Rapid identification and characterization of peroxisomal assembly mutants inYarrowia lipolytica

Peroxisome biogenesis and human peroxisome-deficiency disorders

Peroxisome is a single-membrane-bounded ubiquitous organelle containing a hundred different enzymes that catalyze various metabolic pathways such as β-oxidation of very long-chain fatty acids and synthesis of plasmalogens. To investigate peroxisome biogenesis and human peroxisome biogenesis disorders (PBDs) including Zellweger syndrome, more than a dozen different complementation groups of Chinese hamster ovary (CHO) cell mutants impaired in peroxisome biogenesis are isolated as a model experimental system. By taking advantage of rapid functional complementation assay of the CHO cell mutants, successful cloning of PEX genes encoding peroxins required for peroxisome assembly invaluably contributed to the accomplishment of cloning of pathogenic genes responsible for PBDs. Peroxins are divided into three groups: 1) peroxins including Pex3p, Pex16p and Pex19p, are responsible for peroxisome membrane biogenesis via Pex19p- and Pex3p-dependent class I and Pex19p- and Pex16p-dependent class II pathways; 2) peroxins that function in matrix protein import; 3) those such as Pex11pβ are involved in peroxisome division where DLP1, Mff, and Fis1 coordinately function.

An ancestral role in peroxisome assembly is retained by the divisional peroxin Pex11 in the yeast Yarrowia lipolytica

The peroxin Pex11 has a recognized role in peroxisome division. Pex11p remodels and elongates peroxisomal membranes prior to the recruitment of dynamin-related GTPases that act in membrane scission to divide peroxisomes. We performed a comprehensive comparative genomics survey to understand the significance of the evolution of the Pex11 protein family in yeast and other eukaryotes. Pex11p is highly conserved and ancestral, and has undergone numerous lineage-specific duplications, whereas other Pex11 protein family members are fungal-specific innovations. Functional characterization of the in-silico-predicted Pex11 protein family members of the yeast Yarrowia lipolytica, i.e. Pex11p, Pex11Cp and Pex11/25p, demonstrated that Pex11Cp and Pex11/25p have a role in the regulation of peroxisome size and number characteristic of Pex11 protein family members. Unexpectedly, deletion of PEX11 in Y. lipolytica produces cells that lack morphologically identifiable peroxisomes, mislocalize peroxisomal matrix proteins and preferentially degrade peroxisomal membrane proteins, i.e. they exhibit the classical pex mutant phenotype, which has not been observed previously in cells deleted for the PEX11 gene. Our results are consistent with an unprecedented role for Pex11p in de novo peroxisome assembly.

Peroxisome biogenesis in mammalian cells

To investigate peroxisome assembly and human peroxisome biogenesis disorders (PBDs) such as Zellweger syndrome, thirteen different complementation groups (CGs) of Chinese hamster ovary (CHO) cell mutants defective in peroxisome biogenesis have been isolated and established as a model research system. Successful gene-cloning studies by a forward genetic approach utilized a rapid functional complementation assay of CHO cell mutants led to isolation of human peroxin (PEX) genes. Search for pathogenic genes responsible for PBDs of all 14 CGs is now completed together with the homology search by screening the human expressed sequence tag database using yeast PEX genes. Peroxins are divided into three groups: (1) peroxins including Pex3p, Pex16p, and Pex19p, are responsible for peroxisome membrane biogenesis via classes I and II pathways; (2) peroxins that function in matrix protein import; (3) those such as three forms of Pex11p, Pex11pα, Pex11pβ, and Pex11pγ, are involved in peroxisome proliferation where DLP1, Mff, and Fis1 coordinately function. In membrane assembly, Pex19p forms complexes in the cytosol with newly synthesized PMPs including Pex16p and transports them to the receptor Pex3p, whereby peroxisomal membrane is formed (Class I pathway). Pex19p likewise forms a complex with newly made Pex3p and translocates it to the Pex3p receptor, Pex16p (Class II pathway). In matrix protein import, newly synthesized proteins harboring peroxisome targeting signal type 1 or 2 are recognized by Pex5p or Pex7p in the cytoplasm and are imported to peroxisomes via translocation machinery. In regard to peroxisome-cytoplasmic shuttling of Pex5p, Pex5p initially targets to an 800-kDa docking complex consisting of Pex14p and Pex13p and then translocates to a 500-kDa RING translocation complex. At the terminal step, Pex1p and Pex6p of the AAA family mediate the export of Pex5p, where Cys-ubiquitination of Pex5p is essential for the Pex5p exit.

Dynein light chain interaction with the peroxisomal import docking complex modulates peroxisome biogenesis in yeast

Dynein is a large macromolecular motor complex that moves cargo along microtubules. A motor-independent role for the light chain of dynein, Dyn2p, in peroxisome biology in Saccharomyces cerevisiae was suggested from its interaction with Pex14p, a component of the peroxisomal matrix protein import docking complex. Here we show that cells of the yeast Yarrowia lipolytica deleted for the gene encoding the homologue of Dyn2p are impaired in peroxisome function and biogenesis. These cells exhibit compromised growth on medium containing oleic acid as the carbon source, the metabolism of which requires functional peroxisomes. Their peroxisomes have abnormal morphology, atypical matrix protein localization, and an absence of proteolytic processing of the matrix enzyme thiolase, which normally occurs upon its import into the peroxisome. We also show physical and genetic interactions between Dyn2p and members of the docking complex, particularly Pex17p. Together, our results demonstrate a role for Dyn2p in the assembly of functional peroxisomes and provide evidence that Dyn2p acts in cooperation with the peroxisomal matrix protein import docking complex to effect optimal matrix protein import.

