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.     

Arabidopsis 22-kilodalton peroxisomal membrane protein. Nucleotide sequence analysis and biochemical characterization.

We sequenced and characterized PMP22 (22-kD peroxisomal membrane protein) from Arabidopsis, which shares 28% to 30% amino acid identity and 55% to 57% similarity to two related mammalian peroxisomal membrane proteins, PMP22 and Mpv17. Subcellular fractionation studies confirmed that the Arabidopsis PMP22 is a genuine peroxisomal membrane protein. Biochemical analyses established that the Arabidopsis PMP22 is an integral membrane protein that is completely embedded in the lipid bilayer. In vitro import assays demonstrated that the protein is inserted into the membrane posttranslationally in the absence of ATP, but that ATP stimulates the assembly into the native state. Arabidopsis PMP22 is expressed in all organs of the mature plant and in tissue-cultured cells. Expression of PMP22 is not associated with a specific peroxisome type, as it is detected in seeds and throughout postgerminative growth as cotyledon peroxisomes undergo conversion from glyoxysomes to leaf-type peroxisomes. Although PMP22 shows increased accumulation during the growth of young seedlings, its expression is not stimulated by light.     

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

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

Peroxisomal protein import is conserved between yeast, plants, insects and mammals.

We have previously demonstrated that firefly luciferase can be imported into peroxisomes of both insect and mammalian cells. To determine whether the process of protein transport into the peroxisome is functionally similar in more widely divergent eukaryotes, the cDNA encoding firefly luciferase was expressed in both yeast and plant cells. Luciferase was translocated into peroxisomes in each type of organism. Experiments were also performed to determine whether a yeast peroxisomal protein could be transported to peroxisomes in mammalian cells. We observed that a C-terminal segment of the yeast (Candida boidinii) peroxisomal protein PMP20 could act as a peroxisomal targeting signal in mammalian cells. These results suggest that at least one mechanism of protein translocation into peroxisomes has been conserved throughout eukaryotic evolution.     

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.     

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

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

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

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

Integral membrane polypeptides of rat liver peroxisomes: topology and response to different metabolic states

Rats were treated with clofibrate, a hypolipidemic drug, and with thyroxine. Both drugs which are known to cause peroxisome proliferation, and a concomitant increase in peroxisomal fatty acid beta-oxidation activity in liver increased one of the major integral peroxisomal membrane polypeptides (PMPs), with apparent molecular mass of 69-kDa, six- and twofold, respectively. On the other hand hypothyroidism caused a decrease in peroxisomal fatty acid beta-oxidation activity and considerably lowered the concentration of PMP 69 in the peroxisomal membrane. Two other PMPs with apparent molecular masses of 36 and 22 kDa were not influenced by these treatments. The PMPs with apparent molecular masses of 42, 28, and 26 kDa were shown to be derived from the 69-kDa polypeptide by the activity of a yet uncharacterized endogenous protease during isolation of peroxisomes. Limited proteolysis of intact peroxisomes using proteinase K and subtilisin further substantiated that some portion of the 69-kDa polypeptide extends into the cytoplasm. The 36- and the 22-kDa polypeptides were accessible to proteolytic attack to a much lower extent and, therefore, are supposed to be rather deeply embedded within the peroxisomal membrane. It is demonstrated that peroxisomal acyl-CoA synthetase, an integral PMP extending partially into the cytoplasm, and PMP 69 are not identical polypeptides. Comparison of the peroxisomal membrane with that of mitochondria and microsomes revealed that the 69- and 22-kDa polypeptides as well as the bifunctional protein of the peroxisomal fatty acid beta-oxidation pathway were specifically located only in peroxisomes. Considerable amounts of a polypeptide cross-reacting with the antiserum against the 36-kDa polypeptide were found in mitochondria.     

The 70-kDa peroxisomal membrane protein is a member of the Mdr (P-glycoprotein)-related ATP-binding protein superfamily

The 70-kDa peroxisomal membrane protein (PMP70) is one of the major integral membrane proteins of rat liver peroxisomes. cDNA clones for PMP70 were isolated and sequenced. The predicted amino acid sequence (659 amino acid residues) revealed that the carboxyl-terminal region of PMP70 has strong sequence similarities to a group of ATP-binding proteins such as MalK and Mdr. These proteins form a superfamily and are involved in various biological processes including membrane transport. Limited protease treatment of peroxisomes showed that the ATP-binding domain of PMP70 is exposed to the cytosol. The hydropathy profile, in comparison with those of several other members of the ATP-binding protein superfamily, suggests that PMP70 is a transmembrane protein possibly forming a channel. Based on these results, we propose that PMP70 is involved in active transport across the peroxisomal membrane.     

