Dynamin-related proteins Vps1p and Dnm1p control peroxisome abundance in Saccharomyces cerevisiae

Effects of bursty synthesis in organelle biogenesis

A fundamental question of cell biology is how cells control the number of organelles. The processes of organelle biogenesis, namely de novo synthesis, fission, fusion, and decay, are inherently stochastic, producing cell-to-cell variability in organelle abundance. In addition, experiments suggest that the synthesis of some organelles can be bursty. We thus ask how bursty synthesis impacts intracellular organelle number distribution. We develop an organelle biogenesis model with bursty de novo synthesis by considering geometrically distributed burst sizes. We analytically solve the model in biologically relevant limits and provide exact expressions for the steady-state organelle number distributions and their means and variances. We also present approximate solutions for the whole model, complementing with exact stochastic simulations. We show that bursts generally increase the noise in organelle numbers, producing distinct signatures in noise profiles depending on different mechanisms of organelle biogenesis. We also find different shapes of organelle number distributions, including bimodal distributions in some parameter regimes. Notably, bursty synthesis broadens the parameter regime of observing bimodality compared to the 'non-bursty' case. Together, our framework utilizes number fluctuations to elucidate the role of bursty synthesis in producing organelle number heterogeneity in cells.

A quantitative ultrastructural timeline of nuclear autophagy reveals a role for dynamin-like protein 1 at the nuclear envelope

Autophagic mechanisms that maintain nuclear envelope homeostasis are bulwarks to aging and disease. By leveraging 4D lattice light sheet microscopy and correlative light and electron tomography, we define a quantitative and ultrastructural timeline of a nuclear macroautophagy (nucleophagy) pathway in yeast. Nucleophagy initiates with a rapid local accumulation of the nuclear cargo adaptor Atg39 at the nuclear envelope adjacent to the nucleus-vacuole junction and is delivered to the vacuole in ~300 seconds through an autophagosome intermediate. Mechanistically, nucleophagy incorporates two consecutive and genetically defined membrane fission steps: inner nuclear membrane (INM) fission generates a lumenal vesicle in the perinuclear space followed by outer nuclear membrane (ONM) fission to liberate a double membraned vesicle to the cytosol. ONM fission occurs independently of phagophore engagement and instead relies surprisingly on dynamin-like protein1 (Dnm1), which is recruited to sites of Atg39 accumulation at the nuclear envelope. Loss of Dnm1 compromises nucleophagic flux by stalling nucleophagy after INM fission. Our findings reveal how nuclear and INM cargo are removed from an intact nucleus without compromising its integrity, achieved in part by a non-canonical role for Dnm1 in nuclear envelope remodeling.

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.

Regulatory Mechanism of Peroxisome Number Reduction Caused by FgPex4 and FgPex22-like Deletion in Fusarium graminearum

Peroxisomes are single-membrane-bound organelles that play critical roles in eukaryotic cellular functions. Peroxisome quantity is a key factor influencing the homeostasis and pathogenic processes of pathogenic fungi. The aim of the present study was to investigate the underlying mechanisms of the reduction in number of peroxisomes in Fusarium graminearum consequent to FgPex4 and FgPex22-like deletion. The number of peroxisomes decreased by 40.55% and 39.70% when FgPex4 and FgPex22-like, respectively, were absent. Peroxisome biogenesis-related proteins, as well as inheritance- and division-related dynamin-like proteins were reduced at the transcriptional level in the mutant strains. In addition, the degree of pexophagy was intensified and the accumulation of ubiquitinated FgPex5 was also increased in F. graminearum when FgPex4 or FgPex22-like was absent. The findings suggest that FgPex4 and FgPex22-like influence the number of peroxisomes by influencing peroxisome biogenesis and pexophagy.

Peroxisomal compartmentalization of amino acid biosynthesis reactions imposes an upper limit on compartment size

Cellular metabolism relies on just a few redox cofactors. Selective compartmentalization may prevent competition between metabolic reactions requiring the same cofactor. Is such compartmentalization necessary for optimal cell function? Is there an optimal compartment size? Here we probe these fundamental questions using peroxisomal compartmentalization of the last steps of lysine and histidine biosynthesis in the fission yeast Schizosaccharomyces japonicus. We show that compartmentalization of these NAD+ dependent reactions together with a dedicated NADH/NAD+ recycling enzyme supports optimal growth when an increased demand for anabolic reactions taxes cellular redox balance. In turn, compartmentalization constrains the size of individual organelles, with larger peroxisomes accumulating all the required enzymes but unable to support both biosynthetic reactions at the same time. Our reengineering and physiological experiments indicate that compartmentalized biosynthetic reactions are sensitive to the size of the compartment, likely due to scaling-dependent changes within the system, such as enzyme packing density.

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.

Spindle Position Checkpoint Kinase Kin4 Regulates Organelle Transport in Saccharomyces cerevisiae

Membrane-bound organelles play important, frequently essential, roles in cellular metabolism in eukaryotes. Hence, cells have evolved molecular mechanisms to closely monitor organelle dynamics and maintenance. The actin cytoskeleton plays a vital role in organelle transport and positioning across all eukaryotes. Studies in the budding yeast Saccharomyces cerevisiae (S. cerevisiae) revealed that a block in actomyosin-dependent transport affects organelle inheritance to daughter cells. Indeed, class V Myosins, Myo2, and Myo4, and many of their organelle receptors, have been identified as key factors in organelle inheritance. However, the spatiotemporal regulation of yeast organelle transport remains poorly understood. Using peroxisome inheritance as a proxy to study actomyosin-based organelle transport, we performed an automated genome-wide genetic screen in S. cerevisiae. We report that the spindle position checkpoint (SPOC) kinase Kin4 and, to a lesser extent, its paralog Frk1, regulates peroxisome transport, independent of their role in the SPOC. We show that Kin4 requires its kinase activity to function and that both Kin4 and Frk1 protect Inp2, the peroxisomal Myo2 receptor, from degradation in mother cells. In addition, vacuole inheritance is also affected in kin4/frk1-deficient cells, suggesting a common regulatory mechanism for actin-based transport for these two organelles in yeast. More broadly our findings have implications for understanding actomyosin-based transport in cells.

Sporadic SNCA mutations A18T and A29S exhibit variable effects on protein aggregation, cell viability and oxidative stress

BACKGROUND

α-synuclein aggregation is the hallmark feature of Parkinson's disease. Both familial and sporadic forms of the disease exhibit this feature. Several mutations have been identified in patients and are associated with the disease pathology.

