Protein import into peroxisomes occurs through a nuclear pore-like phase

Bacterial homologs of innate eukaryotic antiviral defenses with anti-phage activity highlight shared evolutionary roots of viral defenses

Prokaryotes have evolved a multitude of defense systems to protect against phage predation. Some of these resemble eukaryotic genes involved in antiviral responses. Here, we set out to systematically project the current knowledge of eukaryotic-like antiviral defense systems onto prokaryotic genomes, using Pseudomonas aeruginosa as a model organism. Searching for phage defense systems related to innate antiviral genes from vertebrates and plants, we uncovered over 450 candidates. We validated six of these phage defense systems, including factors preventing viral attachment, R-loop-acting enzymes, the inflammasome, ubiquitin pathway, and pathogen recognition signaling. Collectively, these defense systems support the concept of deep evolutionary links and shared antiviral mechanisms between prokaryotes and eukaryotes.

Identification of ERAD-dependent degrons for the endoplasmic reticulum lumen

Degrons are minimal protein features that are sufficient to target proteins for degradation. In most cases, degrons allow recognition by components of the cytosolic ubiquitin proteasome system. Currently, all of the identified degrons only function within the cytosol. Using Saccharomyces cerevisiae, we identified the first short linear sequences that function as degrons from the endoplasmic reticulum (ER) lumen. We show that when these degrons are transferred to proteins, they facilitate proteasomal degradation through the ERAD system. These degrons enable degradation of both luminal and integral membrane ER proteins, expanding the types of proteins that can be targeted for degradation in budding yeast and mammalian tissue culture. This discovery provides a framework to target proteins for degradation from the previously unreachable ER lumen and builds toward therapeutic approaches that exploit the highly-conserved ERAD system.

Biomolecular condensates as drivers of membrane trafficking and remodelling

Membrane remodelling is essential for the trafficking of macromolecules throughout the cell, a process that regulates various aspects of cellular health and pathology. Recent studies implicate the role of biomolecular condensates in regulating multiple steps of the membrane trafficking pathway including but not limited to the organization of the trafficking machinery, dynamic remodeling of membranes, spatial and functional regulation, and response to cellular signals. The implicated proteins contain key structural elements, most notably prion-like domains within intrinsically disordered regions that are necessary for biomolecular condensate formation at fusion sites in processes like endocytic assembly, autophagy, organelle biosynthesis and synaptic vesicle fusion. Experimental and theoretical advances in the field continue to demonstrate that protein condensates can perform mechanical work, the implications of which can be extrapolated to diverse areas of membrane biology.

RNA scaffolds the Golgi ribbon by forming condensates with GM130

The mammalian Golgi is composed of stacks that are laterally connected into a continuous ribbon-like structure. The integrity and function of the ribbon is disrupted under stress conditions, but the molecular mechanisms remain unclear. Here we show that the ribbon is maintained by biomolecular condensates of RNA and the Golgi matrix protein GM130 (GOLGA2). We identify GM130 as a membrane-bound RNA-binding protein, which directly recruits RNA and associated RNA-binding proteins to the Golgi membrane. Acute degradation of RNA or GM130 in cells disrupts the ribbon. Under stress conditions, RNA dissociates from GM130 and the ribbon is disjointed, but after the cells recover from stress the ribbon is restored. When overexpressed in cells, GM130 forms RNA-dependent liquid-like condensates. GM130 contains an intrinsically disordered domain at its amino terminus, which binds RNA to induce liquid-liquid phase separation. These co-condensates are sufficient to link purified Golgi membranes, reconstructing lateral linking of stacks into a ribbon-like structure. Together, these studies show that RNA acts as a structural biopolymer that together with GM130 maintains the integrity of the Golgi ribbon.

The Hansenula polymorpha mitochondrial carrier family protein Mir1 is dually localized at peroxisomes and mitochondria