Major ATPases on clofibrate-induced rat liver peroxisomes are not associated with 70 kDa peroxisomal membrane protein (PMP70).

We previously reported that novel Mg(2+)-ATPases were induced in rat liver peroxisomes by clofibrate administration and that these activities consisted of at least two types of enzymes, N-ethylmaleimide (NEM)-sensitive and -resistant. Here we present evidence that neither of these major peroxisomal ATPases is associated with the 70-kDa peroxisomal membrane protein (PMP70), because: (i) proteinase K treatment of peroxisomes resulted in inactivation of only NEM-sensitive ATPase, whereas disappeared PMP70 completely; (ii) NEM-sensitive ATPase activity was barely immunoprecipitated with anti-PMP70 IgG; (iii) the solubilized ATPases behaved differently from PMP70 on native PAGE; and finally (iv), the major peroxisomal ATPases were separated from PMP70 on gel filtration chromatography.     

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.     

Cytosol-dependent peroxisomal protein import in a permeabilized cell system

Using streptolysin-O (SLO) we have developed a permeabilized cell system retaining the competence to import proteins into peroxisomes. We used luciferase and albumin conjugated with a peptide ending in the peroxisomal targeting sequence, SKL, to monitor the import of proteins into peroxisomes. After incubation with SLO-permeabilized cells, these exogenous proteins accumulated within catalase-containing vesicles. The import was strictly signal dependent and could be blocked by a 10-fold excess of peptide containing the SKL-targeting signal, while a control peptide did not affect the import. Peroxisomal accumulation of proteins was time and temperature dependent and required ATP hydrolysis. Dissipation of the membrane potential did not alter the import efficiency. GTP-hydrolyzing proteins were not required for peroxisomal protein targeting. Depletion of endogenous cytosol from permeabilized cells abolished the competence to import proteins into peroxisomes but import was reconstituted by the addition of external cytosol. We present evidence that cytosol contains factors with SKL-specific binding sites. The activity of cytosol is insensitive to N- ethylmaleimide (NEM) treatment, while the cells contain NEM-sensitive membrane-bound or associated proteins which are involved in the import machinery. The cytosol dependence and NEM-sensitivity of peroxisomal protein import should facilitate the purification of proteins involved in the import of proteins into peroxisomes.     

Intraperoxisomal localization of very-long-chain fatty acyl-CoA synthetase: implication in X-adrenoleukodystrophy

X-adrenoleukodystrophy (X-ALD) is a demyelinating disorder characterized by the accumulation of saturated very-long-chain (VLC) fatty acids (>C(22:0)) due to the impaired activity of VLC acyl-CoA synthetase (VLCAS). The gene responsible for X-ALD was found to code for a peroxisomal integral membrane protein (ALDP) that belongs to the ATP binding cassette superfamily of transporters. To understand the function of ALDP and how ALDP and VLCAS interrelate in the peroxisomal beta-oxidation of VLC fatty acids we investigated the peroxisomal topology of VLCAS protein. Antibodies raised against a peptide toward the C-terminus of VLCAS as well as against the N-terminus were used to define the intraperoxisomal localization and orientation of VLCAS in peroxisomes. Indirect immunofluorescent and electron microscopic studies show that peroxisomal VLCAS is localized on the matrix side. This finding was supported by protease protection assays and Western blot analysis of isolated peroxisomes. To further address the membrane topology of VLCAS, Western blot analysis of total membranes or integral membranes prepared from microsomes and peroxisomes indicates that VLCAS is a peripheral membrane-associated protein in peroxisomes, but an integral membrane in microsomes. Moreover, peroxisomes isolated from cultured skin fibroblasts from X-ALD patients with a mutation as well as a deletion in ALDP showed a normal amount of VLCAS. The consequence of VLCAS being localized to the luminal side of peroxisomes suggests that ALDP may be involved in stabilizing VLCAS activity, possibly through protein-protein interactions, and that loss or alterations in these interactions may account for the observed loss of peroxisomal VLCAS activity in X-ALD.     

