A role for the dynamin-like protein Vps1 during endocytosis in yeast

Division of Labor among Fission Dynamins Based on Substrate Size

Membrane fission is necessary for the formation of vesicles in the endolysosomal system and for the division of organelles like peroxisomes, mitochondria, and chloroplasts. In these processes, fission is managed by certain members of the dynamin superfamily of proteins (DSPs). These DSPs are soluble proteins that self-assemble into helical scaffolds that hydrolyze GTP and force the constriction of tubular membrane substrates, leading to their fission. Based on where they function, fission DSPs can be operationally categorized into vesicle dynamins (VDs) or organelle dynamins (ODs). Even though they share conserved domains and display largely similar enzymatic properties, recent results reveal fundamental differences with respect to the size of the tubular membrane substrate that certain VDs and ODs can sever. Substrate sizes encountered during vesicle formation and organelle division are quite different and could have served as physical constraints that forced the evolution of VDs and ODs. Here, we briefly review and rationalize mechanisms for the division of labor among DSPs.The structural basis for substrate size-dependent fission activity among VDs and ODs remains unclear and represents an attractive area for future research.

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 homoeostasis are bulwarks to ageing and disease. Here we define a quantitative and ultrastructural timeline of nuclear macroautophagy (nucleophagy) in yeast by leveraging four-dimensional lattice light sheet microscopy and correlative light and electron tomography. Nucleophagy begins with a rapid accumulation of the selective autophagy receptor Atg39 at the nuclear envelope and finishes in ~300 s with Atg39-cargo delivery to the vacuole. Although there are several routes to the vacuole, at least one pathway incorporates two consecutive membrane fission steps: inner nuclear membrane (INM) fission to generate an INM-derived vesicle in the perinuclear space and outer nuclear membrane fission to liberate a double-membraned vesicle to the cytosol. Outer nuclear membrane fission occurs independently of phagophore engagement and instead relies surprisingly on dynamin-like protein 1 (Dnm1), which is recruited to sites of Atg39 accumulation by Atg11. 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 remodelling.

Cystinosin is involved in Na+/H+ Exchanger 3 trafficking in the proximal tubular cells: new insights in the renal Fanconi syndrome in cystinosis

Cystinosis is a systemic lysosomal storage disease resulting from a defective CTNS gene, leading to the accumulation of cystine in all organs. Despite the ubiquitous expression of cystinosin, the renal Fanconi syndrome (FS) is the first manifestation of cystinosis that presents early in life of the patients while other complications appear years later. Additionally, the cystine reduction therapy, cysteamine, does not prevent the FS. While the matter is still unresolved, it is apparent that specific function(s) of cystinosin in the proximal tubular cells (PTCs) beyond cystine transport explain the early tubular defects in cystinosis. Here, we report a novel interaction of cystinosin with the sodium/hydrogen (Na+/H+) exchanger proteins in the endosomes in both yeast and mammalian cells. One isoform of Na+/H+ exchanger, NHE3, is a major absorptive sodium transporter at the apical membrane of the proximal tubules. Cystinosin was found to play a significant role in NHE3 subcellular localization, trafficking, and resulting sodium uptake in PTCs. Interestingly, introduction of CTNS successfully rescued these defects in CTNS-deficient PTCs, whereas CTNS-LKG, the lysosomal and plasma membrane isoform of cystinosin, did not. NHE3 mislocalization was confirmed in Ctns -/- mice and cystinosis patient kidney. Interestingly, transplantation of wild-type hematopoietic stem and progenitor cells in Ctns -/- mice restored NHE3 expression at the brush border membrane. This study uncovers a new role of cystinosin in the trafficking of NHE3 in the PTCs that is evolutionary conserved, offering new insights in the pathogenesis of the renal FS in cystinosis and potential new therapeutic avenue for this pathology.