Peroxisomes are ubiquitous cell organelles involved in various metabolic pathways. In order to properly function, several cofactors, substrates and products of peroxisomal enzymes need to pass the organellar membrane. So far only a few transporter proteins have been identified. We analysed peroxisomal membrane fractions purified from the yeast Hansenula polymorpha by untargeted label-free quantitation mass spectrometry. As expected, several known peroxisome-associated proteins were enriched in the peroxisomal membrane fraction. In addition, several other proteins were enriched, including mitochondrial transport proteins. Localization studies revealed that one of them, the mitochondrial phosphate carrier Mir1, has a dual localization on mitochondria and peroxisomes. To better understand the molecular mechanisms of dual sorting, we localized Mir1 in cells lacking Pex3 or Pex19, two peroxins that play a role in targeting of peroxisomal membrane proteins. In these cells Mir1 only localized to mitochondria, indicating that Pex3 and Pex19 are required to sort Mir1 to peroxisomes. Analysis of the localization of truncated versions of Mir1 in wild-type H. polymorpha cells revealed that most of them localized to mitochondria, but only one, consisting of the transmembrane domains 3-6, was peroxisomal. Peroxisomal localization of this construct was lost in a MIR1 deletion strain, indicating that full-length Mir1 was required for the localization of the truncated protein to peroxisomes. Our data suggest that only full-length Mir1 sorts to peroxisomes, while Mir1 contains multiple regions with mitochondrial sorting information. Data are available via ProteomeXchange with identifier PXD050324.

Structural anisotropy results in mechano-directional transport of proteins across nuclear pores

The nuclear pore complex regulates nucleocytoplasmic transport by means of a tightly synchronized suite of biochemical reactions. The physicochemical properties of the translocating cargos are emerging as master regulators of their shuttling dynamics. As well as being affected by molecular weight and surface-exposed amino acids, the kinetics of the nuclear translocation of protein cargos also depend on their nanomechanical properties, yet the mechanisms underpinning the mechanoselectivity of the nuclear pore complex are unclear. Here we show that proteins with locally soft regions in the vicinity of the nuclear-localization sequence exhibit higher nuclear-import rates, and that such mechanoselectivity is specifically impaired upon knocking down nucleoporin 153, a key protein in the nuclear pore complex. This allows us to design a short, easy-to-express and chemically inert unstructured peptide tag that accelerates the nuclear-import rate of stiff protein cargos. We also show that U2OS osteosarcoma cells expressing the peptide-tagged myocardin-related transcription factor import this mechanosensitive protein to the nucleus at higher rates and display faster motility. Locally unstructured regions lower the free-energy barrier of protein translocation and might offer a control mechanism for nuclear mechanotransduction.

An essential and highly selective protein import pathway encoded by nucleus-forming phage

Targeting proteins to specific subcellular destinations is essential in prokaryotes, eukaryotes, and the viruses that infect them. Chimalliviridae phages encapsulate their genomes in a nucleus-like replication compartment composed of the protein chimallin (ChmA) that excludes ribosomes and decouples transcription from translation. These phages selectively partition proteins between the phage nucleus and the bacterial cytoplasm. Currently, the genes and signals that govern selective protein import into the phage nucleus are unknown. Here, we identify two components of this protein import pathway: a species-specific surface-exposed region of a phage intranuclear protein required for nuclear entry and a conserved protein, PicA (Protein importer of chimalliviruses A), that facilitates cargo protein trafficking across the phage nuclear shell. We also identify a defective cargo protein that is targeted to PicA on the nuclear periphery but fails to enter the nucleus, providing insight into the mechanism of nuclear protein trafficking. Using CRISPRi-ART protein expression knockdown of PicA, we show that PicA is essential early in the chimallivirus replication cycle. Together, our results allow us to propose a multistep model for the Protein Import Chimallivirus pathway, where proteins are targeted to PicA by amino acids on their surface and then licensed by PicA for nuclear entry. The divergence in the selectivity of this pathway between closely related chimalliviruses implicates its role as a key player in the evolutionary arms race between competing phages and their hosts.

Mammalian pexophagy at a glance

Peroxisomes are highly plastic organelles that are involved in several metabolic processes, including fatty acid oxidation, ether lipid synthesis and redox homeostasis. Their abundance and activity are dynamically regulated in response to nutrient availability and cellular stress. Damaged or superfluous peroxisomes are removed mainly by pexophagy, the selective autophagy of peroxisomes induced by ubiquitylation of peroxisomal membrane proteins or ubiquitin-independent processes. Dysregulated pexophagy impairs peroxisome homeostasis and has been linked to the development of various human diseases. Despite many recent insights into mammalian pexophagy, our understanding of this process is still limited compared to our understanding of pexophagy in yeast. In this Cell Science at a Glance article and the accompanying poster, we summarize current knowledge on the control of mammalian pexophagy and highlight which aspects require further attention. We also discuss the role of ubiquitylation in pexophagy and describe the ubiquitin machinery involved in regulating signals for the recruitment of phagophores to peroxisomes.

