Hansenula polymorpha Pex14p phosphorylated in vivo

Hansenula polymorpha Pex14p is a novel peroxisomal membrane protein essential for peroxisome biogenesis. In vivo labeling experiment of wild-type cells with 32P-orthophosphate and alkaline phosphatase treatment of labeled Pex14p indicate that Pex14p is phosphorylated in vivo. Analysis of the phosphoamino acid in the phosphorylated Pex14p suggested that the major phosphoamino acid was acid labile. Using expression system of several truncated Pex14ps in a PEX14-deletion strain it is suggested that the phosphorylation site of Pex14p resides in the C-terminal 58 residues.

Peroxisomal matrix protein import. Suppression of protein import defects in Hansenula polymorpha pex mutants by overproduction of the PTS1 receptor Pex5p

In the past decade, much progress has been made in understanding the mechanisms that govern sorting of proteins to the peroxisomal lumen. This article summarizes the principal features of how peroxisomal matrix enzymes are thought to reach the peroxisome. In addition, it describes recent data that indicate that, in specific pex mutants of the methylotrophic yeast Hansenula polymorpha, defects in matrix protein import can be (partly) rescued by overproduction of the receptor essential for import of these proteins. The implication of these results on the mechanisms of matrix protein import is discussed.

Pim1, a MAP kinase involved in cell wall integrity in Pichia pastoris

Mitogen-activated protein kinases (MAPKs) are key enzymes in the signal transduction pathways of eukaryotes. We report the isolation of a Pichia pastoris gene, PIM1, which encodes the first MAPK to be identified in this yeast. Pim1 shows the greatest similarity to fungal MAPKs involved in the maintenance of cell integrity. Disruption of the PIM1 gene results in an osmoremediable thermosensitive phenotype reminiscent of that observed in mutants affected in the MAPK Slt2/ Mpk1 of Saccharomyces cerevisiae, which is involved in ensuring cell wall integrity. Furthermore, pim1 mutants are hypersensitive to caffeine and cell wall-destabilising compounds. Pim1 is phosphorylated at two sites, and thereby activated, in response to heat stress, caffeine and agents that alter the fungal cell wall, which is consistent with a role in adaptation to these conditions. These results support the idea that the MAPK-based mechanisms which regulate cell wall integrity are conserved in yeast species. Pim1 is also doubly phosphorylated in S. cerevisiae in response to stimuli that activate the cell integrity pathway in this yeast. In addition, Pim1 is able to activate the transcription of a reporter gene in one-hybrid experiments, as does its S. cerevisiae counterpart, Slt2. Interestingly, however, Pim1 does not rescue the mutant phenotype of an slt2delta strain. This indicates some functional divergence in MAPK modulation and signal transmission by cell integrity pathways and provides a tool that may contribute to a better understanding of MAPK signalling.

Pichia pastoris Pex14p, a phosphorylated peroxisomal membrane protein, is part of a PTS-receptor docking complex and interacts with many peroxins

The peroxisomal protein import machinery plays a central role in the assembly of this organelle in all eukaryotes. Genes encoding components of this machinery, termed peroxins or Pex proteins, have been isolated and characterized in several yeast species and in mammals, including humans. Here we report on one of these components, Pex14p, from the methylotrophic yeast Pichia pastoris. Work in other organisms has shown that Pex14p is located on the cytoplasmic surface of the peroxisomal membrane and binds peroxisomal targeting signal (PTS) receptors carrying proteins bound for the peroxisomal matrix, results that have led to the hypothesis that Pex14p is a receptor-docking protein. P. pastoris Pex14p (PpPex14p) behaves like an integral membrane protein, with its C-terminus exposed on the cytosolic side of the peroxisomal membrane. PpPex14p complexes with many peroxins, including Pex3p (Snyder et al., 1999b), Pex5p, Pex7p, Pex13p, Pex17p, itself, and a previously unreported peroxin, Pex8p. A portion of Pex14p is phosphorylated, but both phosphorylated and unphosphorylated forms of Pex14p interact with several peroxins. The interactions between Pex14p and other peroxins provide clues regarding the function of Pex14p in peroxisomal protein import.

