Formation of irregular giant peroxisomes by overproduction of the crystalloid core protein methanol oxidase in the methylotrophic yeast Hansenula polymorpha

Hydrogen peroxide induced oxidation of peroxisomal malate synthase and catalase

Peroxisomes contain oxidases that produce H(2)O(2), which can result in protein oxidation. To test the vulnerability of peroxisomal proteins to oxidation in vivo the organelles were isolated from castor bean endosperm incubated with H(2)O(2). When peroxisomes were exposed to H(2)O(2)in vivo, the peroxisomal proteins exhibited an increase in carbonylation as detected in avidin blots of biotin hydrazide derivatized samples. Biotin-tagged peptides from trypsin digests of the proteins were analyzed by mass spectroscopy and compared to the masses of peptides from the same protein that had not been biotin-tagged and from proteins not exposed to excess H(2)O(2). H(2)O(2) exposure was found to increase the activity of catalase (CAT), and to increase the number of oxidized peptides found in CAT and malate synthase (MS). CAT had 10 peptides that were affected by in vivo exposure to H(2)O(2) and MS had 8. These sites of oxidation have definable locations within the proteins' structures.

Bioconversion of Methanol to Formaldehyde: 1 by Free Whole Cell and Immobilized Whole Cell of YeastHansenula polymorpha

Modified methylotrophic yeast Hansenula polymorpha (HP A16) was used in this study, which was obtained with recrossing a leucine oxotrophic yeast and a wild type Hansenula polymorpha CB4732 and was discovered by Ogata et al. The yeast is grown with methanol as a sole carbon source in which methanol oxidase (MOX) is a key enzyme of methanol metabolism. Because of its stability and low substrate specificity, alcohol oxidase is of considerable interest for a range of biotechnological processes. Various methanol feeding regimes were evaluated in an effort to increase the biomass concentration and productivity that could be achieved from fermentations using the other Hansenula polymorpha species. This yeast was grown by batch fermentation. The effects of conditions of inoculation media for increasing amount of MOX enzyme in peroksizomes of yeast with MOX activity were observed. The highest MOX activity of yeast was found within optic density of grown media of OD600 1.5, at 0.35 microM of methanol as used an oxotrophic substrate, at 35 degrees C temperature, at pH 7.0 of 0.1 M potassium phosphate buffer (KPB), at 40 microL of buffered yeast cell volume and at the incubation time of 50 minutes. Whole yeast cells were cultivated at above optimized incubation conditions. The cells were immobilized within the polyacyrlamide gels by entrapment method. Free whole cells and immobilized whole cells were compared using bioconversion percentages of methanol to formaldehyde.

Bioconversion of methanol to formaldehyde. II. By purified methanol oxidase from modified yeast, Hansenula polymorpha

Modified methylotrophic yeast Hansenula polymorpha (HP A16) that was obtained by repressing leucine oxotrophic yeast; a wild type of Hansenula polymorpha CB4732 was used in this study. The yeast is grown with methanol, which is used as a sole carbon source, using various methanol concentrations and temperatures, and methanol oxidase (MOX) which is a key enzyme of methanol metabolism; production is maximized. Whole yeast cells were cultivated under optimized inoculation conditions; they were separated into two portions. One portion of these cells was directly used in bioconversion of methanol to formaldehyde. The second portion of the free cells was broken into pieces and a crude enzyme extract was obtained. The MOX enzyme in this extract was purified via salt precipitation, dialysis, and chromatographic methods. The purified MOX enzyme of yeast (HP A16) oxidized the methanol to formaldehyde. Optimization of bioconversion conditions was studied to reach maximum activity of enzyme. The optimum temperature and pH were found to be 35 degrees C and pH 8.0 in boric acid/NaOH buffer, and it was stable over the pH range of 6-9, at the 20 degrees C 15 min. A suitable reaction period was found as 50 min. The enzyme indicated low carbon primary alcohols (C2 to C4), as well as methanol. Initially, MOX activity increased with the increase of methanol concentration, but enzyme activity decreased. The apparent Km and Vmax values for methanol substrate of HP A16 MOX were 0.25 mM and 30 U/mg, respectively. The purified MOX enzyme was applied onto sodium dodecyl sulphate-polyacrylamide gel electrophoresis; molecular weight of the enzyme was calculated to be about 670 kDa. Each MOX enzyme is composed of eight identical subunits, each of whose molecular weight is around 82 kDa and which contain eight moles of FAD as the prosthetic group, and the pI of the natural enzyme is found to be 6.4. The purified MOX enzyme was used in the bioconversion of methanol to formaldehyde as a catalyst; this conversion was compared to the conversion percentages of whole cells in our previous article in terms of catalytic performances.

