The non-specific lipid-transfer protein (sterol carrier protein 2) and its relationship to peroxisomes

Ultrastructural, morphometric, and hormonal analysis of the effects of testosterone treatment on Leydig cells and other interstitial cells in young adult rats

Early effects of testosterone (T) treatment on the ultrastructure of testicular interstitium were analyzed by morphometry. In T-treated young adult rats the T-LH feed-back loop functioned as expected and the marked increase in peripheral T caused almost complete depletion of peripheral LH. Even though the peripheral LH concentration was almost undetectably low, the Leydig cells maintained regulatory interactions with macrophages, peritubular myoid cells and with the seminiferous epithelium lining the tubular lumina as indicated by the high correlations of morphometric parameters between the Leydig and other cell types. The morphometric alterations in the ultrastructure of Leydig cells suggest that the seminiferous tubules may signal by releasing inhibitory paracrine factors affecting the morphology and function of Leydig cells in T-treated young adult rats. The morphometry of Leydig cells in T-treated young adult rats showed a significant quantifiable reduction in nuclei and organelles involved in steroid synthesis and this analysis also offers a good basis for elucidation of the early effects of testosterone in terms of its contraceptive function as well as of different toxic compounds on reproductive functions.

Membrane traffic in sphingolipid storage diseases

In this review, we summarize our studies of membrane lipid transport in sphingolipid storage disease (SLSD) fibroblasts. We recently showed that several fluorescent SL analogs were internalized from the plasma membrane predominantly to the Golgi complex of normal cells, while in ten different SLSD cell types, these lipids accumulated in endosomes and lysosomes (The Lancet 1999;354: 901-905). Additional studies showed that cholesterol homeostasis is perturbed in multiple SLSDs secondary to SL accumulation and that mistargeting of SL analogs was regulated by cholesterol (Nature Cell Biol 1999;1: 386-388). Based on these findings, we hypothesize that endogenous sphingolipids, which accumulate in SLSD cells due to primary defects in lipid catabolism, result in an altered intracellular distribution of cholesterol, and that this alteration in membrane composition then results in defective sorting and transport of SLs. The importance of SL/cholesterol interactions and potential mechanisms underlying the regulation of lipid transport and targeting are also discussed. These studies suggest a new paradigm for regulation of membrane lipid traffic along the endocytic pathway and could have important implications for future studies of protein trafficking as well as lipid transport. This work may also lead to important future clinical developments (e.g. screening tests for SLSD, new methodology for screening drugs which abrogate lipid storage, and possible therapeutic approaches to SLSD).

Role and organization of peroxisomal beta-oxidation

In mammals, peroxisomes are involved in breakdown of very long chain fatty acids, prostanoids, pristanic acid, dicarboxylic fatty acids, certain xenobiotics and bile acid intermediates. Substrate spectrum and specificity studies of the four different beta-oxidation steps in rat and/or in man demonstrate that these substrates are degraded by separate beta-oxidation systems composed of different enzymes. In both species, the enzymes acting on straight chain fatty acids are palmitoyl-CoA oxidase, an L-specific multifunctional protein (MFP-1) and a dimeric thiolase. In liver, bile acid intermediates undergo one cycle of beta-oxidation catalyzed by trihydroxycoprostanoyl-CoA oxidase (in rat), or branched chain acyl-CoA oxidase (in man), a D-specific multifunctional protein (MFP-2) and SCPX-thiolase. Finally, pristanic acid is degraded in rat tissues by pristanoyl-CoA oxidase, the D-specific multifunctional protein-2 and SCPX-thiolase. Although in man a pristanoyl-CoA oxidase gene is present, so far its product has not been found. Hence, pristanoyl-CoA is believed to be desaturated in human tissues by the branched chain acyl-CoA oxidase. Due to the stereospecificity of the oxidases acting on 2-methyl-branched substrates, an additional enzyme, 2-methylacyl-CoA racemase, is required for the degradation of pristanic acid and the formation of bile acids.

