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

Recognition and Chaperoning by Pex19, Followed by Trafficking and Membrane Insertion of the Peroxisome Proliferation Protein, Pex11

Pex11, an abundant peroxisomal membrane protein (PMP), is required for division of peroxisomes and is robustly imported to peroxisomal membranes. We present a comprehensive analysis of how the Pichia pastoris Pex11 is recognized and chaperoned by Pex19, targeted to peroxisome membranes and inserted therein. We demonstrate that Pex11 contains one Pex19-binding site (Pex19-BS) that is required for Pex11 insertion into peroxisomal membranes by Pex19, but is non-essential for peroxisomal trafficking. We provide extensive mutational analyses regarding the recognition of Pex19-BS in Pex11 by Pex19. Pex11 also has a second, Pex19-independent membrane peroxisome-targeting signal (mPTS) that is preserved among Pex11-family proteins and anchors the human HsPex11γ to the outer leaflet of the peroxisomal membrane. Thus, unlike most PMPs, Pex11 can use two mechanisms of transport to peroxisomes, where only one of them depends on its direct interaction with Pex19, but the other does not. However, Pex19 is necessary for membrane insertion of Pex11. We show that Pex11 can self-interact, using both homo- and/or heterotypic interactions involving its N-terminal helical domains. We demonstrate that Pex19 acts as a chaperone by interacting with the Pex19-BS in Pex11, thereby protecting Pex11 from spontaneous oligomerization that would otherwise cause its aggregation and subsequent degradation.

Towards the molecular mechanism of the integration of peroxisomal membrane proteins

The correct topogenesis of peroxisomal membrane proteins is a crucial step for the formation of functioning peroxisomes. Although this process has been widely studied, the exact mechanism with which it occurs has not yet been fully characterized. Nevertheless, it is generally accepted that peroxisomes employ three proteins – Pex3, Pex19 and Pex16 in mammals – for the insertion of peroxisomal membrane proteins into the peroxisomal membrane. Structural biology approaches have been utilized for the elucidation of the mechanistic questions of peroxisome biogenesis, mainly by providing information on the architecture of the proteins significant for this process. This review aims to summarize, compare and put into perspective the structural knowledge that has been generated mainly for Pex3 and Pex19 and their interaction partners in recent years. This article is part of a Special Issue entitled: Peroxisomes edited by Ralf Erdmann.

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Structures of the PMP insertion factors Pex3 and Pex19 and their interactions with other protein ligands

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Structural insights provide a mechanistic understanding of the PMP functional network.

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Functional implications of structural order/disorder transitions

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

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

Multiple pathways for protein transport to peroxisomes

Peroxisomes are unique among the organelles of the endomembrane system. Unlike other organelles that derive most if not all of their proteins from the ER (endoplasmic reticulum), peroxisomes contain dedicated machineries for import of matrix proteins and insertion of membrane proteins. However, peroxisomes are also able to import a subset of their membrane proteins from the ER. One aspect of peroxisome biology that has remained ill defined is the role the various import pathways play in peroxisome maintenance. In this review, we discuss the available data on matrix and membrane protein import into peroxisomes.

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Peroxisomal membrane and matrix proteins require distinct factors for their transport.

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Matrix proteins fold in the cytosol prior to their import.

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Loaded targeting receptors form part of the matrix protein translocation pore.

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Many membrane proteins are directly inserted into the peroxisomal membrane.

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Some peroxisomal membrane proteins are transported via the ER to peroxisomes.

Hydrophobic handoff for direct delivery of peroxisome tail-anchored proteins

Tail-anchored (TA) proteins are inserted into membranes post-translationally through a C-terminal transmembrane domain (TMD). The PEX19 protein binds peroxisome TA proteins in the cytoplasm and delivers them to the membrane through the PEX3 receptor protein. An amphipathic segment in PEX19 promotes docking on PEX3. However, how this leads to substrate insertion is unknown. Here we reconstitute peroxisome TA protein biogenesis into two sequential steps of substrate TMD engagement and membrane insertion. We identify a series of previously uncharacterized amphipathic segments in PEX19 and identify one whose hydrophobicity is required for membrane insertion, but not TMD chaperone activity or PEX3 binding. A membrane-proximal hydrophobic surface of PEX3 promotes an unconventional form of membrane intercalation, and is also required for TMD insertion. Together, these data support a mechanism in which hydrophobic moieties in the TMD chaperone and its membrane-associated receptor act in a concerted manner to prompt TMD release and membrane insertion.

Protein import machineries of peroxisomes

Peroxisomes are a class of structurally and functionally related organelles present in almost all eukaryotic cells. The importance of peroxisomes for human life is highlighted by severe inherited diseases which are caused by defects of peroxins, encoded by PEX genes. To date 32 peroxins are known to be involved in different aspects of peroxisome biogenesis. This review addresses two of these aspects, the translocation of soluble proteins into the peroxisomal matrix and the biogenesis of the peroxisomal membrane. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

Trypanosoma bruceiglycosomal ABC transporters: identification and membrane targeting

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

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

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

Cymbidium ringspot virus defective interfering RNA replication in yeast cells occurs on endoplasmic reticulum-derived membranes in the absence of peroxisomes