Strul--a method for 3D alignment of single-particle projections based on common line correlation in Fourier space

A central problem of 3D reconstruction in single-particle electron microscopy is the determination of relative orientations of the individual projections contributing to the reconstruction. This article describes an implementation of the method of common lines correlation in Fourier space that allows generation of common lines between an arbitrary number of projections which might posses an arbitrary point group symmetry. Based on this method, it is possible to optimize rotational and translational alignment parameters for individual single-particle projections. The underlying philosophy and details of implementation are discussed, and as an illustration a 3D reconstruction in ice of peroxisomal alcohol oxidase from Pichia pastoris, an octameric assembly with 422-symmetry and a molecular weight of 592 kDa is presented.

Cvt9/Gsa9 functions in sequestering selective cytosolic cargo destined for the vacuole

Three overlapping pathways mediate the transport of cytoplasmic material to the vacuole in Saccharomyces cerevisiae. The cytoplasm to vacuole targeting (Cvt) pathway transports the vacuolar hydrolase, aminopeptidase I (API), whereas pexophagy mediates the delivery of excess peroxisomes for degradation. Both the Cvt and pexophagy pathways are selective processes that specifically recognize their cargo. In contrast, macroautophagy nonselectively transports bulk cytosol to the vacuole for recycling. Most of the import machinery characterized thus far is required for all three modes of transport. However, unique features of each pathway dictate the requirement for additional components that differentiate these pathways from one another, including at the step of specific cargo selection.We have identified Cvt9 and its Pichia pastoris counterpart Gsa9. In S. cerevisiae, Cvt9 is required for the selective delivery of precursor API (prAPI) to the vacuole by the Cvt pathway and the targeted degradation of peroxisomes by pexophagy. In P. pastoris, Gsa9 is required for glucose-induced pexophagy. Significantly, neither Cvt9 nor Gsa9 is required for starvation-induced nonselective transport of bulk cytoplasmic cargo by macroautophagy. The deletion of CVT9 destabilizes the binding of prAPI to the membrane and analysis of a cvt9 temperature-sensitive mutant supports a direct role of Cvt9 in transport vesicle formation. Cvt9 oligomers peripherally associate with a novel, perivacuolar membrane compartment and interact with Apg1, a Ser/Thr kinase essential for both the Cvt pathway and autophagy. In P. pastoris Gsa9 is recruited to concentrated regions on the vacuole membrane that contact peroxisomes in the process of being engulfed by pexophagy. These biochemical and morphological results demonstrate that Cvt9 and the P. pastoris homologue Gsa9 may function at the step of selective cargo sequestration.

The human ET(B) endothelin receptor heterologously produced in the methylotrophic yeast Pichia pastoris shows high-affinity binding and induction of stacked membranes

The human endothelin B receptor (ET(B) receptor) was produced in the methylotrophic yeast Pichia pastoris under transcriptional control of the highly inducible alcohol oxidase 1 (AOX1) gene promoter. In the expression plasmids pPIC9KFlagET(B)Bio and pPIC9KFlag deltaGPET(B)Bio the ET(B) receptor coding region was fused in frame to the Saccharomyces cerevisiae alpha-factor prepropeptide and the FLAG-tag. In both constructs, the receptor was also fused to a biotinylation domain. Additionally, in pPIC9KFlag deltaGPET(B)Bio the putative N-glycosylation site and a protease site have been deleted by site directed mutagenesis. Crude membranes prepared from recombinant P. pastoris revealed specific and saturable binding of [125I]ET-1 with a K(D) of about 42 pM. Receptor levels of 60 pmol/mg and 35 pmol/mg for the Flag deltaGPET(B)Bio and the FlagET(B)Bio construct, respectively, were determined. The pharmacological profile for ET-1, ET-2 and ET-3 were as expected for a subtype B endothelin (ET) receptor. Immunoblot analysis showed an apparent molecular mass of 55 kDa for the Kex2-processed and about 74 kDa for the Kex2-unprocessed receptor. Contrary to the Flag deltaGPET(B)Bio construct, the FlagET(B)Bio construct was not correctly processed by the internal Kex2 endopeptidase. As was detected by ultrastructural analysis of recombinant yeast cells, high-level production of the receptor resulted in the formation of stacked membranes.