Growth and Division of Peroxisomes

Peroxisomes are ubiquitous subcellular organelles, which are highly dynamic and display large plasticity in response to cellular and environmental conditions. Novel proteins and pathways that mediate and control peroxisome formation, growth, and division continue to be discovered, and the cellular machineries that act together to regulate peroxisome number and size are under active investigation. Here, advances in the field of peroxisomal dynamics and proliferation in mammals and yeast are reviewed. The authors address the signals, conditions, and proteins that affect, regulate, and control the number and size of this essential organelle, especially the components involved in the division of peroxisomes. Special emphasis is on the function of dynamin-related proteins (DRPs), on Fis1, a putative adaptor for DRPs, on the role of the Pex11 family of peroxisomal membrane proteins, and the cytoskeleton.

The Hansenula polymorpha MOX gene presents two alternative transcription start points differentially utilized and sensitive to respiratory activity

The peroxisomal methanol metabolism of Hansenula polymorpha depends on a group of genes that are coordinately regulated. Methanol oxidase (Mox) plays a key role in this pathway and its synthesis has been shown to be regulated at the transcriptional level. MOX expression is strongly repressed on glucose and activated on glycerol or methanol. In this study we have identified two MOX transcripts that are differentially expressed along MOX derepression. The first one, named l-MOX (for longer MOX), starts at position -425, is only weakly and transiently transcribed and is not translated into the Mox protein. The other is the true MOX mRNA, which initiates around position -25. Using a strain bearing multiple copies of MOX(Q1N) and a reporter gene fused to the MOX promoter, regulation of the two transcripts was investigated. Initiation of the true MOX correlates with repression of l-MOX and conditions that are repressive for MOX transcription, such as the inhibition of mitochondrial activity, lead to higher levels of l-MOX expression. This effect was first observed in a mox mutant (Q1N-M8) unable to grow on nonfermentable carbon sources. No function was detected for l-MOX, but its regulation follows a pattern similar to that of catalase, which is essential for methanol metabolism. This suggests that, l-MOX, although precisely regulated, seems to be a remnant of the evolution of the methanol metabolism network.

Peroxisomes: surprisingly versatile organelles

Peroxisome development is a dynamic process that is not yet completely understood. We use the methylotrophic yeast Hansenula polymorpha as model in our studies on peroxisome homeostasis. Cells of this species may contain different types of peroxisomes that differ in protein composition and capacity to incorporate matrix proteins. This protein import machinery is highly flexible and can accommodate unfolded and complex folded proteins.

Peroxisomal remnant structures in Hansenula polymorpha Pex5 cells can develop into normal peroxisomes upon induction of the PTS2 protein amine oxidase

We have analyzed the properties of peroxisomal remnants in Hansenula polymorpha pex5 cells. In such cells PTS1 matrix protein import is fully impaired. In H. polymorpha pex5 cells, grown on ethanol/ammonium sulfate, conditions that repressed the PTS2 protein amine oxidase (AMO), peroxisomal structures were below the limit of detection. In methanol/ammonium sulfate-grown cells, normal peroxisomes are absent, but a few small membranous structures were observed that apparently represented peroxisomal ghosts since they contained Pex14p. These structures were the target of a Pex10p.myc fusion protein that was produced in pex5 cells under the control of the homologous alcohol oxidase promoter (strain pex5::P(AOX).PEX10.MYC). Glycerol/methanol/ammonium sulfate-grown cells of this transformant were placed in fresh glucose/methylamine media, conditions that fully repress the synthesis of the Pex10p.myc fusion protein but induce the synthesis of AMO. Two hours after the shift Pex10p.myc-containing structures were detectable that had accumulated newly synthesized AMO protein and which during further cultivation developed in normal peroxisomes. Concurrently, the remaining portion of these structures was rapidly degraded. These findings indicate that peroxisomal remnants in pex5 cells can develop into peroxisomes. Also, as for normal peroxisomes in H. polymorpha, apparently a minor portion of these structures actually take part in the development of these organelles.