The replication of Cymbidium ringspot virus (CymRSV) defective interfering (DI) RNA in cells of the yeast Saccharomyces cerevisiae normally takes place in association with the peroxisomal membrane, thus paralleling the replication events in infected plant cells. However, previous results with a peroxisome-deficient mutant strain of yeast had suggested that the presence of peroxisomes is not a strict requirement for CymRSV DI RNA replication. Thus, a novel approach was used to study the putative alternative sites of replication by using S. cerevisiae strain YPH499 which does not contain normal peroxisomes. In this strain, CymRSV p33 and p92 accumulated over portions of the nuclear membrane and on membranous overgrowths which were identified as endoplasmic reticulum (ER) strands, following immunofluorescence and immunoelectron microscope observations. The proteins were not released by high-pH treatment, but were susceptible to proteolytic digestion, thus indicating peripheral and not integrated association. ER-associated p33 and p92 proteins supported in trans the replication of DI RNA. The capacity of plus-strand RNA viruses to replicate in association with different types of cell membranes was thus confirmed.

The ER-peroxisome connection in plants: Development of the “ER semi-autonomous peroxisome maturation and replication” model for plant peroxisome biogenesis

The perceived role of the ER in the biogenesis of plant peroxisomes has evolved significantly from the original "ER vesiculation" model, which portrayed co-translational import of proteins into peroxisomes originating from the ER, to the "ER semi-autonomous peroxisome" model wherein membrane lipids and post-translationally acquired peroxisomal membrane proteins (PMPs) were derived from the ER. Results from more recent studies of various plant PMPs including ascorbate peroxidase, PEX10 and PEX16, as well as a viral replication protein, have since led to the formulation of a more elaborate "ER semi-autonomous peroxisome maturation and replication" model. Herein we review these results in the context of this newly proposed model and its predecessor models. We discuss also key distinct features of the new model pertaining to its central premise that the ER defines the semi-autonomous maturation (maintenance/assembly/differentiation) and duplication (division) features of specialized classes of pre-existing plant peroxisomes. This model also includes a novel peroxisome-to-ER retrograde sorting pathway that may serve as a constitutive protein retrieval/regulatory system. In addition, new plant peroxisomes are envisaged to arise primarily by duplication of the pre-existing peroxisomes that receive essential membrane components from the ER.

Targeting signals in peroxisomal membrane proteins

Peroxisomal membrane proteins (PMPs) are encoded by the nuclear genome and translated on cytoplasmic ribosomes. Newly synthesized PMPs can be targeted directly from the cytoplasm to peroxisomes or travel to peroxisomes via the endoplasmic reticulum (ER). The mechanisms responsible for the targeting of these proteins to the peroxisomal membrane are still rather poorly understood. However, it is clear that the trafficking of PMPs to peroxisomes depends on the presence of cis-acting targeting signals, called mPTSs. These mPTSs show great variability both in the identity and number of requisite residues. An emerging view is that mPTSs consist of at least two functionally distinct domains: a targeting element, which directs the newly synthesized PMP from the cytoplasm to its target membrane, and a membrane-anchoring sequence, which is required for the permanent insertion of the protein into the peroxisomal membrane. In this review, we summarize our knowledge of the mPTSs currently identified.

An intimate collaboration between peroxisomes and lipid bodies

Although peroxisomes oxidize lipids, the metabolism of lipid bodies and peroxisomes is thought to be largely uncoupled from one another. In this study, using oleic acid-cultured Saccharomyces cerevisiae as a model system, we provide evidence that lipid bodies and peroxisomes have a close physiological relationship. Peroxisomes adhere stably to lipid bodies, and they can even extend processes into lipid body cores. Biochemical experiments and proteomic analysis of the purified lipid bodies suggest that these processes are limited to enzymes of fatty acid beta oxidation. Peroxisomes that are unable to oxidize fatty acids promote novel structures within lipid bodies ("gnarls"), which may be organized arrays of accumulated free fatty acids. However, gnarls are suppressed, and fatty acids are not accumulated in the absence of peroxisomal membranes. Our results suggest that the extensive physical contact between peroxisomes and lipid bodies promotes the coupling of lipolysis within lipid bodies with peroxisomal fatty acid oxidation.

Pex19p Binds Pex30p and Pex32p at Regions Required for Their Peroxisomal Localization but Separate from Their Peroxisomal Targeting Signals

The assembly of proteins in the peroxisomal membrane is a multistep process requiring their recognition in the cytosol, targeting to and insertion into the peroxisomal membrane, and stabilization within the lipid bilayer. The peroxin Pex19p has been proposed to be either the receptor that recognizes and targets newly synthesized peroxisomal membrane proteins (PMP) to the peroxisome or a chaperone required for stabilization of PMPs at the peroxisomal membrane. Differentiating between these two roles for Pex19p could be achieved by determining whether the peroxisomal targeting signal (PTS) and the region of Pex19p binding of a PMP are the same or different. We addressed the role for Pex19p in the assembly of two PMPs, Pex30p and Pex32p, of the yeast Saccharomyces cerevisiae. Pex30p and Pex32p control peroxisome size and number but are dispensable for peroxisome formation. Systematic truncations from the carboxyl terminus, together with in-frame deletions of specific regions, have identified PTSs essential for targeting Pex30p and Pex32p to peroxisomes. Both Pex30p and Pex32p interact with Pex19p in regions that do not overlap with their PTSs. However, Pex19p is required for localizing Pex30p and Pex32p to peroxisomes, because mutations that disrupt the interaction of Pex19p with Pex30p and Pex32p lead to their mislocalization to a compartment other than peroxisomes. Mutants of Pex30p and Pex32p that localize to peroxisomes but produce cells exhibiting the peroxisomal phenotypes of cells lacking these proteins demonstrate that the regions in these proteins that control peroxisomal targeting and cell biological activity are separable. Together, our data show that the interaction of Pex19p with Pex30p and Pex32p is required for their roles in peroxisome biogenesis and are consistent with a chaperone role for Pex19p in stabilizing or maintaining membrane proteins in peroxisomes.