Antiviral Mx proteins have an ancient origin and widespread distribution among eukaryotes

Mx proteins, first identified in mammals, encode potent antiviral activity against a wide range of viruses. Mx proteins arose within the Dynamin superfamily of proteins (DSP), which mediate critical cellular processes, such as endocytosis and mitochondrial, plastid, and peroxisomal dynamics. Despite their crucial role, the evolutionary origins of Mx proteins are poorly understood. Through comprehensive phylogenomic analyses with progressively expanded taxonomic sampling, we demonstrate that Mx proteins predate the interferon signaling system in vertebrates. Our analyses find an ancient monophyletic DSP lineage in eukaryotes that groups vertebrate and invertebrate Mx proteins with fungal MxF proteins, the largely uncharacterized plant and algal Dynamin 4A/4C proteins, and representatives from several other eukaryotic lineages, suggesting that Mx-like proteins date back close to the origin of Eukarya. Our phylogenetic analyses also find host-encoded and nucleocytoplasmic large DNA viruses-encoded DSPs interspersed in four distinct DSP lineages, indicating recurrent viral theft of host DSPs. Our analyses thus reveal an ancient history of viral and antiviral functions encoded by the Dynamin superfamily in eukaryotes.

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 nuclear macroautophagy (nucleophagy) in yeast. Nucleophagy begins with a rapid accumulation of the selective autophagy receptor Atg39 at the nuclear envelope and finishes in ~300 seconds with Atg39-cargo delivery to the vacuole. Although there are several routes to the vacuole, at least one pathway incorporates two consecutive membrane fission steps: inner nuclear membrane (INM) fission to generate an INM-derived vesicle in the perinuclear space and 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 1 (Dnm1), which is recruited to sites of Atg39 accumulation by Atg11. 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.

Antiviral Mx proteins have an ancient origin and widespread distribution among eukaryotes

First identified in mammals, Mx proteins are potent antivirals against a broad swathe of viruses. Mx proteins arose within the Dynamin superfamily of proteins (DSP), mediating critical cellular processes, such as endocytosis and mitochondrial, plastid, and peroxisomal dynamics. And yet, the evolutionary origins of Mx proteins are poorly understood. Using a series of phylogenomic analyses with stepwise increments in taxonomic coverage, we show that Mx proteins predate the interferon signaling system in vertebrates. Our analyses find an ancient monophyletic DSP lineage in eukaryotes that groups vertebrate and invertebrate Mx proteins with previously undescribed fungal MxF proteins, the relatively uncharacterized plant and algal Dynamin 4A/4C proteins, and representatives from several early-branching eukaryotic lineages. Thus, Mx-like proteins date back close to the origin of Eukarya. Our phylogenetic analyses also reveal that host-encoded and NCLDV (nucleocytoplasmic large DNA viruses)-encoded DSPs are interspersed in four distinct DSP lineages, indicating recurrent viral theft of host DSPs. Our analyses thus reveal an ancient history of viral and antiviral functions encoded by the Dynamin superfamily in eukaryotes.

Role of the dynamin-related protein 2 family and SH3P2 in clathrin-mediated endocytosis in Arabidopsis thaliana

Clathrin-mediated endocytosis (CME) is vital for the regulation of plant growth and development through controlling plasma membrane protein composition and cargo uptake. CME relies on the precise recruitment of regulators for vesicle maturation and release. Homologues of components of mammalian vesicle scission are strong candidates to be part of the scission machinery in plants, but the precise roles of these proteins in this process are not fully understood. Here, we characterised the roles of the plant dynamin-related protein 2 (DRP2) family (hereafter DRP2s) and SH3-domain containing protein 2 (SH3P2), the plant homologue to recruiters of dynamins, such as endophilin and amphiphysin, in CME by combining high-resolution imaging of endocytic events in vivo and characterisation of the purified proteins in vitro. Although DRP2s and SH3P2 arrive similarly late during CME and physically interact, genetic analysis of the sh3p123 triple mutant and complementation assays with non-SH3P2-interacting DRP2 variants suggest that SH3P2 does not directly recruit DRP2s to the site of endocytosis. These observations imply that, despite the presence of many well-conserved endocytic components, plants have acquired a distinct mechanism for CME.

