Peroxisome biogenesis: Where Arf and coatomer might be involved

Role of the Guanine Nucleotide Exchange Factor GBF1 in the Replication of RNA Viruses

The guanine nucleotide exchange factor GBF1 is a well-known factor that can activate different ADP-ribosylation factor (Arf) proteins during the regulation of different cellular vesicular transport processes. In the last decade, it has become increasingly evident that GBF1 can also regulate different steps of the replication cycle of RNA viruses belonging to different virus families. GBF1 has been shown not only to facilitate the intracellular traffic of different viral and cellular elements during infection, but also to modulate the replication of viral RNA, the formation and maturation of viral replication complexes, and the processing of viral proteins through mechanisms that do not depend on its canonical role in intracellular transport. Here, we review the various roles that GBF1 plays during the replication of different RNA viruses.

The Guanine Nucleotide Exchange Factor GBF1 Participates in Rotavirus Replication

Cellular and viral factors participate in the replication cycle of rotavirus. We report that the guanine nucleotide exchange factor GBF1, which activates the small GTPase Arf1 to induce COPI transport processes, is required for rotavirus replication since knocking down GBF1 expression by RNA interference or inhibiting its activity by treatment with brefeldin A (BFA) or Golgicide A (GCA) significantly reduces the yield of infectious viral progeny. This reduction in virus yield was related to a block in virus assembly, since in the presence of either BFA or GCA, the assembly of infectious mature triple-layered virions was significantly prevented and only double-layered particles were detected. We report that the catalytic activity of GBF1, but not the activation of Arf1, is essential for the assembly of the outer capsid of rotavirus. We show that both BFA and GCA, as well as interfering with the synthesis of GBF1, alter the electrophoretic mobility of glycoproteins VP7 and NSP4 and block the trimerization of the virus surface protein VP7, a step required for its incorporation into virus particles. Although a posttranslational modification of VP7 (other than glycosylation) could be related to the lack of trimerization, we found that NSP4 might also be involved in this process, since knocking down its expression reduces VP7 trimerization. In support, recombinant VP7 protein overexpressed in transfected cells formed trimers only when cotransfected with NSP4.IMPORTANCE Rotavirus, a member of the family Reoviridae, is the major cause of severe diarrhea in children and young animals worldwide. Despite significant advances in the characterization of the biology of this virus, the mechanisms involved in morphogenesis of the virus particle are still poorly understood. In this work, we show that the guanine nucleotide exchange factor GBF1, relevant for COPI/Arf1-mediated cellular vesicular transport, participates in the replication cycle of the virus, influencing the correct processing of viral glycoproteins VP7 and NSP4 and the assembly of the virus surface proteins VP7 and VP4.

Catching Lipid Droplet Contacts by Proteomics

Lipid droplets (LDs), important organelles for energy storage and involved in the development of metabolic disorders, are extremely dynamic and interact with many other cellular compartments to orchestrate lipid metabolism. Little is known about how these organelle contacts are changed according to cellular needs and functions under different metabolic and pathological conditions and which proteins regulate this. Here, we summarize recent exciting discoveries about the reorganization of organelle contacts in steatotic liver, including the identification of novel LD contact site proteins in cell lines and in animals. We also discuss state of the art proteomics workflows that enable the characterization of LD-organelle contacts and tethering proteins and give an outlook how this can inform obesity research.

Pex35 is a regulator of peroxisome abundance

Peroxisomes are cellular organelles with vital functions in lipid, amino acid and redox metabolism. The cellular formation and dynamics of peroxisomes are governed by PEX genes; however, the regulation of peroxisome abundance is still poorly understood. Here, we use a high-content microscopy screen in Saccharomyces cerevisiae to identify new regulators of peroxisome size and abundance. Our screen led to the identification of a previously uncharacterized gene, which we term PEX35, which affects peroxisome abundance. PEX35 encodes a peroxisomal membrane protein, a remote homolog to several curvature-generating human proteins. We systematically characterized the genetic and physical interactome as well as the metabolome of mutants in PEX35, and we found that Pex35 functionally interacts with the vesicle-budding-inducer Arf1. Our results highlight the functional interaction between peroxisomes and the secretory pathway.