Functional Domains and Dynamic Assembly of the Peroxin Pex14p, the Entry Site of Matrix Proteins

The 41-kDa membrane-anchored peroxin Pex14p functions as the peroxisome targeting signal (PTS) receptor-mediated, initial import site for matrix proteins. We here identify the functional domains of Pex14p involved in the assembly of import site subcomplexes. The minimal region of Pex14p required for restoring impaired protein import in pex14 Chinese hamster ovary cell mutant lies at residues 21-260 in the primary sequence. A highly conserved N-terminal region, encompassing residues 21-70, interacts with the PTS1 receptor Pex5p, Pex13p, and Pex19p that is essential for membrane biogenesis. N-terminal residues 21-140, including a hydrophobic segment at 110-138, function as a topogenic sequence. Site-directed mutagenesis, size fractionation, and chemical cross-linking analyses demonstrate that the coiled-coil domain at residues 156-197 regulates homodimerization of Pex14p. Moreover, AXXXA and GXXXG motifs in the transmembrane segment mediate homomeric oligomerization of Pex14p, giving rise to assembly of high molecular mass complexes and thereby assuring Pex13p-dependent localization of Pex14p to peroxisomes. Pex5p, Pex13p, and Pex19p bind to Pex14p homo-oligomers with different molecular masses, whereas cargo-unloaded Pex5p apparently disassembles Pex14p homo-oligomers. Thus, Pex14p most likely forms several distinct peroxin complexes involved in peroxisomal matrix protein import.

Role of Pex19p in the targeting of PMP70 to peroxisome

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

Localization of the tomato bushy stunt virus replication protein p33 reveals a peroxisome-to-endoplasmic reticulum sorting pathway

Tomato bushy stunt virus (TBSV), a positive-strand RNA virus, causes extensive inward vesiculations of the peroxisomal boundary membrane and formation of peroxisomal multivesicular bodies (pMVBs). Although pMVBs are known to contain protein components of the viral membrane-bound RNA replication complex, the mechanisms of protein targeting to peroxisomal membranes and participation in pMVB biogenesis are not well understood. We show that the TBSV 33-kD replication protein (p33), expressed on its own, targets initially from the cytosol to peroxisomes, causing their progressive aggregation and eventually the formation of peroxisomal ghosts. These altered peroxisomes are distinct from pMVBs; they lack internal vesicles and are surrounded by novel cytosolic vesicles that contain p33 and appear to be derived from evaginations of the peroxisomal boundary membrane. Concomitant with these changes in peroxisomes, p33 and resident peroxisomal membrane proteins are relocalized to the peroxisomal endoplasmic reticulum (pER) subdomain. This sorting of p33 is disrupted by the coexpression of a dominant-negative mutant of ADP-ribosylation factor1, implicating coatomer in vesicle formation at peroxisomes. Mutational analysis of p33 revealed that its intracellular sorting is also mediated by several targeting signals, including three peroxisomal targeting elements that function cooperatively, plus a pER targeting signal resembling an Arg-based motif responsible for vesicle-mediated retrieval of escaped ER membrane proteins from the Golgi. These results provide insight into virus-induced intracellular rearrangements and reveal a peroxisome-to-pER sorting pathway, raising new mechanistic questions regarding the biogenesis of peroxisomes in plants.

Endoplasmic Reticulum-directed Pex3p Routes to Peroxisomes and Restores Peroxisome Formation in a Saccharomyces cerevisiae pex3Δ Strain

Recent studies on the sorting of peroxisomal membrane proteins challenge the long-standing model in which peroxisomes are considered to be autonomous organelles that multiply by growth and division. Here, we present data lending support to the idea that the endoplasmic reticulum (ER) is involved in sorting of the peroxisomal membrane protein Pex3p, a protein required early in peroxisome biogenesis. First, we show that the introduction of an artificial glycosylation site into the N terminus of Pex3p leads to partial N-linked core glycosylation, indicative of insertion into the ER membrane. Second, when FLAG-tagged Pex3p is equipped with an ER targeting signal, it can restore peroxisome formation in pex3Delta cells. Importantly, FLAG antibodies that specifically recognize the processed Pex3p show that the signal peptide of the fusion protein is efficiently cleaved off and that the processed protein localizes to peroxisomes. In contrast, a Pex3p construct in which cleavage of the signal peptide is blocked by a mutation localizes to the ER and the cytosol and cannot complement pex3Delta cells. Together, these results strongly suggest that ER-targeted Pex3p indeed routes via the ER to peroxisomes, and we hypothesize that this pathway is also used by endogenous Pex3p.