METHODS AND RESULTS

We have used site-directed mutagenesis to generate α-synuclein mutant variants tagged with GFP. Fluorescence microscopy, flow cytometry, western blotting, cell viability and oxidative stress analysis were performed to investigate the effect of two less studied α-synuclein variants. In this study we characterized two less studied α-synuclein mutations, A18T and A29S, in the well-established yeast model. Our data shows variable expression, distribution and toxicity of the protein in the mutant variants A18T, A29S, A53T and WT. The cells expressing the double mutant variant A18T/A53T showed the most increase in the aggregation phenotype and also depicted reduced viability suggesting a more substantial effect of this variant.

CONCLUSION

The outcome of our study highlights the variable localization, aggregation phenotype and toxicity of the studied α-synuclein variants. This underscores the importance of in-depth analysis of every disease-associated mutation which may result in variable cellular phenotype.

Characterization of the Multiple Domains of Pex30 Involved in Subcellular Localization of the Protein and Regulation of Peroxisome Number

Pex30 is a peroxisomal protein whose role in peroxisome biogenesis via the endoplasmic reticulum has been established. It is a 58 KDa multi-domain protein that facilitates contact site formation between various organelles. The present study aimed to investigate the role of various domains of the protein in its sub-cellular localization and regulation of peroxisome number. For this, we created six truncations of the protein (1-87, 1-250, 1-352, 88-523, 251-523 and 353-523) and tagged GFP at the C-terminus. Biochemical methods and fluorescence microscopy were used to characterize the effect of truncation on expression and localization of the protein. Quantitative analysis was performed to determine the effect of truncation on peroxisome number in these cells. Expression of the truncated variants in cells lacking PEX30 did not cause any effect on cell growth. Interestingly, variable expression and localization of the truncated variants in both peroxisome-inducing and non-inducing medium was observed. Truncated variants depicted different distribution patterns such as punctate, reticulate and cytosolic fluorescence. Interestingly, lack of the complete dysferlin domain or C-Dysf resulted in increased peroxisome number similar to as reported for cells lacking Pex30. No contribution of this domain in the reticulate distribution of the proteins was also observed. Our results show an interesting role for the various domains of Pex30 in localization and regulation of peroxisome number.

Robustness and Universality in Organelle Size Control

One of the grand challenges in cellular biophysics is understanding the precision with which cells assemble and maintain subcellular structures. Organelle sizes, for example, must be flexible enough to allow cells to grow or shrink them as environments demand yet be maintained within homeostatic limits. Despite identification of molecular factors that regulate organelle sizes we lack insight into the quantitative principles underlying organelle size control. Here we show experimentally that cells can robustly control average fluctuations in organelle size. By demonstrating that organelle sizes obey a universal scaling relationship we predict theoretically, our framework suggests that organelles grow in random bursts from a limiting pool of building blocks. Burstlike growth provides a general biophysical mechanism by which cells can maintain on average reliable yet plastic organelle sizes.

Peroxisome biogenesis and inter-organelle communication: an indispensable role for Pex11 and Pex30 family proteins in yeast

Peroxisomes are highly dynamic organelles present in most eukaryotic cells. They also play an important role in human health and the optimum functioning of cells. An extensive repertoire of proteins is associated with the biogenesis and function of these organelles. Two protein families that are involved in regulating peroxisome number in a cell directly or indirectly are Pex11 and Pex30. Interestingly, these proteins are also reported to regulate the contact sites between peroxisomes and other cell organelles such as mitochondria, endoplasmic reticulum and lipid droplets. In this manuscript, we review our current knowledge of the role of these proteins in peroxisome biogenesis in various yeast species. Further, we also discuss in detail the role of these protein families in the regulation of inter-organelle contacts in yeast.

Sharing the wealth: The versatility of proteins targeted to peroxisomes and other organelles

Peroxisomes are eukaryotic organelles with critical functions in cellular energy and lipid metabolism. Depending on the organism, cell type, and developmental stage, they are involved in numerous other metabolic and regulatory pathways. Many peroxisomal functions require factors also relevant to other cellular compartments. Here, we review proteins shared by peroxisomes and at least one different site within the cell. We discuss the mechanisms to achieve dual targeting, their regulation, and functional consequences. Characterization of dual targeting is fundamental to understand how peroxisomes are integrated into the metabolic and regulatory circuits of eukaryotic cells.

Quantitative Characterisation of Low Abundant Yeast Mitochondrial Proteins Reveals Compensation for Haplo-Insufficiency in Different Environments

The quantification of low abundant membrane-binding proteins such as transcriptional factors and chaperones has proven difficult, even with the most sophisticated analytical technologies. Here, we exploit and optimise the non-invasive Fluorescence Correlation Spectroscopy (FCS) for the quantitation of low abundance proteins, and as proof of principle, we choose two interacting proteins involved in the fission of mitochondria in yeast, Fis1p and Mdv1p. In Saccharomyces cerevisiae, the recruitment of Fis1p and Mdv1p to mitochondria is essential for the scission of the organelles and the retention of functional mitochondrial structures in the cell. We use FCS in single GFP-labelled live yeast cells to quantify the protein abundance in homozygote and heterozygote cells and to investigate the impact of the environments on protein copy number, bound/unbound protein state and mobility kinetics. Both proteins were observed to localise predominantly at mitochondrial structures, with the Mdv1p bound state increasing significantly in a strictly respiratory environment. Moreover, a compensatory mechanism that controls Fis1p abundance upon deletion of one allele was observed in Fis1p but not in Mdv1p, suggesting differential regulation of Fis1p and Mdv1p protein expression.

