroxisomal LonP2 protease can selectively remove such oxidatively damaged proteins, thus prolonging the useful lifespan of the organelle. Peroxisomal homeostasis must adapt to the metabolic state of the cell, by a combination of peroxisome proliferation, the removal of excess or badly damaged organelles by autophagy (pexophagy), as well as by processes of peroxisome inheritance and motility. More recently the tumor suppressors ataxia telangiectasia mutate (ATM) and tuberous sclerosis complex (TSC), which regulate mTORC1 signaling, have been found to regulate pexophagy in response to variable levels of certain reactive oxygen and nitrogen species. It is now clear that any significant loss of peroxisome homeostasis can have devastating physiological consequences. Peroxisome dysregulation has been implicated in several metabolic diseases, and increasing evidence highlights the important role of diminished peroxisomal functions in aging processes.

Using Pull Down Strategies to Analyze the Interactome of Peroxisomal Membrane Proteins in Human Cells

Different pull-down strategies were successfully applied to gain novel insight into the interactome of human membrane-associated proteins. Here, we compare the outcome, efficiency and potential of pull-down strategies applied to human peroxisomal membrane proteins. Stable membrane-bound protein complexes can be affinity-purified from genetically engineered human cells or subfractions thereof after detergent solubilization, followed by size exclusion chromatography and analysis by mass spectrometry (MS). As exemplified for Protein A-tagged human PEX14, one of the central constituents of the peroxisomal matrix protein import machinery, MS analyses of the affinity-purified complexes revealed an unexpected association of PEX14 with other protein assemblies like the microtubular network or the insertion apparatus for peroxisomal membrane proteins comprising PEX3, PEX16 and PEX19. The latter association was recently supported by using a different pull-down strategy following in vivo proximity labeling with biotin, named BioID, which enabled the identification of various membrane proteins in close proximity of PEX16 in living cells.

Identification of d-amino acid oxidase and propiverine interaction partners and their potential role in the propiverine-mediated nephropathy

Propiverine, a frequently-prescribed pharmaceutical for the treatment of symptoms associated with overactive bladder syndrome, provoked massive intranuclear and cytosolic protein inclusions in rat proximal tubule epithelium, primarily consisting of the peroxisomal targeting signal 1 (PTS1) containing protein d-amino acid oxidase (DAAO). As this type of nephropathy was also observed for other drugs, the aim was to determine whether propiverine interferes with trafficking and/or import of peroxisomal proteins. To elucidate this, DAAO- and propiverine-specific interaction partners from human HEK293 and rat WKPT cell lines and rat kidney and liver homogenate were determined using co-immunoprecipitation with subsequent nano-ESI-LC-MS/MS analyses. Corroboration of the role of DAAO- and/or propiverine-specific interaction partners in the drug-induced DAAO accumulation was sought via specific immunofluorescence staining of rat kidney sections from control and propiverine-treated rats. Above analyses demonstrated the interaction of propiverine with several protein classes, foremost peroxisomal proteins (DAAO, MFE2, HAOX2) and proteins of the protein quality control system, i.e. chaperones (HSP70 and DnaJ co-chaperones), proteases and proteasomal proteins (regulatory subunits of the 26S proteasome; Rpn1/2). The immunofluorescence analysis revealed mislocalization of many PTS1-proteins (DAAO, CAT, MFE2, ACOX1, EHHADH) in rat renal sections, strongly suggesting that propiverine primarily binds to PTS1 proteins resulting in the formation of PTS1 but not PTS2 or peroxisomal membrane protein (PMP) accumulations. Moreover, chaperones involved in peroxisomal trafficking (HSC70, DnaJB1) and peroxisomal biogenesis factor proteins (PEX3, PEX5, PEX7), also presented with distinct mislocalization patterns. Concomitantly, an increased number of peroxisomes was observed, suggestive of a compensatory mechanism for the presumably suboptimally functioning peroxisomes. Overall, the data presented suggested that propiverine interacts exclusively with DAAO or with a selected number of PTS1 proteins. The consequence of this interaction is the abrogated trafficking and peroxisomal import of PTS1 proteins concomitant with their nuclear and cytosolic accumulation due to inhibited degradation and imbalanced protein homeostasis.

Molecular Chaperone Hsp70 and Its Constitutively Active Form Hsc70 Play an Indispensable Role During Eye Development of Drosophila melanogaster

In the present study, we demonstrate that molecular chaperone Hsp70 and Hsc70 is essential for normal organization and development of ommatidial cells in Drosophila melanogaster eye. An exogenously expressed dominant negative mutant of Hsp70 (K71E) and Hsc70.4 (K71S and D206S) in an eye-specific manner resulted in eye degeneration that includes loss of eye pigment, disorganized ommatidia, abnormality in bristle cell arrangement and reduction in the eye size. The developmental organization of ommatidial cells (cone, photoreceptor, pigment, and bristle cell complex) was disturbed in Hsp70 and Hsc70 mutants. Acridine orange (AO) and caspase 3 staining showed an increased cell death in Hsp70 and Hsc70 mutant eyes. Genetic interaction study of Hsp70 and Hsc70 mutants with candidate genes of JNK signaling pathway and immunocytochemistry study using phospho-JNK antibody suggested that mutation in Hsp70 and Hsc70 results in ectopic activation of JNK signaling in fly eye. Further, anti-PH3 staining in Hsp70 and Hsc70 mutant eyes revealed a reduced number of mitotic cells in second mitotic wave (SMW) of developing eye and anti-Rh1 staining showed reduced Rh1 expression, accumulation of Rh1 in the cytoplasm, and rhabdomere degeneration. Thus, on the basis of results, it was concluded that molecular chaperone Hsp70 and Hsc70 play an indispensable role during Drosophila eye development.