Identification of the transcription factor Znc1p, which regulates the yeast-to-hypha transition in the dimorphic yeast Yarrowia lipolytica

The dimorphic yeast Yarrowia lipolytica is used as a model to study fungal differentiation because it grows as yeast-like cells or forms hyphal cells in response to changes in environmental conditions. Here, we report the isolation and characterization of a gene, ZNC1, involved in the dimorphic transition in Y. lipolytica. The ZNC1 gene encodes a 782 amino acid protein that contains a Zn(II)2C6 fungal-type zinc finger DNA-binding domain and a leucine zipper domain. ZNC1 transcription is elevated during yeast growth and decreases during the formation of mycelium. Cells in which ZNC1 has been deleted show increased hyphal cell formation. Znc1p-GFP localizes to the nucleus, but mutations within the leucine zipper domain of Znc1p, and to a lesser extent within the Zn(II)2C6 domain, result in a mislocalization of Znc1p to the cytoplasm. Microarrays comparing gene expression between znc1::URA3 and wild-type cells during both exponential growth and the induction of the yeast-to-hypha transition revealed 1,214 genes whose expression was changed by 2-fold or more under at least one of the conditions analyzed. Our results suggest that Znc1p acts as a transcription factor repressing hyphal cell formation and functions as part of a complex network regulating mycelial growth in Y. lipolytica.

Disruption of Yarrowia lipolytica TPS1 gene encoding trehalose-6-P synthase does not affect growth in glucose but impairs growth at high temperature

We have cloned the Yarrowia lipolytica TPS1 gene encoding trehalose-6-P synthase by complementation of the lack of growth in glucose of a Saccharomyces cerevisiae tps1 mutant. Disruption of YlTPS1 could only be achieved with a cassette placed in the 3' half of its coding region due to the overlap of its sequence with the promoter of the essential gene YlTFC1. The Yltps1 mutant grew in glucose although the Y. lipolytica hexokinase is extremely sensitive to inhibition by trehalose-6-P. The presence of a glucokinase, insensitive to trehalose-6-P, that constitutes about 80% of the glucose phosphorylating capacity during growth in glucose may account for the growth phenotype. Trehalose content was below 1 nmol/mg dry weight in Y. lipolytica, but it increased in strains expressing YlTPS1 under the control of the YlTEF1 promoter or with a disruption of YALI0D15598 encoding a putative trehalase. mRNA levels of YlTPS1 were low and did not respond to thermal stresses, but that of YlTPS2 (YALI0D14476) and YlTPS3 (YALI0E31086) increased 4 and 6 times, repectively, by heat treatment. Disruption of YlTPS1 drastically slowed growth at 35°C. Homozygous Yltps1 diploids showed a decreased sporulation frequency that was ascribed to the low level of YALI0D20966 mRNA an homolog of the S. cerevisiae MCK1 which encodes a protein kinase that activates early meiotic gene expression.

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

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

Lessons from peroxisome-deficient Chinese hamster ovary (CHO) cell mutants

Cells with a genetic defect affecting a biological activity and/or a cell phenotype are generally called "cell mutants" and are a highly useful tool in genetic, biochemical, as well as cell biological research. To investigate peroxisome biogenesis and human peroxisome biogenesis disorders, more than a dozen complementation groups of Chinese hamster ovary (CHO) cell mutants defective in peroxisome assembly have been successfully isolated and established as a model system. Moreover, successful PEX gene cloning studies by taking advantage of rapid functional complementation assay of CHO cell mutants invaluably contributed to the accomplishment of isolation of pathogenic genes responsible for peroxisome biogenesis diseases. Molecular mechanisms of peroxisome assembly are currently investigated by making use of such mammalian cell mutants.

Yeast and filamentous fungi as model organisms in microbody research

Yeast and filamentous fungi are important model organisms in microbody research. The value of these organisms as models for higher eukaryotes is underscored by the observation that the principles of various aspects of microbody biology are strongly conserved from lower to higher eukaryotes. This has allowed to resolve various peroxisome-related functions, including peroxisome biogenesis disorders in man. This paper summarizes the major advances in microbody research using fungal systems and specifies specific properties and advantages/disadvantages of the major model organisms currently in use.

Yarrowia lipolytica mutants devoid of pyruvate carboxylase activity show an unusual growth phenotype

We have cloned and characterized the gene PYC1, encoding the unique pyruvate carboxylase in the dimorphic yeast Yarrowia lipolytica. The protein putatively encoded by the cDNA has a length of 1,192 amino acids and shows around 70% identity with pyruvate carboxylases from other organisms. The corresponding genomic DNA possesses an intron of 269 bp located 133 bp downstream of the starting ATG. In the branch motif of the intron, the sequence CCCTAAC, not previously found at this place in spliceosomal introns of Y. lipolytica, was uncovered. Disruption of the PYC1 gene from Y. lipolytica did not abolish growth in glucose-ammonium medium, as is the case in other eukaryotic microorganisms. This unusual growth phenotype was due to an incomplete glucose repression of the function of the glyoxylate cycle, as shown by the lack of growth in that medium of double pyc1 icl1 mutants lacking both pyruvate carboxylase and isocitrate lyase activity. These mutants grew when glutamate, aspartate, or Casamino Acids were added to the glucose-ammonium medium. The cDNA from the Y. lipolytica PYC1 gene complemented the growth defect of a Saccharomyces cerevisiae pyc1 pyc2 mutant, but introduction of either the S. cerevisiae PYC1 or PYC2 gene into Y. lipolytica did not result in detectable pyruvate carboxylase activity or in growth on glucose-ammonium of a Y. lipolytica pyc1 icl1 double mutant.

Peroxisome biogenesis occurs in an unsynchronized manner in close association with the endoplasmic reticulum in temperature-sensitive Yarrowia lipolytica Pex3p mutants

Pex3p is a peroxisomal integral membrane protein required early in peroxisome biogenesis, and Pex3p-deficient cells lack identifiable peroxisomes. Two temperature-sensitive pex3 mutant strains of the yeast Yarrowia lipolytica were made to investigate the role of Pex3p in the early stages of peroxisome biogenesis. In glucose medium at 16 degrees C, these mutants underwent de novo peroxisome biogenesis and exhibited early matrix protein sequestration into peroxisome-like structures found at the endoplasmic reticulum-rich periphery of cells or sometimes associated with nuclei. The de novo peroxisome biogenesis seemed unsynchronized, with peroxisomes occurring at different stages of development both within cells and between cells. Cells with peripheral nascent peroxisomes and cells with structures morphologically distinct from peroxisomes, such as semi/circular tubular structures that immunostained with antibodies to peroxisomal matrix proteins and to the endoplasmic reticulum-resident protein Kar2p, and that surrounded lipid droplets, were observed during up-regulation of peroxisome biogenesis in cells incubated in oleic acid medium at 16 degrees C. These structures were not detected in wild-type or Pex3p-deficient cells. Their role in peroxisome biogenesis remains unclear. Targeting of peroxisomal matrix proteins to these structures suggests that Pex3p directly or indirectly sequesters components of the peroxisome biogenesis machinery. Such a role is consistent with Pex3p overexpression producing cells with fewer, larger, and clustered peroxisomes.