OXPHOS deficiencies affect peroxisome proliferation by downregulating genes controlled by the SNF1 signaling pathway

How environmental cues influence peroxisome proliferation, particularly through organelles, remains largely unknown. Yeast peroxisomes metabolize fatty acids (FA), and methylotrophic yeasts also metabolize methanol. NADH and acetyl-CoA, produced by these pathways enter mitochondria for ATP production and for anabolic reactions. During the metabolism of FA and/or methanol, the mitochondrial oxidative phosphorylation (OXPHOS) pathway accepts NADH for ATP production and maintains cellular redox balance. Remarkably, peroxisome proliferation in Pichia pastoris was abolished in NADH-shuttling- and OXPHOS mutants affecting complex I or III, or by the mitochondrial uncoupler, 2,4-dinitrophenol (DNP), indicating ATP depletion causes the phenotype. We show that mitochondrial OXPHOS deficiency inhibits expression of several peroxisomal proteins implicated in FA and methanol metabolism, as well as in peroxisome division and proliferation. These genes are regulated by the Snf1 complex (SNF1), a pathway generally activated by a high AMP/ATP ratio. In OXPHOS mutants, Snf1 is activated by phosphorylation, but Gal83, its interacting subunit, fails to translocate to the nucleus. Phenotypic defects in peroxisome proliferation observed in the OXPHOS mutants, and phenocopied by the Δgal83 mutant, were rescued by deletion of three transcriptional repressor genes (MIG1, MIG2, and NRG1) controlled by SNF1 signaling. Our results are interpreted in terms of a mechanism by which peroxisomal and mitochondrial proteins and/or metabolites influence redox and energy metabolism, while also influencing peroxisome biogenesis and proliferation, thereby exemplifying interorganellar communication and interplay involving peroxisomes, mitochondria, cytosol, and the nucleus. We discuss the physiological relevance of this work in the context of human OXPHOS deficiencies.

Pex30 undergoes phosphorylation and regulates peroxisome number in Saccharomyces cerevisiae

Pex30 is a dysferlin domain-containing protein whose role in peroxisome biogenesis has been studied by several research groups. Notably, recent studies have linked this protein to peroxisomes, endoplasmic reticulum and lipid bodies in Saccharomyces cerevisiae. Phosphoproteome studies of S. cerevisiae have identified several phosphorylation sites in Pex30. In this study we expressed and purified Pex30 from its native host. Analysis of the purified protein by circular dichroism spectroscopy showed that it retained its secondary structure and revealed primarily a helical structure. Further phosphorylation of Pex30 at three residues, Threonine 60, Serine 61 and Serine 511 was identified by mass spectrometry in this study. To understand the importance of this post-translational modification in peroxisome biogenesis, the identified residues were mutated to both non-phosphorylatable (alanine) and phosphomimetic (aspartic acid) variants. Upon analysis of the mutant variants by fluorescence microscopy, no alteration in the localization of the protein to ER and peroxisomes was observed. Interestingly, reduced number of peroxisomes were observed in cells expressing phosphomimetic mutations when cultured in peroxisome-inducing conditions. Our data suggest that phosphorylation and dephosphorylation of Pex30 may promote distinct interactions essential in regulating peroxisome number in a cell.

Simultaneous Determination and Subcellular Localization of Protein-Protein Interactions in Plant Cells Using Bimolecular Fluorescence Complementation Assay

The bimolecular fluorescence complementation (BiFC) assay allows the visualization of protein-protein interactions in their native state within living systems. The BiFC assay is based on the in vivo complementation of nonfluorescent component parts of a fluorescent protein through the interaction or proximity target proteins, each fused to a different component of the fluorescent protein. Expansion of the BiFC toolkit with an increasing spectrum of fluorescence markers and catalog of Gateway-compatible vectors for high-throughput screening, has made BiFC an exceedingly powerful tool in discovering new protein interactions or providing backup evidence for known ones. Apart from the validation of protein-protein interactions, BiFC offers the additional benefit of providing information on the subcellular localization of protein interaction complexes. Subcellular localization to a specific subcellular compartment or organelle may be further validated by the coexpression of a fluorescence-labeled protein marker. Here we describe an efficient yet simple protocol for simultaneous determination and subcellular localization of protein-protein interactions in plant cells.

Modulation of heterologous protein secretion in the thermotolerant methylotrophic yeast Ogataea thermomethanolica TBRC 656 by CRISPR-Cas9 system

The thermotolerant methylotrophic yeast Ogataea thermomethanolica TBRC 656 is a potential host strain for industrial protein production. Heterologous proteins are often retained intracellularly in yeast resulting in endoplasmic reticulum (ER) stress and poor secretion, and despite efforts to engineer protein secretory pathways, heterologous protein production is often lower than expected. We hypothesized that activation of genes involved in the secretory pathway could mitigate ER stress. In this study, we created mutants defective in protein secretory-related functions using clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) tools. Secretion of the model protein xylanase was significantly decreased in loss of function mutants for oxidative stress (sod1Δ) and vacuolar and protein sorting (vps1Δ and ypt7Δ) genes. However, xylanase secretion was unaffected in an autophagy related atg12Δ mutant. Then, we developed a system for sequence-specific activation of target gene expression (CRISPRa) in O. thermomethanolica and used it to activate SOD1, VPS1 and YPT7 genes. Production of both non-glycosylated xylanase and glycosylated phytase was enhanced in the gene activated mutants, demonstrating that CRISPR-Cas9 systems can be used as tools for understanding O. thermomethanolica genes involved in protein secretion, which could be applied for increasing heterologous protein secretion in this yeast.

A PX-BAR protein Mvp1/SNX8 and a dynamin-like GTPase Vps1 drive endosomal recycling

Membrane protein recycling systems are essential for maintenance of the endosome-lysosome system. In yeast, retromer and Snx4 coat complexes are recruited to the endosomal surface, where they recognize cargos. They sort cargo and deform the membrane into recycling tubules that bud from the endosome and target to the Golgi. Here, we reveal that the SNX-BAR protein, Mvp1, mediates an endosomal recycling pathway that is mechanistically distinct from the retromer and Snx4 pathways. Mvp1 deforms the endosomal membrane and sorts cargos containing a specific sorting motif into a membrane tubule. Subsequently, Mvp1 recruits the dynamin-like GTPase Vps1 to catalyze membrane scission and release of the recycling tubule. Similarly, SNX8, the human homolog of Mvp1, which has been also implicated in Alzheimer’s disease, mediates formation of an endosomal recycling tubule. Thus, we present evidence for a novel endosomal retrieval pathway that is conserved from yeast to humans.

Post-translational modifications of proteins associated with yeast peroxisome membrane: An essential mode of regulatory mechanism

Peroxisomes are single membrane‐bound organelles important for the optimum functioning of eukaryotic cells. Seminal discoveries in the field of peroxisomes are made using yeast as a model. Several proteins required for the biogenesis and function of peroxisomes are identified to date. As with proteins involved in other major cellular pathways, peroxisomal proteins are also subjected to regulatory post‐translational modifications. Identification, characterization and mapping of these modifications to specific amino acid residues on proteins are critical toward understanding their functional significance. Several studies have tried to identify post‐translational modifications of peroxisomal proteins and determine their impact on peroxisome structure and function. In this manuscript, we provide an overview of the various post‐translational modifications that govern the peroxisome dynamics in yeast.