Towards solving the mystery of peroxisomal matrix protein import

Peroxisomes are vital metabolic organelles that import their lumenal (matrix) enzymes from the cytosol using mobile receptors. Surprisingly, the receptors can even import folded proteins, but the underlying mechanism has been a mystery. Recent results reveal how import receptors shuttle cargo into peroxisomes. The cargo-bound receptors move from the cytosol across the peroxisomal membrane completely into the matrix by a mechanism that resembles transport through the nuclear pore. The receptors then return to the cytosol through a separate retrotranslocation channel, leaving the cargo inside the organelle. This cycle concentrates imported proteins within peroxisomes, and the energy for cargo import is supplied by receptor export. Peroxisomal protein import thus fundamentally differs from other previously known mechanisms for translocating proteins across membranes.

The proteome of Penicillium expansum during infection of postharvest apple is revealed using Label-Free and Parallel Reaction Monitoring(PRM)Techniques

Penicillium expansum is the main pathogen in the postharvest storage of apples. Penicilliosis caused by P. expansum infection not only seriously affects the appearance and quality of fruits, but also the secondary metabolite Patulin (PAT) can cause harm to human health. Until now, little attention has been paid to the molecular mechanism of P. expansum infecting apples. Studying its molecular mechanism can help us better prevent and control apple postharvest blue mold. In this present investigation, we will use Label-Free technology to perform proteomic sequencing on apple samples at key time points of P. expansum infection, explore and screen key proteins and metabolic pathways during infection, and use Parallel Reaction Monitoring (PRM) technology to thoroughly validate proteomic data. The infection of P. expansum activates the MAPK signaling pathway, plant-pathogen interaction metabolic pathway and phenylpropanoid biosynthesis pathway of apple, participates in the regulation of ROS generation and oxidative stress process, promotes the synthesis of lignin and flavonoids, and the synthesis of Pathogenesis-Related Protein helps apple directly defend against P. expansum infection. This study provides the foundation for relevant postharvest control strategies, paving the way for further exploration of the proteome of pathogens infecting fruit and vegetables. SIGNIFICANCE: Proteins are macromolecules essential to the life of organisms, as they participate in the function and structure of cells. Proteomics technology is currently one of the important means to study the the response mechanism of pathogenic bacteria to plant infection, which can reveal the essence of physiological and pathological processes and help to clarify the possible relationship between protein abundance and plant stress. The present study essentially uses recent proteome analysis technology, namely label-free and PRM techniques, and lays the foundations for studying the of the infection response between P. expansum and apples. In particular, it provides a broad perspective on the molecular mechanism of P. expansum in the early stage of apple infection through detailed functional exploration and verification of associated proteins. Thus, it provides a theoretical basis for preventing and treating apple postharvest blue mold.

Modulation of peroxisomal import by the PEX13 SH3 domain and a proximal FxxxF binding motif

Import of proteins into peroxisomes depends on PEX5, PEX13 and PEX14. By combining biochemical methods and structural biology, we show that the C-terminal SH3 domain of PEX13 mediates intramolecular interactions with a proximal FxxxF motif. The SH3 domain also binds WxxxF peptide motifs in the import receptor PEX5, demonstrating evolutionary conservation of such interactions from yeast to human. Strikingly, intramolecular interaction of the PEX13 FxxxF motif regulates binding of PEX5 WxxxF/Y motifs to the PEX13 SH3 domain. Crystal structures reveal how FxxxF and WxxxF/Y motifs are recognized by a non-canonical surface on the SH3 domain. The PEX13 FxxxF motif also mediates binding to PEX14. Surprisingly, the potential PxxP binding surface of the SH3 domain does not recognize PEX14 PxxP motifs, distinct from its yeast ortholog. Our data show that the dynamic network of PEX13 interactions with PEX5 and PEX14, mediated by diaromatic peptide motifs, modulates peroxisomal matrix import.