The dynamin Vps1 mediates Atg9 transport to the sites of autophagosome formation

Autophagy is a key process in eukaryotes to maintain cellular homeostasis by delivering cellular components to lysosomes/vacuoles for degradation and reuse of the resulting metabolites. Membrane rearrangements and trafficking events are mediated by the core machinery of autophagy-related (Atg) proteins, which carry out a variety of functions. How Atg9, a lipid scramblase and the only conserved transmembrane protein within this core Atg machinery, is trafficked during autophagy remained largely unclear. Here, we addressed this question in yeast Saccharomyces cerevisiae and found that retromer complex and dynamin Vps1 mutants alter Atg9 subcellular distribution and severely impair the autophagic flux by affecting two separate autophagy steps. We provide evidence that Vps1 interacts with Atg9 at Atg9 reservoirs. In the absence of Vps1, Atg9 fails to reach the sites of autophagosome formation, and this results in an autophagy defect. The function of Vps1 in autophagy requires its GTPase activity. Moreover, Vps1 point mutants associated with human diseases such as microcytic anemia and Charcot-Marie-Tooth are unable to sustain autophagy and affect Atg9 trafficking. Together, our data provide novel insights on the role of dynamins in Atg9 trafficking and suggest that a defect in this autophagy step could contribute to severe human pathologies.

From dynamin related proteins structures and oligomers to membrane fusion mediated by mitofusins

Mitochondria assemble in a highly dynamic network where interconnected tubules evolve in length and size through regulated cycles of fission and fusion of mitochondrial membranes thereby adapting to cellular needs. Mitochondrial fusion and fission processes are mediated by specific sets of mechano-chemical large GTPases that belong to the Dynamin-Related Proteins (DRPs) super family. DRPs bind to cognate membranes and auto-oligomerize to drive lipid bilayers remodeling in a nucleotide dependent manner. Although structural characterization and mechanisms of DRPs that mediate membrane fission are well established, the capacity of DRPs to mediate membrane fusion is only emerging. In this review, we discuss the distinct structures and mechanisms of DRPs that trigger the anchoring and fusion of biological membranes with a specific focus on mitofusins that are dedicated to the fusion of mitochondrial outer membranes. In particular, we will highlight oligomeric assemblies of distinct DRPs and confront their mode of action against existing models of mitofusins assemblies with emphasis on recent biochemical, structural and computational reports. As we will see, the literature brings valuable insights into the presumed macro-assemblies mitofusins may form during anchoring and fusion of mitochondrial outer membranes.

The endocytic protein machinery as an actin-driven membrane-remodeling machine

In clathrin-mediated endocytosis, a principal membrane trafficking route of all eukaryotic cells, forces are applied to invaginate the plasma membrane and form endocytic vesicles. These forces are provided by specific endocytic proteins and the polymerizing actin cytoskeleton. One of the best-studied endocytic systems is endocytosis in yeast, known for its simplicity, experimental amenability, and overall similarity to human endocytosis. Importantly, the yeast endocytic protein machinery generates and transmits tremendous force to bend the plasma membrane, making this system beneficial for mechanistic studies of cellular force-driven membrane reshaping. This review summarizes important protein players, molecular functions, applied forces, and open questions and perspectives of this robust, actin-powered membrane-remodeling protein machine.

Regulation of membrane scission in yeast endocytosisDepartment of Biochemistry and National Centre of Competence in Research, Chemical Biology, University of Geneva, Geneva, Switzerland

During clathrin-mediated endocytosis, a flat plasma membrane is shaped into an invagination that undergoes scission to form a vesicle. In mammalian cells, the force that drives the transition from invagination to vesicle is primarily provided by the GTPase dynamin that acts in concert with crescent-shaped BAR domain proteins. In yeast cells, the mechanism of endocytic scission is unclear. The yeast BAR domain protein complex Rvs161/167 (Rvs) nevertheless plays an important role in this process: deletion of Rvs dramatically reduces scission efficiency. A mechanistic understanding of the influence of Rvs on scission however, remains incomplete. We used quantitative live-cell imaging and genetic manipulation to understand the recruitment and function of Rvs and other late-stage proteins at yeast endocytic sites. We found that arrival of Rvs at endocytic sites is timed by interaction of its BAR domain with specific membrane curvature. A second domain of Rvs167 - the SH3 domain - affects localization efficiency of Rvs. We show that Myo3, one of the two type-I myosins in Saccharomyces cerevisiae, has a role in recruiting Rvs167 via the SH3 domain. Removal of the SH3 domain also affects assembly and disassembly of actin and impedes membrane invagination. Our results indicate that both BAR and SH3 domains are important for the role of Rvs as a regulator of scission. We tested other proteins implicated in vesicle formation in Saccharomyces cerevisiae, and found that neither synaptojanins nor dynamin contribute directly to membrane scission. We propose that recruitment of Rvs BAR domains delays scission and allows invaginations to grow by stabilizing them. We also propose that vesicle formation is dependent on the force exerted by the actin network.