Gene expression of peroxisomal beta-oxidation enzymes in rat brain

Despite increasing insight into peroxisomal beta-oxidation, it is still not clear which enzymes catalyze very-long-chain fatty acid degradation. Using the northern blot and RT-PCR techniques, a brain-specific expression is demonstrated for acyl-CoA oxidase 3II mRNA, thiolase-A and trans2,3enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase multifunctional enzyme type 2.

Microsomal long chain fatty acyl-CoA transacylation: differential effect of sterol carrier protein-2

The recent discovery that sterol carrier protein-2 (SCP-2) binds long chain++ (LCFA-CoA) with high affinity (A. Frolov et al., J. Biol. Chem. 271 (1997) 31878-31884) suggests new possible functions of this protein in LCFA-CoA metabolism. The purpose of the present investigation was to determine whether SCP-2 differentially modulated microsomal LCFA-CoA transacylation to cholesteryl esters, triacylglycerols, and phospholipids in vitro. Microsomal acyl-CoA:cholesterol acyltransferase (ACAT) activity measured with liposomal membrane cholesterol donors depended on substrate LCFA-CoA level, mol% cholesterol in the liposomal membrane, and total amount of liposomal cholesterol. As compared to basal activity without liposomes, microsomal ACAT was inhibited 30-50% in the presence of cholesterol poor (1.4 mol%) liposomes. In contrast, cholesterol rich (>25 mol%) liposomes stimulated ACAT up to 6.4-fold compared to basal activity without liposomes and nearly 10-fold as compared to cholesterol pool (1.4 mol%) liposomes. Increasing oleoyl-CoA reversed the inhibition of microsomal ACAT by cholesterol poor (1.4 mol%) liposomes, but did not further stimulate ACAT in the presence of cholesterol rich (35 mol%) liposomes. In contrast, high (100 microM) oleoyl-CoA inhibited ACAT nearly 3-fold. This inhibition was reversed by LCFA-CoA binding proteins, bovine serum albumin (BSA) and SCP-2. SCP-2 was 10-fold more effective (mole for mole) than BSA in reversing LCFA-CoA inhibited microsomal ACAT. Concomitantly, under conditions in which SCP-2 stimulated ACAT it equally enhanced transacylation of oleoyl-CoA into phospholipids, and 5.2-fold enhanced oleoyl-CoA transacylation to triacylglycerols. In summary, SCP-2 appeared to exert its greatest effects on microsomal transacylation in vitro by reversing LCFA-CoA inhibition of ACAT and by differentially targeting LCFA-CoA to triacylglycerols. These data suggest that the high affinity interaction of SCP-2s with LCFA-CoA may be physiologically important in microsomal transacylation reactions.

Alterations in the biosynthesis of cholesterol, dolichol and dolichyl-P in the genetic cholesterol homeostasis disorder, Niemann-Pick type C disease

The biosynthesis of cholesterol, dolichol and dolichyl-P were investigated in a murine model of Niemann-Pick type C disease using both in vitro and in vivo systems. In vivo incorporation of [3H]mevalonate into squalene, dolichol and dolichyl-P decreased. The amount of dolichyl-P was elevated due to a decrease in the rate of degradation. Labeling of squalene and cholesterol of liver homogenates in vitro was decreased in the diseased mice and a lowering of microsomal activities of both HMG-CoA reductase and squalene synthase were also observed. In experiments with brain homogenate, decreased [3H]mevalonate labeling of squalene, cholesterol and dolichol was found in vitro. The decreases in cis-prenyltransferase and squalene synthase activities were observed at a very early phase of the disease. In contrast to the decreased biosynthesis of cholesterol observed in vitro, the labeling of total liver cholesterol was found to be increased in Niemann-Pick type C liver upon in vivo investigation, possibly due to the accumulation of this lipid as a result of a deficient transport process. In the brain, where in vivo labeling reflects only biosynthesis, a decreased rate of cholesterol synthesis was demonstrated.