An essential and highly selective protein import pathway encoded by nucleus-forming phage

Targeting proteins to specific subcellular destinations is essential in prokaryotes, eukaryotes, and the viruses that infect them. Chimalliviridae phages encapsulate their genomes in a nucleus-like replication compartment composed of the protein chimallin (ChmA) that excludes ribosomes and decouples transcription from translation. These phages selectively partition proteins between the phage nucleus and the bacterial cytoplasm. Currently, the genes and signals that govern selective protein import into the phage nucleus are unknown. Here we identify two components of this novel protein import pathway: a species-specific surface-exposed region of a phage intranuclear protein required for nuclear entry and a conserved protein, PicA, that facilitates cargo protein trafficking across the phage nuclear shell. We also identify a defective cargo protein that is targeted to PicA on the nuclear periphery but fails to enter the nucleus, providing insight into the mechanism of nuclear protein trafficking. Using CRISPRi-ART protein expression knockdown of PicA, we show that PicA is essential early in the chimallivirus replication cycle. Together our results allow us to propose a multistep model for the Protein Import Chimallivirus (PIC) pathway, where proteins are targeted to PicA by amino acids on their surface, and then licensed by PicA for nuclear entry. The divergence in the selectivity of this pathway between closely-related chimalliviruses implicates its role as a key player in the evolutionary arms race between competing phages and their hosts.

Significance Statement

The phage nucleus is an enclosed replication compartment built by Chimalliviridae phages that, similar to the eukaryotic nucleus, separates transcription from translation and selectively imports certain proteins. This allows the phage to concentrate proteins required for DNA replication and transcription while excluding DNA-targeting host defense proteins. However, the mechanism of selective trafficking into the phage nucleus is currently unknown. Here we determine the region of a phage nuclear protein that targets it for nuclear import and identify a conserved, essential nuclear shell-associated protein that plays a key role in this process. This work provides the first mechanistic model of selective import into the phage nucleus.

Proteins that carry dual targeting signals can act as tethers between peroxisomes and partner organelles

Peroxisomes are organelles with crucial functions in oxidative metabolism. To correctly target to peroxisomes, proteins require specialized targeting signals. A mystery in the field is the sorting of proteins that carry a targeting signal for peroxisomes and as well as for other organelles, such as mitochondria or the endoplasmic reticulum (ER). Exploring several of these proteins in fungal model systems, we observed that they can act as tethers bridging organelles together to create contact sites. We show that in Saccharomyces cerevisiae this mode of tethering involves the peroxisome import machinery, the ER-mitochondria encounter structure (ERMES) at mitochondria and the guided entry of tail-anchored proteins (GET) pathway at the ER. Our findings introduce a previously unexplored concept of how dual affinity proteins can regulate organelle attachment and communication.

The peroxisome: an update on mysteries 3.0

Peroxisomes are highly dynamic, oxidative organelles with key metabolic functions in cellular lipid metabolism, such as the β-oxidation of fatty acids and the synthesis of myelin sheath lipids, as well as the regulation of cellular redox balance. Loss of peroxisomal functions causes severe metabolic disorders in humans. Furthermore, peroxisomes also fulfil protective roles in pathogen and viral defence and immunity, highlighting their wider significance in human health and disease. This has sparked increasing interest in peroxisome biology and their physiological functions. This review presents an update and a continuation of three previous review articles addressing the unsolved mysteries of this remarkable organelle. We continue to highlight recent discoveries, advancements, and trends in peroxisome research, and address novel findings on the metabolic functions of peroxisomes, their biogenesis, protein import, membrane dynamics and division, as well as on peroxisome-organelle membrane contact sites and organelle cooperation. Furthermore, recent insights into peroxisome organisation through super-resolution microscopy are discussed. Finally, we address new roles for peroxisomes in immune and defence mechanisms and in human disorders, and for peroxisomal functions in different cell/tissue types, in particular their contribution to organ-specific pathologies.

The peroxisome protein translocation machinery is developmentally regulated in the fungus Podospora anserina

IMPORTANCE

Sexual reproduction allows eukaryotic organisms to produce genetically diverse progeny. This process relies on meiosis, a reductional division that enables ploidy maintenance and genetic recombination. Meiotic differentiation also involves the renewal of cell functioning to promote offspring rejuvenation. Research in the model fungus Podospora anserina has shown that this process involves a complex regulation of the function and dynamics of different organelles, including peroxisomes. These organelles are critical for meiosis induction and play further significant roles in meiotic development. Here we show that PEX13-a key constituent of the protein conduit through which the proteins defining peroxisome function reach into the organelle-is subject to a developmental regulation that almost certainly involves its selective ubiquitination-dependent removal and that modulates its abundance throughout meiotic development and at different sexual differentiation processes. Our results show that meiotic development involves a complex developmental regulation of the peroxisome protein translocation system.