Recruiting mechanism of the AAA peroxins, Pex1p and Pex6p, to Pex26p on the peroxisomal membrane

A peroxisomal C-tail-anchored type-II membrane protein, Pex26p, recruits AAA ATPase Pex1p-Pex6p complexes to peroxisomes. We herein attempted to gain mechanistic insight into Pex26p function. Pex26pΔ33-40 truncated in amino-acid residues at 33-40 abolishes the recruiting of Pex1p-Pex6p complex to peroxisomes and fails to complement the impaired phenotype of pex26 CHO cell mutant ZP167, thereby suggesting that peroxisomal localization of Pex1p and Pex6p is indispensable for the transport of matrix proteins. In in vitro transport assay using semipermeabilized CHO cells, Pex1p is targeted to peroxisomes in a manner dependent on ATP hydrolysis, while Pex6p targeting requires ATP but not its hydrolysis. This finding is confirmed by the assay using Walker-motif mutants. Transport of Pex1p and Pex6p is temperature-dependent. In vitro binding assays with glutathione-S-transferase-fused Pex26p, Pex1p and Pex6p bind to Pex26p in a manner dependent on ATP binding but not ATP hydrolysis. These results suggest that ATP hydrolysis is required for stable localization of Pex1p to peroxisomes, but not for binding to Pex26p. Moreover, Pex1p and Pex6p are altered to a more compact conformation upon binding to ATP, as verified by limited proteolysis. Taken together, Pex1p and Pex6p are most likely regulated in their peroxisomal localization onto Pex26p via conformational changes by the ATPase cycle.     

Topographical localization of peroxisomal acyl-CoA ligases: differential localization of palmitoyl-CoA and lignoceroyl-CoA ligases

We found that peroxisomal lignoceroyl-CoA ligase, like palmitoyl-CoA ligase, is present in the peroxisomal membrane whereas the peroxisomal beta-oxidation enzyme system is localized in the matrix. To further define the role of peroxisomal acyl-CoA ligases (membrane component) in providing acyl-CoA for peroxisomal beta-oxidation, we examined the transverse topographical localization of enzymatic sites of palmitoyl-CoA and lignoceroyl-CoA ligases in the peroxisomal membranes. The disruption of peroxisomes by various techniques resulted in the release of a "latent" pool of lignoceroyl-CoA ligase activity while palmitoyl-CoA ligase activity remained the same. Proteolytic enzyme treatment inhibited palmitoyl-CoA ligase activity in intact peroxisomes but had no effect on lignoceroyl-CoA ligase activity. Lignoceroyl-CoA ligase activity was inhibited only if peroxisomes were disrupted with detergent before trypsin treatment. Antibodies to palmitoyl-CoA ligase and to peroxisomal membrane proteins (PMP) inhibited palmitoyl-CoA ligase in intact peroxisomes, and no pool of "latent" activity appeared when antibody-treated peroxisomes were disrupted with detergent. On the other hand, disruption of PMP antibody-treated peroxisomes with detergent resulted in the appearance of a "latent" pool of lignoceroyl-CoA ligase activity. These results demonstrate that the enzymatic site of palmitoyl-CoA ligase is on the cytoplasmic surface whereas that for lignoceroyl-CoA ligase is on the luminal surface of peroxisomal membranes. This implies that palmitoyl-CoA is synthesized on the cytoplasmic surface and is then transferred to the matrix through the peroxisomal membrane for beta-oxidation in the matrix.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cytochemical and biochemical demonstration of an ATPase in membranes of human peroxisomes.

We demonstrated a neutral Mg-ATPase activity in human peroxisomal membranes. To establish the precise experimental conditions for detection of this ATPase, both cytochemical and biochemical characterizations were first carried out in liver peroxisomes from control and cipofibrate-treated rats. The results demonstrated an Mg-ATPase reaction in both normal and proliferated peroxisomes. The nucleotidase activity, with marked preference for ATP, was sensitive to the inhibitors N-ethylmaleimide and 7-chloro-4-nitro-benzo-2-oxadiazole (NBDCl). An ultrastructural cytochemical analysis was developed to evaluate the peroxisomal localization, which localized the reaction product to the peroxisomal membrane. These characteristics can help to differentiate the peroxisomal ATPase from the activity found in mitochondria and endoplasmic reticulum. The conditions established for detecting the rat peroxisomal ATPase were then applied to human peroxisomes isolated from liver and skin fibroblasts in culture. A similar Mg-ATPase activity was readily shown, both cytochemically and biochemically, in the membranes of human peroxisomes. These results, together with previous evidence, strongly support the presence of a specific ATPase in the human peroxisomal membrane. This ATPase may play a crucial role in peroxisome biogenesis.     

