The carboxyl-terminal tripeptide Ala-Lys-Ile is essential for targeting Candida tropicalis trifunctional enzyme to yeast peroxisomes.

The gene encoding Candida tropicalis peroxisomal trifunctional enzyme, hydratase-dehydrogenase-epimerase (HDE), was expressed in both Candida albicans and Saccharomyces cerevisiae. The cellular location of HDE was determined by subcellular fractionation followed by Western blot analysis of peroxisomal and cytosolic fractions using antiserum specific for HDE. HDE was found to be exclusively targeted to and imported into peroxisomes in both heterologous expression systems. Deletion and mutational analyses were used to determine the regions within HDE which are essential for its targeting to peroxisomes. Deletion of a carboxyl-terminal tripeptide Ala-Lys-Ile completely abolished targeting of HDE to peroxisomes, whereas large internal deletions of HDE (amino acids 38-353 or 395-731) had no effect on HDE targeting to peroxisomes in either yeast. This tripeptide is similar to, but distinct from, other tripeptide peroxisomal targeting sequences (PTSs) as identified in peroxisomal firefly luciferase and four mammalian peroxisomal proteins. Substitutions within the carboxyl-terminal tripeptide (Ala----Gly and Lys----Gln) supported targeting of HDE to peroxisomes of C. albicans but not of S. cerevisiae. This is the first detailed analysis of the peroxisomal targeting signal in a yeast peroxisomal protein.     

The tetratricopeptide repeat domains of human, tobacco, and nematode PEX5 proteins are functionally interchangeable with the analogous native domain for peroxisomal import of PTS1-terminated proteins in yeast

In the yeast Saccharomyces cerevisiae, beta-oxidation of fatty acids is compartmentalised in peroxisomes. Most yeast peroxisomal matrix proteins contain a type 1C-terminal peroxisomal targeting signal (PTS1) consisting of the tripeptide SKL or a conservative variant thereof. PTS1-terminated proteins are imported by Pex5p, which interacts with the targeting signal via a tetratricopeptide repeat (TPR) domain. Yeast cells devoid of Pex5p are unable to import PTS1-containing proteins and cannot degrade fatty acids. Here, the PEX5-TPR domains from human, tobacco, and nematode were inserted into a TPR-less yeast Pex5p construct to generate Pex5p chimaeras. These hybrid proteins were examined for functional complementation of the pex5delta mutant phenotype. Expression of the Pex5p chimaeras in pex5delta mutant cells restored peroxisomal import of PTS1-terminated proteins. Chimaera expression also re-established degradation of oleic acid, allowing growth on this fatty acid as a sole carbon source. We conclude that, in the context of Pex5p chimaeras, the human, tobacco, and nematode Pex5p-TPR domains are functionally interchangeable with the native domain for the peroxisomal import of yeast proteins terminating with canonical PTS1s. Non-conserved yeast PTS1s, such as HRL and HKL, did not interact with the tobacco PEX5-TPR domain in the two-hybrid system. HRL occurs at the C-terminus of the peroxisomal protein Eci1p, which is required for growth on unsaturated fatty acids. Although mutant pex5delta cells expressing a yeast/tobacco Pex5p chimaera failed to import a GFP-Eci1p reporter protein, they were able to grow on oleic acid. We reason that this is due to a cryptic PTS in native Eci1p that can function in a redundant system with the C-terminal HRL.     

Transport of fatty acids and metabolites across the peroxisomal membrane

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

The catabolite inactivation of Aspergillus nidulans isocitrate lyase occurs by specific autophagy of peroxisomes

