The return of the peroxisome

An alternative membrane topology permits lipid droplet localization of peroxisomal fatty acyl-CoA reductase 1

Fatty acyl-CoA reductase 1 (Far1) is a ubiquitously expressed peroxisomal membrane protein that generates the fatty alcohols required for the biosynthesis of ether lipids. Lipid droplet localization of exogenously expressed and endogenous human Far1 was observed by fluorescence microscopy under conditions of increased triglyceride synthesis in tissue culture cells. This unexpected finding was supported further by correlative light electron microscopy and subcellular fractionation. Selective permeabilization, protease sensitivity and N-glycosylation tagging suggested that Far1 is able to assume two different membrane topologies, differing in the orientation of the short hydrophilic C-terminus towards the lumen or the cytosol, respectively. Two closely spaced hydrophobic domains are contained within the C-terminal region. When analyzed separately, the second domain was sufficient for the localization of a fluorescent reporter to lipid droplets. Targeting of Far1 to lipid droplets was not impaired in either Pex19 or ASNA1 (also known as TRC40) CRISPR/Cas9 knockout cells. In conclusion, our data suggest that Far1 is a novel member of the rather exclusive group of dual topology membrane proteins. At the same time, Far1 shows lipid metabolism-dependent differential subcellular localizations to peroxisomes and lipid droplets.

Intracellular organelles in health and kidney disease

Subcellular organelles consist of smaller substructures called supramolecular assemblies and these in turn consist of macromolecules. Various subcellular organelles have critical functions that consist of genetic disorders of organelle biogenesis and several metabolic disturbances that occur during non-genetic diseases e.g. infection, intoxication and drug treatments. Mitochondrial damage can cause renal dysfunction as ischemic acute renal injury, chronic kidney disease progression. Moreover, mitochondrial dysfunction is an early event in aldosterone-induced podocyte injury and cardiovascular disease due to oxidative stress in chronic kidney disease. Elevated production of reactive oxygen species could be able to activate NLRP3 inflammasome representing new deregulated biological machinery and a novel therapeutic target in hemodialysis patients. Peroxisomes are actively involved in apoptosis and inflammation, innate immunity, aging and in the pathogenesis of age related diseases, such as diabetes mellitus and cancer. Peroxisomal catalase causes alterations of mitochondrial membrane proteins and stimulates generation of mitochondrial reactive oxygen species. High concentrations of hydrogen peroxide exacerbate organelles and cellular aging. The importance of proper peroxisomal function for the biosynthesis of bile acids has been firmly established. Endoplasmic reticulum stress-induced pathological diseases in kidney cause glomerular injury and tubulointerstitial injury. Furthermore, there is a link between oxidative stress and inflammations in pathological states are associated with endoplasmic reticulum stress. Proteinuria and hyperglycemia in diabetic nephropathy may induce endoplasmic reticulum stress in tubular cells of the kidney. Due to the accumulation in the proximal tubule lysosomes, impaired function of these organelles may be an important mechanism leading to proximal tubular toxicity.

Peroxisomal plant metabolism - an update on nitric oxide, Ca2+ and the NADPH recycling network

Plant peroxisomes are recognized organelles that - with their capacity to generate greater amounts of H₂O₂ than other subcellular compartments - have a remarkable oxidative metabolism. However, over the last 15 years, new information has shown that plant peroxisomes contain other important molecules and enzymes, including nitric oxide (NO), peroxynitrite, a NADPH-recycling system, Ca²⁺ and lipid-derived signals, such as jasmonic acid (JA) and nitro-fatty acid (NO₂-FA). This highlights the potential for complex interactions within the peroxisomal nitro-oxidative metabolism, which also affects the status of the cell and consequently its physiological processes. In this review, we provide an update on the peroxisomal interactions between all these molecules. Particular emphasis will be placed on the generation of the free-radical NO, which requires the presence of Ca²⁺, calmodulin and NADPH redox power. Peroxisomes possess several NADPH regeneration mechanisms, such as those mediated by glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) proteins, which are involved in the oxidative phase of the pentose phosphate pathway, as well as that mediated by NADP-isocitrate dehydrogenase (ICDH). The generated NADPH is also an essential cofactor across other peroxisomal pathways, including the antioxidant ascorbate-glutathione cycle and unsaturated fatty acid β-oxidation, the latter being a source of powerful signaling molecules such as JA and NO₂-FA.