Partial dispensability of Djp1's J domain in peroxisomal protein import in Saccharomyces cerevisiae results from genetic redundancy with another class II J protein, Caj1

J proteins are obligate co-chaperones of Hsp70s. Via their signature J domain, all J proteins interact with their partner Hsp70s and stimulate their weak ATPase activity, which is vital for Hsp70 functions. The dependency of J proteins on their J domain is such that mutations in critical amino acids in the J domain often results into a null phenotype for a particular J protein. Here, we show that the J domain of Djp1, a cytosolic J protein important for peroxisomal protein import in Saccharomyces cerevisiae, is partially dispensable. A complete deletion of Djp1 J domain resulted into only partial loss in peroxisomal protein import function. Instead, the C-terminal domain of Djp1 was found to be essential for proper localization of the peroxisomal targeted GFP-PTS1. Furthermore, we show that Caj1, another cytosolic J protein, also has some role in peroxisomal protein import. Caj1 was found to be partially redundant with Djp1 as cells lacking both Djp1 and Caj1 resulted into a much more severe defect in GFP-PTS1 localization. Based on these results, we propose that dispensability of J domains could be attributed to genetic redundancy between different J proteins sharing common structural topology and cellular localization.

Peroxisomal Import Reduces the Proapoptotic Activity of Deubiquitinating Enzyme USP2

The human deubiquitinating enzyme ubiquitin-specific protease 2 (USP2) regulates multiple cellular pathways, including cell proliferation and apoptosis. As a result of alternative splicing four USP2 isoenzymes are expressed in human cells of which all contain a weak peroxisome targeting signal of type 1 (PTS1) at their C-termini. Here, we systematically analyzed apoptotic effects induced by overexpression and intracellular localization for each isoform. All isoforms exhibit proapoptotic activity and are post-translationally imported into the matrix of peroxisomes in a PEX5-dependent manner. However, a significant fraction of the USP2 pool resides in the cytosol due to a weaker PTS1 and thus low affinity to the PTS receptor PEX5. Blocking of peroxisomal import did not interfere with the proapoptotic activity of USP2, suggesting that the enzyme performs its critical function outside of this compartment. Instead, increase of the efficiency of USP2 import into peroxisomes either by optimization of its peroxisomal targeting signal or by overexpression of the PTS1 receptor did result in a reduction of the apoptotic rate of transfected cells. Our studies suggest that peroxisomal import of USP2 provides additional control over the proapoptotic activity of cytosolic USP2 by spatial separation of the deubiquitinating enzymes from their interaction partners in the cytosol and nucleus.

Multiple Domains in PEX16 Mediate Its Trafficking and Recruitment of Peroxisomal Proteins to the ER

Peroxisomes rely on a diverse array of mechanisms to ensure the specific targeting of their protein constituents. Peroxisomal membrane proteins (PMPs), for instance, are targeted by at least two distinct pathways: directly to peroxisomes from their sites of synthesis in the cytosol or indirectly via the endoplasmic reticulum (ER). However, the extent to which each PMP targeting pathway is involved in the maintenance of pre-existing peroxisomes is unclear. Recently, we showed that human PEX16 plays a critical role in the ER-dependent targeting of PMPs by mediating the recruitment of two other PMPs, PEX3 and PMP34, to the ER. Here, we extend these results by carrying out a comprehensive mutational analysis of PEX16 aimed at gaining insights into the molecular targeting signals responsible for its ER-to-peroxisome trafficking and the domain(s) involved in PMP recruitment function at the ER. We also show that the recruitment of PMPs to the ER by PEX16 is conserved in plants. The implications of these results in terms of the function of PEX16 and the role of the ER in peroxisome maintenance in general are discussed.

Peroxisomal quality control mechanisms

Peroxisomes are ubiquitous organelles that harbor diverse metabolic pathways, which are essential for normal cell performance. Conserved functions of these organelles are hydrogen peroxide metabolism and β-oxidation. Cells employ multiple quality control mechanisms to ensure proper peroxisome function and to protect peroxisomes from damage. These involve the function of molecular chaperones, a peroxisomal Lon protease and autophagic removal of dysfunctional organelles. In addition, multiple mechanisms exist to combat peroxisomal oxidative stress. Here, we outline recent advances in our understanding of peroxisomal quality control, focussing on yeast and filamentous fungi.

Analysis of the Leishmania peroxin 7 interactions with peroxin 5, peroxin 14 and PTS2 ligands

LPEX7 (Leishmania peroxin 7) is essential for targeting newly synthesized proteins with a PTS2 (peroxisome-targeting signal type 2) import signal into the glycosome. In the present paper, we describe the biophysical characterization of a functional LPEX7 isolated from Escherichia coli inclusion bodies. Pull-down assays showed that LPEX7 binds the interacting partners LdPEX5 (Leishmania donovani peroxin 5) and LdPEX14, but, more importantly, this receptor can specifically bind PTS2 cargo proteins in the monomeric and dimeric states. However, in the absence of interacting partners, LPEX7 preferentially adopts a tetrameric structure. Mapping studies localized the LdPEX5- and LdPEX14-binding sites to the N-terminal portion of LPEX7. Deletion of the first 52 residues abolished LdPEX14 association without altering the LdPEX5 interaction. Intrinsic fluorescence techniques suggested that each LPEX7 subunit has a single unique binding site for each of the respective interacting partners LdPEX5, LdPEX14 and PTS2 cargo proteins. Extrinsic fluorescence studies with ANS (8-anilinonaphthalene-1-sulfonic acid) demonstrated that LPEX7 contains a surface-exposed hydrophobic region(s) that was not altered by the binding of a PTS2 protein or LdPEX5. However, in the presence of these ligands, the accessibility of the hydrophobic domain was dramatically restricted, suggesting that both ligands are necessary to induce notable conformational changes in LPEX7. In contrast, binding of LdPEX14 did not alter the hydrophobic domain on LPEX7. It is possible that the hydrophobic surfaces on LPEX7 may be a crucial characteristic for the shuttling of this receptor in and out of the glycosome.