Mimicking human Drp1 disease-causing mutations in yeast Dnm1 reveals altered mitochondrial dynamics

The dynamin-related protein 1 (Drp1) and its homologs in various eukaryotes are essential to maintain mitochondrial morphology and regulate mitochondrial division. Several mutations in different domains of Drp1 have been reported, which result in debilitating conditions. Four such disease-causing mutations of the middle domain of Drp1 were mimicked in the yeast dynamin-related GTPase (Dnm1) and were characterized in this study. Mitochondrial morphology and protein function were observed to be altered to a variable extent in cells expressing the mutated variants of Dnm1. Several aspects related to the protein such as punctate formation, localization to mitochondria, dynamic behavior and structure were analyzed by microscopy, biochemical studies and molecular dynamics simulations. Significant effects on the protein structure and function were observed in cells expressing A430D and G397D mutations. Overall, our data provide insight into the molecular and cellular alterations resulting from middle domain mutations in Dnm1.

Role of CNC1 gene in TDP-43 aggregation-induced oxidative stress-mediated cell death in S. cerevisiae model of ALS

TDP-43 protein is found deposited as inclusions in the amyotrophic lateral sclerosis (ALS) patient's brain. The mechanism of neuron death in ALS is not fully deciphered but several TDP-43 toxicity mechanisms such as mis-regulation of autophagy, mitochondrial impairment and generation of oxidative stress etc., have been implicated. A predominantly nuclear protein, Cyclin C, can regulate the oxidative stress response via transcription of stress response genes and also by translocation to the cytoplasm for the activation of mitochondrial fragmentation-dependent cell death pathway. Using the well-established yeast TDP-43 proteinopathy model, we examined here whether upon TDP-43 aggregation, cell survival depends on the CNC1 gene that encodes the Cyclin C protein or other genes which encode proteins that function in conjunction with Cyclin C, such as DNM1, FIS1 and MED13. We show that the TDP-43's toxicity is significantly reduced in yeast deleted for CNC1 or DNM1 genes and remains unaltered by deletions of genes, FIS1 and MED13. Importantly, this rescue is observed only in presence of functional mitochondria. Also, deletion of the YBH3 gene involved in the mitochondria-dependent apoptosis pathway reduced the TDP-43 toxicity. Deletion of the VPS1 gene involved in the peroxisomal fission pathway did not mitigate the TDP-43 toxicity. Strikingly, Cyclin C-YFP was observed to relocate to the cytoplasm in response to TDP-43's co-expression which was prevented by addition of an anti-oxidant molecule, N-acetyl cysteine. Overall, the Cyclin C, Dnm1 and Ybh3 proteins are found to be important players in the TDP-43-induced oxidative stress-mediated cell death in the S. cerevisiae model.

Characterization of nucleocapsid and matrix proteins of Newcastle disease virus in yeast

Newcastle disease virus is a member of family Paramyxoviridae that infects chicken. Its genome comprises ~15.2 kb negative-sense RNA that encodes six major proteins. The virus encodes various proteins; among all, nucleocapsid (NP) and matrix (M) help in virus replication and its budding from the host cells, respectively. In this study, we investigated the intracellular distribution of NP and M upon expression in the yeast Saccharomyces cerevisiae. We observed nuclear targeting of M, and vacuolar localization of NP was observed in a fraction of yeast cells. Prolonged expression of GFP fused NP or M resulted in altered cell viability and intracellular production of reactive oxygen species in yeast cells. The expression of viral proteins did not alter the morphology and number of the organelles such as nucleus, mitochondria, endoplasmic reticulum, and peroxisomes. However, a significant effect was observed on vacuolar morphology and number in yeast cells. These observations point towards the importance of host cellular reorganization in viral infection. These findings may enable us to understand the conserved pathways affected in eukaryotic cells as a result of viral protein expression.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-020-02624-4.

Balancing the Opposing Principles That Govern Peroxisome Homeostasis

Despite major advances in our understanding of players and mechanisms involved in peroxisome biogenesis and peroxisome degradation, very few studies have focused on unraveling the multi-layered connections between, and the coordination of, these two opposing processes that regulate peroxisome homeostasis. The intersection between these processes also provides exciting avenues for future research. This review highlights the links between peroxisome biogenesis and degradation, incorporating an integrative approach that is critical not only for a mechanistic understanding, but also for manipulating the balance between these processes in relevant disease models.

DNM1, a Dynamin-Related Protein That Contributes to Endocytosis and Peroxisome Fission, Is Required for the Vegetative Growth, Sporulation, and Virulence of Metarhizium robertsii

Although dynamins and dynamin-related proteins (DRPs), a large GTPase superfamily, are involved in the budding of transport vesicles and division of organelles in eukaryotic cells, the function of these proteins in entomopathogenic fungi has not been reported to date. Here, DNM1, a DRP in Metarhizium robertsii, was characterized using gene disruption and complementation strategies. Mutant phenotype assays showed that the ΔDnm1 strain displayed increased defects in radial growth (∼24%) and conidial production (∼42%) compared to those of the wild type (WT), and reduced conidiation levels were accompanied by the repression of several key conidiation-related genes, including flbA, wetA, and flbD Additionally, mutant bioassays revealed that disruption of Dnm1 impaired the virulence (both topical inoculation and injection) of M. robertsii in the insect Galleria mellonella Further analysis demonstrated that deleting Dnm1 in fungi suppressed the transcriptional levels of several virulence genes in the insect hemocoel. Moreover, we found that DNM1 colocalized with peroxisomes and mitochondria. Importantly, disruption of Dnm1 abolished normal fungal endocytosis, resulting in significantly decreased numbers of, as well as morphological changes in, peroxisomes. These findings indicate that deletion of Dnm1 causes significant changes in the vegetative growth, sporulation, and virulence of M. robertsii due to changes in cell function and peroxisomes.IMPORTANCE Dnm1 was found to be involved in fungal development and virulence, mediated peroxisomal fission, and normal endocytosis. This finding provides new insights into the cellular processes and pathogenicity in entomopathogenic fungi.