Integrated systems biology analysis of KSHV latent infection reveals viral induction and reliance on peroxisome mediated lipid metabolism

Kaposi’s Sarcoma associated Herpesvirus (KSHV), an oncogenic, human gamma-herpesvirus, is the etiological agent of Kaposi’s Sarcoma the most common tumor of AIDS patients world-wide. KSHV is predominantly latent in the main KS tumor cell, the spindle cell, a cell of endothelial origin. KSHV modulates numerous host cell-signaling pathways to activate endothelial cells including major metabolic pathways involved in lipid metabolism. To identify the underlying cellular mechanisms of KSHV alteration of host signaling and endothelial cell activation, we identified changes in the host proteome, phosphoproteome and transcriptome landscape following KSHV infection of endothelial cells. A Steiner forest algorithm was used to integrate the global data sets and, together with transcriptome based predicted transcription factor activity, cellular networks altered by latent KSHV were predicted. Several interesting pathways were identified, including peroxisome biogenesis. To validate the predictions, we showed that KSHV latent infection increases the number of peroxisomes per cell. Additionally, proteins involved in peroxisomal lipid metabolism of very long chain fatty acids, including ABCD3 and ACOX1, are required for the survival of latently infected cells. In summary, novel cellular pathways altered during herpesvirus latency that could not be predicted by a single systems biology platform, were identified by integrated proteomics and transcriptomics data analysis and when correlated with our metabolomics data revealed that peroxisome lipid metabolism is essential for KSHV latent infection of endothelial cells.

Kaposi’s Sarcoma herpesvirus (KSHV) is the etiologic agent of Kaposi’s Sarcoma, the most common tumor of AIDS patients. KSHV modulates host cell signaling and metabolism to maintain a life-long latent infection. To unravel the underlying cellular mechanisms modulated by KSHV, we used multiple global systems biology platforms to identify and integrate changes in both cellular protein expression and transcription following KSHV infection of endothelial cells, the relevant cell type for KS tumors. The analysis identified several interesting pathways including peroxisome biogenesis. Peroxisomes are small cytoplasmic organelles involved in redox reactions and lipid metabolism. KSHV latent infection increases the number of peroxisomes per cell and proteins involved in peroxisomal lipid metabolism are required for the survival of latently infected cells. In summary, through integration of multiple global systems biology analyses we were able to identify novel pathways that could not be predicted by one platform alone and found that lipid metabolism in a small cytoplasmic organelle is necessary for the survival of latent infection with a herpesvirus.

II. Capsular vaso-mimicry formed by transgenic mammary tumor spheroids implanted ectopically into mouse dorsal skin fold: implications for cellular mechanisms of metastasis

Most cancer patients die of metastatic disease, not primary tumors, while biological mechanisms leading to metastases remain unclear and effective therapies are missing. Using a mouse dorsal skin chamber model we had observed that tumor growth and vasculature formation could be influenced by the way in vitro cultured (avascular) spheroids of N202 breast tumor cells were implanted; co-implantation of lactating breast tissue created stimulating microenvironment, whereas the absence of the graft resulted in temporary tumor dormancy. This report addressed the issue of cellular mechanisms of the vasculogenic switch that ended the dormancy. In situ ultrastructural analysis revealed that the tumors survived in ectopic microenvironment until some of host and tumor stem cells evolved independently into cells initiating the vasculogenic switch. The tumor cells that survived and proliferated under hypoxic conditions for three weeks were supported by erythrogenic autophagy of others. However, the host microenvironment first responded as it would to non-immunogenic foreign bodies, i.e., by encapsulating the tumor spheroids with collagen-producing fibroblasts. That led to a form of vaso-mimicry consisting of tumor cells amid tumor-derived erythrosomes (synonym of erythrocytes), megakaryocytes and platelets, and encapsulating them all, the host fibroblasts. Such capsular vaso-mimicry could potentially facilitate metastasis by fusing with morphologically similar lymphatic vessels or veins. Once incorporated into the host circulatory system, tumor cells could be carried away passively by blood flow, regardless of their genetic heterogeneity. The fake vascular segment would have permeability properties different from genuine vascular endothelium. The capsular vaso-mimicry was different from vasculogenic mimicry earlier observed in metastases-associated malignant tumors where channels formed by tumor cells were said to contain circulating blood. Structures similar to the vasculogenic mimicry were seen here as well but contained non-circulating erythrosomes formed between tumor nodules. The host's response to the implantation included coordinated formation of new vessels and peripheral nerves.