YlBMH1 encodes a 14-3-3 protein that promotes filamentous growth in the dimorphic yeast Yarrowia lipolytica

Most pathogenic fungi have the ability to alternate between a unicellular yeast form and different filamentous forms (hyphae and pseudohyphae). This attribute is generally regarded as an important virulence factor and has also attracted attention because of its implications in the study of eukaryotic cell differentiation. To identify genes that are involved in the regulation of these events, chemical mutagenesis of the dimorphic yeast Yarrowia lipolytica was performed and morphological mutants that were unable to form hyphal cells were isolated. Screening of a Y. lipolytica genomic DNA library for genes able to complement this defect led to the isolation of YlBMH1, a gene encoding a 14-3-3 protein and whose transcription levels are increased during the yeast-to-hypha transition. Remarkably, overexpression of YlBMH1 was able to enhance pseudohyphae formation in a strain lacking functional YlRAC1 but caused no visible effects in deltamhy1 and deltabem1 cells, thus suggesting that YlBMH1 is involved in the regulation of both hyphal and pseudohyphal growth in Y. lipolytica. The identification of YlBMH2, a gene encoding a second 14-3-3 protein (YlBmh2p) that contains a 19 aa insertion absent in all other members of the 14-3-3 family, is also reported. Differently from YlBMH1, the transcription levels of YlBMH2 do not show any apparent variation during the induction of hyphal growth, and its overexpression has no effects on cells lacking functional MHY1, YlRAC1 or YlBEM1. Taken together, these observations suggest that, in spite of their high conservation, YlBmh1p and YlBmh2p have different cellular functions.

Yarrowia lipolytica cells mutant for the PEX24 gene encoding a peroxisomal membrane peroxin mislocalize peroxisomal proteins and accumulate membrane structures containing both peroxisomal matrix and membrane proteins

Peroxins are proteins required for peroxisome assembly and are encoded by the PEX genes. Functional complementation of the oleic acid-nonutilizing strain mut1-1 of the yeast Yarrowia lipolytica has identified the novel gene, PEX24. PEX24 encodes Pex24p, a protein of 550 amino acids (61,100 Da). Pex24p is an integral membrane protein of peroxisomes that exhibits high sequence homology to two hypothetical proteins encoded by the open reading frames YHR150W and YDR479C of the Saccharomyces cerevisiae genome. Pex24p is detectable in wild-type cells grown in glucose-containing medium, and its levels are significantly increased by incubation of cells in oleic acid-containing medium, the metabolism of which requires intact peroxisomes. pex24 mutants are compromised in the targeting of both matrix and membrane proteins to peroxisomes. Although pex24 mutants fail to assemble functional peroxisomes, they do harbor membrane structures that contain subsets of peroxisomal proteins.

Isolation and characterization of YlBEM1, a gene required for cell polarization and differentiation in the dimorphic yeast Yarrowia lipolytica

The ability to switch between a unicellular yeast form and different filamentous forms (fungal dimorphism) is an important attribute of most pathogenic fungi. Dimorphism involves a series of events that ultimately result in dramatic changes in the polarity of cell growth in response to environmental factors. We have isolated and characterized YlBEM1, a gene encoding a protein of 639 amino acids that is essential for the yeast-to-hypha transition in the yeast Yarrowia lipolytica and whose transcription is significantly increased during this event. Cells with deletions of YlBEM1 are viable but show substantial alterations in morphology, disorganization of the actin cytoskeleton, delocalization of cortical actin and chitin deposition, multinucleation, and loss of mating ability, thus pointing to a major role for YlBEM1 in the regulation of cell polarity and morphogenesis in this fungus. This role is further supported by the localization of YlBemlp, which, like cortical actin, appears to be particularly abundant at sites of growth of yeast, hyphal, and pseudohyphal cells. In addition, the potential involvement of YlBem1p in septum formation and/or cytokinesis is suggested by the concentration of a green fluorescent protein-tagged version of this protein at the mother-bud neck during the last stages of cell division. Interestingly, overexpression of MHY1, YlRAC1, or YlSEC31, three genes involved in filamentous growth of Y. lipolytica, induced hyphal growth of bem1 null mutant cells.

Yarrowia lipolytica cells mutant for the peroxisomal peroxin Pex19p contain structures resembling wild-type peroxisomes

PEX genes encode peroxins, which are proteins required for peroxisome assembly. The PEX19 gene of the yeast Yarrowia lipolytica was isolated by functional complementation of the oleic acid-nonutilizing strain pex19-1 and encodes Pex19p, a protein of 324 amino acids (34,822 Da). Subcellular fractionation and immunofluorescence microscopy showed Pex19p to be localized primarily to peroxisomes. Pex19p is detected in cells grown in glucose-containing medium, and its levels are not increased by incubation of cells in oleic acid-containing medium, the metabolism of which requires intact peroxisomes. pex19 cells preferentially mislocalize peroxisomal matrix proteins and the peripheral intraperoxisomal membrane peroxin Pex16p to the cytosol, although small amounts of these proteins could be reproducibly localized to a subcellular fraction enriched for peroxisomes. In contrast, the peroxisomal integral membrane protein Pex2p exhibits greatly reduced levels in pex19 cells compared with its levels in wild-type cells. Importantly, pex19 cells were shown by electron microscopy to contain structures that resemble wild-type peroxisomes in regards to size, shape, number, and electron density. Subcellular fractionation and isopycnic density gradient centrifugation confirmed the presence of vesicular structures in pex19 mutant strains that were similar in density to wild-type peroxisomes and that contained profiles of peroxisomal matrix and membrane proteins that are similar to, yet distinct from, those of wild-type peroxisomes. Because peroxisomal structures form in pex19 cells, Pex19p apparently does not function as a peroxisomal membrane protein receptor in Y. lipolytica. Our results are consistent with a role for Y. lipolytica Pex19p in stabilizing the peroxisomal membrane.