Defective peroxisomal catabolism of branched fatty acyl coenzyme A in mice lacking the sterol carrier protein-2/sterol carrier protein-x gene function

Gene targeting in mice was used to investigate the unknown function of Scp2, encoding sterol carrier protein-2 (SCP2; a peroxisomal lipid carrier) and sterol carrier protein-x (SCPx; a fusion protein between SCP2 and a peroxisomal thiolase). Complete deficiency of SCP2 and SCPx was associated with marked alterations in gene expression, peroxisome proliferation, hypolipidemia, impaired body weight control, and neuropathy. Along with these abnormalities, catabolism of methyl-branched fatty acyl CoAs was impaired. The defect became evident from up to 10-fold accumulation of the tetramethyl-branched fatty acid phytanic acid in Scp2(-/-) mice. Further characterization supported that the gene disruption led to inefficient import of phytanoyl-CoA into peroxisomes and to defective thiolytic cleavage of 3-ketopristanoyl-CoA. These results corresponded to high-affinity binding of phytanoyl-CoA to the recombinant rat SCP2 protein, as well as high 3-ketopristanoyl-CoA thiolase activity of the recombinant rat SCPx protein.

Phospholipid transfer proteins revisited

Phosphatidylinositol transfer protein (PI-TP) and the non-specific lipid transfer protein (nsL-TP) (identical with sterol carrier protein 2) belong to the large and diverse family of intracellular lipid-binding proteins. Although these two proteins may express a comparable phospholipid transfer activity in vitro, recent studies in yeast and mammalian cells have indicated that they serve completely different functions. PI-TP (identical with yeast SEC14p) plays an important role in vesicle flow both in the budding reaction from the trans-Golgi network and in the fusion reaction with the plasma membrane. In yeast, vesicle budding is linked to PI-TP regulating Golgi phosphatidylcholine (PC) biosynthesis with the apparent purpose of maintaining an optimal PI/PC ratio of the Golgi complex. In mammalian cells, vesicle flow appears to be dependent on PI-TP stimulating phosphatidylinositol 4,5-bisphosphate (PIP2) synthesis. This latter process may also be linked to the ability of PI-TP to reconstitute the receptor-controlled PIP2-specific phospholipase C activity. The nsL-TP is a peroxisomal protein which, by its ability to bind fatty acyl-CoAs, is most likely involved in the beta-oxidation of fatty acids in this organelle. This protein constitutes the N-terminus of the 58 kDa protein which is one of the peroxisomal 3-oxo-acyl-CoA thiolases. Further studies on these and other known phospholipid transfer proteins are bound to reveal new insights in their important role as mediators between lipid metabolism and cell functions.

Peroxisomal impairment in Niemann-Pick type C disease

Niemann-Pick type C disease (NPC) belongs to the group of lysosomal storage diseases characterized by an accumulation of cholesterol and sphingomyelin. Using a mutant mouse strain, enzymatic markers for lysosomes, mitochondria, microsomes, and peroxisomes were investigated in the liver and brain. Aside from lysosomal changes, we found a sizable decrease of peroxisomal beta-oxidation of fatty acids and catalase activity in the brain and liver. Isolated peroxisomes displayed a significant decrease of these enzyme activities. Furthermore, the only phospholipid change in brain was a decreased content of the plasmalogen form of phosphatidylethanolamine, and the dimethylacetal pattern was also modified. The electron microscopical appearance of peroxisomes did not display any large changes. The defect of peroxisomal enzymes was already present 18 days before the onset of the disease. In contrast, the lysosomal marker enzyme increased in activity only 6 days after appearance of the symptoms. The events of the studied process have previously been considered to be elicited by a lysosomal deficiency, but this study demonstrates disturbances similar to those in a number of peroxisomal diseases. It appears that the peroxisomal impairment is an early event in the process and could be a factor in the development of Niemann-Pick type C disease.