Arabidopsis peroxisomes possess functionally redundant membrane and matrix isoforms of monodehydroascorbate reductase

The H2O2 byproduct of fatty acid catabolism in plant peroxisomes is removed in part by a membrane-associated antioxidant system that involves both an ascorbate peroxidase and a monodehydroascorbate reductase (MDAR). Despite descriptions of 32-kDa MDAR polypeptides in pea and castor peroxisomal membranes and cDNA sequences for several 'cytosolic' MDARs, the genetic and protein factors responsible for peroxisomal MDAR function have yet to be elucidated. Of the six MDAR polypeptides in the Arabidopsis proteome, named AtMDAR1 to AtMDAR6 in this study, 47-kDa AtMDAR1 and 54-kDa AtMDAR4 possess amino acid sequences that resemble matrix (PTS1) and membrane peroxisomal targeting signals, respectively. Epitope-tagged versions of these two MDARs and a pea 47-kDa MDAR (PsMDAR) sorted in vivo directly from the cytosol to peroxisomes in Arabidopsis and BY-2 suspension cells, whereas AtMDAR2 and AtMDAR3 accumulated in the cytosol. The PTS1-dependent sorting of AtMDAR1 and PsMDAR to peroxisomes was incomplete (inefficient?), but was improved for PsMDAR after changing its PTS1 sequence from -SKI to the canonical tripeptide -SKL. A C-terminal transmembrane domain and basic cluster of AtMDAR4 were necessary and sufficient for targeting directly to peroxisomes. MDAR activity in isolated Arabidopsis peroxisomes was distributed among both water-soluble matrix and KCl-insoluble membrane subfractions that contained respectively 47- and 54-kDa MDAR polypeptides. Notably, a 32-kDa MDAR was not identified. Combined with membrane association and topological orientation findings, these results indicate that ascorbate recycling in Arabidopsis (and probably other plant) peroxisomes is coordinated through functionally redundant MDARs that reside in the membrane and the matrix of the organelle.

Biogenesis of peroxisomes

Genetic and proteomic approaches have led to the identification of 32 proteins, collectively called peroxins, which are required for the biogenesis of peroxisomes. Some are responsible for the division and inheritance of peroxisomes; however, most peroxins have been implicated in the topogenesis of peroxisomal proteins. Peroxisomal membrane and matrix proteins are synthesized on free ribosomes in the cytosol and are imported post-translationally into pre-existing organelles (Lazarow PB & Fujiki Y (1985) Annu Rev Cell Biol1, 489-530). Progress has been made in the elucidation of how these proteins are targeted to the organelle. In addition, the understanding of the composition of the peroxisomal import apparatus and the order of events taking place during the cascade of peroxisomal protein import has increased significantly. However, our knowledge on the basic principles of peroxisomal membrane protein insertion or translocation of peroxisomal matrix proteins across the peroxisomal membrane is rather limited. The latter is of particular interest as the peroxisomal import machinery accommodates folded, even oligomeric, proteins, which distinguishes this apparatus from the well characterized translocons of other organelles. Furthermore, the origin of the peroxisomal membrane is still enigmatic. Recent observations suggest the existence of two classes of peroxisomal membrane proteins. Newly synthesized class I proteins are directly targeted to and inserted into the peroxisomal membrane, while class II proteins reach their final destination via the endoplasmic reticulum or a subcompartment thereof, which would be in accord with the idea that the peroxisomal membrane might be derived from the endoplasmic reticulum.

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

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

Biogenesis of peroxisomes and glycosomes: trypanosomatid glycosome assembly is a promising new drug target

In trypanosomatids (Trypanosoma and Leishmania), protozoa responsible for serious diseases of mankind in tropical and subtropical countries, core carbohydrate metabolism including glycolysis is compartmentalized in peculiar peroxisomes called glycosomes. Proper biogenesis of these organelles and the correct sequestering of glycolytic enzymes are essential to these parasites. Biogenesis of glycosomes in trypanosomatids and that of peroxisomes in other eukaryotes, including the human host, occur via homologous processes involving proteins called peroxins, which exert their function through multiple, transient interactions with each other. Decreased expression of peroxins leads to death of trypanosomes. Peroxins show only a low level of sequence conservation. Therefore, it seems feasible to design compounds that will prevent interactions of proteins involved in biogenesis of trypanosomatid glycosomes without interfering with peroxisome formation in the human host cells. Such compounds would be suitable as lead drugs against trypanosomatid-borne diseases.

Peroxisomal disorders I: biochemistry and genetics of peroxisome biogenesis disorders

The peroxisomal disorders represent a group of genetic diseases in humans in which there is an impairment in one or more peroxisomal functions. The peroxisomal disorders are usually subdivided into two subgroups including (i) the peroxisome biogenesis disorders (PBDs) and (ii) the single peroxisomal (enzyme-) protein deficiencies. The PBD group is comprised of four different disorders including Zellweger syndrome (ZS), neonatal adrenoleukodystrophy (NALD), infantile Refsum's disease (IRD), and rhizomelic chondrodysplasia punctata (RCDP). ZS, NALD, and IRD are clearly distinct from RCDP and are usually referred to as the Zellweger spectrum with ZS being the most severe and NALD and IRD the less severe disorders. Studies in the late 1980s had already shown that the PBD group is genetically heterogeneous with at least 12 distinct genetic groups as concluded from complementation studies. Thanks to the much improved knowledge about peroxisome biogenesis notably in yeasts and the successful extrapolation of this knowledge to humans, the genes responsible for all these complementation groups have been identified making molecular diagnosis of PBD patients feasible now. It is the purpose of this review to describe the current stage of knowledge about the clinical, biochemical, cellular, and molecular aspects of PBDs, and to provide guidelines for the post- and prenatal diagnosis of PBDs. Less progress has been made with respect to the pathophysiology and therapy of PBDs. The increasing availability of mouse models for these disorders is a major step forward in this respect.