Dictyostelium Dynamin Superfamily GTPases Implicated in Vesicle Trafficking and Host-Pathogen Interactions

The haploid social amoeba Dictyostelium discoideum is a powerful model organism to study vesicle trafficking, motility and migration, cell division, developmental processes, and host cell-pathogen interactions. Dynamin superfamily proteins (DSPs) are large GTPases, which promote membrane fission and fusion, as well as membrane-independent cellular processes. Accordingly, DSPs play crucial roles for vesicle biogenesis and transport, organelle homeostasis, cytokinesis and cell-autonomous immunity. Major progress has been made over the last years in elucidating the function and structure of mammalian DSPs. D. discoideum produces at least eight DSPs, which are involved in membrane dynamics and other processes. The function and structure of these large GTPases has not been fully explored, despite the elaborate genetic and cell biological tools available for D. discoideum. In this review, we focus on the current knowledge about mammalian and D. discoideum DSPs, and we advocate the use of the genetically tractable amoeba to further study the role of DSPs in cell and infection biology. Particular emphasis is put on the virulence mechanisms of the facultative intracellular bacterium Legionella pneumophila.

Candida albicans ENT2 Contributes to Efficient Endocytosis, Cell Wall Integrity, Filamentation, and Virulence

Epsins play a pivotal role in the formation of endocytic vesicles and potentially provide a linkage between endocytic and other trafficking pathways. We identified a Candida albicans epsin, ENT2, that bears homology to the Saccharomyces cerevisiae early endocytosis genes ENT1 and ENT2 and studied its functions by a reverse genetic approach utilizing CRISPR-Cas9-mediated gene deletion. The C. albicans ent2Δ/Δ null mutant displayed cell wall defects and altered antifungal drug sensitivity. To define the role of C. albicans ENT2 in endocytosis, we performed assays with the lipophilic dye FM4-64 that revealed greatly reduced uptake in the ent2Δ/Δ mutant. Next, we showed that the C. albicans ent2Δ/Δ mutant was unable to form hyphae and biofilms. Assays for virulence properties in an in vitro keratinocyte infection model demonstrated reduced damage of mammalian adhesion zippers and host cell death from the ent2Δ/Δ mutant. We conclude that C. albicans ENT2 has a role in efficient endocytosis, a process that is required for maintaining cell wall integrity, hyphal formation, and virulence-defining traits. IMPORTANCE The opportunistic fungal pathogen Candida albicans is an important cause of invasive infections in hospitalized patients and a source of considerable morbidity and mortality. Despite its clinical importance, we still need to improve our ability to diagnose and treat this common pathogen. In order to support these advancements, a greater understanding of the biology of C. albicans is needed. In these studies, we are focused on the fundamental biological process of endocytosis, of which little is directly known in C. albicans. In addition to studying the function of a key gene in this process, we are examining the role of endocytosis in the virulence-related processes of filamentation, biofilm formation, and tissue invasion. These studies will provide greater insight into the role of endocytosis in causing invasive fungal infections.

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.

The Lack of SNARE Protein Homolog Syn8 Influences Biofilm Formation of Candida glabrata

Biofilm formation of Candida species is considered to be a pathogenic factor of host infection. Since biofilm formation of Candida glabrata has not been as well studied as that of Candida albicans, we performed genetic screening of C. glabrata, and three candidate genes associated with biofilm formation were identified. Candida glabrata SYN8 (CAGL0H06325g) was selected as the most induced gene in biofilm cells for further research. Our results indicated that the syn8Δ mutant was defective not only in biofilm metabolic activity but also in biofilm morphological structure and biomass. Deletion of SYN8 seemed to have no effect on extracellular matrix production, but it led to a notable decrease in adhesion ability during biofilm formation, which may be linked to the repression of two adhesin genes, EPA10 and EPA22. Furthermore, hypersensitivity to hygromycin B and various ions in addition to the abnormal vacuolar morphology in the syn8Δ mutant suggested that active vacuolar function is required for biofilm formation of C. glabrata. These findings enhance our understanding of biofilm formation in this fungus and provide information for the development of future clinical treatments.