Peroxisome degradation in mammals: mechanisms of action, recent advances, and perspectives

Peroxisomes are remarkably dynamic organelles that participate in a diverse array of cellular processes, including the metabolism of lipids and reactive oxygen species. In order to regulate peroxisome function in response to changing nutritional and environmental stimuli, new organelles need to be formed and superfluous and dysfunctional organelles have to be selectively removed. Disturbances in any of these processes have been associated with the etiology and progression of various congenital neurodegenerative and age-related human disorders. The aim of this review is to critically explore our current knowledge of how peroxisomes are degraded in mammalian cells and how defects in this process may contribute to human disease. Some of the key issues highlighted include the current concepts of peroxisome removal, the peroxisome quality control mechanisms, the initial triggers for peroxisome degradation, the factors for dysfunctional peroxisome recognition, and the regulation of peroxisome homeostasis. We also dissect the functional and mechanistic relationship between different forms of selective organelle degradation and consider how lysosomal dysfunction may lead to defects in peroxisome turnover. In addition, we draw lessons from studies on other organisms and extrapolate this knowledge to mammals. Finally, we discuss the potential pathological implications of dysfunctional peroxisome degradation for human health.

Pex5p imports folded tetrameric catalase by interaction with Pex13p

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

A novel family of dehydrin-like proteins is involved in stress response in the human fungal pathogen Aspergillus fumigatus

During a search for genes controlling conidial dormancy in Aspergillus fumigatus, two dehydrin-like genes, DprA and DprB, were identified. The deduced proteins had repeated stretches of 23 amino acids that contained a conserved dehydrin-like protein (DPR) motif. Disrupted DprAΔ mutants were hypersensitive to oxidative stress and to phagocytic killing, whereas DprBΔ mutants were impaired in osmotic and pH stress responses. However, no effect was observed on their pathogenicity in our experimental models of invasive aspergillosis. Molecular dissection of the signaling pathways acting upstream showed that expression of DprA was dependent on the stress-activated kinase SakA and the cyclic AMP-protein kinase A (cAMP-PKA) pathways, which activate the bZIP transcription factor AtfA, while expression of DprB was dependent on the SakA mitogen-activated protein kinase (MAPK) pathway, and the zinc finger transcription factor PacC. Fluorescent protein fusions showed that both proteins were associated with peroxisomes and the cytosol. Accordingly, DprA and DprB were important for peroxisome function. Our findings reveal a novel family of stress-protective proteins in A. fumigatus and, potentially, in filamentous ascomycetes.

Proteome analysis of Aspergillus fumigatus total membrane proteins identifies proteins associated with the glycoconjugates and cell wall biosynthesis using 2D LC-MS/MS

We attempted to identify membrane proteins associated with the glycoconjugates and cell wall biosynthesis in the total membrane preparations of Aspergillus fumigatus. The total membrane preparations were first run on 1D gels, and then the stained gels were cut and submitted to in-gel digestion followed by 2D LC-MS/MS and database search. A total of 530 proteins were identified with at least two peptides detected with MS/MS spectra. Seventeen integral membrane proteins were involved in N-, O-glycosylation or GPI anchor biosynthesis. Nine membrane proteins were involved in cell wall biosynthesis. Eight proteins were identified as enzymes involved in sphingolipid synthesis. In addition, the proteins involved in cell wall and ergosterol biosynthesis can potentially be used as antifungal drug targets. Our method, for the first time, clearly provided a global view of the membrane proteins associated with glycoconjugates and cell wall biosynthesis in the total membrane proteome of A. fumigatus.

Peroxisomal targeting of PTS2 pre-import complexes in the yeast Saccharomyces cerevisiae

Posttranslational matrix protein import into peroxisomes uses either one of the two peroxisomal targeting signals (PTS), PTS1 and PTS2. Unlike the PTS1 receptor Pex5p, the PTS2 receptor Pex7p is necessary but not sufficient to target cargo proteins into the peroxisomal matrix and requires coreceptors. Saccharomyces cerevisiae possesses two coreceptors, Pex18p and Pex21p, with a redundant but not a clearly defined function. To gain further insight into the early events of this import pathway, PTS2 pre-import complexes of S. cerevisiae were isolated and characterized by determination of size and protein composition in wild-type and different mutant strains. Mass spectrometric analysis of the cytosolic PTS2 pre-import complex indicates that Fox3p is the only abundant PTS2 protein under oleate growth conditions. Our data strongly suggest that the formation of the ternary cytosolic PTS2 pre-import complex occurs hierarchically. First, Pex7p recognizes cargo proteins through its PTS2 in the cytosol. In a second step, the coreceptor binds to this complex, and finally, this ternary 150 kDa pre-import complex docks at the peroxisomal membrane, where both the PTS1 and the PTS2 import pathways converge. Gel filtration analysis of membrane-bound subcomplexes suggests that Pex13p provides the initial binding partner at the peroxisomal membrane, whereas Pex14p assembles with Pex18p in high-molecular-weight complexes after or during dissociation of the PTS2 receptor.

Shuttles and cycles: transport of proteins into the peroxisome matrix (review)

Peroxisomes are organelles that carry out diverse biochemical processes in eukaryotic cells, including the core pathways of beta-oxidation of lipid molecules and detoxification of reactive oxygen species. In multicellular organisms defects in peroxisome assembly result in multiple biochemical and developmental abnormalities. As peroxisomes do not contain genetic material, their protein content, and therefore function, is determined by the import of nuclearly encoded proteins from the cytosol and, presumably, removal of damaged or obsolete proteins. Import of matrix proteins can be broken down into four steps: targeting signal recognition by the cycling import receptors; receptor-cargo docking at the peroxisome membrane; translocation and cargo unloading; and receptor recycling. Import is mediated by a set of evolutionarily conserved proteins called peroxins that have been identified primarily via genetic screens, but knowledge of their biochemical activities remains largely unresolved. Recent studies have filled in some of the blanks regarding receptor recycling and the role of ubiquitination but outstanding questions remain concerning the nature of the translocon and its ability to accommodate folded, even oligomeric proteins, and the mechanism of cargo unloading and turnover of peroxisomal proteins. This review seeks to integrate recent findings from yeast, mammalian and plant systems to present an up to date account of how proteins enter the peroxisome matrix.