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.

Mutations affecting the expression of the MOX gene encoding peroxisomal methanol oxidase in Hansenula polymorpha

In this study, aimed at identifying genetic factors acting positively upon the MOX gene, we report the isolation and characterisation of several methanol utilisation-defective (Mut-) mutants of Hansenula polymorpha. These fall into 12 complementation groups, eight of which show significant reductions in alcohol (methanol) oxidase activity in methanol. Three of these groups, identifying the MUT3, MUT5 and MUT10 loci, exhibit extremely low levels of MOX promoter activity, not only in methanol medium, but also during growth in glycerol or methylamine. We suggest that these loci play a significant role in the derepression of the MOX gene expression. One of these genes (MUT10) also seems to be involved in the utilisation of carbon sources other than methanol, and it is apparent that the same gene plays some role in the biogenesis or in the enlargement of the peroxisome. Three other genes (MUT7, MUT8 and MUT9) appear to be involved in peroxisome biogenesis, whereas most other mutants harbour lesions that leave the peroxisome biogenesis and proliferation unaffected.

The peroxisome biogenesis factors pex4p, pex22p, pex1p, and pex6p act in the terminal steps of peroxisomal matrix protein import

Peroxisomes are independent organelles found in virtually all eukaryotic cells. Genetic studies have identified more than 20 PEX genes that are required for peroxisome biogenesis. The role of most PEX gene products, peroxins, remains to be determined, but a variety of studies have established that Pex5p binds the type 1 peroxisomal targeting signal and is the import receptor for most newly synthesized peroxisomal matrix proteins. The steady-state abundance of Pex5p is unaffected in most pex mutants of the yeast Pichia pastoris but is severely reduced in pex4 and pex22 mutants and moderately reduced in pex1 and pex6 mutants. We used these subphenotypes to determine the epistatic relationships among several groups of pex mutants. Our results demonstrate that Pex4p acts after the peroxisome membrane synthesis factor Pex3p, the Pex5p docking factors Pex13p and Pex14p, the matrix protein import factors Pex8p, Pex10p, and Pex12p, and two other peroxins, Pex2p and Pex17p. Pex22p and the interacting AAA ATPases Pex1p and Pex6p were also found to act after Pex10p. Furthermore, Pex1p and Pex6p were found to act upstream of Pex4p and Pex22p. These results suggest that Pex1p, Pex4p, Pex6p, and Pex22p act late in peroxisomal matrix protein import, after matrix protein translocation. This hypothesis is supported by the phenotypes of the corresponding mutant strains. As has been shown previously for P. pastoris pex1, pex6, and pex22 mutant cells, we show here that pex4Delta mutant cells contain peroxisomal membrane protein-containing peroxisomes that import residual amounts of peroxisomal matrix proteins.

Pisum sativum mitochondrial pyruvate dehydrogenase can be assembled as a functional alpha(2)beta(2) heterotetramer in the cytoplasm of Pichia pastoris

Pea (Pisum sativum) mitochondrial pyruvate dehydrogenase (E1) was produced by coexpression of the mature alpha and beta subunits in the cytoplasm of the yeast Pichia pastoris. Size-exclusion chromatography of recombinant E1, using a Superose 12 column, yielded a peak at M(r) 160,000 that contained both alpha and beta subunits as well as E1 activity. This corresponds to the size of native alpha(2)beta(2) E1. Recombinant E1 alpha (His(6))-E1 beta was purified by affinity chromatography using immobilized Ni(+), with a yield of 2.8 mg L(-1). The pyruvate-decarboxylating activity of recombinant E1 was dependent upon added Mg(2+) and thiamin-pyrophosphate and was enhanced by the oxidant potassium ferricyanide. Native pea mitochondrial E1-kinase catalyzed phosphorylation of Ser residues in the alpha-subunit of recombinant E1, with concomitant loss of enzymatic activity. Thus, mitochondrial pyruvate dehydrogenase can be assembled in the cytoplasm of P. pastoris into an alpha(2)beta(2) heterotetramer that is both catalytically active and competent for regulatory phosphorylation.