Characterization of a non-specific lipid transfer protein associated with the peroxisomal membrane of the yeast, Saccharomyces cerevisiae

A lipid transfer protein with a broad substrate specificity is associated with the peroxisomal membrane of the yeast Saccharomyces cerevisiae. The protein catalyzes in vitro the transfer of various phospholipids, phosphatidylinositol and phosphatidylserine being translocated at the highest rates. The transfer protein can be released from peroxisomal membranes by treatment with 0.25 M KCl and highly enriched using conventional chromatographic techniques. It is inactivated by heat, detergents, divalent cations and proteinases. During various steps of purification this lipid transfer protein co-fractionated with peroxisomal acyl-CoA oxidase (Pox1p). In a pox1 disruptant peroxisomal lipid transfer activity was still present, although at a reduced level. The peroxisomal lipid transfer protein from the pox1 mutant exhibited different chromatographic properties as compared to the wild-type strain suggesting that acyl-CoA oxidase and the peroxisomal lipid transfer protein may from a complex.

Association of insulin-degrading enzyme with a 70 kDa cytosolic protein in hepatoma cells

We have investigated the biosynthesis, subcellular location and expression of insulin-degrading enzyme (IDE). a type-I peroxisomal protease, in semi-permeabilized hepatoma cells using pulse-chase experiments, non-denaturing immunoprecipitation protocols and Northern-blot analyses. In HcpG2 cell lysates prepared from cells radiolabelled with Tran[35S]-label, immunoprecipitated IDE was observed immediately after a 5 min pulse and subsequently declined during chase with t1/2 of approx. 33 h. In addition to the 110 kDa IDE protein, a protein of 70 kDa (p70) was identified in radiolabelled immunoprecipitates when using a monoclonal anti-IDE antibody 9B12 under non-denaturing conditions. This same antibody did not recognize p70 on Western blots of whole-cell lysates nor in sequential immunoprecipitates of immunocomplex-bead eluates from anti-IDE immunoprecipitations. Likewise, cross-linking studies performed on intact HepG2 and H35 hepatoma cells in vivo revealed the existence of a hetero-oligomeric complex of 180 kDa in which IDE and p70 were physically associated. Digitonin-permeabilization studies in normal and 35S-labelled HepG2 cells have defined a predominant association of IDE and its associated protein p70 with cytosol (supernatant); only a minor amount of the protein IDE was detected in peroxisomes (cellular pellet). Immunoprecipitation of IDE from 35S-labelled cell lysates of normal and stably transfected Chinese hamster ovary cells overexpressing IDE failed to detect p70. Treatment of HepG2 cells with clofibrate, a peroxisome proliferator, resulted in a dose-dependent increase of the two human IDE transcripts of 3.6 and 3.2 kb. This effect was not accompanied by a similar change at the protein level, nor by a change in the subcellular location of the proteins IDE and p70. Based on these findings we propose that in hepatoma cells: (1) IDE mainly exists in a stable cytoplasmic pool that is unchanged in cells undergoing peroxisomal proliferation; and (2) p70 binding to IDE may serve to maintain the dual cytosolic and peroxisomal pools of IDE in a stable equilibrium.

Further characterization of the peroxisomal 3-hydroxyacyl-CoA dehydrogenases from rat liver. Relationship between the different dehydrogenases and evidence that fatty acids and the C27 bile acids di- and tri-hydroxycoprostanic acids are metabolized by separate multifunctional proteins