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.

Immunogold labeling of yeast cells: an efficient tool for the study of protein targeting and morphological alterations due to overexpression and inactivation of genes

Immunogold labeling on Lowicryl HM20 resin sections is a valuable complement to biochemical methods as well as methods of molecular biology in the study of basic mechanisms in the yeast system. This contribution presents an overview of the state of the art. Emphasis is put on the explanation of caveats and pitfalls rather than on detailed bench protocols. In the Applications section the morphological aspect of genetic manipulation is accentuated and links to human pathology are indicated. The morphological consequences of genetic manipulations may gain importance in view of the efforts made to establish gene therapies. In particular, the contribution of immunoelectron microscopy to the elucidation of peroxisomal targeting signals and to the detection and identification of morphological alteration due to overexpressed, mutated or deleted genes in the context of peroxisome biogenesis is described.

Conserved regulation of the Hansenula polymorpha MOX promoter in Saccharomyces cerevisiae reveals insights in the transcriptional activation by Adr1p

The Hansenula polymorpha MOX gene encodes a peroxisomal enzyme that catalyzes the first step of the highly specialized methanol metabolism. MOX is strongly transcribed in cells growing in methanol and completely repressed in glucose. We show here that the MOX promoter confers a glucose-repressible expression upon a lacZ reporter gene in Saccharomyces cerevisiae, an unrelated yeast species that lacks the methanol metabolism. Repression was mediated by a 200-bp region of the MOX promoter, termed MOX-B, and was counteracted by Adr1p, a transcription factor involved in the derepression of S. cerevisiae genes encoding peroxisomal proteins, the class to which MOX belongs. Binding of Adr1p to MOX-B was demonstrated by gel retardation and DNaseI-footprinting, and Adr1p was shown to interact with a DNA region containing only a half of the putative Adr1p consensus binding site. Our findings suggest that Adr1p is a conserved regulator for genes encoding peroxisomal proteins at least in other yeast species, and that its interaction with the DNA is dependent on the promoter context.

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.

The PAH2 gene is required for peroxisome assembly in the methylotrophic yeast Hansenula polymorpha and encodes a member of the tetratricopeptide repeat family of proteins

Peroxisome assembly mutants in the methylotrophic yeast, Hansenula polymorpha, were selected by a novel procedure involving the inability of mutants to use both oleic acid and methanol as carbon sources. These compounds are both metabolized within peroxisomes through two different enzymatic pathways. 15 mutant strains called mut (methanol non-utilizing) were isolated. These strains were assigned to ten genetic complementation groups. Subcellular fractionation analysis showed that peroxisomal matrix enzymes were mislocalized to the cytoplasm in mut strains. Electron microscopy confirmed that the inability of mut strains to grow on oleic acid and methanol was due to defects in peroxisome assembly. Functional complementation of a mutant strain, mut2, with a plasmid library of H. polymorpha genomic DNA sequences has identified a gene, PAH2, that restores growth on methanol and the correct localization of matrix enzymes to the peroxisome. PAH2 encodes Pah2p, a polypeptide of 569 amino acids that is a member of the tetratricopeptide repeat (TPR) family of proteins. Pah2p shows identity with Pas8p and Pas10p, two proteins required for peroxisome assembly in the yeasts Pichia pastoris and Saccharomyces cerevisiae, respectively, and which have been suggested to be receptors that recognize peroxisomal targeting signal-1 (PTS1) motifs.