Molecular Basis of Mitochondrial and Peroxisomal Division Machineries

Mitochondria and peroxisomes are ubiquitous subcellular organelles that are highly dynamic and possess a high degree of plasticity. These organelles proliferate through division of pre-existing organelles. Studies on yeast, mammalian cells, and unicellular algae have led to a surprising finding that mitochondria and peroxisomes share the components of their division machineries. At the heart of the mitochondrial and peroxisomal division machineries is a GTPase dynamin-like protein, Dnm1/Drp1, which forms a contractile ring around the neck of the dividing organelles. During division, Dnm1/Drp1 functions as a motor protein and constricts the membrane. This mechanochemical work is achieved by utilizing energy from GTP hydrolysis. Over the last two decades, studies have focused on the structure and assembly of Dnm1/Drp1 molecules around the neck. However, the regulation of GTP during the division of mitochondrion and peroxisome is not well understood. Here, we review the current understanding of Dnm1/Drp1-mediated divisions of mitochondria and peroxisomes, exploring the mechanisms of GTP regulation during the Dnm1/Drp1 function, and provide new perspectives on their potential contribution to mitochondrial and peroxisomal biogenesis.

Distinct Contributions of the Peroxisome-Mitochondria Fission Machinery During Sexual Development of the Fungus Podospora anserina

Mitochondria and peroxisomes are organelles whose activity is intimately associated and that play fundamental roles in development. In the model fungus Podospora anserina, peroxisomes and mitochondria are required for different stages of sexual development, and evidence indicates that their activity in this process is interrelated. Additionally, sexual development involves precise regulation of peroxisome assembly and dynamics. Peroxisomes and mitochondria share the proteins mediating their division. The dynamin-related protein Dnm1 (Drp1) along with its membrane receptors, like Fis1, drives this process. Here we demonstrate that peroxisome and mitochondrial fission in P. anserina depends on FIS1 and DNM1. We show that FIS1 and DNM1 elimination affects the dynamics of both organelles throughout sexual development in a developmental stage-dependent manner. Moreover, we discovered that the segregation of peroxisomes, but not mitochondria, is affected upon elimination of FIS1 or DNM1 during the division of somatic hyphae and at two central stages of sexual development—the differentiation of meiocytes (asci) and of meiotic-derived spores (ascospores). Furthermore, we found that FIS1 and DNM1 elimination results in delayed karyogamy and defective ascospore differentiation. Our findings reveal that sexual development relies on complex remodeling of peroxisomes and mitochondria, which is driven by their common fission machinery.

Cell-to-cell variability in organelle abundance reveals mechanisms of organelle biogenesis

How cells regulate the number of organelles is a fundamental question in cell biology. While decades of experimental work have uncovered four fundamental processes that regulate organelle biogenesis, namely, de novo synthesis, fission, fusion, and decay, a comprehensive understanding of how these processes together control organelle abundance remains elusive. Recent fluorescence microscopy experiments allow for the counting of organelles at the single-cell level. These measurements provide information about the cell-to-cell variability in organelle abundance in addition to the mean level. Motivated by such measurements, we build upon a recent study and analyze a general stochastic model of organelle biogenesis. We compute the exact analytical expressions for the probability distribution of organelle numbers, their mean, and variance across a population of single cells. It is shown that different mechanisms of organelle biogenesis lead to distinct signatures in the distribution of organelle numbers which allow us to discriminate between these various mechanisms. By comparing our theory against published data for peroxisome abundance measurements in yeast, we show that a widely believed model of peroxisome biogenesis that involves de novo synthesis, fission, and decay is inadequate in explaining the data. Also, our theory predicts bimodality in certain limits of the model. Overall, the framework developed here can be harnessed to gain mechanistic insights into the process of organelle biogenesis.

Peroxisome Maintenance Depends on De Novo Peroxisome Formation in Yeast Mutants Defective in Peroxisome Fission and Inheritance

There is an ongoing debate on how peroxisomes form: by growth and fission of pre-existing peroxisomes or de novo from another membrane. It has been proposed that, in wild type yeast cells, peroxisome fission and careful segregation of the organelles over mother cells and buds is essential for organelle maintenance. Using live cell imaging we observed that cells of the yeast Hansenula polymorpha, lacking the peroxisome fission protein Pex11, still show peroxisome fission and inheritance. Also, in cells of mutants without the peroxisome inheritance protein Inp2 peroxisome segregation can still occur. In contrast, peroxisome fission and inheritance were not observed in cells of a pex11 inp2 double deletion strain. In buds of cells of this double mutant, new organelles likely appear de novo. Growth of pex11 inp2 cells on methanol, a growth substrate that requires functional peroxisomes, is retarded relative to the wild type control. Based on these observations we conclude that in H. polymorpha de novo peroxisome formation is a rescue mechanism, which is less efficient than organelle fission and inheritance to maintain functional peroxisomes.

Mutation of key lysine residues in the Insert B region of the yeast dynamin Vps1 disrupts lipid binding and causes defects in endocytosis

The yeast dynamin-like protein Vps1 has roles at multiple stages of membrane trafficking including Golgi to vacuole transport, endosomal recycling, endocytosis and in peroxisomal fission. While the majority of the Vps1 amino acid sequence shows a high level of identity with the classical mammalian dynamins, it does not contain a pleckstrin homology domain (PH domain). The Dyn1 PH domain has been shown to bind to lipids with a preference for PI(4,5)P2 and it is considered central to the function of Dyn1 in endocytosis. The lack of a PH domain in Vps1 has raised questions as to whether the protein can function directly in membrane fusion or fission events. Here we demonstrate that the region Insert B, located in a position equivalent to the dynamin PH domain, is able to bind directly to lipids and that mutation of three lysine residues reduces its capacity to interact with lipids, and in particular with PI(4,5)P2. The Vps1 KKK-AAA mutant shows more diffuse staining but does still show some localization to compartments adjacent to vacuoles and to endocytic sites suggesting that other factors are also involved in its recruitment. This mutant selectively blocks endocytosis, but is functional in other processes tested. While mutant Vps1 can localise to endocytic sites, the mutation results in a significant increase in the lifetime of the endocytic reporter Sla2 and a high proportion of defective scission events. Together our data indicate that the lipid binding capacity of the Insert B region of Vps1 contributes to the ability of the protein to associate with membranes and that its capacity to interact with PI(4,5)P2 is important in facilitating endocytic scission.

Srv2 Is a Pro-fission Factor that Modulates Yeast Mitochondrial Morphology and Respiration by Regulating Actin Assembly

Dynamic processes such as fusion, fission, and trafficking are important in the regulation of cellular organelles, with an abundant literature focused on mitochondria. Mitochondrial dynamics not only help shape its network within cells but also are involved in the modulation of respiration and integrity. Disruptions of mitochondrial dynamics are associated with neurodegenerative disorders. Although proteins that directly bind mitochondria to promote membrane fusion/fission have been studied intensively, machineries that regulate dynamic mitochondrial processes remain to be explored. We have identified an interaction between the mitochondrial fission GTPase Dnm1/DRP1 and the actin-regulatory protein Srv2/CAP at mitochondria. Deletion of Srv2 causes elongated-hyperfused mitochondria and reduces the reserved respiration capacity in yeast cells. Our results further demonstrate that the irregular network morphology in Δsrv2 cells derives from disrupted actin assembly at mitochondria. We suggest that Srv2 functions as a pro-fission factor in shaping mitochondrial dynamics and regulating activity through its actin-regulatory effects.