Molecular basis for peroxisomal localization of tetrameric carbonyl reductase

Pig heart peroxisomal carbonyl reductase (PerCR) belongs to the short-chain dehydrogenase/reductase family, and its sequence comprises a C-terminal SRL tripeptide, which is a variant of the type 1 peroxisomal targeting signal (PTS1) Ser-Lys-Leu. PerCR is imported into peroxisomes of HeLa cells when the cells are transfected with vectors expressing the enzyme. However, PerCR does not show specific targeting when introduced into the cells with a protein transfection reagent. To understand the structural basis for peroxisomal localization of PerCR, we determined the crystal structure of PerCR. Our data revealed that the C-terminal PTS1 of each subunit of PerCR was involved in intersubunit interactions and was buried in the interior of the tetrameric molecule. These findings indicate that the PTS1 receptor Pex5p in the cytosol recognizes the monomeric form of PerCR whose C-terminal PTS1 is exposed, and that this PerCR is targeted into the peroxisome, thereby forming a tetramer.

Peroxisomal matrix protein receptor ubiquitination and recycling

The peroxisomal targeting signal type1 (PTS1) receptor Pex5 is required for the peroxisomal targeting of most matrix proteins. Pex5 recognises target proteins in the cytosol and directs them to the peroxisomal membrane where cargo is released into the matrix, and the receptor shuttles back to the cytosol. Recently, it has become evident that the membrane-bound Pex5 can be modified by mono- and polyubiquitination. This review summarises recent results on Pex5 ubiquitination and on the role of the AAA peroxins Pex1 and Pex6 as dislocases required for the release of Pex5 from the membrane to the cytosol where the receptor is either degraded by proteasomes or made available for another round of protein import into peroxisomes.

Uniqueness of the mechanism of protein import into the peroxisome matrix: transport of folded, co-factor-bound and oligomeric proteins by shuttling receptors

Based on earlier suggestions that peroxisomes may have arisen from endosymbionts that later lost their DNA, it was expected that protein transport into this organelle would have parallels to systems found in other organelles of endosymbiont origin, such as mitochondria and chloroplasts. This review highlights three features of peroxisomal matrix protein import that make it unique in comparison with these other subcellular compartments - the ability of this organelle to transport folded, co-factor-bound and oligomeric proteins, the dynamics of the import receptors during the matrix protein import cycle and the existence of a peroxisomal quality-control pathway, which insures that the peroxisome membrane is cleared of cargo-free receptors.

Peroxisomes and aging

Peroxisomes are indispensable for proper functioning of human cells. They efficiently compartmentalize enzymes responsible for a number of metabolic processes, including the absolutely essential beta-oxidation of specific fatty acid chains. These and other oxidative reactions produce hydrogen peroxide, which is, in most instances, immediately processed in situ to water and oxygen. The responsible peroxidase is the heme-containing tetrameric enzyme, catalase. What has emerged in recent years is that there are circumstances in which the tightly regulated balance of hydrogen peroxide producing and degrading activities in peroxisomes is upset-leading to the net production and accumulation of hydrogen peroxide and downstream reactive oxygen species. The factor most essentially involved is catalase, which is missorted in aging, missing or present at reduced levels in certain disease states, and inactivated in response to exposure to specific xenobiotics. The overall goal of this review is to summarize the molecular events associated with the development and advancement of peroxisomal hypocatalasemia and to describe its effects on cells. In addition, results of recent efforts to increase levels of peroxisomal catalase and restore oxidative balance in cells will be discussed.

Molecular organization of peroxisomal enzymes: protein-protein interactions in the membrane and in the matrix

The beta-oxidation of fatty acids in peroxisomes produces hydrogen peroxide (H2O2), a toxic metabolite, as a bi-product. Fatty acids beta-oxidation activity is deficient in X-linked adrenoleukodystrophy (X-ALD) because of mutation in ALD-gene resulting in loss of very long chain acyl-CoA synthetase (VLCS) activity. It is also affected in disease with catalase negative peroxisomes as a result of inactivation by H2O2. Therefore, the following studies were undertaken to delineate the molecular interactions between both the ALD-gene product (adrenoleukodystrophy protein, ALDP) and VLCS as well as H2O2 degrading enzyme catalase and proteins of peroxisomal beta-oxidation. Studies using a yeast two hybrid system and surface plasmon resonance techniques indicate that ALDP, a peroxisomal membrane protein, physically interacts with VLCS. Loss of these interactions in X-ALD cells may result in a deficiency in VLCS activity. The yeast two-hybrid system studies also indicated that catalase physically interacts with L-bifunctional enzyme (L-BFE). Interactions between catalase and L-BFE were further supported by affinity purification, using a catalase-linked resin. The affinity bound 74-kDa protein, was identified as L-BFE by Western blot with specific antibodies and by proteomic analysis. Additional support for their interaction comes from immunoprecipitation of L-BFE with antibodies against catalase as a catalase- L-BFE complex. siRNA for L-BFE decreased the specific activity and protein levels of catalase without changing its subcellular distribution. These observations indicate that L-BFE might help in oligomerization and possibly in the localization of catalase at the site of H2O2 production in the peroxisomal beta-oxidation pathway.