Transformation in the methylotrophic yeast Pichia methanolica utilizing homologous ADE1 and heterologous Saccharomyces cerevisiae ADE2 and LEU2 genes as genetic markers

Development of transformation systems for methylotrophic yeasts is the starting point for research aimed at developing molecular genetics of these genera and will be the key to their further successful use in biotechnology. We transformed Pichia methanolica using selector genes ADE2 and LEU2 from Saccharomyces cerevisiae and ADE1 (homologue of S. cerevisiae ADE2 gene) from P. methanolica which was cloned and sequenced in our laboratory (Hiep et al., 1991). Lithium transformation of P. methanolica strains was inefficient with intact plasmids. Linearization of plasmids at a unique restriction site within the ADE1 gene prior to transformation substantially increased its frequency. Transformation with linear ADE1, ADE2 or LEU2 gene fragments was even more effective. Introduced DNA fragments either circularized in vivo, irrespective of the structures of their ends, giving unstable transformants; or integrated at different sites of the host genome. Using this transformation system, we obtained a disruption of the ADE1 gene on the chromosome by inserting the S. cerevisiae LEU2 gene. The disruption mutation ade1::LEU2 was used to study the mechanism of intragenic recombination in P. methanolica.

Targeting of glyoxysomal proteins to peroxisomes in leaves and roots of a higher plant

Higher plants possess several classes of peroxisomes that are present at distinct developmental stages and serve different metabolic roles. To investigate the cellular processes that regulate developmental transitions of peroxisomal function, we analyzed the targeting of glyoxysomal proteins to leaf-type and root peroxisomes. We transferred genes encoding the glyoxysome-specific enzymes isocitrate lyase (IL) and malate synthase into Arabidopsis plants and showed, in cell fractionation and immunogold localization experiments, that the glyoxysomal proteins were imported into leaf-type and root peroxisomes. We next defined the sequences that target IL to peroxisomes and asked whether the same targeting determinant is recognized by different classes of the organelle. By localizing deletion and fusion derivatives of IL, we showed that the polypeptide's carboxyl terminus is both necessary for its transport to peroxisomes and sufficient to redirect a passenger protein from the cytosol to both glyoxysomes and leaf-type peroxisomes. Thus, glyoxysomal proteins are transported into several classes of peroxisomes using a common targeting determinant, suggesting that protein import does not play a regulatory role in determining a peroxisome's function. Rather, the specific metabolic role of a peroxisome appears to be determined primarily by processes that regulate the synthesis and/or stability of its constituent proteins. These processes are specified by the differentiated state of the cells in which the organelles are found.

Targeting of glyoxysomal proteins to peroxisomes in leaves and roots of a higher plant

Higher plants possess several classes of peroxisomes that are present at distinct developmental stages and serve different metabolic roles. To investigate the cellular processes that regulate developmental transitions of peroxisomal function, we analyzed the targeting of glyoxysomal proteins to leaf-type and root peroxisomes. We transferred genes encoding the glyoxysome-specific enzymes isocitrate lyase (IL) and malate synthase into Arabidopsis plants and showed, in cell fractionation and immunogold localization experiments, that the glyoxysomal proteins were imported into leaf-type and root peroxisomes. We next defined the sequences that target IL to peroxisomes and asked whether the same targeting determinant is recognized by different classes of the organelle. By localizing deletion and fusion derivatives of IL, we showed that the polypeptide's carboxyl terminus is both necessary for its transport to peroxisomes and sufficient to redirect a passenger protein from the cytosol to both glyoxysomes and leaf-type peroxisomes. Thus, glyoxysomal proteins are transported into several classes of peroxisomes using a common targeting determinant, suggesting that protein import does not play a regulatory role in determining a peroxisome's function. Rather, the specific metabolic role of a peroxisome appears to be determined primarily by processes that regulate the synthesis and/or stability of its constituent proteins. These processes are specified by the differentiated state of the cells in which the organelles are found.