Interaction of peroxisomes with microtubules. In vitro studies using a novel peroxisome-microtubule binding assay.

The association of membrane-bounded cell organelles to microtubules is crucial for determination of their shape, intracellular localization and translocation. We have shown previously the high affinity binding of peroxisomes to microtubules which appears to be of static nature as in vivo studies indicate that only a few peroxisomes move along the microtubular tracks. In order to characterize the interactions of peroxisomes with microtubules, we have developed a semiquantitative in vitro binding assay, which is based on the association of highly purified rat liver peroxisomes to microtubules coated onto microtiterplates. The binding was visualized by differential interference contrast and immunofluorescence using a confocal laser scanning microscope. The binding was concentration dependent and saturable, being affected by time, temperature, and pH. Addition of ATP or the motor proteins kinesin and dynein increased the binding capacity, while ATP-depletion or microtubule associated proteins (MAPs) decreased it. KCl treatment of peroxisomes reduced the binding, which was restored by dialyzed KCl-stripping eluate as well as by rat liver cytosol. The reconstituting effect of cytosol was abolished by its pretreatment with proteases or N-ethylmaleimide. Moreover, the treatment of peroxisomes with proteases or N-ethylmaleimide reduced their binding, which was not reversed by cytosol. These results suggest the involvement of a peroxisomal membrane protein and cytosolic factor(s) in the binding of peroxisomes to microtubules. This notion is supported by the observation that distinct subfractions of dialyzed KCl-stripping eluate obtained by gel chromatography augmented the binding. Those subfractions, as well as purified peroxisome fractions, exhibited strong immunoreactivity with an antibody to cytoplasmic linker protein (CLIP)-115, revealing a 70-kDa polypeptide. Moreover, immunodepletion of KCl-stripping eluate and its subfractions with an antibody to the conserved microtubule binding domain of CLIPs, abolished their promoting effect on the binding, thus suggesting the involvement of a CLIP-related protein in the binding of peroxisomes to microtubules.     

Transport of microinjected proteins into peroxisomes of mammalian cells: inability of Zellweger cell lines to import proteins with the SKL tripeptide peroxisomal targeting signal.

Previous work has shown that the firefly (Photinus pyralis) luciferase contains a C-terminal peroxisomal targeting signal consisting of the tripeptide Ser-Lys-Leu. This report describes the microinjection of two proteins, (i) luciferase and (ii) albumin conjugated to a peptide ending in the sequence Ser-Lys-Leu, into mammalian cells grown in tissue culture. Following microinjection, incubation of the cells at 37 degrees C resulted in peroxisomal transport of these exogenous proteins into catalase-containing vesicles. The translocation was both time and temperature dependent. The transport could be inhibited by coinjection of synthetic peptides bearing various peroxisomal targeting signal motifs. These proteins could be transported into peroxisomes in normal human fibroblast cell lines but not in cell lines derived from patients with Zellweger syndrome. These results demonstrate that microinjection of peroxisomal proteins yields an authentic in vivo system with which to study peroxisomal transport. Furthermore, these results reveal that the process of peroxisomal transport does not involve irreversible modification of the protein, that artificial hybrid substrates can be transported and used as tools to study peroxisomal transport, and that the defect in Zellweger syndrome is indeed the inability to transport proteins containing the Ser-Lys-Leu targeting signal into the peroxisomal lumen.     

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.     

Conservation of PEX19-binding motifs required for protein targeting to mammalian peroxisomal and trypanosome glycosomal membranes

Glycosomes are divergent peroxisomes found in trypanosomatid protozoa, including those that cause severe human diseases throughout much of the world. While peroxisomes are dispensable for both yeast (Saccharomyces cerevisiae and others) and mammalian cells in vitro, glycosomes are essential for trypanosomes and hence are viewed as a potential drug target. The import of proteins into the matrix of peroxisomes utilizes multiple peroxisomal membrane proteins which require the peroxin PEX19 for insertion into the peroxisomal membrane. In this report, we show that the specificity of peroxisomal membrane protein binding for Trypanosoma brucei PEX19 is very similar to those previously identified for human and yeast PEX19. Our studies show that trafficking is conserved across these distant phyla and that both a PEX19 binding site and a transmembrane domain are required for the insertion of two test proteins into the glycosomal membrane. However, in contrast to T. brucei PEX10 and PEX12, T. brucei PEX14 does not traffic to human peroxisomes, indicating that it is not recognized by the human PEX14 import mechanism.