Monomeric inducible nitric oxide synthase localizes to peroxisomes in hepatocytes

Hepatocytes are capable of repeated inducible NO synthase (iNOS) expression, which occurs under inflammatory and stress conditions. This iNOS expression regulates a number of cellular functions as well as cell viability. To better understand the posttranslational mechanisms that regulate the fate of iNOS in these cells, we characterized the iNOS distributed within peroxisomes. The selective permeabilization of membranes (plasma vs. peroxisomal) confirmed that there are cytosolic and peroxisomal pools of iNOS in cytokine-stimulated hepatocytes and that the iNOS protein associates with peroxisome. Detergent solubilization of the membrane fraction released iNOS to the soluble fraction. iNOS localized to membrane fraction is predominantly monomeric, but dimerization is partially reconstituted rapidly upon incubation with tetrahydrobiopterin. The reconstituted iNOS exhibits a lower specific activity than iNOS isolated from the soluble pool. Depletion of intracellular tetrahydrobiopterin with an inhibitor of de novo pterin synthesis resulted in a predominance of monomeric iNOS without a greater relative distribution of iNOS to the peroxisomal pool. Thus, iNOS exists in a least two pools in hepatocytes: a soluble pool composed of both active dimer and monomer and a peroxisomal pool of monomeric iNOS. iNOS might localize to peroxisomes in long-lived cells such as hepatocytes as a protective mechanism to remove incompetent enzyme.

Requirement for Microtubules and Dynein Motors in the Earliest Stages of Peroxisome Biogenesis

Our aim was to determine the role of microtubules in the biogenesis of peroxisomes. Fusion experiments between human PEX16- and PEX1-mutant cells in the presence of nocodazol implied that microtubules were not required for import of proteins into the peroxisomal matrix after cell fusion complementation. We further studied the importance of microtubules in the early stages of peroxisome biogenesis following the microinjection complementation of PEX16-mutant cells. In the absence of nocodazol, nuclear microinjection of plasmids expressing EGFP-SKL and Pex16p in PEX16-mutant cells resulted in the accumulation of EGFP-SKL into newly formed peroxisomes. However, pretreatment of the cells with nocodazol, prior to microinjection, resulted in the inhibition of complementation of the PEX16 mutant and the cytosolic location of the EGFP-SKL. In addition, coexpression of a dominant-negative CC1 subunit of the dynein/dynactin motor complex resulted in the inability to complement PEX16-mutant cells. Both of these treatments resulted in the cytosolic localization of expressed Pex16p. Our results demonstrate that the formation of peroxisomes via the preperoxisomal compartment is dependent upon microtubules and minus-end-directed motor proteins and that the inhibition described above occurs at a step that precedes the association of Pex16p with the vesicles that would otherwise become the peroxisomes.

Peroxisome biogenesis

Peroxisome biogenesis conceptually consists of the (a) formation of the peroxisomal membrane, (b) import of proteins into the peroxisomal matrix and (c) proliferation of the organelles. Combined genetic and biochemical approaches led to the identification of 25 PEX genes-encoding proteins required for the biogenesis of peroxisomes, so-called peroxins. Peroxisomal matrix and membrane proteins are synthesized on free ribosomes in the cytosol and posttranslationally imported into the organelle in an unknown fashion. The protein import into the peroxisomal matrix and the targeting and insertion of peroxisomal membrane proteins is performed by distinct machineries. At least three peroxins have been shown to be involved in the topogenesis of peroxisomal membrane proteins. Elaborate peroxin complexes form the machinery which in a concerted action of the components transports folded, even oligomeric matrix proteins across the peroxisomal membrane. The past decade has significantly improved our knowledge of the involvement of certain peroxins in the distinct steps of the import process, like cargo recognition, docking of cargo-receptor complexes to the peroxisomal membrane, translocation, and receptor recycling. This review summarizes our knowledge of the functional role the known peroxins play in the biogenesis and maintenance of peroxisomes. Ideas on the involvement of preperoxisomal structures in the biogenesis of the peroxisomal membrane are highlighted and special attention is paid to the concept of cargo protein aggregation as a presupposition for peroxisomal matrix protein import.

More than folding: localized functions of cytosolic chaperones

Compared with other chaperone systems, heat shock proteins Hsp70 and Hsp90 interact with a larger variety of co-chaperone proteins that regulate their activity or aid in the folding of specific substrate proteins. Although many co-chaperones are soluble cytosolic proteins, co-chaperone domains are also found in modular adaptor proteins, which are often localized to intracellular membranes or elements of the cytoskeleton. These specialized co-chaperones include auxilin, cysteine string protein, Tom70, UNC-45 and homologs of Bag-1. The localized co-chaperones can harness the ATP-dependent mechanisms of Hsp70 and Hsp90 to do conformational work in diverse functional contexts, including vesicle secretion and recycling, protein transport and the regulated assembly and/or disassembly of protein complexes. Such flexibility is unique to the cytosolic Hsp70 and Hsp90 chaperone system.

The energetics of Pex5p-mediated peroxisomal protein import

Most newly synthesized peroxisomal matrix proteins are targeted to the organelle by Pex5p, the peroxisomal cycling receptor. According to current models of peroxisomal biogenesis, Pex5p interacts with cargo proteins in the cytosol and transports them to the peroxisomal membrane. After delivering the passenger protein into the peroxisomal matrix, Pex5p returns to the cytosol to catalyze additional rounds of transportation. Obviously, such cyclic pathway must require energy, and indeed, data confirming this need are already available. However, the exact step(s) of this cycle where energy input is necessary remains unclear. Here, we present data suggesting that insertion of Pex5p into the peroxisomal membrane does not require ATP hydrolysis. This observation raises the possibility that at the peroxisomal membrane ATP is needed predominantly (if not exclusively) downstream of the protein translocation step to reset the Pex5p-mediated transport system.