In Aspergillus nidulans, activity of the glyoxylate cycle enzyme isocitrate lyase is finely regulated. Isocitrate lyase is induced by growth on C2 compounds and long-chain fatty acids and repressed by glucose. In addition, activity of isocitrate lyase is subject to a second mechanism of catabolite control, glucose-induced inactivation. Here, we demonstrate that the catabolite inactivation of A. nidulans isocitrate lyase, a process that takes place during glucose adaptation of cells grown under gluconeogenic conditions, occurs by proteolysis of the enzyme. Ultrastructural analyses were carried out in order to investigate the cellular processes that govern the catabolite inactivation of this peroxisomal enzyme. Addition of glucose to oleate-induced cells triggered the specific engulfment and sequestration of peroxisomes by the vacuoles. Sequestration of various peroxisomes by a single vacuole was a frequently observed phenomenon. Results obtained by immunoelectron microscopy using antibodies against A. nidulans isocitrate lyase showed that degradation of this peroxisomal enzyme occurred inside the vacuole. In addition, ultrastructural studies demonstrated that microautophagy was the autophagic pathway involved in degradation of redundant peroxisomes during glucose adaptation of oleate-induced cells of A. nidulans.     

Peroxisomal localization and function of NADP-specific isocitrate dehydrogenases in yeast

Yeast peroxisomal NADP⁺-specific isocitrate dehydrogenase (IDP3) contains a canonical type I peroxisomal targeting sequence (a carboxyl-terminal Cys-Lys-Leu tripeptide), and provides the NADPH required for β-oxidation of some fatty acids in that organelle. Cytosolic yeast IDP2 carrying a PTS1 (IDP2^+CKL) was only partially localized to peroxisomes, and the enzyme was able to function in lieu of either peroxisomal IDP3 or cytosolic IDP2. The analogous isocitrate dehydrogenase enzyme (IDPA) from Aspergillus nidulans, irrespective of the presence or absence of a putative PTS1, was found to exhibit patterns of dual compartmental distribution and of dual function in yeast similar to those observed for IDP2^+CKL. To test a potential cellular limit on peroxisomal levels, authentic yeast IDP3, which is normally strictly peroxisomal, was over-expressed. This also resulted in dual distribution and function of the enzyme in both the cytosol and in peroxisomes, supporting the possibility of a restriction on organellar amounts of IDP.     

Existence of catalase-less peroxisomes in Sf21 insect cells

Catalase activity, a peroxisomal marker enzyme, was not detectable in any of the subcellular fractions of Spodoptera frugiperda (Sf) 21 insect cells, although marker enzymes in other organelles were distributed in the fractions in a manner similar to that seen in mammalian cells. When a green fluorescent protein fused with peroxisome targeting signal 1 at the C-terminal (GFP-SKL) was expressed in Sf21 cells, punctate fluorescent dots were observed in the cytoplasm. The fraction where GFP-SKL was concentrated exhibited long-chain and very-long-chain fatty acid beta-oxidation activities in the presence of KCN and the density of this fraction was slightly higher than that of mitochondria. Immunoelectron microscopy studies with anti-SKL antibody demonstrated that Sf21 cells have immunoreactive peroxisome-like organelles which are structurally distinct from mitochondria, endoplasmic reticulum, and lysosomes. In contrast to peroxisomal matrix proteins, adrenoleukodystrophy protein, a peroxisomal membrane protein, was not located to peroxisomes. This suggests that the targeting signal for PMP in insect cells is distinct from that in mammalian cells. These results demonstrate that Sf21 insect cells have unique catalase-less peroxisomes capable of beta-oxidation of fatty acids.     

Identification of peroxisomal acyl-CoA thioesterases in yeast and humans

A computer-based screen of the Saccharomyces cerevisiae genome identified YJR019C as a candidate oleate-induced gene. YJR019C mRNA levels were increased significantly during growth on fatty acids, suggesting that it may play a role in fatty acid metabolism. The YJR019C product is highly similar to tesB, a bacterial acyl-CoA thioesterase, and carries a tripeptide sequence, alanine-lysine-phenylalanineCOOH, that closely resembles the consensus sequence for type-1 peroxisomal targeting signals. YJR019C directed green fluorescence protein to peroxisomes, and biochemical studies revealed that YJR019C is an abundant component of purified yeast peroxisomes. Disruption of the YJR019C gene caused a significant decrease in total cellular thioesterase activity, and recombinant YJR019C was found to exhibit intrinsic acyl-CoA thioesterase activity of 6 units/mg. YJR019C also shared significant sequence similarity with hTE, a human thioesterase that was previously identified because of its interaction with human immunodeficiency virus-Nef in the yeast two-hybrid assay. We report here that hTE is also a peroxisomal protein, demonstrating that thioesterase activity is a conserved feature of peroxisomes. We propose that YJR019C and hTE be renamed as yeast and human PTE1 to reflect the fact that they encode peroxisomal thioesterases. The physical segregation of yeast and human PTE1 from the cytosolic fatty acid synthase suggests that these enzymes are unlikely to play a role in formation of fatty acids. Instead, the observation that PTE1 contributes to growth on fatty acids implicates this thioesterase in fatty acid oxidation.     