PEX19 is a predominantly cytosolic chaperone and import receptor for class 1 peroxisomal membrane proteins

Integral peroxisomal membrane proteins (PMPs) are synthesized in the cytoplasm and imported posttranslationally. Here, we demonstrate that PEX19 binds and stabilizes newly synthesized PMPs in the cytosol, binds to multiple PMP targeting signals (mPTSs), interacts with the hydrophobic domains of PMP targeting signals, and is essential for PMP targeting and import. These results show that PEX19 functions as both a chaperone and an import receptor for newly synthesized PMPs. We also demonstrate the existence of two PMP import mechanisms and two classes of mPTSs: class 1 mPTSs, which are bound by PEX19 and imported in a PEX19-dependent manner, and class 2 mPTSs, which are not bound by PEX19 and mediate protein import independently of PEX19.

Expression of the Cymbidium ringspot virus 33-kilodalton protein in Saccharomyces cerevisiae and molecular dissection of the peroxisomal targeting signal

Open reading frame 1 in the viral genome of Cymbidium ringspot virus encodes a 33-kDa protein (p33), which was previously shown to localize to the peroxisomal membrane in infected and transgenic plant cells. To determine the sequence requirements for the organelle targeting and membrane insertion, the protein was expressed in the yeast Saccharomyces cerevisiae in native form (33K) or fused to the green fluorescent protein (33KGFP). Cell organelles were identified by immunolabeling of marker proteins. In addition, peroxisomes were identified by simultaneous expression of the red fluorescent protein DsRed containing a peroxisomal targeting signal and mitochondria by using the dye MitoTracker. Fluorescence microscopy showed the 33KGFP fusion protein concentrated in a few large bodies colocalizing with peroxisomes. These bodies were shown by electron microscopy to be composed by aggregates of peroxisomes, a few mitochondria and endoplasmic reticulum (ER) strands. In immunoelectron microscopy, antibodies to p33 labeled the peroxisomal clumps. Biochemical analysis suggested that p33 is anchored to the peroxisomal membrane through a segment of ca. 7 kDa, which corresponds to the sequence comprising two hydrophobic transmembrane domains and a hydrophilic interconnecting loop. Analysis of deletion mutants confirmed these domains as essential components of the p33 peroxisomal targeting signal, together with a cluster of three basic amino acids (KRR). In yeast mutants lacking peroxisomes p33 was detected in the ER. The possible involvement of the ER as an intermediate step for the integration of p33 into the peroxisomal membrane is discussed.

Peroxisomal membrane proteins contain common Pex19p-binding sites that are an integral part of their targeting signals

Targeting of peroxisomal membrane proteins (PMPs) is a multistep process that requires not only recognition of PMPs in the cytosol but also their insertion into the peroxisomal membrane. As a consequence, targeting signals of PMPs (mPTS) are rather complex. A candidate protein for the PMP recognition event is Pex19p, which interacts with most PMPs. However, the respective Pex19p-binding sites are ill-defined and it is currently disputed whether these sites are contained within mPTS. By using synthetic peptide scans and yeast two-hybrid analyses, we determined and characterized Pex19p-binding sites in Pex11p and Pex13p, two PMPs from Saccharomyces cerevisiae. The sites turned out to be composed of a short helical motif with a minimal length of 11 amino acids. With the acquired data, it proved possible to predict and experimentally verify Pex19p-binding sites in several other PMPs by applying a pattern search and a prediction matrix. A peroxisomally targeted Pex13p fragment became mislocalized to the endoplasmic reticulum in the absence of its Pex19p-binding site. By adding the heterologous binding site of Pex11p, peroxisomal targeting of the Pex13p fragment was restored. We conclude that Pex19p-binding sites are well-defined entities that represent an essential part of the mPTS.

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

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

Peroxisome biogenesis and the role of protein import

Peroxisomes are metabolic organelles with enzymatic content that are found in virtually all cells and are involved in beta-oxidation of fatty acids, hydrogen peroxide-based respiration and defence against oxidative stress. The steps of their biogenesis involves "peroxins", proteins encoded by PEX genes. Peroxins are involved in three key stages of peroxisome development: (1). import of peroxisomal membrane proteins; (2). import of peroxisomal matrix proteins and (3). peroxisome proliferation. Of these three areas, peroxisomal matrix-protein import is by far the best understood and accounts for most of the available published data on peroxisome biogenesis. Defects in peroxisome biogenesis result in peroxisome biogenesis disorders (PBDs), which although rare, have no known cure to-date. This review explores current understanding of each key area in peroxisome biogenesis, paying particular attention to the role of protein import.