PEROXISOMEBIOGENESISDISORDERS

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

Peroxisome assembly in yeast

Peroxisomes are essential organelles that may be involved in various functions, dependent on organism, cell type, developmental stage of the cell, and the environment. Until recently, peroxisomes were viewed as a class of static organelles that developed by growth and fission from pre-existing organelles. Recent observations have challenged this view by providing evidence that peroxisomes may be part of the endomembrane system and constitute a highly dynamic population of organelles that arises and is removed upon environmental demands. Additionally, evidence is now accumulating that peroxisomes may arise by alternative methods. This review summarizes relevant recent data on this subject. In addition, the progress in the understanding of the principles of the peroxisomal matrix protein import machinery is discussed.

Protein structure and import into the peroxisomal matrix

Proteins destined for the peroxisomal matrix are synthesized in the cytosol, and imported post-translationally. It has been previously demonstrated that stably folded proteins are substrates for peroxisomal import. Mammalian peroxisomes do not contain endogenous chaperone molecules. Therefore, it is possible that proteins are required to fold into their stable, tertiary conformation in order to be imported into the peroxisome. These investigations were undertaken to determine whether proteins rendered incapable of folding were also substrates for import into peroxisomes. Reduction of albumin resulted in a less compact tertiary structure as measured by analytical centrifugation. Microinjection of unfolded albumin molecules bearing the PTS1 targeting signal resulted in their import into peroxisomes. Kinetic analysis indicated that native and unfolded molecules were imported into peroxisomes at comparable rates. While import was unaffected by treatment with cycloheximide, hsc70 molecules were observed to be imported along with the unfolded albumin molecules. These results indicate that proteins, which are incapable of assuming their native conformation, are substrates for peroxisomal import. When combined with previous observations demonstrating the import of stably folded proteins, these results support the model that tertiary structure has no effect on protein import into the peroxisomal matrix.

Peroxisome senescence in human fibroblasts

The molecular mechanisms of peroxisome biogenesis have begun to emerge; in contrast, relatively little is known about how the organelle functions as cells age. In this report, we characterize age-related changes in peroxisomes of human cells. We show that aging compromises peroxisomal targeting signal 1 (PTS1) protein import, affecting in particular the critical antioxidant enzyme catalase. The number and appearance of peroxisomes are altered in these cells, and the organelles accumulate the PTS1-import receptor, Pex5p, on their membranes. Concomitantly, cells produce increasing amounts of the toxic metabolite hydrogen peroxide, and we present evidence that this increased load of reactive oxygen species may further reduce peroxisomal protein import and exacerbate the effects of aging.

Chloroplast protein import inhibition by a soluble factor from wheat germ lysate

Protein import into chloroplasts occurs post-translationally in vitro. The precursor proteins are generally synthesised in a reticulocyte lysate- or wheat germ lysate-derived system and imported out of this system into chloroplast. These complex soluble protein mixtures are likely to contain factors, which influence somehow the import competence and import efficiency. Here we describe a heat-stable soluble proteinaceaous factor, which inhibits protein import into chloroplasts in vitro. The inhibitor interacts directly with the precursor protein and renders it import incompetent. This mode of action is supported by two observations: firstly, binding of the precursor to the chloroplast surface is diminished in the presence of the inhibitor. Secondly, when chloroplasts were loaded with precursor proteins under conditions, which allow only binding but not import the inhibitor was unable to abolish the subsequent translocation step.

Opinion: peroxisomal-protein import: is it really that complex?

Peroxisomal enzymes are synthesized in the cytoplasm and imported post-translationally across the peroxisome membrane. Unlike other organelles with a sealed membrane, peroxisomes can import folded enzymes, and they seem to lack intraperoxisomal chaperones. Here, we propose a mechanistic model for the early steps in peroxisomal-matrix-enzyme import, which might help to explain the unusual features of this process.

Monomeric alcohol oxidase is preferentially digested by a novel protease from Candida boidinii

A protease activity has been partially purified from peroxisomal matrix fractions of the methylotrophic yeast Candida boidinii. The enzyme migrates as a single peak on a sucrose velocity gradient with an apparent native molecular mass of approximately 80-90 kDa. Activity can be recovered from nonreducing sodium dodecyl sulfate gels as a approximately 20 kDa species, suggesting it is an oligomer. The protein exhibits chymotrypsin-like activity and cleaves the model compound suc-L-L-V-Y-AMC. Additionally, monomers of alcohol oxidase (AO), an abundant protein of C. boidinii peroxisomes, generated in vitro or in pulse-radiolabeled cells, are preferentially sensitive to degradation by the protease. Sensitivity is lost over time in vivo as AO folds and matures into octamers, suggesting that the protease may be involved in these processes.

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.

Heat shock proteins and cardiovascular pathophysiology

In the eukaryotic cell an intrinsic mechanism is present providing the ability to defend itself against external stressors from various sources. This defense mechanism probably evolved from the presence of a group of chaperones, playing a crucial role in governing proper protein assembly, folding, and transport. Upregulation of the synthesis of a number of these proteins upon environmental stress establishes a unique defense system to maintain cellular protein homeostasis and to ensure survival of the cell. In the cardiovascular system this enhanced protein synthesis leads to a transient but powerful increase in tolerance to such endangering situations as ischemia, hypoxia, oxidative injury, and endotoxemia. These so-called heat shock proteins interfere with several physiological processes within several cell organelles and, for proper functioning, are translocated to different compartments following stress-induced synthesis. In this review we describe the physiological role of heat shock proteins and discuss their protective potential against various stress agents in the cardiovascular system.

Protein translocation machineries of peroxisomes

Targeting of peroxisomal matrix and membrane proteins is performed by distinct transport machineries and requires the concerted action of at least 23 peroxins. Cargo recognition takes place in the cytosol and the multiple binding sites for peroxisomal signal sequence receptors at the peroxisomal membrane reflect the existence of an import cascade where the cargo-loaded receptors successively interact with different components of the import machinery. These interactions are likely to trigger conformational changes of the proteins within the import cascade which are required for the consecutive steps of peroxisomal protein import: docking, translocation, cargo release and receptor recycling.