Insertional mutagenesis in the n-alkane-assimilating yeast Yarrowia lipolytica: generation of tagged mutations in genes involved in hydrophobic substrate utilization

Tagged mutants affected in the degradation of hydrophobic compounds (HC) were generated by insertion of a zeta-URA3 mutagenesis cassette (MTC) into the genome of a zeta-free and ura3 deletion-containing strain of Yarrowia lipolytica. MTC integration occurred predominantly at random by nonhomologous recombination. A total of 8,600 Ura(+) transformants were tested by replica plating for (i) growth on minimal media with alkanes of different chain lengths (decane, dodecane, and hexadecane), oleic acid, tributyrin, or ethanol as the C source and (ii) colonial defects on different glucose-containing media (YPD, YNBD, and YNBcas). A total of 257 mutants were obtained, of which about 70 were affected in HC degradation, representing different types of non-alkane-utilizing (Alk(-)) mutants (phenotypic classes alkA to alkE) and tributyrin degradation mutants. Among Alk(-) mutants, growth defects depending on the alkane chain length were observed (alkAa to alkAc). Furthermore, mutants defective in yeast-hypha transition and ethanol utilization and selected auxotrophic mutants were isolated. Flanking borders of the integrated MTC were sequenced to identify the disrupted genes. Sequence analysis indicated that the MTC was integrated in the LEU1 locus in N083, a leucine-auxotrophic mutant, in the isocitrate dehydrogenase gene of N156 (alkE leaky), in the thioredoxin reductase gene in N040 (alkAc), and in a peroxine gene (PEX14) in N078 (alkD). This indicates that MTC integration is a powerful tool for generating and analyzing tagged mutants in Y. lipolytica.

The genetics of peroxisome biogenesis

The segregation of metabolic functions within discrete organelles is a hallmark of eukaryotic cells. These compartments allow for the concentration of related metabolic functions, the separation of competing metabolic functions, and the formation of unique chemical microenvironments. However, such organization is not spontaneous and requires an array of genes that are dedicated to the assembly and maintenance of these structures. In this review we focus on the genetics of peroxisome biogenesis and on how defects in this process cause human disease.

A role for the peroxin Pex8p in Pex20p-dependent thiolase import into peroxisomes of the yeast Yarrowia lipolytica

Peroxins are proteins required for peroxisome assembly. The cytosolic peroxin Pex20p binds directly to the beta-oxidation enzyme thiolase and is necessary for its dimerization and peroxisomal targeting. The intraperoxisomal peroxin Pex8p has a role in the import of peroxisomal matrix proteins, including thiolase. We report the results of yeast two-hybrid analyses with various peroxins of the yeast Yarrowia lipolytica and characterize more fully the interaction between Pex8p and Pex20p. Coimmunoprecipitation showed that Pex8p and Pex20p form a complex, while in vitro binding studies demonstrated that the interaction between Pex8p and Pex20p is specific, direct, and autonomous. Pex8p fractionates with peroxisomes in cells of a PEX20 disruption strain, indicating that Pex20p is not necessary for the targeting of Pex8p to peroxisomes. In cells of a PEX8 disruption strain, thiolase is mostly cytosolic, while Pex20p and a small amount of thiolase associate with peroxisomes, suggesting the involvement of Pex8p in the import of thiolase after docking of the Pex20p-thiolase complex to the membrane. In the absence of Pex8p, peroxisomal thiolase and Pex20p are protected from the action of externally added protease. This finding, together with the fact that Pex8p is intraperoxisomal, suggests that Pex20p may accompany thiolase into peroxisomes during import.

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.

A rac homolog is required for induction of hyphal growth in the dimorphic yeast Yarrowia lipolytica

Dimorphism in fungi is believed to constitute a mechanism of response to adverse conditions and represents an important attribute for the development of virulence by a number of pathogenic fungal species. We have isolated YlRAC1, a gene encoding a 192-amino-acid protein that is essential for hyphal growth in the dimorphic yeast Yarrowia lipolytica and which represents the first Rac homolog described for fungi. YlRAC1 is not an essential gene, and its deletion does not affect the ability to mate or impair actin polarization in Y. lipolytica. However, strains lacking functional YlRAC1 show alterations in cell morphology, suggesting that the function of YlRAC1 may be related to some aspect of the polarization of cell growth. Northern blot analysis showed that transcription of YlRAC1 increases steadily during the yeast-to-hypha transition, while Southern blot analysis of genomic DNA suggested the presence of several RAC family members in Y. lipolytica. Interestingly, strains lacking functional YlRAC1 are still able to grow as the pseudohyphal form and to invade agar, thus pointing to a function for YlRAC1 downstream of MHY1, a previously isolated gene encoding a C(2)H(2)-type zinc finger protein with the ability to bind putative stress response elements and whose activity is essential for both hyphal and pseudohyphal growth in Y. lipolytica.

Mutants of the Yarrowia lipolytica PEX23 gene encoding an integral peroxisomal membrane peroxin mislocalize matrix proteins and accumulate vesicles containing peroxisomal matrix and membrane proteins

pex mutants are defective in peroxisome assembly. The mutant strain pex23-1 of the yeast Yarrowia lipolytica lacks morphologically recognizable peroxisomes and mislocalizes all peroxisomal matrix proteins investigated preferentially to the cytosol. pex23 strains accumulate vesicular structures containing both peroxisomal matrix and membrane proteins. The PEX23 gene was isolated by functional complementation of the pex23-1 strain and encodes a protein, Pex23p, of 418 amino acids (47,588 Da). Pex23p exhibits high sequence similarity to two hypothetical proteins of the yeast Saccharomyces cerevisiae. Pex23p is an integral membrane protein of peroxisomes that is completely, or nearly completely, sequestered from the cytosol. Pex23p is detected at low levels in cells grown in medium containing glucose, and its levels are significantly increased by growth in medium containing oleic acid, the metabolism of which requires intact peroxisomes.