Recently, we purified five 3-hydroxyacyl-CoA dehydrogenases from isolated rat liver peroxisomal fractions. The enzymes were designated I-V according to their order of elution from the first column used in the purification procedure. Determination of the substrate (L- or D-hydroxyacyl-CoA) stereo-specificity and (de)hydratase measurements with the different 3-hydroxyacyl-CoA stereoisomers of straight-chain fatty acids and the bile acid intermediate trihydroxycoprostanic acid, immunoblotting analysis with antibodies raised against the different enzymes and peptide sequencing, all performed on enzymes I-V and molecular cloning of enzyme III revealed the following picture. Rat liver peroxisomes contain two multifunctional beta-oxidation proteins: (a) multifunctional protein 1 (the classical multifunctional protein; MFP-1) displaying 2-enoyl-CoA hydratase, L-3-hydroxyacyl-CoA dehydrogenase and delta 3, delta 2-enoyl-CoA isomerase activity (enzyme IV) and (b) multifunctional protein 2 (MFP-2) displaying 2-enoyl-CoA hydratase and D-3-hydroxyacyl-CoA dehydrogenase activity (enzyme III). Because of their substrate stereospecificity and because of the stereochemical configuration of the naturally occurring beta-oxidation intermediates, MFP-1 and MFP-2 appear to be involved in the beta-oxidation of fatty acids and bile acids intermediates, respectively. The deduced amino acid sequence of the cloned MFP-2 cDNA is highly similar to that of the recently described porcine endometrial estradiol 17 beta-dehydrogenase [Leenders, F., Adamski, J., Husen, B., Thole, H. H. & Jungblut, P. W. (1994) Eur. J. Biochem. 222, 221-227]. In agreement, MFP-2 also displayed estradiol 17 beta-dehydrogenase activity, indicating that MFP-2 and the steroid dehydrogenase are identical enzymes. MFP-2 is partially cleaved, most probably in vivo, in a estradiol 17 beta-dehydrogenase/D-3-hydroxyacyl-CoA dehydrogenase that forms a dimeric complex (enzyme I) and a hydratase. The physiological significance of enzyme I in bile acid synthesis (and steroid metabolism) remains to be determined. MFP-1 (enzyme IV) is artefactually cleaved during purification giving rise to 3-hydroxyacyl-CoA dehydrogenase V. 3-Hydroxyacyl-CoA dehydrogenase II is a mitochondrial contaminant similar to porcine and murine mitochondrial 3-hydroxyacyl-CoA dehydrogenase.

Molecular cloning of human phosphomevalonate kinase and identification of a consensus peroxisomal targeting sequence

Two overlapping cDNAs which encode human liver phosphomevalonate kinase (PMKase) were isolated. The human PMKase cDNAs predict a 191-amino acid protein with a molecular weight of 21,862, consistent with previous reports for mammalian PMKase (Mr = 21,000-22,500). Further verification of the clones was obtained by expression of PMKase activity in bacteria using a composite 1024-base pair cDNA clone. Northern blot analysis of several human tissues revealed a doublet of transcripts at approximately 1 kilobase (kb) in heart, liver, skeletal muscle, kidney, and pancreas and lower but detectable transcript levels in brain, placenta, and lung. Analysis of transcripts from human lymphoblasts subcultured in lipid-depleted sera (LDS) and LDS supplemented with lovastatin indicated that PMKase gene expression is subject to regulation by sterol at the level of transcription. Southern blotting indicated that PMKase is a single copy gene covering less than 15 kb in the human genome. The human PMKase amino acid sequence contains a consensus peroxisomal targeting sequence (PTS-1), Ser-Arg-Leu, at the C terminus of the protein. This is the first report of a cholesterol biosynthetic protein which contains a consensus PTS-1, providing further evidence for the concept that early cholesterol and nonsterol isoprenoid biosynthesis may occur in the peroxisome.

Peroxisomes and sterol carrier protein-2 in luteal cell steroidogenesis: a possible role in cholesterol transport from lipid droplets to mitochondria