Protein translocation across membranes

Cellular membranes act as semipermeable barriers to ions and macromolecules. Specialized mechanisms of transport of proteins across membranes have been developed during evolution. There are common mechanistic themes among protein translocation systems in bacteria and in eukaryotic cells. Here we review current understanding of mechanisms of protein transport across the bacterial plasma membrane as well as across several organelle membranes of yeast and mammalian cells. We consider a variety of organelles including the endoplasmic reticulum, outer and inner membranes of mitochondria, outer, inner, and thylakoid membranes of chloroplasts, peroxisomes, and lysosomes. Several common principles are evident: (a) multiple pathways of protein translocation across membranes exist, (b) molecular chaperones are required in the cytosol, inside the organelle, and often within the organelle membrane, (c) ATP and/or GTP hydrolysis is required, (d) a proton-motive force across the membrane is often required, and (e) protein translocation occurs through gated, aqueous channels. There are exceptions to each of these common principles indicating that our knowledge of how proteins translocate across membranes is not yet complete.

Hsp70 regulates the interaction between the peroxisome targeting signal type 1 (PTS1)-receptor Pex5p and PTS1

The peroxisome targeting signal type 1 (PTS1) receptor, Pex5p, of the tetratricopeptide repeat (TPR) motif family is located mostly in the cytosol and mediates the translocation of PTS1 proteins to peroxisomes. As a step towards understanding the mechanisms of protein import into peroxisomes, we investigated the molecular mechanisms involved in PTS1 recognition by Pex5p with regard to requirement of energy and cytosolic factors, using cell-free synthesized acyl-CoA oxidase (AOx) as a PTS1 cargo protein, together with Pex5p and heat-shock protein (Hsp)70 from rat liver. Pex5p was partly associated with peroxisomes of rat liver, was resistant to washing with a high concentration of salt and to alkaline extraction and was inaccessible to protease added externally. Pex5p bound to AOx in an ATP-dependent manner. AOx synthesized in a cell-free translating system from rabbit reticulocyte lysate was imported into peroxisomes without being supplemented with Pex5p and Hsp70, implying that peroxisome-associated Pex5p was released from the membranes and functional in this in vitro import assay. Antibodies against Pex5p and Hsp70 inhibited AOx import. In contrast, AOx synthesized in a wheat-germ lysate required the external addition of Pex5p for import, in which Hsp70 augmented the AOx import. The TPR domain of Pex5p was revealed to bind to the N-terminal part in an Hsp70-independent manner, whereas mutual interaction of the TPR region was noted in the presence of Hsp70. Hsp70 interacted with the TPR domain of Pex5p. Moreover, Hsp70 and ATP synergistically enhanced the binding of Pex5p to the C-terminal PTS1-containing part of AOx, implying that Pex5p recognizes its cargo PTS1 protein by chaperone-assisted as well as energy-dependent mechanisms in vivo.

The life cycle of the peroxisome

Peroxisomes are highly adaptable organelles that carry out oxidative reactions. Distinct cellular machineries act together to coordinate peroxisome formation, growth, division, inheritance, turnover, movement and function. Soluble and membrane-associated components of these machineries form complex networks of physical and functional interactions that provide supramolecular control of the precise dynamics of peroxisome biogenesis.

PTS2 protein import into mammalian peroxisomes

Peroxisome targeting signal (PTS)2 directs proteins from their site of synthesis in the cytosol to the lumen of the peroxisome. Unlike PTS1 which is present in the great majority of peroxisomal matrix proteins and whose import mechanics have been dissected in considerable detail, PTS2 is a relatively rare topogenic signal whose import mechanisms are far less well understood. However, as is the case for PTS1 proteins, an inability to import PTS2 proteins leads to human disease. In this report, we describe the biochemical characterization of mammalian PTS2 protein import using a semi-permeabilized cell system. We show that a PTS2-containing reporter molecule is taken up by peroxisomes in a reaction that is time-, temperature-, ATP-, and cytosol-dependent. Furthermore, the import process is specific, saturable, and requires action of the chaperone Hsc70, the cochaperone Hsp40, and the peroxins Pex5p and Pex14p. We also demonstrate peroxisomal translocation of PTS2 reporter/antibody complexes confirming the import competence of higher order structures. Importantly, cultured fibroblasts from patients with the rhizomelic form of chondrodysplasia punctata (RCDP) which are deficient for the PTS2 receptor protein, Pex7p, are unable to import the PTS2 reporter in this assay. The ability to monitor PTS2 import in vitro will permit, for the first time, a detailed comparison of the biochemical properties of PTS1 and PTS2 protein import.

Import of peroxisomal matrix and membrane proteins

This review summarizes the progress made in our understanding of peroxisome biogenesis in the last few years, during which the functional roles of many of the 23 peroxins (proteins involved in peroxisomal protein import and peroxisome biogenesis) have become clearer. Previous reviews in the field have focussed on the metabolic functions of peroxisomes, aspects of import/biogenesis the role of peroxins in human disease, and involvement of the endoplasmic reticulum in peroxisome membrane biogenesis as well as the degradation of this organelle. This review refers to some of the earlier work for the sake of introduction and continuity but deals primarily with the more recent progress. The principal areas of progress are the identification of new peroxins, definition of protein-protein interactions among peroxins leading to the recognition of complexes involved in peroxisomal protein import, insight into the biogenesis of peroxisomal membrane proteins, and, of most importance, the elucidation of the role of many conserved peroxins in human disease. Given the rapid progress in the field, this review also highlights some of the unanswered questions that remain to be tackled.