Presenilin I expression in yeast lowers secretion of the amyloid precursor protein

Presenilin (PS) mutations are associated with early-onset Alzheimer's disease and PS proteins are involved with gamma-secretase cleavage of the amyloid precursor protein, APP. We have shown previously that alpha-, beta- and gamma-secretase cleavages of APP are conserved in Pichia pastoris. Here, we report co-expression of APP and PSI in P. pastoris and show by immunoelectron microscopy colocalization of these two proteins in expanded endoplasmic reticulum. Western blot analysis indicates a drastic reduction of both alpha- and beta-secretase products. A relative increase in beta-secretase product derived from immature APP is also observed, pointing to a beta-secretase activity of P. pastoris associated with the early secretory pathway.

Pex17p is required for import of both peroxisome membrane and lumenal proteins and interacts with Pex19p and the peroxisome targeting signal-receptor docking complex in Pichia pastoris

Pichia pastoris PEX17 was cloned by complementation of a peroxisome-deficient strain obtained from a novel screen for mutants disrupted in the localization of a peroxisomal membrane protein (PMP) reporter. PEX17 encodes a 267-amino-acid protein with low identity (18%) to the previously characterized Saccharomyces cerevisiae Pex17p. Like ScPex17p, PpPex17p contains a putative transmembrane domain near the amino terminus and two carboxyl-terminal coiled-coil regions. PpPex17p behaves as an integral PMP with a cytosolic carboxyl-terminal domain. pex17Delta mutants accumulate peroxisomal matrix proteins and certain integral PMPs in the cytosol, suggesting a critical role for Pex17p in their localization. Peroxisome remnants were observed in the pex17Delta mutant by morphological and biochemical means, suggesting that Pex17p is not absolutely required for remnant formation. Yeast two-hybrid analysis demonstrated that the carboxyl terminus of Pex19p was required for interaction with Pex17p lacking the carboxyl-terminal coiled-coil domains. Biochemical evidence confirmed the interaction between Pex19p and Pex17p. Additionally, Pex17p cross-linked to components of the peroxisome targeting signal-receptor docking complex, which unexpectedly contained Pex3p. Our evidence suggests the existence of distinct subcomplexes that contain separable pools of Pex3p, Pex19p, Pex17p, Pex14p, and the peroxisome targeting signal receptors. These distinct pools may serve different purposes for the import of matrix proteins or PMPs.

A Pichia pastoris VPS15 homologue is required in selective peroxisome autophagy

Methylotrophic yeasts contain large peroxisomes during growth on methanol. Upon exposure to excess glucose or ethanol these organelles are selectively degraded by autophagy. Here we describe the cloning of a Pichia pastoris gene (PpVPS15) involved in peroxisome degradation, which is homologous to Saccharomyces cerevisiae VPS15. In methanol-grown cells of a P. pastoris VPS15 deletion strain, the levels of peroxisomal marker enzymes remained high after addition of excess glucose or ethanol. Electron microscopic studies revealed that the organelles were not taken up by vacuoles, suggesting that PpVPS15 is required at an early stage in peroxisome degradation.

Characterization of the oligosaccharides assembled on the Pichia pastoris-expressed recombinant aspartic protease

Aspartic protease, widely used as a milk-coagulating agent in industrial cheese production, contains three potential N-glycosylation sites. In this study, we report the characterization of N-linked oligosaccharides on recombinant aspartic protease secreted from the methylotrophic yeast Pichia pastoris using a combination of mass spectrometric, 2D chromatographic, chemical and enzymatic methods. The carbohydrates from site I (Asn79) were found to range from Man6-17GlcNAc2 with 50% bearing a phospho-diester-motif, site II (Asn113) was not occupied and site III (Asn188) contained mostly uncharged species ranging from Man-13GlcNAc2. These charged groups are not affecting the transport through the secretion pathway of the recombinant glycoprotein. Changes from a molasses-based medium to a minimal salts-based medium led to a clear reduction of the degree of phosphorylation of the N-glycan population.