Emerging Mechanisms of Endocytosis in Toxoplasma gondii

Eukaryotes critically rely on endocytosis of autologous and heterologous material to maintain homeostasis and to proliferate. Although mechanisms of endocytosis have been extensively identified in mammalian and plant systems along with model systems including budding yeast, relatively little is known about endocytosis in protozoan parasites including those belonging to the phylum Apicomplexa. Whereas it has been long established that the apicomplexan agents of malaria (Plasmodium spp.) internalize and degrade hemoglobin from infected red blood cells to acquire amino acids for growth, that the related and pervasive parasite Toxoplasma gondii has a functional and active endocytic system was only recently discovered. Here we discuss emerging and hypothesized mechanisms of endocytosis in Toxoplasma gondii with reference to model systems and malaria parasites. Establishing a framework for potential mechanisms of endocytosis in Toxoplasma gondii will help guide future research aimed at defining the molecular basis and biological relevance of endocytosis in this tractable and versatile parasite.

Syntaxin 5 Is Required for the Formation and Clearance of Protein Inclusions during Proteostatic Stress

Spatial sorting to discrete quality control sites in the cell is a process harnessing the toxicity of aberrant proteins. We show that the yeast t-snare phosphoprotein syntaxin5 (Sed5) acts as a key factor in mitigating proteotoxicity and the spatial deposition and clearance of IPOD (insoluble protein deposit) inclusions associates with the disaggregase Hsp104. Sed5 phosphorylation promotes dynamic movement of COPII-associated Hsp104 and boosts disaggregation by favoring anterograde ER-to-Golgi trafficking. Hsp104-associated aggregates co-localize with Sed5 as well as components of the ER, trans Golgi network, and endocytic vesicles, transiently during proteostatic stress, explaining mechanistically how misfolded and aggregated proteins formed at the vicinity of the ER can hitchhike toward vacuolar IPOD sites. Many inclusions become associated with mitochondria in a HOPS/vCLAMP-dependent manner and co-localize with Vps39 (HOPS/vCLAMP) and Vps13, which are proteins providing contacts between vacuole and mitochondria. Both Vps39 and Vps13 are required also for efficient Sed5-dependent clearance of aggregates.

Genes and lipids that impact uptake and assimilation of exogenous coenzyme Q in Saccharomyces cerevisiae

Coenzyme Q (CoQ) is an essential player in the respiratory electron transport chain and is the only lipid-soluble antioxidant synthesized endogenously in mammalian and yeast cells. In humans, genetic mutations, pathologies, certain medical treatments, and aging, result in CoQ deficiencies, which are linked to mitochondrial, cardiovascular, and neurodegenerative diseases. The only strategy available for these patients is CoQ supplementation. CoQ supplements benefit a small subset of patients, but the poor solubility of CoQ greatly limits treatment efficacy. Consequently, the efficient delivery of CoQ to the mitochondria and restoration of respiratory function remains a major challenge. A better understanding of CoQ uptake and mitochondrial delivery is crucial to make this molecule a more efficient and effective therapeutic tool. In this study, we investigated the mechanism of CoQ uptake and distribution using the yeast Saccharomyces cerevisiae as a model organism. The addition of exogenous CoQ was tested for the ability to restore growth on non-fermentable medium in several strains that lack CoQ synthesis (coq mutants). Surprisingly, we discovered that the presence of CoQ biosynthetic intermediates impairs assimilation of CoQ into a functional respiratory chain in yeast cells. Moreover, a screen of 40 gene deletions considered to be candidates to prevent exogenous CoQ from rescuing growth of the CoQ-less coq2Δ mutant, identified six novel genes (CDC10, RTS1, RVS161, RVS167, VPS1, and NAT3) as necessary for efficient trafficking of CoQ to mitochondria. The proteins encoded by these genes represent essential steps in the pathways responsible for transport of exogenously supplied CoQ to its functional sites in the cell, and definitively associate CoQ distribution with endocytosis and intracellular vesicular trafficking pathways conserved from yeast to human cells.