Cloning, sequencing, and characterization of five genes coding for acyl-CoA oxidase isozymes in the yeast Yarrowia lipolytica

The Acyl-CoA oxidase (AOX) isozymes catalyze the first steps of peroxisomal beta-oxidation, which is important for the degradation of fatty acids. Using conserved blocks in previously identified yeast POX genes encoding AOXs, the authors have shown that five POX genes are present in the yeast Yarrowia lipolytica. These genes show approx 63% identity among themselves, and 42% identity with the POX genes from other yeasts. Mono-disrupted Y. lipolytica strains were constructed using a variation of the sticky-end polymerase chain reaction method. AOX activity in the mono-disrupted strains revealed that a long-chain oxidase is encoded by the POX2 gene and a short-chain oxidase by the POX3 gene.

Molecular cloning and characterization of the gene HXK1 encoding the hexokinase from Yarrowia lipolytica

We have cloned the gene HXK1 from the dimorphic yeast Yarrowia lipolytica that encodes the unique hexokinase of this yeast. The gene has an intron located 39 base pairs after the A of the first ATG. The putative protein contains a sequence of 40 amino acids which is absent from other known hexokinase sequences. Y. lipolytica strains devoid of hexokinase grew in glucose slower than wild-type. This growth was due to the existence of a glucokinase. The hexokinase from Y. lipolytica substituted effectively for hexokinase II from S. cerevisiae in catabolite repression of invertase. The hexokinases from Schizosaccharomyces pombe or Kluyveromyces lactis were much less effective in this role. The K(m) for glucose and fructose of hexokinase was 0.38 mM and 3.56 mM, respectively. The K(m) of glucokinase for glucose was 0.17 mM. While the hexokinase was strongly inhibited by trehalose-6-phosphate (K(i)=3.6 microM), glucokinase was not affected by this compound.

Evaluation of acyl coenzyme A oxidase (Aox) isozyme function in the n-alkane-assimilating yeast Yarrowia lipolytica

We have identified five acyl coenzyme A (CoA) oxidase isozymes (Aox1 through Aox5) in the n-alkane-assimilating yeast Yarrowia lipolytica, encoded by the POX1 through POX5 genes. The physiological function of these oxidases has been investigated by gene disruption. Single, double, triple, and quadruple disruptants were constructed. Global Aox activity was determined as a function of time after induction and of substrate chain length. Single null mutations did not affect growth but affected the chain length preference of acyl-CoA oxidase activity, as evidenced by a chain length specificity for Aox2 and Aox3. Aox2 was shown to be a long-chain acyl-CoA oxidase and Aox3 was found to be active against short-chain fatty acids, whereas Aox5 was active against molecules of all chain lengths. Mutations in Aox4 and Aox5 resulted in an increase in total Aox activity. The growth of mutant strains was analyzed. In the presence of POX1 only, strains did not grow on fatty acids, whereas POX4 alone elicited partial growth, and the growth of the double POX2-POX3-deleted mutant was normal excepted on plates containing oleic acid as the carbon source. The amounts of Aox protein detected by Western blotting paralleled the Aox activity levels, demonstrating the regulation of Aox in cells according to the POX genotype.

MHY1 encodes a C2H2-type zinc finger protein that promotes dimorphic transition in the yeast Yarrowia lipolytica

The yeast-to-hypha morphological transition (dimorphism) is typical of many pathogenic fungi. Dimorphism has been attributed to changes in temperature and nutritional status and is believed to constitute a mechanism of response to adverse conditions. We have isolated and characterized a gene, MHY1, whose transcription is dramatically increased during the yeast-to-hypha transition in Yarrowia lipolytica. Deletion of MHY1 is viable and has no effect on mating, but it does result in a complete inability of cells to undergo mycelial growth. MHY1 encodes a C2H2-type zinc finger protein, Mhy1p, which can bind putative cis-acting DNA stress response elements, suggesting that Mhy1p may act as a transcription factor. Interestingly, Mhy1p tagged with a hemagglutinin epitope was concentrated in the nuclei of actively growing cells found at the hyphal tip.

Cloning and characterization of the peroxisomal acyl CoA oxidase ACO3 gene from the alkane-utilizing yeast Yarrowia lipolytica

The ACO3 gene, which encodes one of the acyl-CoA oxidase isoenzymes, was isolated from the alkane-utilizing yeast Yarrowia lipolytica as a 10 kb genomic fragment. It was sequenced and found to encode a 701-amino acid protein very similar to other ACOs, 67.5% identical to Y. lipolytica Aco1p and about 40% identical to S. cerevisiae Pox1p. Haploid strains with a disrupted allele were able to grow on fatty acids. The levels of acyl-CoA oxidase activity in the ACO3 deleted strain, in an ACO1 deleted strain and in the wild-type strain, suggested that ACO3 encodes a short chain acyl-CoA oxidase isoenzyme. This narrow substrate spectrum was confirmed by expression of Aco3p in E. coli.

Pex20p of the yeast Yarrowia lipolytica is required for the oligomerization of thiolase in the cytosol and for its targeting to the peroxisome

Pex mutants are defective in peroxisome assembly. In the pex20-1 mutant strain of the yeast Yarrowia lipolytica, the peroxisomal matrix protein thiolase is mislocalized exclusively to the cytosol, whereas the import of other peroxisomal proteins is unaffected. The PEX20 gene was isolated by functional complementation of the pex20-1 strain and encodes a protein, Pex20p, of 424 amino acids (47,274 D). Despite its role in the peroxisomal import of thiolase, which is targeted by an amino-terminal peroxisomal targeting signal-2 (PTS2), Pex20p does not exhibit homology to Pex7p, which acts as the PTS2 receptor. Pex20p is mostly cytosolic, whereas 4-8% is associated with high-speed (200,000 g) pelletable peroxisomes. In the wild-type strain, all newly synthesized thiolase is associated with Pex20p in a heterotetrameric complex composed of two polypeptide chains of each protein. This association is independent of PTS2. Pex20p is required for both the oligomerization of thiolase in the cytosol and its targeting to the peroxisome. Our data suggest that monomeric Pex20p binds newly synthesized monomeric thiolase in the cytosol and promotes the formation of a heterotetrameric complex of these two proteins, which could further bind to the peroxisomal membrane. Translocation of the thiolase homodimer into the peroxisomal matrix would release Pex20p monomers back to the cytosol, thereby permitting a new cycle of binding-oligomerization-targeting-release for Pex20p and thiolase.