II. Capsular vaso-mimicry formed by transgenic mammary tumor spheroids implanted ectopically into mouse dorsal skin fold: cellular mechanisms of metastasis

Most cancer patients die of metastatic disease, not primary tumors, while biological mechanisms leading to metastases remain unclear and effective therapies are missing. Using a mouse dorsal skin chamber model we had observed that tumor growth and vasculature formation could be influenced by the way in vitro cultured (avascular) spheroids of N202 breast tumor cells were implanted; co-implantation of lactating breast tissue created stimulating microenvironment, whereas the absence of the graft resulted in temporary tumor dormancy. This report addressed the issue of cellular mechanisms of the vasculogenic switch that ended the dormancy. In situ ultrastructural analysis revealed that the tumors survived in ectopic microenvironment until some of host and tumor stem cells evolved independently into cells initiating the vasculogenic switch. The tumor cells that survived and proliferated under hypoxic conditions for three weeks were supported by erythrogenic autophagy of others. However, the host microenvironment first responded as it would to non-immunogenic foreign bodies, i.e., by encapsulating the tumor spheroids with collagen-producing fibroblasts. That led to a form of vaso-mimicry consisting of tumor cells amid tumor-derived erythrosomes (synonym of erythrocytes), megakaryocytes and platelets, and encapsulating them all, the host fibroblasts. Such capsular vaso-mimicry could potentially facilitate metastasis by fusing with morphologically similar lymphatic vessels or veins. Once incorporated into the host circulatory system, tumor cells could be carried away passively by blood flow, regardless of their genetic heterogeneity. The fake vascular segment would have permeability properties different from genuine vascular endothelium. The capsular vaso-mimicry was different from vasculogenic mimicry earlier observed in metastases-associated malignant tumors where channels formed by tumor cells were said to contain circulating blood. Structures similar to the vasculogenic mimicry were seen here as well but contained non-circulating erythrosomes formed between tumor nodules. The host's response to the implantation included coordinated formation of new vessels and peripheral nerves.

The versatility of peroxisome function in filamentous fungi

Peroxisomes are ubiquitous and versatile cell organelles. They consist of a single membrane that encloses a proteinaceous matrix. Conserved functions are fatty acid β-oxidation and hydrogen peroxide metabolism. In filamentous fungi, many other metabolic functions have been identified. Also, they contain highly specialized peroxisome-derived structures termed Woronin bodies, which have a structural function in plugging septal pores in order to prevent cytoplasmic bleeding of damaged hyphae.In filamentous fungi peroxisomes play key roles in the production of a range of secondary metabolites such as antibiotics. Most likely the atlas of fungal peroxisomal metabolic pathways is still far from complete. Relative recently discovered functions include their role in biotin biosynthesis as well as in the production of several toxins, among which polyketides. Finally, in filamentous fungi peroxisomes are important for development and pathogenesis.In this contribution we present an overview of our current knowledge on fungal peroxisome formation as well as on their functional diversity.

Pex11pbeta-mediated growth and division of mammalian peroxisomes follows a maturation pathway

Peroxisomes are ubiquitous subcellular organelles, which multiply by growth and division but can also form de novo via the endoplasmic reticulum. Growth and division of peroxisomes in mammalian cells involves elongation, membrane constriction and final fission. Dynamin-like protein (DLP1/Drp1) and its membrane adaptor Fis1 function in the later stages of peroxisome division, whereas the membrane peroxin Pex11pbeta appears to act early in the process. We have discovered that a Pex11pbeta-YFP(m) fusion protein can be used as a specific tool to further dissect peroxisomal growth and division. Pex11pbeta-YFP(m) inhibited peroxisomal segmentation and division, but resulted in the formation of pre-peroxisomal membrane structures composed of globular domains and tubular extensions. Peroxisomal matrix and membrane proteins were targeted to distinct regions of the peroxisomal structures. Pex11pbeta-mediated membrane formation was initiated at pre-existing peroxisomes, indicating that growth and division follows a multistep maturation pathway and that formation of mammalian peroxisomes is more complex than simple division of a pre-existing organelle. The implications of these findings on the mechanisms of peroxisome formation and membrane deformation are discussed.