A General Group-Protection Synthesis Strategy to Fabricate Covalent Organic Framework Gels

Fabricating insoluble and infusible porous materials into gels for advanced applications is of great importance but has formidable challenges. Here, we present a general, facile, and scalable protocol to fabricate covalent organic framework (COF) gels using a group-protection synthesis strategy. To prove the generality of this strategy, we successfully prepared 10 types of COF organohydrogels with high crystallinity, porosity, good mechanical properties, and excellent solvent and freezing resistance. Notably, these COF organohydrogels can easily transform into hydrogels, organogels, and aerogels, breaking the gaps between different types of COF gels. An in-depth mechanism investigation unveils that the group-protection strategy effectively slows down the formation rate and regulates the morphology of COFs, benefiting the formation of cross-linked nanofibers/nanosheets to produce COF gels. We also find that the hydrogen bond network formed by the organic/water binary solvent and functional groups in the COF skeletons plays a vital role in creating organohydrogels and maintaining frost resistance and solvent resistance. As an application demonstration, COF gels installed with photoresponsive azobenzene groups show excellent solar energy absorption, photothermal conversion, and water transmission performances, demonstrating great potential in solar desalination. This work enriches the synthesis toolboxes for COF gels and expands the application scope of COFs.

Puzzling out nuclear pore complex assembly

Nuclear pore complexes (NPCs) are sophisticated multiprotein assemblies embedded within the nuclear envelope and controlling the exchanges of molecules between the cytoplasm and the nucleus. In this review, we summarize the mechanisms by which these elaborate complexes are built from their subunits, the nucleoporins, based on our ever-growing knowledge of NPC structural organization and on the recent identification of additional features of this process. We present the constraints faced during the production of nucleoporins, their gathering into oligomeric complexes, and the formation of NPCs within nuclear envelopes, and review the cellular strategies at play, from co-translational assembly to the enrolment of a panel of cofactors. Remarkably, the study of NPCs can inform our perception of the biogenesis of multiprotein complexes in general - and vice versa.

Glutathione and peroxisome redox homeostasis

Despite intensive research on peroxisome biochemistry, the role of glutathione in peroxisomal redox homeostasis has remained a matter of speculation for many years, and only recently has this issue started to be experimentally addressed. Here, we summarize and compare data from several organisms on the peroxisome-glutathione topic. It is clear from this comparison that the repertoire of glutathione-utilizing enzymes in peroxisomes of different organisms varies widely. In addition, the available data suggest that the kinetic connectivity between the cytosolic and peroxisomal pools of glutathione may also be different in different organisms, with some possessing a peroxisomal membrane that is promptly permeable to glutathione whereas in others this may not be the case. However, regardless of the differences, the picture that emerges from all these data is that glutathione is a crucial component of the antioxidative system that operates inside peroxisomes in all organisms.

Oxidative regulation of TDP-43 self-association by a β-to-α conformational switch

An evolutionarily conserved region of the TDP-43 low-complexity domain (LCD) twenty residues in length can adopt either an α-helical or β-strand conformation. When in the latter conformation, TDP-43 self-associates via the formation of a labile, cross-β structure. Self-association can be monitored via the formation of phase-separated protein droplets. Exposure of droplets to hydrogen peroxide leads to oxidation of conserved methionine residues distributed throughout the LCD. Oxidation disassembles the cross-β structure, thus eliminating both self-association and phase separation. Here, we demonstrate that this process reciprocally enables formation of α-helical structure in precisely the same region formerly functioning to facilitate β-strand-mediated self-association. We further observe that the α-helical conformation allows interaction with a lipid-like detergent and that exposure to lipids enhances the β-to-α conformational switch. We hypothesize that regulation of this oxidative switch will prove to be important to the control of localized translation within vertebrate cells. The experimental observations reported herein were heavily reliant on studies of 1,6-hexanediol, a chemical agent that selectively dissolves labile structures formed via the self-association of protein domains of low sequence complexity. This aliphatic alcohol is shown to exert its dissociative activity primarily via hydrogen-bonding interactions with carbonyl oxygen atoms of the polypeptide backbone. Such observations underscore the central importance of backbone-mediated protein:protein interactions that facilitate the self-association and phase separation of LCDs.