Multiple targeting modules on peroxisomal proteins are not redundant: discrete functions of targeting signals within Pmp47 and Pex8p

Several peroxisomal proteins have two nonoverlapping targeting signals. These signals have been termed "redundant" because targeting can still occur with only one signal. We now report that separate targeting motifs within both Pmp47 and Pex8 provide complementary function. Pmp47 is an ATP translocator that contains six transmembrane domains (TMDs). We had previously shown that the TMD2 region (termed TMD2R, consisting of TMD2 and a short adjacent segment of cytosolic loop) was required for targeting to proliferated peroxisomes in Saccharomyces cerevisiae. We now report that the analogous TMD4R, which cannot target to proliferated peroxisomes, targets at least as well, or much better (depending on strain and growth conditions) in cells containing only basal (i.e., nonproliferated) peroxisomes. These data suggest differences in the targeting pathway among peroxisome populations. Pex8p, a peripheral protein facing the matrix, contains a typical carboxy terminal targeting sequence (PTS1) that has been shown to be nonessential for targeting, indicating the existence of a second targeting domain (not yet defined in S. cerevisiae); thus, its function was unknown. We show that targeting to basal peroxisomes, but not to proliferated peroxisomes, is more efficient with the PTS1 than without it. Our results indicate that multiple targeting signals within peroxisomal proteins extend coverage among heterogeneous populations of peroxisomes and increase efficiency of targeting in some metabolic states.

Sorting pathway and molecular targeting signals for the Arabidopsis peroxin 3

Peroxin 3 (Pex3p) has been identified and characterized as a peroxisomal membrane protein in yeasts and mammals. We identified two putative homologs in Arabidopsis (AtPex3p, forms 1 and 2), both with an identical cluster of positively charged amino acid residues (RKHRRK) immediately preceding one of the two predicted transmembrane domains (TMD1). In transiently transformed Arabidopsis and tobacco BY-2 suspension-cultured cells, epitope-tagged AtPex3p (form 2) sorted post-translationally from the cytosol directly to peroxisomes, the first sorting pathway described for any peroxin in plants. TMD1 and RKHRRK were necessary for targeting form 2 to peroxisomes and sufficient for directing chloramphenicol acetyltransferase to peroxisomes in both cell types. The N and C termini of AtPex3p (form 2) extend into the peroxisomal matrix, different from mammal and yeast Pex3 proteins. Thus, two authentic peroxisomal membrane-bound Pex3p homologs possessing a membrane peroxisomal targeting signal, the first one defined for a plant peroxin and for any Pex3p homolog, exist in plant cells.

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.

Pex30p, Pex31p, and Pex32p form a family of peroxisomal integral membrane proteins regulating peroxisome size and number in Saccharomyces cerevisiae

The peroxin Pex23p of the yeast Yarrowia lipolytica exhibits high sequence similarity to the hypothetical proteins Ylr324p, Ygr004p, and Ybr168p encoded by the Saccharomyces cerevisiae genome. Ylr324p, Ygr004p, and Ybr168p are integral to the peroxisomal membrane and act to control peroxisome number and size. Synthesis of Ylr324p and Ybr168p, but not of Ygr004p, is induced during incubation of cells in oleic acid-containing medium, the metabolism of which requires intact peroxisomes. Cells deleted for YLR324w exhibit increased numbers of peroxisomes, whereas cells deleted for YGR004w or YBR168w exhibit enlarged peroxisomes. Ylr324p and Ybr168p cannot functionally substitute for one another or for Ygr004p, whereas Ygr004p shows partial functional redundancy with Ylr324p and Ybr168p. Ylr324p, Ygr004p, and Ybr168p interact within themselves and with Pex28p and Pex29p, which have been shown also to regulate peroxisome size and number. Systematic deletion of genes demonstrated that PEX28 and PEX29 function upstream of YLR324w, YGR004w, and YBR168w in the regulation of peroxisome proliferation. Our data suggest a role for Ylr324p, Ygr004p, and Ybr168p--now designated Pex30p, Pex31p, and Pex32p, respectively--together with Pex28p and Pex29p in controlling peroxisome size and proliferation in Saccharomyces cerevisiae.

Characterization of the targeting signal of the Arabidopsis 22-kD integral peroxisomal membrane protein

Using a combination of in vivo and in vitro assays, we characterized the sorting pathway and molecular targeting signal for the Arabidopsis 22-kD peroxisome membrane protein (PMP22), an integral component of the membrane of all peroxisomes in the mature plant. We show that nascent PMP22 is sorted directly from the cytosol to peroxisomes and that it is inserted into the peroxisomal boundary membrane with its N- and C-termini facing the cytosol. This direct sorting of PMP22 to peroxisomes contrasts with the indirect sorting reported previously for cottonseed (Gossypium hirsutum) ascorbate peroxidase, an integral PMP that sorts to peroxisomes via a subdomain of the endoplasmic reticulum. Thus, at least two different sorting pathways for PMPs exist in plant cells. At least four distinct regions within the N-terminal one-half of PMP22, including a positively charged domain present in most peroxisomal integral membrane-destined proteins, functions in a cooperative manner in efficient peroxisomal targeting and insertion. In addition, targeting with high fidelity to peroxisomes requires all four membrane-spanning domains in PMP22. Together, these results illustrate that the PMP22 membrane peroxisomal targeting signal is complex and that different elements within the signal may be responsible for mediating unique aspects of PMP22 biogenesis, including maintaining the solubility before membrane insertion, targeting to peroxisomes, and ensuring proper assembly in the peroxisomal boundary membrane.