Peroxisome diversity and evolution

Peroxisomes are organelles bounded by a single membrane that can be found in all major groups of eukaryotes. A single evolutionary origin of this cellular compartment is supported by the presence, in diverse organisms, of a common set of proteins implicated in peroxisome biogenesis and maintenance. Their enzymatic content, however, can vary substantially across species, indicating a high level of evolutionary plasticity. Proteomic analyses have greatly expanded our knowledge on peroxisomes in some model organisms, including plants, mammals and yeasts. However, we still have a limited knowledge about the distribution and functionalities of peroxisomes in the vast majority of groups of microbial eukaryotes. Here, I review recent advances in our understanding of peroxisome diversity and evolution, with a special emphasis on peroxisomes in microbial eukaryotes.

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.

Targeting sequences of UBXD8 and AAM-B reveal that the ER has a direct role in the emergence and regression of lipid droplets

Lipid droplets are sites of neutral lipid storage thought to be actively involved in lipid homeostasis. A popular model proposes that droplets are formed in the endoplasmic reticulum (ER) by a process that begins with the deposition of neutral lipids between the membrane bilayer. As the droplet grows, it becomes surrounded by a monolayer of phospholipid derived from the outer half of the ER membrane, which contains integral membrane proteins anchored by hydrophobic regions. This model predicts that for an integral droplet protein inserted into the outer half of the ER membrane to reach the forming droplet, it must migrate in the plane of the membrane to sites of lipid accumulation. Here, we report the results of experiments that directly test this hypothesis. Using two integral droplet proteins that contain unique hydrophobic targeting sequences (AAM-B and UBXD8), we present evidence that both proteins migrate from their site of insertion in the ER to droplets that are forming in response to fatty acid supplementation. Migration to droplets occurs even when further protein synthesis is inhibited or dominant-negative Sar1 blocks transport to the Golgi complex. Surprisingly, when droplets are induced to disappear from the cell, both proteins return to the ER as the level of neutral lipid declines. These data suggest that integral droplet proteins form from and regress to the ER as part of a cyclic process that does not involve traffic through the secretory pathway.

How peroxisomes multiply

With every cell division, peroxisomes duplicate and are segregated between progeny cells. Here, we discuss the different modes of peroxisome multiplication and the machinery that is involved in each case. Peroxisomes have been considered by many to be peripheral to mainstream cell biology. However, this is changing in response to the recent finding that peroxisomes obtain membrane constituents from the endoplasmic reticulum, making them the latest branch of the endomembrane system to be identified. Furthermore, the observations that peroxisome and mitochondrial biogenesis can occur in a coordinated manner, and that these organelles share factors for their multiplication, demonstrate previously unanticipated aspects of cellular organisation.

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.

Autophagy in the liver

A great part of our current understanding of mammalian macroautophagy is derived from studies of the liver. The term "autophagy" was introduced by Christian de Duve in part based on ultrastructural changes in rat liver following glucagon injection. Subsequent morphological, biochemical, and kinetics studies of autophagy in the liver defined the basic process of autophagosome formation, maturation, and degradation and the regulation of autophagy by hormones, phosphoinositide 3-kinases, and mammalian target of rapamycin. It is now clear that macroautophagy in the liver is important for the balance of energy and nutrients for basic cell functions, the removal of misfolded proteins resulting from genetic mutations or pathophysiological stimulations, and the turnover of major subcellular organelles such as mitochondria, endoplasmic reticulum, and peroxisomes under both normal and pathophysiological conditions. Disturbance of autophagy function in the liver could thus have a major impact on liver physiology and liver disease.

Vesicle, mitochondrial, and plastid division machineries with emphasis on dynamin and electron-dense rings

The original eukaryotic cells contained at least one set of double-membrane-bounded organelles (cell nucleus and mitochondria) and single-membrane-bounded organelles [endoplasmic reticulum, Golgi apparatus, lysosomes (vacuoles), and microbodies (peroxisomes)]. An increase in the number of organelles accompanied the evolution of these cells into Amoebozoa and Opisthokonta. Furthermore, the basic cells, containing mitochondria, engulfed photosynthetic Cyanobacteria, which were converted to plastids, and the cells thereby evolved into cells characteristic of the Bikonta. How did basic single- and double-membrane-bounded organelles originate from bacteria-like cells during early eukaryotic evolution? To answer this question, the important roles of the GTPase dynamin- and electron-dense rings in the promotion of diverse cellular activities in eukaryotes, including endocytosis, vesicular transport, mitochondrial division, and plastid division, must be considered. In this review, vesicle division, mitochondrial division, and plastid division machineries, including the dynamin- and electron-dense rings, and their roles in the origin and biogenesis of organelles in eukaryote cells are summarized.