Hansenula polymorpha Pex1p and Pex6p are peroxisome-associated AAA proteins that functionally and physically interact

We have cloned the Hansenula polymorpha PEX1 and PEX6 genes by functional complementation of the corresponding peroxisome-deficient (pex) mutants. The gene products, HpPex1p and HpPex6p, are ATPases which both belong to the AAA protein family. Cells deleted for either gene (Deltapex1 or Deltapex6) were characterized by the presence of small peroxisomal remnants which contained peroxisomal membrane proteins and minor amounts of matrix proteins. The bulk of the matrix proteins, however, resided in the cytosol. In cell fractionation studies HpPex1p and HpPex6p co-sedimented with the peroxisomal membrane protein HpPex3p in both wild-type cells and in Deltapex4, Deltapex8 or Deltapex14 cells. Both proteins are loosely membrane-bound and face the cytosol. Furthermore, HpPex1p and HpPex6p physically and functionally interact in vivo. Overexpression of PEX6 resulted in defects in peroxisomal matrix protein import. By contrast, overexpression of PEX1 was not detrimental to the cells. Interestingly, co-overproduction of HpPex1p rescued the protein import defect caused by HpPex6p overproduction. Overproduced HpPex1p and HpPex6p remained predominantly membrane-bound, but only partially co-localized with the peroxisomal membrane protein HpPex3p. Our data indicate that HpPex1p and HpPex6p function in a protein complex associated with the peroxisomal membrane and that overproduced, mislocalized HpPex6p prevents HpPex1p from reaching its site of activity.

Pex22p of Pichia pastoris, essential for peroxisomal matrix protein import, anchors the ubiquitin-conjugating enzyme, Pex4p, on the peroxisomal membrane

We isolated a Pichia pastoris mutant that was unable to grow on the peroxisome-requiring media, methanol and oleate. Cloning the gene by complementation revealed that the encoded protein, Pex22p, is a new peroxin. A Deltapex22 strain does not grow on methanol or oleate and is unable to import peroxisomal matrix proteins. However, this strain targets peroxisomal membrane proteins to membranes, most likely peroxisomal remnants, detectable by fluorescence and electron microscopy. Pex22p, composed of 187 amino acids, is an integral peroxisomal membrane protein with its NH2 terminus in the matrix and its COOH terminus in the cytosol. It contains a 25-amino acid peroxisome membrane-targeting signal at its NH2 terminus. Pex22p interacts with the ubiquitin-conjugating enzyme Pex4p, a peripheral peroxisomal membrane protein, in vivo, and in a yeast two-hybrid experiment. Pex22p is required for the peroxisomal localization of Pex4p and in strains lacking Pex22p, the Pex4p is cytosolic and unstable. Therefore, Pex22p anchors Pex4p at the peroxisomal membrane. Strains that do not express Pex4p or Pex22p have similar phenotypes and lack Pex5p, suggesting that Pex4p and Pex22p act at the same step in peroxisome biogenesis. The Saccharomyces cerevisiae hypothetical protein, Yaf5p, is the functional homologue of P. pastoris Pex22p.

The Hansenula polymorpha PDD1 gene product, essential for the selective degradation of peroxisomes, is a homologue of Saccharomyces cerevisiae Vps34p

Via functional complementation we have isolated the Hansenula polymorpha PDD1 gene essential for selective, macroautophagic peroxisome degradation. HpPDD1 encodes a 116 kDa protein with high similarity (42% identity) to Saccharomyces cerevisiae Vps34p, which has been implicated in vacuolar protein sorting and endocytosis. Western blotting experiments revealed that HpPDD1 is expressed constitutively. In a H. polymorpha pdd1 disruption strain peroxisome degradation is fully impaired. Sequestered peroxisomes, typical for the first stage of peroxisome degradation in H. polymorpha, were never observed, suggesting that HpPdd1p plays a role in the tagging of redundant peroxisomes and/or sequestration of these organelles from the cytosol. Possibly, HpPdd1p is the functional homologue of ScVps34p, because-like S. cerevisiae vps34 mutants-H. polymorpha pdd1 mutants are temperature-sensitive for growth and are impaired in the sorting of vacuolar carboxypeptidase Y. Moreover, HpPdd1p is associated to membranes, as was also observed for ScVps34p.