Comprehensive proteomics analysis of glycosomes from Leishmania donovani

Leishmania donovani is a kinetoplastid protozoan that causes a severe and fatal disease kala-azar, or visceral leishmaniasis. L. donovani infects human host after the phlebotomine sandfly takes a blood meal and resides within the phagolysosome of infected macrophages. Previous studies on host-parasite interactions have not focused on Leishmania organelles and the role that they play in the survival of this parasite within macrophages. Leishmania possess glycosomes that are unique and specialized subcellular microbody organelles. Glycosomes are known to harbor most peroxisomal enzymes and, in addition, they also possess nine glycolytic enzymes. In the present study, we have carried out proteomic profiling using high resolution mass spectrometry of a sucrose density gradient-enriched glycosomal fraction isolated from L. donovani promastigotes. This study resulted in the identification of 4022 unique peptides, leading to the identification of 1355 unique proteins from a preparation enriched in L. donovani glycosomes. Based on protein annotation, 566 (41.8%) were identified as hypothetical proteins with no known function. A majority of the identified proteins are involved in metabolic processes such as carbohydrate, lipid, and nucleic acid metabolism. Our present proteomic analysis is the most comprehensive study to date to map the proteome of L. donovani glycosomes.

Localization of Rab proteins to peroxisomes: a proteomics and immunofluorescence study

A proteomics screen was initiated to identify Rab proteins regulating transport to and away from peroxisomes. Mass spectrometry-based protein correlation profiling of rat liver organelles and immunofluorescence analysis of the peroxisome candidate Rab proteins revealed Rab6, Rab10, Rab14 and Rab18 to associate with the peroxisomal membrane. While Rab14 localized to peroxisomes predominantly in its dominant-active form, other Rab proteins associated with peroxisomes in both their GTP- and GDP-bound state. In summary, our data suggest that Rab6, Rab10, Rab14 and Rab18 associate with the peroxisomal compartment and similar as previously shown for Rab8, Rab18 in its GDP-bound state favors peroxisome proliferation.

The 31-kDa antigen of Angiostrongylus cantonensis comprises distinct antigenic glycoproteins

Human angiostrongyliasis results from accidental infection with Angiostrongylus, an intra-arterial nematode. Angiostrongylus cantonensis infections result in eosinophilic meningitis, and A. costaricensis infections cause eosinophilic enteritis. Immunological methodologies are critical to the diagnosis of both infections, since these parasites cannot be isolated from fecal matter and are rarely found in cerebrospinal fluid samples. A. costaricensis and A. cantonensis share common antigenic epitopes which elicit antibodies that recognize proteins present in either species. Detection of antibodies to a 31-kDa A. cantonensis protein present in crude adult worm extracts is a sensitive and specific method for immunodiagnosis of cerebral angiostrongyliasis. The objective of the present work was to isolate and characterize the 31-kDa proteins using soluble protein extracts derived from adult female worms using both one- (1DE) and two-dimensional (2DE) gel electrophoresis. Separated proteins were blotted onto nitrocellulose and probed using sera from infected and non-infected controls. The 31-kDa band present in 1DE gels and the 4 spots identified in 2DE gels were excised and analyzed by electrospray ionization mass spectrometry. Using the highest scores obtained following Mascot analysis, amino acid sequences were obtained that matched four unique proteins: tropomyosin, the 14-3-3 phosphoserine-binding protein, a protein containing a nascent polypeptide-associated complex domain, and the putative epsilon subunit of coatomer protein complex isoform 2. Oxidative cleavage of diols using sodium m-periodate demonstrated that carbohydrate moieties are essential for the antigenicity of all four spots of the 31-kDa antigen. In this article we describe the identification of the 31-kDa antigen, and provide DNA sequencing of the targets. In conclusion, these data suggest that reactivity to the 31-kDa proteins may represent antibody recognition of more than one protein, and recombinant protein-based assays for cerebral angiostrongyliasis diagnosis may require eukaryotic expression systems to maintain antigenicity.