Ceramide regulates interaction of Hsd17b4 with Pex5 and function of peroxisomes

The sphingolipid ceramide regulates beta-oxidation of medium and long chain fatty acids in mitochondria. It is not known whether it also regulates oxidation of very long chain fatty acids (VLCFAs) in peroxisomes. Using affinity chromatography, co-immunoprecipitation, and proximity ligation assays we discovered that ceramide interacts with Hsd17b4, an enzyme critical for peroxisomal VLCFA oxidation and docosahexaenoic acid (DHA) generation. Immunocytochemistry showed that Hsd17b4 is distributed to ceramide-enriched mitochondria-associated membranes (CEMAMs). Molecular docking and in vitro mutagenesis experiments showed that ceramide binds to the sterol carrier protein 2-like domain in Hsd17b4 adjacent to peroxisome targeting signal 1 (PTS1), the C-terminal signal for interaction with peroxisomal biogenesis factor 5 (Pex5), a peroxin mediating transport of Hsd17b4 into peroxisomes. Inhibition of ceramide biosynthesis induced translocation of Hsd17b4 from CEMAMs to peroxisomes, interaction of Hsd17b4 with Pex5, and upregulation of DHA. This data indicates a novel role of ceramide as a molecular switch regulating interaction of Hsd17b4 with Pex5 and peroxisomal function.     

Analyses in transfected cells and in vitro of a putative peroxisomal targeting signal of rat liver serine:pyruvate aminotransferase

Serine:pyruvate aminotransferase (SPT; EC 2.6.1.51) of rat liver is a unique enzyme in that it is located in both mitochondria and peroxisomes. To analyze a peroxisomal targeting signal (PTS) of SPT, we constructed in this study various peroxisomal SPT clones having mutations at the C-terminal 20-amino acid region in which a putative PTS is located, and we examined subcellular localization of mutated products expressed in transfected COS-1 cells. When the mutant SPTs were unstable in transfected COS-1 cells, their translocation into peroxisomes was examined using an in vitro peroxisomal import system. Deletion of the C-terminal tripeptide, NKL, and amino acid substitution of K2 (the second lysine from the C-terminus), K4, or E15 abolished or impaired the peroxisomal import of the translated product, resulting in cytosolic accumulation in the cell. In the cases of mutation of R18G, D19A, or K2Q and the conversion to proline of L9, L13, V17, or A20, no products were detected in transfected cells. However, the results of an in vitro peroxisomal import experiment showed that the mutation of L9P, L13P, V17P, and A20P caused loss of the PTS function. When serine was introduced instead of N3 to generate a typical PTS1, the SKL motif, at the C-terminus, all of the proteins having mutations at P5, E11, R12, or E15 showed extensive localization in peroxisomes. These results suggest that the putative C-terminal PTS of SPT is not equivalent to the typical PTS1 shown in acyl-CoA oxidase and urate oxidase, because the PTS of SPT is not restricted to the C-terminal tripeptide. The results also suggest that the alpha-helical structure of the C-terminal region of SPT is important for the stable conformation of the enzyme and the peroxisomal targeting function of its PTS.     