Mutants of the yeast Yarrowia lipolytica defective in protein exit from the endoplasmic reticulum are also defective in peroxisome biogenesis

Mutations in the SEC238 and SRP54 genes of the yeast Yarrowia lipolytica not only cause temperature-sensitive defects in the exit of the precursor form of alkaline extracellular protease and of other secretory proteins from the endoplasmic reticulum and in protein secretion but also lead to temperature-sensitive growth in oleic acid-containing medium, the metabolism of which requires the assembly of functionally intact peroxisomes. The sec238A and srp54KO mutations at the restrictive temperature significantly reduce the size and number of peroxisomes, affect the import of peroxisomal matrix and membrane proteins into the organelle, and significantly delay, but do not prevent, the exit of two peroxisomal membrane proteins, Pex2p and Pex16p, from the endoplasmic reticulum en route to the peroxisomal membrane. Mutations in the PEX1 and PEX6 genes, which encode members of the AAA family of N-ethylmaleimide-sensitive fusion protein-like ATPases, not only affect the exit of precursor forms of secretory proteins from the endoplasmic reticulum but also prevent the exit of the peroxisomal membrane proteins Pex2p and Pex16p from the endoplasmic reticulum and cause the accumulation of an extensive network of endoplasmic reticulum membranes. None of the peroxisomal matrix proteins tested associated with the endoplasmic reticulum in sec238A, srp54KO, pex1-1, and pex6KO mutant cells. Our data provide evidence that the endoplasmic reticulum is required for peroxisome biogenesis and suggest that in Y. lipolytica, the trafficking of some membrane proteins, but not matrix proteins, to the peroxisome occurs via the endoplasmic reticulum, results in their glycosylation within the lumen of the endoplasmic reticulum, does not involve transport through the Golgi, and requires the products encoded by the SEC238, SRP54, PEX1, and PEX6 genes.

Components involved in peroxisome import, biogenesis, proliferation, turnover, and movement

In the decade that has elapsed since the discovery of the first peroxisomal targeting signal (PTS), considerable information has been obtained regarding the mechanism of protein import into peroxisomes. The PTSs responsible for the import of matrix and membrane proteins to peroxisomes, the receptors for several of these PTSs, and docking proteins for the PTS1 and PTS2 receptors are known. Many peroxins involved in peroxisomal protein import and biogenesis have been characterized genetically and biochemically. These studies have revealed important new insights regarding the mechanism of protein translocation across the peroxisomal membrane, the conservation of PEX genes through evolution, the role of peroxins in fatal human peroxisomal disorders, and the biogenesis of the organelle. It is clear that peroxisomal protein import and biogenesis have many features unique to this organelle alone. More recent studies on peroxisome degradation, division, and movement highlight newer aspects of the biology of this organelle that promise to be just as exciting and interesting as import and biogenesis.

Enlarged peroxisomes are present in oleic acid-grown Yarrowia lipolytica overexpressing the PEX16 gene encoding an intraperoxisomal peripheral membrane peroxin

Pex mutants of the yeast Yarrowia lipolytica are defective in peroxisome assembly. The mutant strain pex16-1 lacks morphologically recognizable peroxisomes. Most peroxisomal proteins are mislocalized to a subcellular fraction enriched for cytosol in pex16 strains, but a subset of peroxisomal proteins is localized at, or near, wild-type levels to a fraction typically enriched for peroxisomes. The PEX16 gene was isolated by functional complementation of the pex16-1 strain and encodes a protein, Pex16p, of 391 amino acids (44,479 D). Pex16p has no known homologues. Pex16p is a peripheral protein located at the matrix face of the peroxisomal membrane. Substitution of the carboxylterminal tripeptide Ser-Thr-Leu, which is similar to the consensus sequence of peroxisomal targeting signal 1, does not affect targeting of Pex16p to peroxisomes. Pex16p is synthesized in wild-type cells grown in glucose-containing media, and its levels are modestly increased by growth of cells in oleic acid-containing medium. Overexpression of the PEX16 gene in oleic acid- grown Y. lipolytica leads to the appearance of a small number of enlarged peroxisomes, which contain the normal complement of peroxisomal proteins at levels approaching those of wild-type peroxisomes.

Peroxisome biogenesis

Peroxisomes are eukaryotic organelles that are the subcellular location of important metabolic reactions. In humans, defects in the organelle's function are often lethal. Yet, relative to other organelles, little is known about how cells maintain and propagate peroxisomes or how they direct specific sets of newly synthesized proteins to these organelles (peroxisome biogenesis/assembly). In recent years, substantial progress has been made in elucidating aspects of peroxisome biogenesis and in identifying PEX genes whose products, peroxins, are essential for one or more of these processes. The most progress has been made in understanding the mechanism by which peroxisome matrix proteins are imported into the organelles. Signal sequences responsible for targeting proteins to the organelle have been defined. Potential signal receptor proteins, a receptor docking protein and other components of the import machinery have been identified, along with insights into how they operate. These studies indicate that multiple peroxisomal protein-import mechanisms exist and that these mechanisms are novel, not simply variations of those described for other organelles.

Signals, receptors, and cytosolic factors involved in peroxisomal protein import

Peroxisomes are ubiquitous eukaryotic organelles which function in a wide variety of metabolic processes. The many lethal human disorders associated with defects in peroxisomal protein import underscore the importance of this organelle. In recent years, the evolutionarily conserved molecular mechanisms of protein targeting to, and translocation across, peroxisomal membranes have begun to emerge. Signals which route proteins to the organelle have been identified, as have cytosolic, membrane-associated, and lumenal components of the import machinery. The goal of this brief review was to summarize our current knowledge of some of these molecules and to describe several potential mechanisms by which peroxisomes selectively import their constituent proteins. Aspects of these mechanisms that distinguish peroxisomal protein import from protein targeting to other organelles are highlighted.