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.

Involvement of the endoplasmic reticulum in peroxisome formation

The traditional view holds that peroxisomes are autonomous organelles multiplying by growth and division. More recently, new observations have challenged this concept. Herein, we present evidence supporting the involvement of the endoplasmic reticulum (ER) in peroxisome formation by electron microscopy, immunocytochemistry and three-dimensional image reconstruction of peroxisomes and associated compartments in mouse dendritic cells. We found the peroxisomal membrane protein Pex13p and the ATP-binding cassette transporter protein PMP70 present in specialized subdomains of the ER that were continuous with a peroxisomal reticulum from which mature peroxisomes arose. The matrix proteins catalase and thiolase were only detectable in the reticula and peroxisomes. Our results suggest the existence of a maturation pathway from the ER to peroxisomes and implicate the ER as a major source from which the peroxisomal membrane is derived.

Saccharomyces cerevisiae contains a Type II phosphoinositide 4-kinase

The yeast Saccharomyces cerevisiae contains two known phosphoinositide 4-kinases (PI 4-kinases), which are encoded by PIK1 and STT4; both are essential. Pik1p is important for exocytic transport from the Golgi, whereas Stt4p plays a role in cell-wall integrity and cytoskeletal rearrangements. In the present study, we report that cells have a third PI 4-kinase activity encoded by LSB6, a protein identified previously in a two-hybrid screen as interacting with LAS17p. Although Pik1p and Stt4p are closely related members of the Type III class of PI 4-kinases, Lsb6p belongs to the distinct Type II class, based on its amino acid sequence, its sensitivity to inhibition by adenosine and its insensitivity to wortmannin. Lsb6p is the first fungal Type II enzyme cloned. The protein was expressed and purified from Sf9 cells and used to define kinetic parameters. As commonly observed for surface-active enzymes, activities varied both with substrate concentration and lipid/detergent molar ratios. Maximal activities of approx. 100 min(-1) were obtained at the PI/Triton X-100 ratio of 1:5. The K (m) value for ATP was 266 microM, intermediate between the values reported for mammalian Type II and III kinases. Epitope-tagged protein, expressed in yeast, was entirely particulate, and about half of it could be extracted with non-ionic detergent. Lsb6p-green fluorescent protein was found both on vacuolar membranes and on the plasma membrane, suggesting a role in endocytic or exocytic pathways.

Involvement of the endoplasmic reticulum in peroxisome formation

The traditional view holds that peroxisomes are autonomous organelles multiplying by growth and division. More recently, new observations have challenged this concept. Herein, we present evidence supporting the involvement of the endoplasmic reticulum (ER) in peroxisome formation by electron microscopy, immunocytochemistry and three-dimensional image reconstruction of peroxisomes and associated compartments in mouse dendritic cells. We found the peroxisomal membrane protein Pex13p and the ATP-binding cassette transporter protein PMP70 present in specialized subdomains of the ER that were continuous with a peroxisomal reticulum from which mature peroxisomes arose. The matrix proteins catalase and thiolase were only detectable in the reticula and peroxisomes. Our results suggest the existence of a maturation pathway from the ER to peroxisomes and implicate the ER as a major source from which the peroxisomal membrane is derived.

The peroxisomal membrane targeting elements of human peroxin 2 (PEX2)

Peroxin 2 (PEX2) is a 35-kDa integral peroxisomal membrane protein with two transmembrane regions and a zinc RING domain within its cytoplasmically exposed C-terminus. Although its role in peroxisome biogenesis and function is poorly understood, it seems to be involved in peroxisomal matrix protein import. PEX2 is synthesized on free cytosolic ribosomes and is posttranslationally imported into the peroxisome membrane by specific targeting information. While a clear picture of the basic targeting mechanisms for peroxisomal matrix proteins has emerged over the past years, the targeting processes for peroxisomal membrane proteins are less well understood. We expressed various deletion constructs of PEX2 in fusion with the green fluorescent protein in COS-7 cells and determined their intracellular localization. We found that the minimum peroxisomal targeting signal of human PEX2 consists of an internal protein region of 30 amino acids (AA130 to AA159) and the first transmembrane domain, and that adding the second transmembrane domain increases targeting efficiency. Within the minimum targeting region we identified the motif "KX6(I/L)X(L/F/I)LK(L/F/I)" that includes important targeting information and is also present in the targeting regions of the 22-kDa peroxisomal membrane protein (PMP22) and the 70-kDa peroxisomal membrane protein (PMP70). Mutations in this targeting motif mislocalize PEX2 to the cytosol. In contrast, the second transmembrane domain does not seem to have specific peroxisomal membrane targeting information. Replacing the second transmembrane domain of human PEX2 with the transmembrane domain of human cytochrome c oxidase subunit IV does not alter PEX2 peroxisome targeting function and efficiency.