Regulation of peroxisome size and number by fatty acid beta -oxidation in the yeast yarrowia lipolytica

The Yarrowia lipolytica MFE2 gene encodes peroxisomal beta-oxidation multifunctional enzyme type 2 (MFE2). MFE2 is peroxisomal in a wild-type strain but is cytosolic in a strain lacking the peroxisomal targeting signal-1 (PTS1) receptor. MFE2 has a PTS1, Ala-Lys-Leu, that is essential for targeting to peroxisomes. MFE2 lacking a PTS1 can apparently oligomerize with full-length MFE2 to enable targetting to peroxisomes. Peroxisomes of an oleic acid-induced MFE2 deletion strain, mfe2-KO, are larger and more abundant than those of the wild-type strain. Under growth conditions not requiring peroxisomes, peroxisomes of mfe2-KO are larger but less abundant than those of the wild-type strain, suggesting a role for MFE2 in the regulation of peroxisome size and number. A nonfunctional version of MFE2 did not restore normal peroxisome morphology to mfe2-KO cells, indicating that their phenotype is not due to the absence of MFE2. mfe2-KO cells contain higher amounts of beta-oxidation enzymes than do wild-type cells. We also show that increasing the level of the beta-oxidation enzyme thiolase results in enlarged peroxisomes. Our results implicate peroxisomal beta-oxidation in the control of peroxisome size and number in yeast.     

Tail-anchored PEX26 targets peroxisomes via a PEX19-dependent and TRC40-independent class I pathway

Tail-anchored (TA) proteins are anchored into cellular membranes by a single transmembrane domain (TMD) close to the C terminus. Although the targeting of TA proteins to peroxisomes is dependent on PEX19, the mechanistic details of PEX19-dependent targeting and the signal that directs TA proteins to peroxisomes have remained elusive, particularly in mammals. The present study shows that PEX19 formed a complex with the peroxisomal TA protein PEX26 in the cytosol and translocated it directly to peroxisomes by interacting with the peroxisomal membrane protein PEX3. Unlike in yeast, the adenosine triphosphatase TRC40, which delivers TA proteins to the endoplasmic reticulum, was dispensable for the peroxisomal targeting of PEX26. Moreover, the basic amino acids within the luminal domain of PEX26 were essential for binding to PEX19 and thereby for peroxisomal targeting. Finally, our results suggest that a TMD that escapes capture by TRC40 and is followed by a highly basic luminal domain directs TA proteins to peroxisomes via the PEX19-dependent route.     

Preliminary characterization of Yor180Cp: identification of a novel peroxisomal protein of saccharomyces cerevisiae involved in fatty acid metabolism.

Here we report the preliminary characterization of Yor180Cp, a novel peroxisomal protein involved in fatty acid metabolism in the yeast Saccharomyces cerevisiae. A computer-based screen identified Yor180Cp as a putative peroxisomal protein, and Yor180Cp targeted GFP to peroxisomes in a PEX8-dependent manner. Yor180Cp was also detected by mass spectrometric analysis of an HPLC-separated extract of yeast peroxisomal matrix proteins. YOR180C is upregulated during growth on oleic acid, and deletion of YOR180C from the yeast genome resulted in a mild but significant growth defect on oleic acid, indicating a role for Yor180Cp in fatty acid metabolism. In addition, we observed that yor180cDelta cells fail to efficiently import the enzyme Delta3,Delta2-enoyl-CoA isomerase (Eci1p) to peroxisomes. This result suggested that Yor180Cp might associate with Eci1p in vivo, and a Yor180Cp-Eci1p interaction was detected using the yeast two-hybrid system. Potential roles for Yor180Cp in peroxisomal fatty acid metabolism are discussed.     

Arabidopsis peroxisomal citrate synthase is required for fatty acid respiration and seed germination

We tested the hypothesis that peroxisomal citrate synthase (CSY) is required for carbon transfer from peroxisomes to mitochondria during respiration of triacylglycerol in Arabidopsis thaliana seedlings. Two genes encoding peroxisomal CSY are expressed in Arabidopsis seedlings, and seeds from plants with both CSY genes disrupted were dormant and did not metabolize triacylglycerol. Germination was achieved by removing the seed coat and supplying sucrose, but the seedlings still did not use triacylglycerol. The mutant seedlings were resistant to 2,4-dichlorophenoxybutyric acid, indicating a block in peroxisomal beta-oxidation, and were unable to develop further after transfer to soil. The mutant phenotype was complemented with a cDNA encoding CSY with either its native peroxisomal targeting sequence (PTS2) or a heterologous PTS1 sequence from pumpkin (Cucurbita pepo) malate synthase. These results suggest that peroxisomal CSY in Arabidopsis is not only a key enzyme of the glyoxylate cycle but also catalyzes an essential step in the respiration of fatty acids. We conclude that citrate is exported from the peroxisome during fatty acid respiration, whereas in yeast, acetylcarnitine is exported.     