The Yarrowia lipolytica gene PAY5 encodes a peroxisomal integral membrane protein homologous to the mammalian peroxisome assembly factor PAF-1

Pay mutants of the yeast Yarrowia lipolytica fail to assemble functional peroxisomes. One mutant strain, pay5-1, lacks normal peroxisomes and instead contains irregular vesicular structures surrounded by multiple unit membranes. The pay5-1 mutant is not totally deficient in peroxisomal matrix protein targeting, as a subset of matrix proteins continues to localize to a subcellular fraction enriched for peroxisomes. The functionally complementing gene PAY5 encodes a protein, Pay5p, of 380 amino acids (41,720 Da). Pay5p is a peroxisomal integral membrane protein homologous to mammalian PAF-1 proteins, which are essential for peroxisome assembly and whose mutation in humans results in Zellweger syndrome. Pay5p is targeted to mammalian peroxisomes, demonstrating the evolutionary conservation of the targeting mechanism for peroxisomal membrane proteins. Our results suggest that in pay5 mutants, normal peroxisome assembly is blocked, which leads to the accumulation of the membranous vesicular structures observed.

Mutations in the PAY5 gene of the yeast Yarrowia lipolytica cause the accumulation of multiple subpopulations of peroxisomes

We previously reported the cloning of the PAY5 gene of the yeast Yarrowia lipolytica by complementation of the peroxisome assembly mutant pay5-1 (Eitzen, G. A., Titorenko, V. I., Smith, J. J., Veenhuis, M., Szilard, R. K., and Rachubinski, R. A. (1996) J. Biol. Chem. 271, 20300-20306). The peroxisomal integral membrane protein Pay5p is a homologue of mammalian PAF-1 proteins, which are essential for peroxisome assembly and whose mutation in humans results in peroxisome biogenesis disorders. Mutations in the PAY5 gene result in the accumulation of three distinct peroxisomal subpopulations. These subpopulations are characterized by differences in 1) buoyant density, 2) the relative distribution of peroxisomal matrix and membrane proteins, 3) the efficiency of import of several peroxisomal matrix proteins, and 4) the phospholipid levels of peroxisomal membranes. These data, together with the analysis of temporal changes in the relative abundance of individual peroxisomal subpopulations in pay5 mutants, suggest that these subpopulations represent intermediates in a multistep peroxisome assembly pathway normally operating in yeast cells.

Isolation and characterization of Pas2p, a peroxisomal membrane protein essential for peroxisome biogenesis in the methylotrophic yeast Pichia pastoris

The pas2 mutant of the methylotrophic yeast Pichia pastoris is characterized by a deficiency in peroxisome biogenesis. We have cloned the PpPAS2 gene by functional complementation and show that it encodes a protein of 455 amino acids with a molecular mass of 52 kDa. In a Pppas2 null mutant, import of both peroxisomal targeting signal 1 (PTS1)- and PTS2-containing proteins is impaired as shown by biochemical fractionation and fluorescence microscopy. No morphologically distinguishable peroxisomal structures could be detected by electron microscopy in Pppas2 null cells induced on methanol and oleate, suggesting that PpPas2p is involved in the early stages of peroxisome biogenesis. PpPas2p is a peroxisomal membrane protein (PMP) and is resistant to extraction by 1 M NaCl or alkaline sodium carbonate, suggesting that it is a peroxisomal integral membrane protein. Two hydrophobic domains can be distinguished which may be involved in anchoring PpPas2p to the peroxisomal membrane. PpPas2p is homologous to the Saccharomyces cerevisiae Pas3p. The first 40 amino acids of PpPas2p, devoid of the hydrophobic domains, are sufficient to target a soluble fluorescent reporter protein to the peroxisomal membrane, with which it associates tightly. A comparison with the membrane peroxisomal targeting signal of PMP47 of Candida boidinii revealed a stretch of positively charged amino acids common to both sequences. The role of peroxisomal membrane targeting signals and transmembrane domains in anchoring PMPs to the peroxisomal membrane is discussed.

Convergence of model systems for peroxisome biogenesis

Receptors for the two peroxisomal targeting signals PTS1 and PTS2 have recently been identified in yeasts. The disparate subcellular locations of PTS receptor homologs have led to proposals of receptor shuttling between the cytosol and the peroxisomal membrane, and to the suggestion that some of these molecules may even reside normally in the peroxisomal matrix. A peroxisomal membrane protein that interacts with the PTS1 receptor in Saccharomyces cerevisiae may serve as the receptor-docking site on the organelle. The conservation of the PTS1 receptor in several yeasts has led to the cloning and characterization of the gene encoding its human homolog, PTS1R, which is mutated in a group of patients afflicted with fatal peroxisomal disorders. The identification of peroxisome assembly genes in yeasts is likely to lead to the cloning of additional human genes responsible for other generalized peroxisomal disorders. Protein unfolding is not a prerequisite for import of peroxisomal matrix proteins, suggesting novel mechanisms for the translocation of polypeptides across the peroxisomal membrane.

Maintenance of the peroxisomal compartment in glucose-repressed and anaerobically grown Saccharomyces cerevisiae cells

According to the current model of peroxisome biogenesis, the inheritance of this compartment requires the growth and division of pre-existing organelles followed by their distribution between mother and daughter cells. However, no known peroxisomal functions are present nor required for Saccharomyces cerevisiae cells grown under glucose repression and in anaerobiosis and the peroxisomal compartment becomes virtually indistinguishable under such conditions. This raised the question of the fate of this compartment in such cells. Is it maintained throughout prolonged growth under glucose repression or does it disappear from the cell and then reassemble on demand? To study the maintenance of putatively functional peroxisomes in S cerevisiae cells grown under conditions of glucose repression and anaerobiosis, we applied the vector-mediated overexpression of peroxisome matrix enzyme's catalase A and acyl-CoA oxidase. Evidence is presented that in S cerevisiae the peroxisomal import machinery responsible for targeting of matrix enzymes into this compartment is preserved under glucose repression and in the absence of oxygen.