Quantitative analysis of peroxisomal protein import in vitro

Protein import into the peroxisome matrix is mediated by peroxisome-targeting signals (PTSs). We have developed a novel, quantitative, in vitro assay for measuring peroxisomal import of PTS1-containing proteins. This enzyme-linked immunosorbent assay-based system utilizes semi-intact human A431 cells or fibroblasts and a biotinylated version of the PTS1-containing import substrate, luciferase. We show that biotinylated luciferase accumulated in peroxisomes in a time- and temperature-dependent fashion, in a reaction stimulated by exogenously added ATP, cytosol, and zinc. No import was detected in fibroblasts from a human patient belonging to complementation group 2, who suffered from the fatal peroxisomal disorder Zellweger syndrome and lacked a functional PTS1 receptor, Pex5p. Also, the reaction was significantly inhibited by antibodies to the zinc-finger protein, Pex2p. Several lines of evidence demonstrate that biotinylated luciferase was imported into the lumen of bona fide peroxisomes. (a) Biochemical fractionation of cells after the import reaction showed a time-dependent accumulation of the import substrate within intracellular organelles. (b) Confocal fluorescence microscopy indicated that imported biotinylated luciferase colocalized with the peroxisomal protein PMP70. (c) Visualization of the imported biotinylated luciferase by indirect fluorescence or indirect immunofluorescence required disruption of the peroxisomal membrane, indicating true import rather than binding to the outside of the organelle.

Peroxisome biogenesis and degradation in yeast: a structure/function analysis

In yeast, peroxisomes are the site of specific catabolic pathways that characteristically include hydrogen peroxide producing oxidases and catalase. During the last 10 years, much progress has been made in unravelling the molecular mechanisms involved in the biogenesis of this organelle. At present, 23 different genes (PEX genes) have been identified that are involved in different aspects of peroxisome biogenesis (e.g., proliferation, formation of the peroxisomal membrane, import of matrix proteins). The principles of peroxisome degradation are still much less understood. Recently, the first yeast mutants affected in this process have become available and used to clone corresponding genes by functional complementation. In this paper, an overview is presented of the research on yeast peroxisomes, focusing on recent achievements in the molecular aspects of peroxisome development, function, and turnover.

Cell biology of peroxisomes and their characteristics in aquatic organisms

The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.

How peroxisomes arise

Peroxisomes are formed by the synthesis and assembly of membrane proteins and lipids, the selective import of proteins from the cytosol, and the growth and division of resultant organelles. To date, 23 proteins, called peroxins, are known to participate in these processes. This review summarizes recent progress in peroxin characterization and examines the underlying molecular mechanisms of peroxisome biosynthesis.

Cytosolic Hsp70s are involved in the transport of aminopeptidase 1 from the cytoplasm into the vacuole

Eukaryotic 70 kDa heat shock proteins (Hsp70s) are localized in various cellular compartments and exhibit functions such as protein translocation across membranes, protein folding and assembly. Here we demonstrate that the constitutively expressed members of the yeast cytoplasmic Ssa subfamily, Ssa1/2p, are involved in the transport of the vacuolar hydrolase aminopeptidase 1 from the cytoplasm into the vacuole. The Ssap family members displayed overlapping functions in the transport of aminopeptidase 1. In SSAI and SSAII deletion mutants the precursor of aminopeptidase 1 accumulated in a dodecameric complex that is packaged in prevacuolar transport vesicles. Ssa1/2p was prominently localized to the vacuolar membrane, consistent with the role we propose for Ssa proteins in the fusion of transport vesicles with the vacuolar membrane.

Peroxisomal membrane ascorbate peroxidase is sorted to a membranous network that resembles a subdomain of the endoplasmic reticulum

The peroxisomal isoform of ascorbate peroxidase (APX) is a novel membrane isoform that functions in the regeneration of NAD(+) and protection against toxic reactive oxygen species. The intracellular localization and sorting of peroxisomal APX were examined both in vivo and in vitro. Epitope-tagged peroxisomal APX, which was expressed transiently in tobacco BY-2 cells, localized to a reticular/circular network that resembled endoplasmic reticulum (ER; 3,3'-dihexyloxacarbocyanine iodide-stained membranes) and to peroxisomes. The reticular network did not colocalize with other organelle marker proteins, including three ER reticuloplasmins. However, in vitro, peroxisomal APX inserted post-translationally into the ER but not into other purified organelle membranes (including peroxisomal membranes). Insertion into the ER depended on the presence of molecular chaperones and ATP. These results suggest that regions of the ER serve as a possible intermediate in the sorting pathway of peroxisomal APX. Insight into this hypothesis was obtained from in vivo experiments with brefeldin A (BFA), a toxin that blocks vesicle-mediated protein export from ER. A transiently expressed chloramphenicol acetyltransferase-peroxisomal APX (CAT-pAPX) fusion protein accumulated only in the reticular/circular network in BFA-treated cells; after subsequent removal of BFA from these cells, the CAT-pAPX was distributed to preexisting peroxisomes. Thus, plant peroxisomal APX, a representative enzymatic peroxisomal membrane protein, is sorted to peroxisomes through an indirect pathway involving a preperoxisomal compartment with characteristics of a distinct subdomain of the ER, possibly a peroxisomal ER subdomain.

Import of proteins into peroxisomes

Peroxisomes are organelles that confine an important set of enzymes within their single membrane boundaries. In man, a wide variety of genetic disorders is caused by loss of peroxisome function. In the most severe cases, the clinical phenotype indicates that abnormalities begin to appear during embryological development. In less severe cases, the quality of life of adults is affected. Research on yeast model systems has contributed to a better understanding of peroxisome formation and maintenance. This framework of knowledge has made it possible to understand the molecular basis of most of the peroxisome biogenesis disorders. Interestingly, most peroxisome biogenesis disorders are caused by a failure to target peroxisomal proteins to the organellar matrix or membrane, which classifies them as protein targeting diseases. Here we review recent fundamental research on peroxisomal protein targeting and discuss a few burning questions in the field concerning the origin of peroxisomes.