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Srv2 interacts with fission protein Dnm1 on mitochondria in yeast cells

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Srv2 deletion causes an irregular, hyperfused-elongated mitochondrial network

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The irregular network derives from loss of Srv2-mediated actin assembly at mitochondria

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Srv2 modulates both mitochondrial dynamics and activity

A bacteria-derived tail anchor localizes to peroxisomes in yeast and mammalian cells

Prokaryotes can provide new genetic information to eukaryotes by horizontal gene transfer (HGT), and such transfers are likely to have been particularly consequential in the era of eukaryogenesis. Since eukaryotes are highly compartmentalized, it is worthwhile to consider the mechanisms by which newly transferred proteins might reach diverse organellar destinations. Toward this goal, we have focused our attention upon the behavior of bacteria-derived tail anchors (TAs) expressed in the eukaryote Saccharomyces cerevisiae. In this study, we report that a predicted membrane-associated domain of the Escherichia coli YgiM protein is specifically trafficked to peroxisomes in budding yeast, can be found at a pre-peroxisomal compartment (PPC) upon disruption of peroxisomal biogenesis, and can functionally replace an endogenous, peroxisome-directed TA. Furthermore, the YgiM(TA) can localize to peroxisomes in mammalian cells. Since the YgiM(TA) plays no endogenous role in peroxisomal function or assembly, this domain is likely to serve as an excellent tool allowing further illumination of the mechanisms by which TAs can travel to peroxisomes. Moreover, our findings emphasize the ease with which bacteria-derived sequences might target to organelles in eukaryotic cells following HGT, and we discuss the importance of flexible recognition of organelle targeting information during and after eukaryogenesis.

The peroxisome: an update on mysteries 2.0

Peroxisomes are key metabolic organelles, which contribute to cellular lipid metabolism, e.g. the β-oxidation of fatty acids and the synthesis of myelin sheath lipids, as well as cellular redox balance. Peroxisomal dysfunction has been linked to severe metabolic disorders in man, but peroxisomes are now also recognized as protective organelles with a wider significance in human health and potential impact on a large number of globally important human diseases such as neurodegeneration, obesity, cancer, and age-related disorders. Therefore, the interest in peroxisomes and their physiological functions has significantly increased in recent years. In this review, we intend to highlight recent discoveries, advancements and trends in peroxisome research, and present an update as well as a continuation of two former review articles addressing the unsolved mysteries of this astonishing organelle. We summarize novel findings on the biological functions of peroxisomes, their biogenesis, formation, membrane dynamics and division, as well as on peroxisome-organelle contacts and cooperation. Furthermore, novel peroxisomal proteins and machineries at the peroxisomal membrane are discussed. Finally, we address recent findings on the role of peroxisomes in the brain, in neurological disorders, and in the development of cancer.

Yeast peroxisomes: How are they formed and how do they grow?

Peroxisomes are single membrane enclosed cell organelles, which are present in almost all eukaryotic cells. In addition to the common peroxisomal pathways such as β-oxidation of fatty acids and decomposition of H₂O₂, these organelles fulfil a range of metabolic and non-metabolic functions. Peroxisomes are very important since various human disorders exist that are caused by a defect in peroxisome function. Here we describe our current knowledge on the molecular mechanisms of peroxisome biogenesis in yeast, including peroxisomal protein sorting, organelle dynamics and peroxisomal membrane contact sites.

Structures of the fungal dynamin-related protein Vps1 reveal a unique, open helical architecture

How specific dynamin-related proteins (DRPs) are tailored to their cellular targets is an open question. Varlakhanova et al. present structures of the fungal DRP Vps1, which functions at the endosomal compartment. The crystal and cryoEM structures reveal a unique DRP architecture that highlights structural flexibilities of DRP self-assembly.

Dynamin-related proteins (DRPs) are large multidomain GTPases required for diverse membrane-remodeling events. DRPs self-assemble into helical structures, but how these structures are tailored to their cellular targets remains unclear. We demonstrate that the fungal DRP Vps1 primarily localizes to and functions at the endosomal compartment. We present crystal structures of a Vps1 GTPase–bundle signaling element (BSE) fusion in different nucleotide states to capture GTP hydrolysis intermediates and concomitant conformational changes. Using cryoEM, we determined the structure of full-length GMPPCP-bound Vps1. The Vps1 helix is more open and flexible than that of dynamin. This is due to further opening of the BSEs away from the GTPase domains. A novel interface between adjacent GTPase domains forms in Vps1 instead of the contacts between the BSE and adjacent stalks and GTPase domains as seen in dynamin. Disruption of this interface abolishes Vps1 function in vivo. Hence, Vps1 exhibits a unique helical architecture, highlighting structural flexibilities of DRP self-assembly.

Peroxisomes: role in cellular ageing and age related disorders

Peroxisomes are dynamic organelles essential for optimum functioning of a eukaryotic cell. Biogenesis of these organelles and the diverse functions performed by them have been extensively studied in the past decade. Their ability to perform functions depending on the cell type and growth conditions is unique and remarkable. Oxidation of fatty acids and reactive oxygen species metabolism are the two most important functions of these ubiquitous organelles. They are often referred to as both source and sink of reactive oxygen species in a cell. Recent research connects peroxisome dysfunction to fatal oxidative damage associated with ageing-related diseases/disorders. It is now widely accepted that mitochondria and peroxisomes are required to maintain oxidative balance in a cell. However, our understanding on the inter-dependence of these organelles to maintain cellular homeostasis of reactive oxygen species is still in its infancy. Herein, we summarize findings that highlight the role of peroxisomes in cellular reactive oxygen species metabolism, ageing and age-related disorders.