Golgi structure correlates with transitional endoplasmic reticulum organization in Pichia pastoris and Saccharomyces cerevisiae

Golgi stacks are often located near sites of "transitional ER" (tER), where COPII transport vesicles are produced. This juxtaposition may indicate that Golgi cisternae form at tER sites. To explore this idea, we examined two budding yeasts: Pichia pastoris, which has coherent Golgi stacks, and Saccharomyces cerevisiae, which has a dispersed Golgi. tER structures in the two yeasts were visualized using fusions between green fluorescent protein and COPII coat proteins. We also determined the localization of Sec12p, an ER membrane protein that initiates the COPII vesicle assembly pathway. In P. pastoris, Golgi stacks are adjacent to discrete tER sites that contain COPII coat proteins as well as Sec12p. This arrangement of the tER-Golgi system is independent of microtubules. In S. cerevisiae, COPII vesicles appear to be present throughout the cytoplasm and Sec12p is distributed throughout the ER, indicating that COPII vesicles bud from the entire ER network. We propose that P. pastoris has discrete tER sites and therefore generates coherent Golgi stacks, whereas S. cerevisiae has a delocalized tER and therefore generates a dispersed Golgi. These findings open the way for a molecular genetic analysis of tER sites.

Ultrastructural and immunocytochemical evidences of core-particle formation in the methylotrophic Pichia pastoris yeast when expressing HCV structural proteins (core-E1)

Particulate antigens of the Hepatitis C virus (HCV) are reported for the first time by transmission electron microscopy in Pichia pastoris. The yeast was cloned to express the first 339 NH2-terminal amino acids of the HCV polyprotein (C-E1.339 polypeptide). The C-E1.339 polypeptide covers the putative 191 aa of the core protein (aa 1-191) and 148 aa of the E1 envelope antigen (aa 192-339). Virus-like particles (VLP) with diameters ranging from 20 nm to 30 nm were specifically observed in those cells expressing the HCV polyprotein. The VLP appeared along the membrane of the endoplasmic reticulum, but were fundamentally localized in vacuoles, either free or inside autophagic bodies. Clustered particles, chains of particles, high-density reticular structures, and crystalloid bodies were also detected, the last one being an orderly arrangement of particles with 20 nm diameters. The crystal-associated particles are well differentiated from the intracellular VLP because of their uniform size and shape. We argue that membrane components are retained in the architecture of the VLP, conferring to this particle certain heterogeneity. Both kinds of particles, the VLP formed after treatment with NP-40 and the crystal-associated particles, were core protein-positives. Whether they reflect mature HCV nucleocapsid or intermediary states in the viral nucleocapsid morphogenesis remains unknown. We conclude that, like mammalian cell lines, the P. pastoris yeast could be an appropriate host for the analysis of HCV polyprotein processing and, eventually, virus assembly.

Positive selection of novel peroxisome biogenesis-defective mutants of the yeast Pichia pastoris

We have developed two novel schemes for the direct selection of peroxisome-biogenesis-defective (pex) mutants of the methylotrophic yeast Pichia pastoris. Both schemes take advantage of our observation that methanol-induced pex mutants contain little or no alcohol oxidase (AOX) activity. AOX is a peroxisomal matrix enzyme that catalyzes the first step in the methanol-utilization pathway. One scheme utilizes allyl alcohol, a compound that is not toxic to cells but is oxidized by AOX to acrolein, a compound that is toxic. Exposure of mutagenized populations of AOX-induced cells to allyl alcohol selectively kills AOX-containing cells. However, pex mutants without AOX are able to grow. The second scheme utilizes a P. pastoris strain that is defective in formaldehyde dehydrogenase (FLD), a methanol pathway enzyme required to metabolize formaldehyde, the product of AOX. AOX-induced cells of fld1 strains are sensitive to methanol because of the accumulation of formaldehyde. However, fld1 pex mutants, with little active AOX, do not efficiently oxidize methanol to formaldehyde and therefore are not sensitive to methanol. Using these selections, new pex mutant alleles in previously identified PEX genes have been isolated along with mutants in three previously unidentified PEX groups.