Review: methylotrophic yeasts as factories for the production of foreign proteins

In this contribution we discuss the potential of methylotrophic yeasts as hosts for the high level production of valuable foreign proteins. Recent relevant achievements on the intracellular production or secretion of proteins are summarized. Special attention is paid to a specific advantage of the use of methylotrophic yeasts, namely the possibility of accumulating the foreign gene products inside peroxisomes. This approach may be of major advantage when the protein product is toxic for the host cell and, also, to protect these proteins from undesired side-effects such as proteolysis or aggregation.

Identification of peroxisomal membrane ghosts with an epitope-tagged integral membrane protein in yeast mutants lacking peroxisomes

Many yeast peroxisome biogenesis mutants have been isolated in which peroxisomes appear to be completely absent. Introduction of a wild-type copy of the defective gene causes the reappearance of peroxisomes, despite the fact that new peroxisomes are thought to form only from pre-existing peroxisomes. This apparent paradox has been explained for similar human mutant cell lines (from patients with Zellweger syndrome) by the discovery of peroxisomal membrane ghosts in the mutant cells (Santos, M. J., T. Imanaka, H. Shio, G. M. Small and P. B. Lazarow. 1988. Science 239, 1536-1538). Introduction of a wild-type gene is thought to restore to the ghosts the ability to import matrix proteins, and thus lead to the refilling of the peroxisomes. It is vitally important to our understanding of peroxisome biogenesis to determine whether the yeast mutants contain ghosts. We have solved this problem by introducing an epitope-tagged version of Pas3p, a peroxisome integral membrane protein (that is essential for peroxisome biogenesis). Nucleotides encoding a nine amino acid HA epitope were added to the PAS3 gene immediately before the stop codon. The tagged gene (PAS3HA) was inserted in the genome, replacing the wild-type gene at its normal locus. It was fully functional (the cells assembled peroxisomes normally and grew on oleic acid) but the expression level was too low to detect the protein with monoclonal antibody 12CA5. PAS3HA was expressed in greater quantity from an episomal plasmid with the CUP1 promoter. The gene product, Pas3pHA, was detected by immunogold labelling on the membranes of individual and clustered peroxisomes; the clusters appeared as large spots in immunofluorescence. PAS3HA was similarly expressed in peroxisome biogenesis mutants peb2 and peb4, which lack morphologically recognizable peroxisomes. Gold-labelled membranes were clearly visible in both mutants: in peb2 the labelled membrane vesicles were generally much smaller than those in peb4, which resembled normal peroxisomes in size.

Characterization of peroxisome-deficient mutants of Hansenula polymorpha

In the methylotrophic yeast Hansenula polymorpha, approximately 25% of all methanol-utilization-defective (Mut-) mutants are affected in genes required for peroxisome biogenesis (PER genes). Previously, we reported that one group of per mutants, termed Pim-, are characterized by the presence of a few small peroxisomes with the bulk of peroxisomal enzymes located in the cytosol. Here, we describe a second major group of per mutants that were observed to be devoid of any peroxisome-like structure (Per-). In each Per- mutant, the peroxisomal methanol-pathway enzymes alcohol oxidase, catalase and dihydroxyacetone synthase were present and active but located in the cytosol. Together, the Pim- and Per- mutant collections involved mutations in 14 different PER genes. Two of the genes, PER5 and PER7, were represented by both dominant-negative and recessive alleles. Diploids resulting from crosses of dominant per strains and wild-type H. polymorpha were Mut- and harbored peroxisomes with abnormal morphology. This is the first report of dominant-negative mutations affecting peroxisome biogenesis.

The Yarrowia lipolytica gene PAY2 encodes a 42-kDa peroxisomal integral membrane protein essential for matrix protein import and peroxisome enlargement but not for peroxisome membrane proliferation

PAY genes are required for peroxisome assembly in the yeast Yarrowia lipolytica. Here we show that a mutant strain, pay2, is disrupted for the import of proteins targeted by either peroxisomal targeting signal-1 or -2. Electron microscopy of pay2 cells revealed the presence of small peroxisomal "ghosts," similar to the vesicular structures found in fibroblasts of patients with the human peroxisome assembly disorder, Zellweger syndrome. Functional complementation of pay2 with a plasmid library of Y. lipolytica genomic DNA identified a gene, PAY2, that restores growth of pay2 on oleic acid, import of catalase and multifunctional enzyme into peroxisomes, and formation of wild type peroxisomes. The PAY2 gene encodes Pay2p, a hydrophobic polypeptide of 404 amino acids. An antibody raised against Pay2p recognizes a polypeptide of approximately 42-kDa whose synthesis is induced by growth of Y. lipolytica on oleic acid. Pay2p is a peroxisomal integral membrane protein, as it localizes to carbonate-stripped peroxisomal membranes. Pay2p shows no identity to any known protein. Our results suggest that Pay2p is essential for the activity of the peroxisomal import machinery but does not affect the initial steps of peroxisomal membrane proliferation.

Effect of site-directed mutagenesis of conserved lysine residues upon Pas1 protein function in peroxisome biogenesis

The Pas1 protein (Pas1p) is required for peroxisome biogenesis in Saccharomyces cerevisiae and contains two putative ATP-binding sites, each within a domain which is conserved among members of the recently characterized AAA-family. To elucidate whether both putative ATP-binding sites are essential for Pas1p function, lysine 467 of the first and lysine 744 of the second putative ATP-binding site were each changed to glutamate by site-directed mutagenesis. While replacement of lysine 744 abolished the function of the Pas1 protein in peroxisome biogenesis, replacement of lysine 467 had no obvious effect.

Stabilization of methionine-rich protein in Saccharomyces cerevisiae: targeting of BZN protein into the peroxisome

We have constructed a gene coding for the 12-kDa intermediate form of the 2s methionine-rich protein from Bertholletia excelsa seeds. This protein, expressed intracellularly in yeast, is characterised by a 20-min half-life. By adding 11 amino acids corresponding to the peroxisome-targeting sequence (PTSc) of luciferase, we have significantly increased its half-life. This stabilization allowed accumulation of the BZN protein into the peroxisome as judged by cell fractionation. Accumulation of the 12-kDa protein results in a significant increase of the total methionine content in yeast cells (30%) indicating that such a microorganism could represent a practicable protected shuttle for an animal-feed additive.