Targeting sequences of the two major peroxisomal proteins in the methylotrophic yeast Hansenula polymorpha

Dihydroxyacetone synthase (DAS) and methanol oxidase (MOX) are the major enzyme constituents of the peroxisomal matrix in the methylotrophic yeast Hansenula polymorpha when grown on methanol as a sole carbon source. In order to characterize their topogenic signals the localization of truncated polypeptides and hybrid proteins was analysed in transformed yeast cells by subcellular fractionation and electron microscopy. The C-terminal part of DAS, when fused to the bacterial beta-lactamase or mouse dihydrofolate reductase, directed these hybrid polypeptides to the peroxisome compartment. The targeting signal was further delimited to the extreme C-terminus, comprising the sequence N-K-L-COOH, similar to the recently identified and widely distributed peroxisomal targeting signal (PTS) S-K-L-COOH in firefly luciferase. By an identical approach, the extreme C-terminus of MOX, comprising the tripeptide A-R-F-COOH, was shown to be the PTS of this protein. Furthermore, on fusion of a C-terminal sequence from firefly luciferase including the PTS, beta-lactamase was also imported into the peroxisomes of H. polymorpha. We conclude that, besides the conserved PTS (or described variants), other amino acid sequences with this function have evolved in nature.

Overexpression of alcohol oxidase in Pichia pastoris

The protein import capacity of peroxisomes in methylotrophic yeasts was studied using Pichia pastoris containing one or two extra copies of the gene encoding the peroxisomal protein alcohol oxidase. The alcohol oxidase overproduced in this strain was only partially imported and assembled into the active, octameric form of the protein. The excess remained in the cytosol as protein aggregates composed of monomers. These results are discussed in view of the possible application of peroxisomes as storage compartments for heterologous proteins.

Biosynthesis and assembly of alcohol oxidase, a peroxisomal matrix protein in methylotrophic yeasts: a review

Alcohol oxidase (AO) catalyses the first step of methanol metabolism in yeasts. In vivo the enzyme is compartmentalized in special cell compartments, called peroxisomes. The enzyme along with the organelles are induced during growth of methylotrophic yeasts on methanol as the sole carbon source. Like all other peroxisomal matrix proteins, AO is encoded by a nuclear gene. Expression of the protein is regulated by a repression/derepression mechanism, but also by induction. Inactive monomeric precursor protein is synthesized in the cytosol and subsequently imported post-translationally into peroxisomes without further processing. Assembly into the active homo-octameric enzyme and binding of the prosthetic group flavin adenine dinucleotide occurs inside the organelle. When enhanced concentration of octameric alcohol oxidase are present in the organelles, the enzyme may form a crystalloid. Oligomerization is not dependent on translocation of AO precursors into their target organelle since octameric, active AO is detected in the cytosol and nucleus of peroxisome-deficient mutants of Hansenula polymorpha: at high expression rates large cytosolic AO crystalloids are formed, which occasionally are also encountered inside the nucleus of such mutants. This paper summarizes recent findings and views on the mechanisms involved in synthesis, import, assembly and crystallization of this important peroxisomal enzyme.

Functional complementation of catalase-defective peroxisomes in a methylotrophic yeast by import of the catalase A from Saccharomyces cerevisiae

A mutant of the methanol-utilizing yeast Hansenula polymorpha defective in catalase was isolated. It lacks the ability to grow on methanol as the sole source of carbon and energy due to a loss of peroxisomal function that is required for the dissimilation and assimilation of this substrate. Growth of the mutant on glucose or glycerol was not impaired. Transformation of mutant cells with the gene coding for catalase A from Saccharomyces cerevisiae [Cohen, G., Fessl, F., Traczyk, J., Rytka, J. & Ruis, H. (1985) Mol. Gen. Genet. 200, 74-79] conferred constitutive expression of catalase activity. When the gene was placed under control of the regulatory methanol oxidase promoter from H. polymorpha, high levels of activity subject to glucose repression were obtained. In both cases efficient targeting of catalase A to the heterologous peroxisomes and assembly into an active form could be demonstrated. Concomitantly, growth on methanol was restored in the transformed mutant. The results are in line with a high conservation of transport signals on peroxisomal proteins. Expression of a cytosolic catalase in H. polymorpha did not confer the ability to grow on methanol. Therefore, proper localization of the catalase activity is a prerequisite for peroxisomal function.