Structure of the peroxisomal Pex1/Pex6 ATPase complex bound to a substrate

The double-ring AAA+ ATPase Pex1/Pex6 is required for peroxisomal receptor recycling and is essential for peroxisome formation. Pex1/Pex6 mutations cause severe peroxisome associated developmental disorders. Despite its pathophysiological importance, mechanistic details of the heterohexamer are not yet available. Here, we report cryoEM structures of Pex1/Pex6 from Saccharomyces cerevisiae, with an endogenous protein substrate trapped in the central pore of the catalytically active second ring (D2). Pairs of Pex1/Pex6(D2) subdomains engage the substrate via a staircase of pore-1 loops with distinct properties. The first ring (D1) is catalytically inactive but undergoes significant conformational changes resulting in alternate widening and narrowing of its pore. These events are fueled by ATP hydrolysis in the D2 ring and disengagement of a "twin-seam" Pex1/Pex6(D2) heterodimer from the staircase. Mechanical forces are propagated in a unique manner along Pex1/Pex6 interfaces that are not available in homo-oligomeric AAA-ATPases. Our structural analysis reveals the mechanisms of how Pex1 and Pex6 coordinate to achieve substrate translocation.

The Pex6 N1 domain is required for Pex15 binding and proper assembly with Pex1

The heterohexameric AAA-ATPase Pex1/Pex6 is essential for the formation and maintenance of peroxisomes. Pex1/Pex6, similar to other AAA-ATPases, uses the energy from ATP hydrolysis to mechanically thread substrate proteins through its central pore, thereby unfolding them. In related AAA-ATPase motors, substrates are recruited through binding to the motor's N-terminal domains or N-terminally bound co-factors. Here we use structural and biochemical techniques to characterize the function of the N1 domain in Pex6 from budding yeast, S. cerevisiae. We found that although Pex1/ΔN1-Pex6 is an active ATPase in vitro, it does not support Pex1/Pex6 function at the peroxisome in vivo. An X-ray crystal structure of the isolated Pex6 N1 domain shows that the Pex6 N1 domain shares the same fold as the N terminal domains of PEX1, CDC48, or NSF, despite poor sequence conservation. Integrating this structure with a cryo-EM reconstruction of Pex1/Pex6, AlphaFold2 predictions, and biochemical assays shows that Pex6 N1 mediates binding to both the peroxisomal membrane tether Pex15 and an extended loop from the D2 ATPase domain of Pex1 that influences Pex1/Pex6 heterohexamer stability. Given the direct interactions with both Pex15 and the D2 ATPase domains, the Pex6 N1 domain is poised to coordinate binding of co-factors and substrates with Pex1/Pex6 ATPase activity.

Molecular basis of the glycosomal targeting of PEX11 and its mislocalization to mitochondrion in trypanosomes

PEX19 binding sites are essential parts of the targeting signals of peroxisomal membrane proteins (mPTS). In this study, we characterized PEX19 binding sites of PEX11, the most abundant peroxisomal and glycosomal membrane protein from Trypanosoma brucei and Saccharomyces cerevisiae. TbPEX11 contains two PEX19 binding sites, one close to the N-terminus (BS1) and a second in proximity to the first transmembrane domain (BS2). The N-terminal BS1 is highly conserved across different organisms and is required for maintenance of the steady-state concentration and efficient targeting to peroxisomes and glycosomes in both baker's yeast and Trypanosoma brucei. The second PEX19 binding site in TbPEX11 is essential for its glycosomal localization. Deletion or mutations of the PEX19 binding sites in TbPEX11 or ScPEX11 results in mislocalization of the proteins to mitochondria. Bioinformatic analysis indicates that the N-terminal region of TbPEX11 contains an amphiphilic helix and several putative TOM20 recognition motifs. We show that the extreme N-terminal region of TbPEX11 contains a cryptic N-terminal signal that directs PEX11 to the mitochondrion if its glycosomal transport is blocked.

Import and quality control of peroxisomal proteins

Peroxisomes are involved in a multitude of metabolic and catabolic pathways, as well as the innate immune system. Their dysfunction is linked to severe peroxisome-specific diseases, as well as cancer and neurodegenerative diseases. To ensure the ability of peroxisomes to fulfill their many roles in the organism, more than 100 different proteins are post-translationally imported into the peroxisomal membrane and matrix, and their functionality must be closely monitored. In this Review, we briefly discuss the import of peroxisomal membrane proteins, and we emphasize an updated view of both classical and alternative peroxisomal matrix protein import pathways. We highlight different quality control pathways that ensure the degradation of dysfunctional peroxisomal proteins. Finally, we compare peroxisomal matrix protein import with other systems that transport folded proteins across membranes, in particular the twin-arginine translocation (Tat) system and the nuclear pore.