Regulation of dynamin family proteins by post-translational modifications

Dynamin superfamily proteins comprising classical dynamins and related proteins are membrane remodelling agents involved in several biological processes such as endocytosis, maintenance of organelle morphology and viral resistance. These large GTPases couple GTP hydrolysis with membrane alterations such as fission, fusion or tubulation by undergoing repeated cycles of self-assembly/disassembly. The functions of these proteins are regulated by various post-translational modifications that affect their GTPase activity, multimerization or membrane association. Recently, several reports have demonstrated variety of such modifications providing a better understanding of the mechanisms by which dynamin proteins influence cellular responses to physiological and environmental cues. In this review, we discuss major post-translational modifications along with their roles in the mechanism of dynamin functions and implications in various cellular processes.

The N-terminal amphipathic helix of Pex11p self-interacts to induce membrane remodelling during peroxisome fission

Pex11p plays a crucial role in peroxisome fission. Previously, it was shown that a conserved N-terminal amphipathic helix in Pex11p, termed Pex11-Amph, was necessary for peroxisomal fission in vivo while in vitro studies revealed that this region alone was sufficient to bring about tubulation of liposomes with a lipid consistency resembling the peroxisomal membrane. However, molecular details of how Pex11-Amph remodels the peroxisomal membrane remain unknown. Here we have combined in silico, in vitro and in vivo approaches to gain insights into the molecular mechanisms underlying Pex11-Amph activity. Using molecular dynamics simulations, we observe that Pex11-Amph peptides form linear aggregates on a model membrane. Furthermore, we identify mutations that disrupted this aggregation in silico, which also abolished the peptide's ability to remodel liposomes in vitro, establishing that Pex11p oligomerisation plays a direct role in membrane remodelling. In vivo studies revealed that these mutations resulted in a strong reduction in Pex11 protein levels, indicating that these residues are important for Pex11p function. Taken together, our data demonstrate the power of combining in silico techniques with experimental approaches to investigate the molecular mechanisms underlying Pex11p-dependent membrane remodelling.

Yeast cells contain a heterogeneous population of peroxisomes that segregate asymmetrically during cell division

Here, we used fluorescence microscopy and a peroxisome-targeted tandem fluorescent protein timer to determine the relative age of peroxisomes in yeast. Our data indicate that yeast cells contain a heterogeneous population of relatively old and young peroxisomes. During budding, the peroxisome retention factor inheritance of peroxisomes protein 1 (Inp1) selectively associates to the older organelles, which are retained in the mother cells. Inp2, a protein required for transport of peroxisomes to the bud, preferentially associates to younger organelles. Using a microfluidics device, we demonstrate that the selective segregation of younger peroxisomes to the buds is carefully maintained during multiple budding events. The replicative lifespan of mother cells increased upon deletion of INP2, which resulted in the retention of all organelles in mother cells. These data suggest that, in wild-type yeast, transport of aged and deteriorated peroxisomes to the bud is prevented, whereas the young and vital organelles are preferably transported to the newly forming buds.

Anilinopyrimidine Resistance in Botrytis cinerea Is Linked to Mitochondrial Function

Crop protection anilinopyrimidine (AP) fungicides were introduced more than 20 years ago for the control of a range of diseases caused by ascomycete plant pathogens, and in particular for the control of gray mold caused by Botrytis cinerea. Although early mode of action studies suggested an inhibition of methionine biosynthesis, the molecular target of this class of fungicides was never fully clarified. Despite AP-specific resistance having been described in B. cinerea field isolates and in multiple other targeted species, the underlying resistance mechanisms were unknown. It was therefore expected that the genetic characterization of resistance mechanisms would permit the identification of the molecular target of these fungicides. In order to explore the widest range of possible resistance mechanisms, AP-resistant B. cinerea UV laboratory mutants were generated and the mutations conferring resistance were determined by combining whole-genome sequencing and reverse genetics. Genetic mapping from a cross between a resistant field isolate and a sensitive reference isolate was used in parallel and led to the identification of an additional molecular determinant not found from the characterized UV mutant collection. Together, these two approaches enabled the characterization of an unrivaled diversity of resistance mechanisms. In total, we report the elucidation of resistance-conferring mutations within nine individual genes, two of which are responsible for almost all instances of AP resistance in the field. All identified resistance-conferring genes encode proteins that are involved in mitochondrial processes, suggesting that APs primarily target the mitochondria. The functions of these genes and their possible interactions are discussed in the context of the potential mode of action for this important class of fungicides.

Pro- and Antioxidant Functions of the Peroxisome-Mitochondria Connection and Its Impact on Aging and Disease

Peroxisomes and mitochondria are the main intracellular sources for reactive oxygen species. At the same time, both organelles are critical for the maintenance of a healthy redox balance in the cell. Consequently, failure in the function of both organelles is causally linked to oxidative stress and accelerated aging. However, it has become clear that peroxisomes and mitochondria are much more intimately connected both physiologically and structurally. Both organelles share common fission components to dynamically respond to environmental cues, and the autophagic turnover of both peroxisomes and mitochondria is decisive for cellular homeostasis. Moreover, peroxisomes can physically associate with mitochondria via specific protein complexes. Therefore, the structural and functional connection of both organelles is a critical and dynamic feature in the regulation of oxidative metabolism, whose dynamic nature will be revealed in the future. In this review, we will focus on fundamental aspects of the peroxisome-mitochondria interplay derived from simple models such as yeast and move onto discussing the impact of an impaired peroxisomal and mitochondrial homeostasis on ROS production, aging, and disease in humans.

Mitochatting - If only we could be a fly on the cell wall

Mitochondria, cellular metabolic hubs, perform many essential processes and are required for the production of metabolites such as ATP, iron-sulfur clusters, heme, amino acids and nucleotides. To fulfill their multiple roles, mitochondria must communicate with all other organelles to exchange small molecules, ions and lipids. Since mitochondria are largely excluded from vesicular trafficking routes, they heavily rely on membrane contact sites. Contact sites are areas of close proximity between organelles that allow efficient transfer of molecules, saving the need for slow and untargeted diffusion through the cytosol. More globally, multiple metabolic pathways require coordination between mitochondria and additional organelles and mitochondrial activity affects all other cellular entities and vice versa. Therefore, uncovering the different means of mitochondrial communication will allow us a better understanding of mitochondria and may illuminate disease processes that occur in the absence of proper cross-talk. In this review we focus on how mitochondria interact with all other organelles and emphasize how this communication is essential for mitochondrial and cellular homeostasis. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

Peroxisomal fission is induced during appressorium formation and is required for full virulence of the rice blast fungus