Hitchhiking of Cu/Zn Superoxide Dismutase to Peroxisomes – Evidence for a Natural Piggyback Import Mechanism in Mammals

Most newly synthesized peroxisomal proteins are imported in a receptor-mediated fashion, depending on the interaction of a peroxisomal targeting signal (PTS) with its cognate targeting receptor Pex5 or Pex7 located in the cytoplasm. Apart from this classic mechanism, heterologous protein complexes that have been proposed more than a decade ago are also to be imported into peroxisomes. However, it remains still unclear if this so-called piggyback import is of physiological relevance in mammals. Here, we show that Cu/Zn superoxide dismutase 1 (SOD1), an enzyme without an endogenous PTS, is targeted to peroxisomes using its physiological interaction partner 'copper chaperone of SOD1' (CCS) as a shuttle. Both proteins have been identified as peroxisomal constituents by 2D-liquid chromatography mass spectrometry of isolated rat liver peroxisomes. Yet, while a major fraction of CCS was imported into peroxisomes in a PTS1-dependent fashion in CHO cells, overexpressed SOD1 remained in the cytoplasm. However, increasing the concentrations of both CCS and SOD1 led to an enrichment of SOD1 in peroxisomes. In contrast, CCS-mediated SOD1 import into peroxisomes was abolished by deletion of the SOD domain of CCS, which is required for heterodimer formation. SOD1/CCS co-import is the first demonstration of a physiologically relevant piggyback import into mammalian peroxisomes.     

Characterization of a novel component of the peroxisomal protein import apparatus using fluorescent peroxisomal proteins.

Fluorescent peroxisomal probes were developed by fusing green fluorescent protein (GFP) to the matrix peroxisomal targeting signals PTS1 and PTS2, as well as to an integral peroxisomal membrane protein (IPMP). These proteins were used to identify and characterize novel peroxisome assembly (pas) mutants in the yeast Pichia pastoris. Mutant cells lacking the PAS10 gene mislocalized both PTS1-GFP and PTS2-GFP to the cytoplasm but did incorporate IPMP-GFP into peroxisome membranes. Similar distributions were observed for endogenous peroxisomal matrix and membrane proteins. While peroxisomes from translocation-competent pas mutants sediment in sucrose gradients at the density of normal peroxisomes, >98% of peroxisomes from pas10 cells migrated to a much lower density and had an extremely low ratio of matrix:membrane protein. These data indicate that Pas10p plays an important role in protein translocation across the peroxisome membrane. Consistent with this hypothesis, we find that Pas10p is an integral protein of the peroxisome membrane. In addition, Pas10p contains a cytoplasmically-oriented C3HC4 zinc binding domain that is essential for its biological activity.     

Targeting of hFis1 to peroxisomes is mediated by Pex19p

The processes of peroxisome formation and proliferation are still a matter of debate. We have previously shown that peroxisomes share some components of their division machinery with mitochondria. hFis1, a tail-anchored membrane protein, regulates the membrane fission of both organelles by DLP1/Drp1 recruitment, but nothing is known about the mechanisms of the dual targeting of hFis1. Here we demonstrate for the first time that peroxisomal targeting of hFis1 depends on Pex19p, a peroxisomal membrane protein import factor. hFis1/Pex19p binding was demonstrated by expression and co-immunoprecipitation studies. Using mutated versions of hFis1 an essential binding region for Pex19p was located within the last 26 C-terminal amino acids of hFis1, which are required for proper targeting to both mitochondria and peroxisomes. The basic amino acids in the very C terminus are not essential for Pex19p binding and peroxisomal targeting, but are instead required for mitochondrial targeting. Silencing of Pex19p by small interference RNA reduced the targeting of hFis1 to peroxisomes, but not to mitochondria. In contrast, overexpression of Pex19p alone was not sufficient to shift the targeting of hFis1 to peroxisomes. Our findings indicate that targeting of hFis1 to peroxisomes and mitochondria are independent events and support a direct, Pex19p-dependent targeting of peroxisomal tail-anchored proteins.     