COPI budding within the Golgi stack

The Golgi serves as a hub for intracellular membrane traffic in the eukaryotic cell. Transport within the early secretory pathway, that is within the Golgi and from the Golgi to the endoplasmic reticulum, is mediated by COPI-coated vesicles. The COPI coat shares structural features with the clathrin coat, but differs in the mechanisms of cargo sorting and vesicle formation. The small GTPase Arf1 initiates coating on activation and recruits en bloc the stable heptameric protein complex coatomer that resembles the inner and the outer shells of clathrin-coated vesicles. Different binding sites exist in coatomer for membrane machinery and for the sorting of various classes of cargo proteins. During the budding of a COPI vesicle, lipids are sorted to give a liquid-disordered phase composition. For the release of a COPI-coated vesicle, coatomer and Arf cooperate to mediate membrane separation.

Membrane elongation factors in organelle maintenance: the case of peroxisome proliferation

Separation of metabolic pathways in organelles is critical for eukaryotic life. Accordingly, the number, morphology and function of organelles have to be maintained through processes linked with membrane remodeling events. Despite their acknowledged significance and intense study many questions remain about the molecular mechanisms by which organellar membranes proliferate. Here, using the example of peroxisome proliferation, we give an overview of how proteins elongate membranes. Subsequent membrane fission is achieved by dynamin-related proteins shared with mitochondria. We discuss basic criteria that membranes have to fulfill for these fission factors to complete the scission. Because peroxisome elongation is always associated with unequal distribution of matrix and membrane proteins, we propose peroxisomal division to be non-stochastic and asymmetric. We further show that these organelles need not be functional to carry on membrane elongation and present the most recent findings concerning members of the Pex11 protein family as membrane elongation factors. These factors, beside known proteins such as BAR-domain proteins, represent another family of proteins containing an amphipathic α-helix with membrane bending activity.

Urban planning of the endoplasmic reticulum (ER): how diverse mechanisms segregate the many functions of the ER

The endoplasmic reticulum (ER) is the biggest organelle in most cell types, but its characterization as an organelle with a continuous membrane belies the fact that the ER is actually an assembly of several, distinct membrane domains that execute diverse functions. Almost 20 years ago, an essay by Sitia and Meldolesi first listed what was known at the time about domain formation within the ER. In the time that has passed since, additional ER domains have been discovered and characterized. These include the mitochondria-associated membrane (MAM), the ER quality control compartment (ERQC), where ER-associated degradation (ERAD) occurs, and the plasma membrane-associated membrane (PAM). Insight has been gained into the separation of nuclear envelope proteins from the remainder of the ER. Research has also shown that the biogenesis of peroxisomes and lipid droplets occurs on specialized membranes of the ER. Several studies have shown the existence of specific marker proteins found on all these domains and how they are targeted there. Moreover, a first set of cytosolic ER-associated sorting proteins, including phosphofurin acidic cluster sorting protein 2 (PACS-2) and Rab32 have been identified. Intra-ER targeting mechanisms appear to be superimposed onto ER retention mechanisms and rely on transmembrane and cytosolic sequences. The crucial roles of ER domain formation for cell physiology are highlighted with the specific targeting of the tumor metastasis regulator gp78 to ERAD-mediating membranes or of the promyelocytic leukemia protein to the MAM.

Dual function of Sec16B: Endoplasmic reticulum-derived protein secretion and peroxisome biogenesis in mammalian cells

The origin of peroxisomes has long been disputed. However, recent evidence suggests that peroxisomes can be formed de novo from the endoplasmic reticulum (ER) in yeast and higher eukaryotes. Sec16A and Sec16B, mammalian orthologs of yeast Sec16, are scaffold proteins that organize ER exit sites by interacting with COPII components. We recently demonstrated that Sec16B, but not Sec16A, regulates the transport of peroxisomal biogenesis factors from the ER to peroxisomes in mammalian cells. The C-terminal region of Sec16B, which is not conserved in Sec16A, is required for this function. The data suggest that Sec16B in ER areas other than ER exit sites plays this role. Our findings provide an unexpected connection between at least part of the COPII machinery and the formation of preperoxisomal vesicles at the ER, and offer an explanation of how secretory and peroxisomal trafficking from the ER are distinguished.