In the present investigation, we have studied peroxisomes and sterol carrier protein-2 (SCP2) in control and luteinizing hormone stimulated rat luteal cells. Superovulated immature rats in mid-luteal phase (8 days after ovulation) were divided into two groups (n = 4/group) and treated with vehicle (0.2 ml saline), or luteinizing hormone (LH, 20 micrograms/rat). In this animal model, LH acutely stimulates steroidogenesis. Thirty minutes later, corpora lutea were fixed by whole body perfusion and processed for (1) electron microscopic immunocytochemistry to localize SCP2 via the protein A gold immunolabeling technique, and for (2) electron microscopic histochemistry to stain peroxisomal catalase via the alkaline 3,3'-diaminobenzidine tetrahydrochloride method. In the steroidogenic, mid-phase luteal cells of vehicle treated rats (controls), SCP2 was highly concentrated in peroxisomes and sparsely scattered on mitochondria, but no labeling was observed in lipid droplets. In the luteal cells of rats acutely stimulated with LH, peroxisomes immunolabeled for SCP2 were observed within the luteal cell lipid droplets and mitochondria, and in union with lipid droplets and mitochondria. Moreover, in contrast to control luteal cells, significant immunolabeling for SCP2 was detected within the lipid droplets and mitochondria in luteal cells of LH-treated rats. As SCP2 binds cholesterol to 1:1 molar ratio and is known to be involved in the intracellular movement of cholesterol, these findings suggest that peroxisomes and SCP2 may possibly be involved in delivering cholesterol from lipid droplets to the mitochondria when luteal cell steroidogenesis is acutely stimulated by LH.

Degradation of the cleaved leader peptide of thiolase by a peroxisomal proteinase

A peroxisomal location for insulin-degrading enzyme (IDE) has been defined by confocal immunofluorescence microscopy of stably transfected CHO cells overexpressing IDE and digitonin-permeabilization studies in normal nontransfected fibroblasts. The functional significance of IDE in degrading cleaved leader peptides of peroxisomal proteins targeted by the type II motif was evaluated with a synthetic peptide corresponding to the type II leader peptide of prethiolase. The peptide effectively competed for degradation and cross-linking of the high-affinity substrate 125I-labeled insulin to IDE. Direct proteolysis of the leader peptide of prethiolase was confirmed by HPLC; degradation was inhibited by immunodepletion with an antibody to IDE. Phylogenetic analysis of proteinases related to IDE revealed sequence similarity to mitochondrial processing peptidases.

Concentration of intracellular hepatic apolipoprotein E in Golgi apparatus saccular distensions and endosomes

The intrahepatic distribution of apolipoprotein E has been assessed by immunogold labeling of cryosections as well as by Western blotting of organelles isolated from liver homogenates. Both techniques supported the prior analytical fractionation studies of Wong (1989) who concluded that intrahepatic apoE was largely endosomal. All endosomal components decorated by gold particles indicative of apoE antigenicity in cryosections appeared filled with lipoprotein-like particles thereby accounting for this prominent morphological feature of isolated liver endosomes. The distribution of gold particles about the hepatic Golgi apparatus revealed a high content of apoE in closely apposed endosomes, ca. 400 nm in diameter, double labeled for apoE and internalized HRP. Remarkably, apoE (but not internalized HRP) was also observed within saccular distensions of all saccules of stacked Golgi cisternae but absent from the flattened saccular components as was also observed for apoB. This contrasted with albumin, the major secretory protein, which was uniformly distributed throughout the hepatic Golgi apparatus. These observations support a growing body of evidence for intra-Golgi sorting of secretory material in hepatic Golgi apparatus. The lack of any immunoreactive apoE or albumin in small 70-90 nm vesicles about the Golgi cisternae suggests limits to current models of vesicle-mediated intra-Golgi transport.

Near-stoichiometric interaction between the non-specific lipid-transfer protein of the yeast Candida tropicalis and peroxisomal acyl-coenzyme A oxidase prevents the thermal denaturation of the enzyme in vitro