Peroxisomes are involved in various metabolic processes and are important for virulence in different pathogenic fungi. How peroxisomes rapidly emerge in the appressorium during fungal infection is poorly understood. Here, we describe a gene, PEF1, which can regulate peroxisome formation in the appressorium by controlling peroxisomal fission, and is required for plant infection in the rice blast fungus Magnaporthe oryzae. Targeted deletion of PEF1 resulted in a reduction in virulence and a delay in penetration and invasive growth in host cells. PEF1 was particularly expressed during appressorial development, and its encoding protein was co-localized with peroxisomes during appressorial development. Compared with the massive vesicle-shaped peroxisomes formed in the wild-type appressorium, the Δpef1 mutant could only form stringy linked immature peroxisomes, suggesting that PEF1 was involved in peroxisomal fission during appressorium formation. We also found that the Δpef1 mutant could not utilize fatty acids efficiently, which can improve significantly the expression level of PEF1 and induce peroxisomal fission. As expected, the Δpef1 mutant showed reduced intracellular production of reactive oxygen species (ROS) during appressorium formation and induced ROS accumulation in host cells during infection. Taken together, PEF1-mediated peroxisomal fission is important for fungal infection by controlling the number of peroxisomes in the appressorium.

Evidence for Amino Acid Snorkeling from a High-Resolution, In Vivo Analysis of Fis1 Tail-Anchor Insertion at the Mitochondrial Outer Membrane

Proteins localized to mitochondria by a carboxyl-terminal tail anchor (TA) play roles in apoptosis, mitochondrial dynamics, and mitochondrial protein import. To reveal characteristics of TAs that may be important for mitochondrial targeting, we focused our attention upon the TA of the Saccharomyces cerevisiaeFis1 protein. Specifically, we generated a library of Fis1p TA variants fused to the Gal4 transcription factor, then, using next-generation sequencing, revealed which Fis1p TA mutations inhibited membrane insertion and allowed Gal4p activity in the nucleus. Prompted by our global analysis, we subsequently analyzed the ability of individual Fis1p TA mutants to localize to mitochondria. Our findings suggest that the membrane-associated domain of the Fis1p TA may be bipartite in nature, and we encountered evidence that the positively charged patch at the carboxyl terminus of Fis1p is required for both membrane insertion and organelle specificity. Furthermore, lengthening or shortening of the Fis1p TA by up to three amino acids did not inhibit mitochondrial targeting, arguing against a model in which TA length directs insertion of TAs to distinct organelles. Most importantly, positively charged residues were more acceptable at several positions within the membrane-associated domain of the Fis1p TA than negatively charged residues. These findings, emerging from the first high-resolution analysis of an organelle targeting sequence by deep mutational scanning, provide strong, in vivo evidence that lysine and arginine can “snorkel,” or become stably incorporated within a lipid bilayer by placing terminal charges of their side chains at the membrane interface.

Mechanistic insights into selective autophagy pathways: lessons from yeast

Autophagy has burgeoned rapidly as a field of study because of its evolutionary conservation, the diversity of intracellular cargoes degraded and recycled by this machinery, the mechanisms involved, as well as its physiological relevance to human health and disease. This self-eating process was initially viewed as a non-selective mechanism used by eukaryotic cells to degrade and recycle macromolecules in response to stress; we now know that various cellular constituents, as well as pathogens, can also undergo selective autophagy. In contrast to non-selective autophagy, selective autophagy pathways rely on a plethora of selective autophagy receptors (SARs) that recognize and direct intracellular protein aggregates, organelles and pathogens for specific degradation. Although SARs themselves are not highly conserved, their modes of action and the signalling cascades that activate and regulate them are. Recent yeast studies have provided novel mechanistic insights into selective autophagy pathways, revealing principles of how various cargoes can be marked and targeted for selective degradation.

MoDnm1 Dynamin Mediating Peroxisomal and Mitochondrial Fission in Complex with MoFis1 and MoMdv1 Is Important for Development of Functional Appressorium in Magnaporthe oryzae

Dynamins are large superfamily GTPase proteins that are involved in various cellular processes including budding of transport vesicles, division of organelles, cytokinesis, and pathogen resistance. Here, we characterized several dynamin-related proteins from the rice blast fungus Magnaporthe oryzae and found that MoDnm1 is required for normal functions, including vegetative growth, conidiogenesis, and full pathogenicity. In addition, we found that MoDnm1 co-localizes with peroxisomes and mitochondria, which is consistent with the conserved role of dynamin proteins. Importantly, MoDnm1-dependent peroxisomal and mitochondrial fission involves functions of mitochondrial fission protein MoFis1 and WD-40 repeat protein MoMdv1. These two proteins display similar cellular functions and subcellular localizations as MoDnm1, and are also required for full pathogenicity. Further studies showed that MoDnm1, MoFis1 and MoMdv1 are in complex to regulate not only peroxisomal and mitochondrial fission, pexophagy and mitophagy progression, but also appressorium function and host penetration. In summary, our studies provide new insights into how MoDnm1 interacts with its partner proteins to mediate peroxisomal and mitochondrial functions and how such regulatory events may link to differentiation and pathogenicity in the rice blast fungus.

Dynamin superfamily members are involved in budding of transport vesicles and division of organelles in eukaryotic cells. To further understand how dynamins function in phytopathogenic fungi, we characterized several dynamin-related proteins from the rice blast fungus M. oryzae. In addition to revealing major conserved dynamin functions, we described how MoDnm1 interacts with mitochondrial fission protein MoFis1 and WD repeat adaptor protein MoMdv1 to mediate peroxisomal and mitochondrial fission, pexophagy and mitophagy. Importantly, we provided evidence to demonstrate that MoDnm1-, MoFis1- and MoMdv1-dependent peroxisomal and mitochondrial functions are linked to differentiation and pathogenicity of the rice blast fungus.

Programmed Cell Death Initiation and Execution in Budding Yeast

Apoptosis or programmed cell death (PCD) was initially described in metazoans as a genetically controlled process leading to intracellular breakdown and engulfment by a neighboring cell . This process was distinguished from other forms of cell death like necrosis by maintenance of plasma membrane integrity prior to engulfment and the well-defined genetic system controlling this process. Apoptosis was originally described as a mechanism to reshape tissues during development. Given this context, the assumption was made that this process would not be found in simpler eukaryotes such as budding yeast. Although basic components of the apoptotic pathway were identified in yeast, initial observations suggested that it was devoid of prosurvival and prodeath regulatory proteins identified in mammalian cells. However, as apoptosis became extensively linked to the elimination of damaged cells, key PCD regulatory proteins were identified in yeast that play similar roles in mammals. This review highlights recent discoveries that have permitted information regarding PCD regulation in yeast to now inform experiments in animals.