Peroxisome reintroduction in Hansenula polymorpha requires Pex25 and Rho1

We identified two proteins, Pex25 and Rho1, which are involved in reintroduction of peroxisomes in peroxisome-deficient yeast cells. These are, together with Pex3, the first proteins identified as essential for this process. Of the three members of the Hansenula polymorpha Pex11 protein family-Pex11, Pex25, and Pex11C-only Pex25 was required for reintroduction of peroxisomes into a peroxisome-deficient mutant strain. In peroxisome-deficient pex3 cells, Pex25 localized to structures adjacent to the ER, whereas in wild-type cells it localized to peroxisomes. Pex25 cells were not themselves peroxisome deficient but instead contained a slightly increased number of peroxisomes. Interestingly, pex11 pex25 double deletion cells, in which both peroxisome fission (due to the deletion of PEX11) and reintroduction (due to deletion of PEX25) was blocked, did display a peroxisome-deficient phenotype. Peroxisomes reappeared in pex11 pex25 cells upon synthesis of Pex25, but not of Pex11. Reintroduction in the presence of Pex25 required the function of the GTPase Rho1. These data therefore provide new and detailed insight into factors important for de novo peroxisome formation in yeast.

A proteomic approach towards the identification of the matrix protein content of the two types of microbodies in Neurospora crassa

Microbodies (peroxisomes) comprise a class of organelles with a similar biogenesis but remarkable biochemical heterogeneity. Here, we purified the two distinct microbody family members of filamentous fungi, glyoxysomes and Woronin bodies, from Neurospora crassa and analyzed their protein content by HPLC/ESI-MS/MS. In the purified Woronin bodies, we unambiguously identified only hexagonal 1 (HEX1), suggesting that the matrix is probably exclusively filled with the HEX1 hexagonal crystal. The proteomic analysis of highly purified glyoxysomes allowed the identification of 191 proteins. Among them were 16 proteins with a peroxisomal targeting signal type 1 (PTS1) and three with a PTS2. The collection also contained the previously described N. crassa glyoxysomal matrix proteins FOX2 and ICL1 that lack a typical PTS. Three PTS1 proteins were identified that likely represent the long sought glyoxysomal acyl-CoA dehydrogenases of filamentous fungi. Two of them were demonstrated by subcellular localization studies to be indeed glyoxysomal. Furthermore, two PTS proteins were identified that are suggested to be involved in the detoxification of nitroalkanes. Since the glyoxysomal localization was experimentally demonstrated for one of these enzymes, a new biochemical reaction is expected to be associated with microbody function.

RhoA regulates peroxisome association to microtubules and the actin cytoskeleton

The current view of peroxisome inheritance provides for the formation of new peroxisomes by both budding from the endoplasmic reticulum and autonomous division. Here we investigate peroxisome-cytoskeleton interactions and show by proteomics, biochemical and immunofluorescence analyses that actin, non-muscle myosin IIA (NMM IIA), RhoA, Rho kinase II (ROCKII) and Rab8 associate with peroxisomes. Our data provide evidence that (i) RhoA in its inactive state, maintained for example by C. botulinum toxin exoenzyme C3, dissociates from peroxisomes enabling microtubule-based peroxisomal movements and (ii) dominant-active RhoA targets to peroxisomes, uncouples the organelles from microtubules and favors Rho kinase recruitment to peroxisomes. We suggest that ROCKII activates NMM IIA mediating local peroxisomal constrictions. Although our understanding of peroxisome-cytoskeleton interactions is still incomplete, a picture is emerging demonstrating alternate RhoA-dependent association of peroxisomes to the microtubular and actin cytoskeleton. Whereas association of peroxisomes to microtubules clearly serves bidirectional, long-range saltatory movements, peroxisome-acto-myosin interactions may support biogenetic functions balancing peroxisome size, shape, number, and clustering.