A 14-kDa peroxisomal-matrix protein, named PXP-18, of the yeast Candida tropicalis is a structural and functional homologue of the mammalian nonspecific lipid-transfer protein (identical to sterol carrier protein-2). PXP-18 protected acyl-coenzyme A oxidase (ACO), the rate limiting enzyme of the peroxisomal beta-oxidation of fatty acids, from thermal inactivation at 48 degrees C or 70 degrees C. This effect was dose-dependent and not replaceable either by chicken egg white lysozyme, which is similar to PXP-18 (insofar as it is basic, small, and monomeric), or by bovine serum albumin, a carrier of lipids in the blood. ACO was irreversibly denatured by heat treatment at 70 degrees C for 15 min. However, when ACO and PXP-18 were similarly heat-treated, they formed a large complex at a molar ratio of PXP-18 to ACO subunit that was about one, independent of their initial ratio. This near-stoichiometric complex had ACO activity after a 500-fold dilution and was accompanied by ACO that was free of PXP-18 and indistinguishable from native ACO in size and activity. PXP-18 also protected urate oxidase, another peroxisomal enzyme, from inactivation at 66 degrees C for 15 min and facilitated the renaturation of ACO denatured by 2 M urea. These results indicated that PXP-18 is active in modulating the structure of peroxisomal enzymes in vitro. It is possible that PXP-18 functions as a stress protein or as a part of the system that keeps peroxisomal proteins intact.

The sequence of porcine 80 kDa 17 beta-estradiol dehydrogenase reveals similarities to the short chain alcohol dehydrogenase family, to actin binding motifs and to sterol carrier protein 2

The cDNA of porcine 17 beta-estradiol dehydrogenase codes for a polypeptide of 737 amino acids. The dehydrogenase activity of the 80 kDa translation product is located in its N-terminal 32 kDa fragment, which is the major form isolated from endometrial epithelium. beta-Actin co-purifies with some of the 32 kDa enzyme, which contains actin-binding motifs and is homologous to hydratase-dehydrogenase-epimerase of Candida tropicalis. The microbody-targeting signal AKI and sequences resembling sterol carrier protein 2 are present in the C-terminal part of the 80 kDa protein. The N- and C-terminal parts are connected by a sequence containing the putative protease recognition signal AAP.

Predominant localization of non-specific lipid-transfer protein of the yeast Candida tropicalis in the matrix of peroxisomes

PXP-18 is a 14-kDa major peroxisomal protein of the yeast Candida tropicalis and a homologue of the non-specific lipid-transfer protein (nsLTP) of mammals. Mammalian nsLTP is thought to facilitate the contact of membranes, to stimulate lipid-transfer between them. If PXP-18 functions like nsLTP, it must be present on organelle membranes. Immunoelectron microscopy of C. tropicalis cells indicated that gold particles, which visualized PXP-18, localized exclusively in the matrix of peroxisomes. Subcellular fractionation followed by Western blotting revealed the association of PXP-18 with peroxisomes in C. tropicalis cells. An enzyme-linked immunosorbent assay revealed that almost all the PXP-18 associated with peroxisomes was detectable after the solubilization of the organelle but not before, implying the predominance of PXP-18 inside peroxisomes. This differential assay was applied to the intracellular import of the intact and truncated PXP-18s expressed in Saccharomyces cerevisiae cells. Most of the intact PXP-18 was shown to be imported into the matrix of host-cell peroxisomes, whereas the truncated PXP-18, which lacked the C-terminal tripeptide Pro-Lys-Leu, no longer targeted peroxisomes. These results are consistent with the view that PXP-18 is the matrix protein of peroxisomes and must function in a system other than that of lipid transfer.

Phospholipid-transfer proteins and their mRNAs in developing rat lung and in alveolar type-II cells

Gene expression of non-specific lipid-transfer protein (nsL-TP; identical with sterol carrier protein 2) and phosphatidylinositol-transfer protein (PI-TP) was investigated in developing rat lung. During the late prenatal period (between days 17 and 22) there is a 7-fold increase in the level of nsL-TP and a 2-fold rise in that of PI-TP. The prenatal increases in the levels of nsL-TP and PI-TP are accompanied by parallel increases in the levels of their mRNAs, indicating pretranslational regulation. Compared with whole lung, isolated alveolar type-II cells are enriched in nsL-TP and its mRNA, but not in PI-TP and its mRNA. The observation that the levels of nsL-TP and its mRNA in rat lung show a pronounced increase in the period of accelerated surfactant formation, together with the observation that the surfactant-producing type-II cells are enriched in nsL-TP and its mRNA, suggest that nsL-TP plays a role in the metabolism of pulmonary surfactant.