Maintaining peroxisome populations: a story of division and inheritance

Eukaryotic cells divide their metabolic labor between functionally distinct, membrane-enveloped organelles, each precisely tailored for a specific set of biochemical reactions. Peroxisomes are ubiquitous, endoplasmic reticulum-derived organelles that perform requisite biochemical functions intimately connected to lipid metabolism. Upon cell division, cells have to strictly control peroxisome division and inheritance to maintain an appropriate number of peroxisomes in each cell. Peroxisome division follows a specific sequence of events that include peroxisome elongation, membrane constriction, and peroxisome fission. Pex11 proteins mediate the elongation step of peroxisome division, whereas dynamin-related proteins execute the final fission. The mechanisms responsible for peroxisome membrane constriction are poorly understood. Molecular players involved in peroxisome inheritance are just beginning to be elucidated. Inp1p and Inp2p are two recently identified peroxisomal proteins that perform antagonistic functions in regulating peroxisome inheritance in budding yeast. Inp1p promotes the retention of peroxisomes in mother cells and buds by attaching peroxisomes to as-yet-unidentified cortical structures. Inp2p is implicated in the motility of peroxisomes by linking them to the Myo2p motor, which then propels their movement along actin cables. The functions of Inp1p and Inp2p are cell cycle regulated and coordinated to ensure a fair distribution of peroxisomes at cytokinesis.

Dynamin-dependent biogenesis, cell cycle regulation and mitochondrial association of peroxisomes in fission yeast

Peroxisomes were visualized for the first time in living fission yeast cells. In small, newly divided cells, the number of peroxisomes was low but increased in parallel with the increase in cell length/volume that accompanies cell cycle progression. In cells grown in oleic acid, both the size and the number of peroxisomes increased. The peroxisomal inventory of cells lacking the dynamin-related proteins Dnm1 or Vps1 was similar to that in wild type. By contrast, cells of the double mutant dnm1Delta vps1Delta contained either no peroxisomes at all or a small number of morphologically aberrant organelles. Peroxisomes exhibited either local Brownian movement or longer-range linear displacements, which continued in the absence of either microtubules or actin filaments. On the contrary, directed peroxisome motility appeared to occur in association with mitochondria and may be an indirect function of intrinsic mitochondrial dynamics. We conclude that peroxisomes are present in fission yeast and that Dnm1 and Vps1 act redundantly in peroxisome biogenesis, which is under cell cycle control. Peroxisome movement is independent of the cytoskeleton but is coupled to mitochondrial dynamics.

Yeast peroxisomes multiply by growth and division

Peroxisomes can arise de novo from the endoplasmic reticulum (ER) via a maturation process. Peroxisomes can also multiply by fission. We have investigated how these modes of multiplication contribute to peroxisome numbers in Saccharomyces cerevisiae and the role of the dynamin-related proteins (Drps) in these processes. We have developed pulse-chase and mating assays to follow the fate of existing peroxisomes, de novo-formed peroxisomes, and ER-derived preperoxisomal structures. We find that in wild-type (WT) cells, peroxisomes multiply by fission and do not form de novo. A marker for the maturation pathway, Pex3-GFP, is delivered from the ER to existing peroxisomes. Strikingly, cells lacking peroxisomes as a result of a segregation defect do form peroxisomes de novo. This process is slower than peroxisome multiplication in WT cells and is Drp independent. In contrast, peroxisome fission is Drp dependent. Our results show that peroxisomes multiply by growth and division under our assay conditions. We conclude that the ER to peroxisome pathway functions to supply existing peroxisomes with essential membrane constituents.

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.