Hitchhiking on vesicles: a way to harness age-related proteopathies?

Central to proteopathies and leading to most age-related neurodegenerative disorders is a failure in protein quality control (PQC). To harness the toxicity of misfolded and damaged disease proteins, such proteins are either refolded, degraded by temporal PQC, or sequestered by spatial PQC into specific, organelle-associated, compartments within the cell. Here, we discuss the impact of vesicle trafficking pathways in general, and syntaxin 5 in particular, as key players in spatial PQC directing misfolded proteins to the surface of vacuole and mitochondria, which facilitates their clearance and detoxification. Since boosting vesicle trafficking genetically can positively impact on spatial PQC and make cells less sensitive to misfolded disease proteins, we speculate that regulators of such trafficking might serve as therapeutic targets for age-related neurological disorders.

The Role of Secretory Pathways in Candida albicans Pathogenesis

Candida albicans is a fungus that is a commensal organism and a member of the normal human microbiota. It has the ability to transition into an opportunistic invasive pathogen. Attributes that support pathogenesis include secretion of virulence-associated proteins, hyphal formation, and biofilm formation. These processes are supported by secretion, as defined in the broad context of membrane trafficking. In this review, we examine the role of secretory pathways in Candida virulence, with a focus on the model opportunistic fungal pathogen, Candida albicans.

Evolutionarily unique mechanistic framework of clathrin-mediated endocytosis in plants

In plants, clathrin mediated endocytosis (CME) represents the major route for cargo internalisation from the cell surface. It has been assumed to operate in an evolutionary conserved manner as in yeast and animals. Here we report characterisation of ultrastructure, dynamics and mechanisms of plant CME as allowed by our advancement in electron microscopy and quantitative live imaging techniques. Arabidopsis CME appears to follow the constant curvature model and the bona fide CME population generates vesicles of a predominantly hexagonal-basket type; larger and with faster kinetics than in other models. Contrary to the existing paradigm, actin is dispensable for CME events at the plasma membrane but plays a unique role in collecting endocytic vesicles, sorting of internalised cargos and directional endosome movement that itself actively promote CME events. Internalized vesicles display a strongly delayed and sequential uncoating. These unique features highlight the independent evolution of the plant CME mechanism during the autonomous rise of multicellularity in eukaryotes.

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.

Identification of insect genes involved in baculovirus AcMNPV entry into insect cells

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a model enveloped DNA virus that infects and replicates in lepidopteran insect cells, and can efficiently enter a wide variety of non-host cells. Budded virions of AcMNPV enter cells by endocytosis and traffic to the nucleus where the virus initiates gene expression and genome replication. While trafficking of nucleocapsids by actin propulsion has been studied in detail, other important components of trafficking during entry remain poorly understood. We used a recombinant AcMNPV virus expressing an EGFP reporter in combination with an RNAi screen in Drosophila DL1 cells, to identify host proteins involved in AcMNPV entry. The RNAi screen targeted 86 genes involved in vesicular trafficking, including genes coding for VPS and ESCRT proteins, Rab GTPases, Exocyst proteins, and Clathrin adaptor proteins. We identified 24 genes required for efficient virus entry and reporter expression, and 4 genes that appear to restrict virus entry.

BAR scaffolds drive membrane fission by crowding disordered domains

Cylindrical protein scaffolds are thought to stabilize membrane tubules, preventing membrane fission. In contrast, Snead et al. find that when scaffold proteins assemble, bulky disordered domains within them become acutely concentrated, generating steric pressure that destabilizes tubules, driving fission.

Cellular membranes are continuously remodeled. The crescent-shaped bin-amphiphysin-rvs (BAR) domains remodel membranes in multiple cellular pathways. Based on studies of isolated BAR domains in vitro, the current paradigm is that BAR domain–containing proteins polymerize into cylindrical scaffolds that stabilize lipid tubules. But in nature, proteins that contain BAR domains often also contain large intrinsically disordered regions. Using in vitro and live cell assays, here we show that full-length BAR domain–containing proteins, rather than stabilizing membrane tubules, are instead surprisingly potent drivers of membrane fission. Specifically, when BAR scaffolds assemble at membrane surfaces, their bulky disordered domains become crowded, generating steric pressure that destabilizes lipid tubules. More broadly, we observe this behavior with BAR domains that have a range of curvatures. These data suggest that the ability to concentrate disordered domains is a key driver of membrane remodeling and fission by BAR domain–containing proteins.