Lipid droplets: a dynamic organelle moves into focus

Lipid droplets (LDs) were perceived as static storage deposits, which passively participate in the energy homeostasis of both cells and entire organisms. However, this view has changed recently after the realization of a complex and highly dynamic LD proteome. The proteome contains key components of the fat mobilization system and proteins that suggest LD interactions with a variety of cell organelles, including the endoplasmic reticulum, mitochondria and peroxisomes. The study of LD cell biology, including cross-talk with other organelles, the trafficking of LDs in the cell and regulatory events involving the LD coat proteins is now on the verge of leaving its infancy and unfolds that LDs are highly dynamic cellular organelles.

Suppression of peroxisome biogenesis factor 10 reduces cuticular wax accumulation by disrupting the ER network in Arabidopsis thaliana

Peroxisome biogenesis factor 10 (PEX10) is a component of the peroxisomal matrix protein import machinery. To analyze the physiological function of PEX10, we used transgenic AtPEX10i Arabidopsis plants that had suppressed expression of the PEX10 gene due to RNA interference. AtPEX10i plants had patches of paleness on leaves, and abnormal floral organs that were typical of cuticular wax-deficient mutants. Quantitative analysis of cuticular wax revealed that the amount of wax in AtPEX10i plants was indeed lower than that in control plants. This result was confirmed by toluidine blue staining and scanning electron microscopic analysis of AtPEX10i. The CER1, CER4, WAX2 and SHN1 genes are known to be responsible for wax biosynthesis in Arabidopsis. Of these, CER1, CER4 and WAX2 were found to be localized on the endoplasmic reticulum (ER). In AtPEX10i plants, the expression of these genes was down-regulated, and CER1, CER4 and WAX2 were mislocalized to the cytosol. We also found that AtPEX10i plants had defects in ER morphology. Based on these results, we propose that PEX10 is essential for the maintenance of ER morphology and for the expression of CER1, CER4, WAX2 and SHN1 genes, which contribute to the biosynthesis of cuticular wax.

Small G proteins in peroxisome biogenesis: the potential involvement of ADP-ribosylation factor 6

BACKGROUND

Peroxisomes execute diverse and vital functions in virtually every eukaryote. New peroxisomes form by budding from pre-existing organelles or de novo by vesiculation of the ER. It has been suggested that ADP-ribosylation factors and COPI coatomer complexes are involved in these processes.

RESULTS

Here we show that all viable Saccharomyces cerevisiae strains deficient in one of the small GTPases which have an important role in the regulation of vesicular transport contain functional peroxisomes, and that the number of these organelles in oleate-grown cells is significantly upregulated in the arf1 and arf3 null strains compared to the wild-type strain. In addition, we provide evidence that a portion of endogenous Arf6, the mammalian orthologue of yeast Arf3, is associated with the cytoplasmic face of rat liver peroxisomes. Despite this, ablation of Arf6 did neither influence the regulation of peroxisome abundance nor affect the localization of peroxisomal proteins in cultured fetal hepatocytes. However, co-overexpression of wild-type, GTP hydrolysis-defective or (dominant-negative) GTP binding-defective forms of Arf1 and Arf6 caused mislocalization of newly-synthesized peroxisomal proteins and resulted in an alteration of peroxisome morphology.

CONCLUSION

These observations suggest that Arf6 is a key player in mammalian peroxisome biogenesis. In addition, they also lend strong support to and extend the concept that specific Arf isoform pairs may act in tandem to regulate exclusive trafficking pathways.

Spatiotemporal dynamics of the ER-derived peroxisomal endomembrane system

Recent studies have provided evidence that peroxisomes constitute a multicompartmental endomembrane system. The system begins to form with the targeting of certain peroxisomal membrane proteins to the ER and their exit from the ER via preperoxisomal carriers. These carriers undergo a multistep maturation into metabolically active peroxisomes containing the entire complement of peroxisomal membrane and matrix proteins. At each step, the import of a subset of proteins and the uptake of certain membrane lipids result in the formation of a distinct, more mature compartment of the peroxisomal endomembrane system. Individual peroxisomal compartments proliferate by undergoing one or several rounds of division. Herein, we discuss various strategies that evolutionarily diverse organisms use to coordinate compartment formation, maturation, and division in the peroxisomal endomembrane system. We also critically evaluate the molecular and cellular mechanisms governing these processes, outline the most important unanswered questions, and suggest directions for future research.