The ubiquitin-protein ligase MIEL1 localizes to peroxisomes to promote seedling oleosin degradation and lipid droplet mobilization

Lipid droplets are organelles conserved across eukaryotes that store and release neutral lipids to regulate energy homeostasis. In oilseed plants, fats stored in seed lipid droplets provide fixed carbon for seedling growth before photosynthesis begins. As fatty acids released from lipid droplet triacylglycerol are catabolized in peroxisomes, lipid droplet coat proteins are ubiquitinated, extracted, and degraded. In Arabidopsis seeds, the predominant lipid droplet coat protein is OLEOSIN1 (OLE1). To identify genes modulating lipid droplet dynamics, we mutagenized a line expressing mNeonGreen-tagged OLE1 expressed from the OLE1 promoter and isolated mutants with delayed oleosin degradation. From this screen, we identified four miel1 mutant alleles. MIEL1 (MYB30-interacting E3 ligase 1) targets specific MYB transcription factors for degradation during hormone and pathogen responses [D. Marino et al., Nat. Commun. 4, 1476 (2013); H. G. Lee and P. J. Seo, Nat. Commun. 7, 12525 (2016)] but had not been implicated in lipid droplet dynamics. OLE1 transcript levels were unchanged in miel1 mutants, indicating that MIEL1 modulates oleosin levels posttranscriptionally. When overexpressed, fluorescently tagged MIEL1 reduced oleosin levels, causing very large lipid droplets. Unexpectedly, fluorescently tagged MIEL1 localized to peroxisomes. Our data suggest that MIEL1 ubiquitinates peroxisome-proximal seed oleosins, targeting them for degradation during seedling lipid mobilization. The human MIEL1 homolog (PIRH2; p53-induced protein with a RING-H2 domain) targets p53 and other proteins for degradation and promotes tumorigenesis [A. Daks et al., Cells 11, 1515 (2022)]. When expressed in Arabidopsis, human PIRH2 also localized to peroxisomes, hinting at a previously unexplored role for PIRH2 in lipid catabolism and peroxisome biology in mammals.

Development of novel PEX5-PEX14 protein-protein interaction (PPI) inhibitors based on an oxopiperazine template

Protein-protein interactions (PPIs) constitute an important but challenging class of molecular targets for small molecules. The PEX5-PEX14 PPI has been shown to play a critical role in glycosome biogenesis and its disruption impairs the metabolism in Trpanosoma parasites, eventually leading to their death. Therefore, this PPI is a potential molecular target for new drugs against diseases caused by Trypanosoma infections. Here, we report a new class of peptidomimetic scaffolds to target the PEX5-PEX14 PPI. The molecular design was based on an oxopiperazine template for the α-helical mimetics. A structural simplification along with modifications of the central oxopiperazine scaffold and addressing the lipophilic interactions led to the development of peptidomimetics that inhibit PEX5-TbPEX14 PPI and display cellular activity against T. b. brucei. This approach provides an alternative approach towards the development of trypanocidal agents and may be generally useful for the design of helical mimetics as PPI inhibitors.

Peroxisome Deficiency in Cochlear Hair Cells Causes Hearing Loss by Deregulating BK Channels

The peroxisome is a ubiquitous organelle in rodent cells and plays important roles in a variety of cell types and tissues. It is previously indicated that peroxisomes are associated with auditory function, and patients with peroxisome biogenesis disorders (PBDs) are found to have hearing dysfunction, but the specific role of peroxisomes in hearing remains unclear. In this study, two peroxisome-deficient mouse models (Atoh1-Pex5^-/- and Pax2-Pex5^-/- ) are established and it is found that peroxisomes mainly function in the hair cells of cochleae. Furthermore, peroxisome deficiency-mediated negative effects on hearing do not involve mitochondrial dysfunction and oxidative damage. Although the mammalian target of rapamycin complex 1 (mTORC1) signaling is shown to function through peroxisomes, no changes are observed in the mTORC1 signaling in Atoh1-Pex5^-/- mice when compared to wild-type (WT) mice. However, the expression of large-conductance, voltage-, and Ca2⁺ -activated K⁺ (BK) channels is less in Atoh1-Pex5^-/- mice as compared to the WT mice, and the administration of activators of BK channels (NS-1619 and NS-11021) restores the auditory function in knockout mice. These results suggest that peroxisomes play an essential role in cochlear hair cells by regulating BK channels. Hence, BK channels appear as the probable target for treating peroxisome-related hearing diseases such as PBDs.