Role of clathrin-mediated endocytosis in the use of heme and hemoglobin by the fungal pathogen Cryptococcus neoformans

Heme is a major source of iron for pathogens of humans, and its use is critical in determining the outcome of infection and disease. Cryptococcus neoformans is an encapsulated fungal pathogen that causes life-threatening infections in immunocompromised individuals. C. neoformans effectively uses heme as an iron source, but the underlying mechanisms are poorly defined. Non-iron metalloporphyrins (MPPs) are toxic analogues of heme and are thought to enter microbial cells via endogenous heme acquisition systems. We therefore carried out a mutant screen for susceptibility against manganese MPP (MnMPP) to identify new components for heme uptake in C. neoformans. We identified several genes involved in signalling, DNA repair, sugar metabolism, and trafficking that play important roles in susceptibility to MnMPP and in the use of heme as an iron source. We focused on investigating the role of clathrin-mediated endocytosis (CME) and found that several components of CME including Chc1, Las17, Rvs161, and Rvs167 are required for growth on heme and hemoglobin and for endocytosis and intracellular trafficking of these molecules. We show that the hemoglobin uptake process in C. neoformans involves clathrin heavy chain, Chc1, which appears to colocalise with hemoglobin-containing vesicles and to potentially assist in proper delivery of hemoglobin to the vacuole. Additionally, C. neoformans strains lacking Chc1, Las17, Rvs161, or Rvs167 were defective in the elaboration of several key virulence factors, and a las17 mutant was avirulent in a mouse model of cryptococcosis. Overall, this study unveils crucial functions of CME in the use of heme iron by C. neoformans and reveals a role for CME in fungal pathogenesis.

Molecular mechanisms of force production in clathrin-mediated endocytosis

During clathrin-mediated endocytosis (CME), a flat patch of membrane is invaginated and pinched off to release a vesicle into the cytoplasm. In yeast CME, over 60 proteins-including a dynamic actin meshwork-self-assemble to deform the plasma membrane. Several models have been proposed for how actin and other molecules produce the forces necessary to overcome the mechanical barriers of membrane tension and turgor pressure, but the precise mechanisms and a full picture of their interplay are still not clear. In this review, we discuss the evidence for these force production models from a quantitative perspective and propose future directions for experimental and theoretical work that could clarify their various contributions.

Yeast dynamin and Ypt6 function in parallel for the endosome-to-Golgi retrieval of Snc1

Protein recycling is an important cellular process required for cell homeostasis. Results from prior studies have shown that vacuolar sorting protein-1 (Vps1), a dynamin homolog in yeast, is implicated in protein recycling from the endosome to the trans-Golgi Network (TGN). However, the function of Vps1 in relation to Ypt6, a master GTPase in the recycling pathway, remains unknown. The present study reveals that Vps1 physically interacts with Ypt6 if at least one of them is full-length. We found that overexpression of full-length Vps1, but not GTP hydrolysis-defective Vps1 mutants, is sufficient to rescue abnormal phenotypes of Snc1 distribution provoked by the loss of Ypt6, and vice versa. This suggests that Vps1 and Ypt6 function in parallel pathways instead of in a sequential pathway and that GTP binding/hydrolysis of Vps1 is required for proper traffic of Snc1 toward the TGN. Additionally, we identified two novel Vps1-binding partners, Vti1 and Snc2, which function for the endosome-derived vesicle fusion at the TGN. Taken together, the present study demonstrates that Vps1 plays a role in later stages of the endosome-to-TGN traffic.

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.

Stochastic simulations of nanoparticle internalization through transferrin receptor dependent clathrin-mediated endocytosis

BACKGROUND

Receptor dependent clathrin-mediated endocytosis (CME) is one of the most important endocytic pathways for the internalization of bioparticles into cells. During CME, the ligand-receptor interactions, development of clathrin-coated pit (CCP) and membrane evolution all act together to drive the internalization of bioparticles. In this work, we develop a stochastic computational model to investigate the CME based on the Metropolis Monte Carlo simulations.