Regulation of peroxisome dynamics

Peroxisomes are single-membraned organelles ubiquitous to eukaryotic cells that house metabolic reactions that generate and destroy harmful oxidative intermediates. They are dynamic structures whose morphology, abundance, composition, and function depend on the cell type and environment. Perhaps due to the potentially damaging and protective metabolic roles of peroxisomes and their dynamic presence in the cell, peroxisome biogenesis is emerging as a process that involves complex underlying mechanisms of regulated formation and maintenance. There are roughly 30 known peroxins, proteins involved in peroxisome biogenesis, many of which have been conserved from yeast to mammals. This review focuses on the biogenesis of peroxisomes with an emphasis on the regulation of peroxisome formation and the import of peroxisomal matrix proteins in the model organism Saccharomyces cerevisiae.

gammaCOP is required for apical protein secretion and epithelial morphogenesis in Drosophila melanogaster

Background

There is increasing evidence that tissue-specific modifications of basic cellular functions play an important role in development and disease. To identify the functions of COPI coatomer-mediated membrane trafficking in Drosophila development, we were aiming to create loss-of-function mutations in the γCOP gene, which encodes a subunit of the COPI coatomer complex.

Principal Findings

We found that γCOP is essential for the viability of the Drosophila embryo. In the absence of zygotic γCOP activity, embryos die late in embryogenesis and display pronounced defects in morphogenesis of the embryonic epidermis and of tracheal tubes. The coordinated cell rearrangements and cell shape changes during tracheal tube morphogenesis critically depend on apical secretion of certain proteins. Investigation of tracheal morphogenesis in γCOP loss-of-function mutants revealed that several key proteins required for tracheal morphogenesis are not properly secreted into the apical lumen. As a consequence, γCOP mutants show defects in cell rearrangements during branch elongation, in tube dilation, as well as in tube fusion. We present genetic evidence that a specific subset of the tracheal defects in γCOP mutants is due to the reduced secretion of the Zona Pellucida protein Piopio. Thus, we identified a critical target protein of COPI-dependent secretion in epithelial tube morphogenesis.

Conclusions/Significance

These studies highlight the role of COPI coatomer-mediated vesicle trafficking in both general and tissue-specific secretion in a multicellular organism. Although COPI coatomer is generally required for protein secretion, we show that the phenotypic effect of γCOP mutations is surprisingly specific. Importantly, we attribute a distinct aspect of the γCOP phenotype to the effect on a specific key target protein.

The peroxisome: still a mysterious organelle

More than half a century of research on peroxisomes has revealed unique features of this ubiquitous subcellular organelle, which have often been in disagreement with existing dogmas in cell biology. About 50 peroxisomal enzymes have so far been identified, which contribute to several crucial metabolic processes such as β-oxidation of fatty acids, biosynthesis of ether phospholipids and metabolism of reactive oxygen species, and render peroxisomes indispensable for human health and development. It became obvious that peroxisomes are highly dynamic organelles that rapidly assemble, multiply and degrade in response to metabolic needs. However, many aspects of peroxisome biology are still mysterious. This review addresses recent exciting discoveries on the biogenesis, formation and degradation of peroxisomes, on peroxisomal dynamics and division, as well as on the interaction and cross talk of peroxisomes with other subcellular compartments. Furthermore, recent advances on the role of peroxisomes in medicine and in the identification of novel peroxisomal proteins are discussed.

Evolution of insect proteomes: insights into synapse organization and synaptic vesicle life cycle

BACKGROUND

The molecular components in synapses that are essential to the life cycle of synaptic vesicles are well characterized. Nonetheless, many aspects of synaptic processes, in particular how they relate to complex behaviour, remain elusive. The genomes of flies, mosquitoes, the honeybee and the beetle are now fully sequenced and span an evolutionary breadth of about 350 million years; this provides a unique opportunity to conduct a comparative genomics study of the synapse.