The carboxyl-terminal tripeptide serine-lysine-leucine of firefly luciferase is necessary but not sufficient for peroxisomal import in yeast.

Firefly luciferase is imported into peroxisomes in insects, mammals, plants, and yeast, which implies that the mechanism of protein translocation into peroxisomes has been conserved during eukaryotic evolution. The carboxyl-terminal tripeptide serine-lysine-leucine in luciferase acts as a peroxisomal import signal in mammalian cells. We have investigated whether this tripeptide is also involved in translocation of firefly luciferase into peroxisomes in yeast (Saccharomyces cerevisiae). We show by gene fusion experiments that the carboxyl-terminal 104 amino acids of luciferase can direct a heterologous protein to yeast peroxisomes. Luciferase mutant proteins were tested for their ability to be imported into yeast peroxisomes in vivo. We demonstrate that mutations in the carboxyl-terminal serine-lysine-leucine tripeptide abolish translocation of the protein into yeast peroxisomes. However, when a passenger protein was tagged at its carboxyl terminus with this tripeptide the fusion protein did not go to peroxisomes. These results indicate that, in yeast, the tripeptide is necessary but not sufficient for peroxisomal import.     

Pex19p contributes to peroxisome inheritance in the association of peroxisomes to Myo2p

During budding of yeast cells peroxisomes are distributed over mother cell and bud, a process that involves the myosin motor protein Myo2p and the peroxisomal membrane protein Inp2p. Here, we show that Pex19p, a peroxin implicated in targeting and complex formation of peroxisomal membrane proteins, also plays a role in peroxisome partitioning. Binding studies revealed that Pex19p interacts with the cargo-binding domain of Myo2p. We identified mutations in Myo2p that specifically reduced binding to Pex19p, but not to Inp2p. The interaction between Myo2p and Pex19p was also reduced by a mutation that blocked Pex19p farnesylation. Microscopy revealed that the Pex19p-Myo2p interaction is important for peroxisome inheritance, because mutations that affect this interaction hamper peroxisome inheritance in vivo. Together these data suggest that both Inp2p and Pex19p are required for proper association of peroxisomes to Myo2p.     

Phytol-induced pathology in 2-hydroxyacyl-CoA lyase (HACL1) deficient mice. Evidence for a second non-HACL1-related lyase

2-Hydroxyacyl-CoA lyase (HACL1) is a key enzyme of the peroxisomal α-oxidation of phytanic acid. To better understand its role in health and disease, a mouse model lacking HACL1 was investigated. Under normal conditions, these mice did not display a particular phenotype. However, upon dietary administration of phytol, phytanic acid accumulated in tissues, mainly in liver and serum of KO mice. As a consequence of phytanic acid (or a metabolite) toxicity, KO mice displayed a significant weight loss, absence of abdominal white adipose tissue, enlarged and mottled liver and reduced hepatic glycogen and triglycerides. In addition, hepatic PPARα was activated. The central nervous system of the phytol-treated mice was apparently not affected. In addition, 2OH-FA did not accumulate in the central nervous system of HACL1 deficient mice, likely due to the presence in the endoplasmic reticulum of an alternate HACL1-unrelated lyase. The latter may serve as a backup system in certain tissues and account for the formation of pristanic acid in the phytol-fed KO mice. As the degradation of pristanic acid is also impaired, both phytanoyl- and pristanoyl-CoA levels are increased in liver, and the ω-oxidized metabolites are excreted in urine. In conclusion, HACL1 deficiency is not associated with a severe phenotype, but in combination with phytanic acid intake, the normal situation in man, it might present with phytanic acid elevation and resemble a Refsum like disorder.     

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.