Peroxisome biogenesis initiated by protein phase separation

Peroxisomes are organelles that carry out β-oxidation of fatty acids and amino acids. Both rare and prevalent diseases are caused by their dysfunction1. Among disease-causing variant genes are those required for protein transport into peroxisomes. The peroxisomal protein import machinery, which also shares similarities with chloroplasts2, is unique in transporting folded and large, up to 10 nm in diameter, protein complexes into peroxisomes3. Current models postulate a large pore formed by transmembrane proteins4; however, so far, no pore structure has been observed. In the budding yeast Saccharomyces cerevisiae, the minimum transport machinery includes the membrane proteins Pex13 and Pex14 and the cargo-protein-binding transport receptor, Pex5. Here we show that Pex13 undergoes liquid-liquid phase separation (LLPS) with Pex5-cargo. Intrinsically disordered regions in Pex13 and Pex5 resemble those found in nuclear pore complex proteins. Peroxisomal protein import depends on both the number and pattern of aromatic residues in these intrinsically disordered regions, consistent with their roles as 'stickers' in associative polymer models of LLPS5,6. Finally, imaging fluorescence cross-correlation spectroscopy shows that cargo import correlates with transient focusing of GFP-Pex13 and GFP-Pex14 on the peroxisome membrane. Pex13 and Pex14 form foci in distinct time frames, suggesting that they may form channels at different saturating concentrations of Pex5-cargo. Our findings lead us to suggest a model in which LLPS of Pex5-cargo with Pex13 and Pex14 results in transient protein transport channels7.

Improving the hole picture: towards a consensus on the mechanism of nuclear transport

Nuclear pore complexes (NPCs) mediate the exchange of materials between the nucleoplasm and cytoplasm, playing a key role in the separation of nucleic acids and proteins into their required compartments. The static structure of the NPC is relatively well defined by recent cryo-EM and other studies. The functional roles of dynamic components in the pore of the NPC, phenylalanyl-glycyl (FG) repeat rich nucleoporins, is less clear because of our limited understanding of highly dynamic protein systems. These proteins form a 'restrained concentrate' which interacts with and concentrates nuclear transport factors (NTRs) to provide facilitated nucleocytoplasmic transport of cargoes. Very rapid on- and off-rates among FG repeats and NTRs supports extremely fast facilitated transport, close to the rate of macromolecular diffusion in cytoplasm, while complexes without specific interactions are entropically excluded, though details on several aspects of the transport mechanism and FG repeat behaviors remain to be resolved. However, as discussed here, new technical approaches combined with more advanced modeling methods will likely provide an improved dynamic description of NPC transport, potentially at the atomic level in the near future. Such advances are likely to be of major benefit in comprehending the roles the malfunctioning NPC plays in cancer, ageing, viral diseases, and neurodegeneration.

Barrier properties of Nup98 FG phases ruled by FG motif identity and inter-FG spacer length

Nup98 FG repeat domains comprise hydrophobic FG motifs linked through uncharged spacers. FG motifs capture nuclear transport receptors (NTRs) during nuclear pore complex (NPC) passage, confer inter-repeat cohesion, and condense the domains into a selective phase with NPC-typical barrier properties. We show that shortening inter-FG spacers enhances cohesion, increases phase density, and tightens such barrier - all consistent with a sieve-like phase. Phase separation tolerates mutating the Nup98-typical GLFG motifs, provided domain-hydrophobicity remains preserved. NTR-entry, however, is sensitive to (certain) deviations from canonical FG motifs, suggesting co-evolutionary adaptation. Unexpectedly, we observed that arginines promote FG-phase-entry apparently also by hydrophobic interactions/ hydrogen-bonding and not just through cation-π interactions. Although incompatible with NTR·cargo complexes, a YG phase displays remarkable transport selectivity, particularly for engineered GFP^NTR-variants. GLFG to FSFG mutations make the FG phase hypercohesive, precluding NTR-entry. Extending spacers relaxes this hypercohesion. Thus, antagonism between cohesion and NTR·FG interactions is key to transport selectivity.