RESULTS

We compiled a list of 120 gene prototypes that comprise the core of presynaptic structures in insects. Insects lack several scaffolding proteins in the active zone, such as bassoon and piccollo, and the most abundant protein in the mammalian synaptic vesicle, namely synaptophysin. The pattern of evolution of synaptic protein complexes is analyzed. According to this analysis, the components of presynaptic complexes as well as proteins that take part in organelle biogenesis are tightly coordinated. Most synaptic proteins are involved in rich protein interaction networks. Overall, the number of interacting proteins and the degrees of sequence conservation between human and insects are closely correlated. Such a correlation holds for exocytotic but not for endocytotic proteins.

CONCLUSION

This comparative study of human with insects sheds light on the composition and assembly of protein complexes in the synapse. Specifically, the nature of the protein interaction graphs differentiate exocytotic from endocytotic proteins and suggest unique evolutionary constraints for each set. General principles in the design of proteins of the presynaptic site can be inferred from a comparative study of human and insect genomes.

Peroxisome proliferation in Hansenula polymorpha requires Dnm1p which mediates fission but not de novo formation

We show that the dynamin-like proteins Dnm1p and Vps1p are not required for re-introduction of peroxisomes in Hansenula polymorpha pex3 cells upon complementation with PEX3-GFP. Instead, Dnm1p, but not Vps1p, plays a crucial role in organelle proliferation via fission. In H. polymorpha DNM1 deletion cells (dnm1) a single peroxisome is present that forms long extensions, which protrude into developing buds and divide during cytokinesis. Budding pex11.dnm1 double deletion cells lack these peroxisomal extensions, suggesting that the peroxisomal membrane protein Pex11p is required for their formation. Life cell imaging revealed that fluorescent Dnm1p-GFP spots fluctuate between peroxisomes and mitochondria. On the other hand Pex11p is present over the entire organelle surface, but concentrates during fission at the basis of the organelle extension in dnm1 cells. Our data indicate that peroxisome fission is the major pathway for peroxisome multiplication in H. polymorpha.

Peroxisomal dynamics

Peroxisomes are a dynamic compartment in almost all eukaryotic cells and have diverse metabolic roles in response to environmental changes and cellular demands. The accompanying changes in enzyme content or abundance of peroxisomes are accomplished by dynamically operating membrane- and matrix-protein transport machineries. This review discusses recent progress in understanding peroxisomal proliferation and maintenance, insertion of peroxisomal membrane proteins, compartmentalization of peroxisomal matrix proteins and selective degradation of peroxisomes via pexophagy.

The peroxisomal protein import machinery

Peroxisomes are unique organelles whose physiological functions vary depending on the cellular environment or metabolic and developmental state of the organism. These changes in enzyme content are accomplished by the dynamically operating membrane and matrix protein import machineries of peroxisomes that rely on the concerted function of at least 20 peroxins. The import of folded matrix proteins is mediated by cycling receptors that shuttle between the cytosol and peroxisomal lumen. Receptor release back to the cytosol represents the ATP-dependent step of peroxisomal matrix protein import, which consists of two energy-consuming reactions: receptor ubiquitination and dislocation.

Phosphoinositide synthesis and degradation in isolated rat liver peroxisomes

Analyzing peroxisomal phosphoinositide (PId(#)) synthesis in highly purified rat liver peroxisomes we found synthesis of phosphatidylinositol 4-phosphate (PtdIns4P), PtdIns(4,5)P(2) and PtdIns(3,5)P(2). PtdIns3P was hardly detected in vitro, however, was observed in vivo after [(32)P]-phosphate labeling of primary rat hepatocytes. In comparison with other subcellular organelles peroxisomes revealed a unique PId pattern suggesting peroxisomal specificity of the observed synthesis. Use of phosphatase inhibitors enhanced the amount of PtdIns4P. The results obtained provide evidence that isolated rat liver peroxisomes synthesize PIds and suggest the association of PId 4-kinase and PId 5-kinase and PId 4-phosphatase activities with the peroxisomal membrane.

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.