Peroxisomicine A1 (plant toxin-514) affects normal peroxisome assembly in the yeast Hansenula polymorpha

Previously we demonstrated that peroxisomicine A1 (T-514), a plant toxin isolated from Karwinskia species, has a deteriorating effect on the integrity of peroxisomes of methylotrophic yeasts. Here we describe two strains of Hansenula polymorpha, affected in the normal utilization of methanol as sole source of carbon and energy due to peroxisomicine A1 treatment. The two strains isolated (L17 and RV31) grew poorly on methanol, apparently due to malfunctioning of their peroxisomes. Moreover, the cells displayed a high peroxisome turnover rate. We argue that the peroxisomicine A1 induced phenotype of both strains is due to a genomic mutation. Strain L17 was functionally complemented after transformation with a H. polymorpha genomic library. The complementing 2.8 kb DNA fragment did not contain a well-defined ORF and led us to speculate that it may contain regulatory sequences that, when present in multiple copies in the cell, result in a change of expression of specific genes, thus causing restoration of normal methylotrophic growth.

Kodamaea nitidulidarum, Candida restingae and Kodamaea anthophila, three new related yeast species from ephemeral flowers

Three new yeast species were discovered during studies of yeasts associated with ephemeral flowers in Brazil, Australia and Hawaii. Their physiological and morphological similarity to Kodamaea (Pichia) ohmeri suggested a possible relationship to that species, which was confirmed by rDNA sequencing. Kodamaea nitidulidarum and Candida restingae were found in cactus flowers and associated nitidulid beetles in sand dune ecosystems (restinga) of South-eastern Brazil. Over 350 strains of Kodamaea anthophila were isolated from Hibiscus and morning glory flowers (Ipomoea spp.) in Australia, and from associated nitidulid beetles and Drosophila hibisci. A single isolate came from a beach morning glory in Hawaii. Expansion of the genus Kodamaea to three species modified the existing definition of the genus only slightly. The type and isotype strains are as follows: K. nitidulidarum strains UFMG96-272T (h+; CBS 8491T) and UFMG96-394I (h-; CBS 8492I); Candida restingae UFMG96-276T (CBS 8493T); K. anthophila strains UWO(PS)95-602.1T (h+; CBS 8494T), UWO(PS)91-893.2I (h-; CBS 8495I) and UWO(PS)95-725.1I (h-; CBS 8496I).

The plant PTS1 receptor: similarities and differences to its human and yeast counterparts

Two targeting signals, PTS1 and PTS2, mediate import of proteins into the peroxisomal matrix. We have cloned and sequenced the watermelon (Citrullus vulgaris) cDNA homologue to the PTS1 receptor gene (PEX5). Its gene product, CvPex5p, belongs to the family of tetratricopeptide repeat (TPR) containing proteins like the human and yeast counterparts, and exhibits 11 repeats of the sequence W-X2-(E/S)-(Y/F/Q) in its N-terminal half. According to fractionation studies the plant Pex5p is located mainly in the cytosolic fraction and therefore could function as a cycling receptor between the cytosol and glyoxysomes, as has been proposed for the Pex5p of human and some yeast peroxisomes. Transformation of the Hansenula polymorpha peroxisome deficient pex5 mutant with watermelon PEX5 resulted in restoration of peroxisome formation and the synthesis of additional membranes surrounding the peroxisomes. These structures are labeled in immunogold experiments using antibodies against the Hansenula polymorpha integral membrane protein Pex3p, confirming their peroxisomal nature. The plant Pex5p was localized by immunogold labelling mainly in the cytosol of the yeast, but also inside the newly formed peroxisomes. However, import of the PTS1 protein alcohol oxidase is only partially restored by CvPex5p.