M-LP, Mpv17-like protein, has a peroxisomal membrane targeting signal comprising a transmembrane domain and a positively charged loop and up-regulates expression of the manganese superoxide dismutase gene

M-LP (Mpv17-like protein) has been identified as a new protein that has high sequence homology with Mpv17 protein, a peroxisomal membrane protein involved in the development of early onset glomerulosclerosis. In this study, we verified the peroxisomal localization of M-LP by performing dual-color confocal analysis of COS-7 cells cotransfected with green fluorescent protein-tagged M-LP and DsRED2-PTS1, a red fluorescent peroxisomal marker. To characterize the peroxisomal membrane targeting signal, we examined the intracellular localizations of several green fluorescent protein-tagged deletion mutants and demonstrated that, of the three transmembrane segments predicted, the first near the NH(2) terminus and NH(2)-terminal half of the following loop region, which is abundant in positively charged amino acids, were necessary and sufficient for peroxisomal targeting. To elucidate the function of M-LP, we examined the activities of several enzymes involved in reactive oxygen species metabolism in COS-7 cells and found that transfection with M-LP increased the superoxide dismutase activity significantly. Quantitative real-time PCR analysis revealed that the manganese SOD (SOD2) mRNA level of COS-7 cells transfected with M-LP was elevated. These results indicate that M-LP participates in reactive oxygen species metabolism.

The membrane biogenesis peroxin Pex16p. Topogenesis and functional roles in peroxisomal membrane assembly

Previously we isolated human PEX16 encoding 336-amino acid-long peroxin Pex16p and showed that its dysfunction was responsible for Zellweger syndrome of complementation group D (group 9). Here we have determined the membrane topology of Pex16p by differential permeabilization method: both N- and C-terminal parts are exposed to the cytosol. In the search for Pex16p topogenic sequence, basic amino acids clustered sequence, RKELRKKLPVSLSQQK, at positions 66-81 and the first transmembrane segment locating far downstream, nearly by 40 amino acids, of this basic region were defined to be essential for integration into peroxisome membranes. Localization to peroxisomes of membrane proteins such as Pex14p, Pex13p, and PMP70 was interfered with in CHO-K1 cells by a higher level expression of the pex16 patient-derived dysfunctional but topogenically active Pex16pR176ter comprising resides 1-176 or of the C-terminal cytoplasmic part starting from residues at 244 to the C terminus. Furthermore, Pex16p C-terminal cytoplasmic part severely abrogated peroxisome restoration in pex mutants such as matrix protein import-defective pex12 and membrane assembly impaired pex3 by respective PEX12 and PEX3 expression, whereas the N-terminal cytosolic region did not affect restoration. These results imply that Pex16p functions in peroxisome membrane assembly, more likely upstream of Pex3p.

Two different targeting signals direct human peroxisomal membrane protein 22 to peroxisomes

The 22-kDa peroxisomal membrane protein (PMP22) is a major component of peroxisomal membranes in mammals. Although its precise role in peroxisome function is poorly understood, it seems to be involved in pore forming activity and may contribute to the unspecific permeability of the organelle membrane. PMP22 is synthesized on free cytosolic ribosomes and then directed to the peroxisome membrane by specific targeting information. Previous studies in rats revealed that PMP22 contains one distinct peroxisomal membrane targeting signal in the amino-terminal cytoplasmic tail. We cloned and characterized the targeting signal of human PMP22 and compared it with the already described characteristics of the corresponding rat protein. Amino acid sequence alignment of rat and human protein revealed 77% identity including a high conservation of several protein motifs. We expressed various deletion constructs of PMP22 in fusion with the green fluorescent protein in COS-7 cells and determined their intracellular localization. In contrast to previous studies on rat PMP22 and most other peroxisomal membrane proteins, we showed that human as well as rat PMP22 contains two distinct and nonoverlapping peroxisomal membrane targeting signals, one in the amino-terminal and the other in the carboxyl-terminal protein region. They consist of two transmembrane domains and adjacent protein loops with almost identical basic clusters. Both of these peroxisomal targeting regions interact with PEX19, a factor required for peroxisome membrane synthesis. In addition, we observed that fusing the green fluorescent protein immediately adjacent to the targeting region completely abolishes targeting function and mislocalizes PMP22 to the cytosol.

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

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

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.

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

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

Multiple distinct targeting signals in integral peroxisomal membrane proteins

Peroxisomal proteins are synthesized on free polysomes and then transported from the cytoplasm to peroxisomes. This process is mediated by two short well-defined targeting signals in peroxisomal matrix proteins, but a well-defined targeting signal has not yet been described for peroxisomal membrane proteins (PMPs). One assumption in virtually all prior studies of PMP targeting is that a given protein contains one, and only one, distinct targeting signal. Here, we show that the metabolite transporter PMP34, an integral PMP, contains at least two nonoverlapping sets of targeting information, either of which is sufficient for insertion into the peroxisome membrane. We also show that another integral PMP, the peroxin PEX13, also contains two independent sets of peroxisomal targeting information. These results challenge a major assumption of most PMP targeting studies. In addition, we demonstrate that PEX19, a factor required for peroxisomal membrane biogenesis, interacts with the two minimal targeting regions of PMP34. Together, these results raise the interesting possibility that PMP import may require novel mechanisms to ensure the solubility of integral PMPs before their insertion in the peroxisome membrane, and that PEX19 may play a central role in this process.