The ER-peroxisome connection in plants: Development of the “ER semi-autonomous peroxisome maturation and replication” model for plant peroxisome biogenesis

The perceived role of the ER in the biogenesis of plant peroxisomes has evolved significantly from the original "ER vesiculation" model, which portrayed co-translational import of proteins into peroxisomes originating from the ER, to the "ER semi-autonomous peroxisome" model wherein membrane lipids and post-translationally acquired peroxisomal membrane proteins (PMPs) were derived from the ER. Results from more recent studies of various plant PMPs including ascorbate peroxidase, PEX10 and PEX16, as well as a viral replication protein, have since led to the formulation of a more elaborate "ER semi-autonomous peroxisome maturation and replication" model. Herein we review these results in the context of this newly proposed model and its predecessor models. We discuss also key distinct features of the new model pertaining to its central premise that the ER defines the semi-autonomous maturation (maintenance/assembly/differentiation) and duplication (division) features of specialized classes of pre-existing plant peroxisomes. This model also includes a novel peroxisome-to-ER retrograde sorting pathway that may serve as a constitutive protein retrieval/regulatory system. In addition, new plant peroxisomes are envisaged to arise primarily by duplication of the pre-existing peroxisomes that receive essential membrane components from the ER.

Lipids and lipid domains in the peroxisomal membrane of the yeast Yarrowia lipolytica

Biological membranes have unique and highly diverse compositions of their lipid constituents. At present, we have only partial understanding of how membrane lipids and lipid domains regulate the structural integrity and functionality of cellular organelles, maintain the unique molecular composition of each organellar membrane by orchestrating the intracellular trafficking of membrane-bound proteins and lipids, and control the steady-state levels of numerous signaling molecules generated in biological membranes. Similar to other organellar membranes, a single lipid bilayer enclosing the peroxisome, an organelle known for its essential role in lipid metabolism, has a unique lipid composition and organizes some of its lipid and protein components into distinctive assemblies. This review highlights recent advances in our knowledge of how lipids and lipid domains of the peroxisomal membrane regulate the processes of peroxisome assembly and maintenance in the yeast Yarrowia lipolytica. We critically evaluate the molecular mechanisms through which lipid constituents of the peroxisomal membrane control these multistep processes and outline directions for future research in this field.

The import competence of a peroxisomal membrane protein is determined by Pex19p before the docking step

Biogenesis of the mammalian peroxisomal membrane requires the action of Pex3p and Pex16p, two proteins present in the organelle membrane, and Pex19p, a protein that displays a dual subcellular distribution (peroxisomal and cytosolic). Pex19p interacts with most peroxisomal intrinsic membrane proteins, but whether this property reflects its role as an import receptor for this class of proteins or a chaperone-like function in the assembly/disassembly of peroxisomal membrane proteins has been the subject of much controversy. Here, we describe an in vitro system particularly suited to address this issue. It is shown that insertion of a reporter protein into the peroxisomal membrane is a Pex3p-dependent process that does not require ATP/GTP hydrolysis. The system can be programmed with recombinant versions of Pex19p, allowing us to demonstrate that Pex19p-cargo protein complexes formed in the absence of peroxisomes are the substrates for the peroxisomal docking/insertion machinery. Data suggesting that cargo-loaded Pex19p displays a much higher affinity for Pex3p than Pex19p alone are also provided. These results suggest that soluble Pex19p participates in the targeting of newly synthesized peroxisomal membrane proteins to the organelle membrane and support the existence of a cargo-induced peroxisomal targeting mechanism for Pex19p.

Pex19p-dependent targeting of Pex17p, a peripheral component of the peroxisomal protein import machinery

Pex19p is required for the topogenesis of peroxisomal membrane proteins (PMPs). Here we have demonstrated that Pex19p is also required for the peroxisomal targeting and stability of Pex17p, a peripheral component of the docking complex of the peroxisomal protein import machinery. We have demonstrated that Pex17p is associated with the peroxisomal Pex13p-Pex14p complex as well as with Pex19p. We have identified the corresponding binding sites for Pex14p and Pex19p and demonstrated that a specific loss of the Pex19p interaction resulted in mistargeting of Pex17p. We have shown that a construct consisting only of the Pex19p- and Pex14p-binding sites of Pex17p is sufficient to direct an otherwise cytosolic reporter protein to the peroxisomal membrane in a Pex19p-dependent manner. Our data show that the function of Pex19p as chaperone or import receptor is not restricted to integral membrane proteins but may also include peripheral PMPs. As a consequence of our data, the previous definition of a targeting signal for PMPs (mPTS) as a Pex19p-binding motif in conjunction with a transmembrane segment should be extended to regions comprising a Pex19p-binding motif and a peroxisomal anchor sequence.