METHODS

The model is based on the combination of a stochastic particle binding model with a membrane model. The energetic costs of membrane bending, CCP formation and ligand-receptor interactions are systematically linked together.

RESULTS

We implement our model to investigate the effects of particle size, ligand density and membrane stiffness on the overall process of CME from the drug delivery perspectives. Consistent with some experiments, our results show that the intermediate particle size and ligand density favor the particle internalization. Moreover, our results show that it is easier for a particle to enter a cell with softer membrane.

CONCLUSIONS

The model presented here is able to provide mechanistic insights into CME and can be readily modified to include other important factors, such as actins. The predictions from the model will aid in the therapeutic design of intracellular/transcellular drug delivery and antiviral interventions.

Ribosomal DNA status inferred from DNA cloud assays and mass spectrometry identification of agarose-squeezed proteins interacting with chromatin (ASPIC-MS)

Ribosomal RNA-encoding genes (rDNA) are the most abundant genes in eukaryotic genomes. To meet the high demand for rRNA, rDNA genes are present in multiple tandem repeats clustered on a single or several chromosomes and are vastly transcribed. To facilitate intensive transcription and prevent rDNA destabilization, the rDNA-encoding portion of the chromosome is confined in the nucleolus. However, the rDNA region is susceptible to recombination and DNA damage, accumulating mutations, rearrangements and atypical DNA structures. Various sophisticated techniques have been applied to detect these abnormalities. Here, we present a simple method for the evaluation of the activity and integrity of an rDNA region called a “DNA cloud assay”. We verified the efficacy of this method using yeast mutants lacking genes important for nucleolus function and maintenance (RAD52, SGS1, RRM3, PIF1, FOB1 and RPA12). The DNA cloud assay permits the evaluation of nucleolus status and is compatible with downstream analyses, such as the chromosome comet assay to identify DNA structures present in the cloud and mass spectrometry of agarose squeezed proteins (ASPIC-MS) to detect nucleolar DNA-bound proteins, including Las17, the homolog of human Wiskott-Aldrich Syndrome Protein (WASP).

Intersection of endocytic and exocytic systems in Toxoplasma gondii

Host cytosolic proteins are endocytosed by Toxoplasma gondii and degraded in its lysosome-like compartment, the vacuolar compartment (VAC), but the dynamics and route of endocytic trafficking remain undefined. Conserved endocytic components and plant-like features suggest T. gondii endocytic trafficking involves transit through early and late endosome-like compartments (ELCs) and potentially the trans-Golgi network (TGN) as in plants. However, exocytic trafficking to regulated secretory organelles, micronemes and rhoptries, also proceeds through ELCs and requires classical endocytic components, including a dynamin-related protein, DrpB. Here, we show that host cytosolic proteins are endocytosed within 7 minutes post-invasion, trafficked through ELCs en route to the VAC, and degraded within 30 minutes. We could not definitively interpret if ingested protein is trafficked through the TGN. We also found that parasites ingest material from the host cytosol throughout the parasite cell cycle. Ingested host proteins colocalize with immature microneme proteins, proM2AP and proMIC5, in transit to the micronemes, but not with the immature rhoptry protein proRON4, indicating that endocytic trafficking of ingested protein intersects with exocytic trafficking of microneme proteins. Finally, we show that conditional expression of a DrpB dominant negative mutant increases T. gondii ingestion of host-derived proteins, suggesting that DrpB is not required for parasite endocytosis.

Dynamin-Like Proteins Are Potentially Involved in Membrane Dynamics within Chloroplasts and Cyanobacteria

Dynamin-like proteins (DLPs) are a family of membrane-active proteins with low sequence identity. The proteins operate in different organelles in eukaryotic cells, where they trigger vesicle formation, membrane fusion, or organelle division. As discussed here, representatives of this protein family have also been identified in chloroplasts and DLPs are very common in cyanobacteria. Since cyanobacteria and chloroplasts, an organelle of bacterial origin, have similar internal membrane systems, we suggest that DLPs are involved in membrane dynamics in cyanobacteria and chloroplasts. Here, we discuss the features and activities of DLPs with a focus on their potential presence and activity in chloroplasts and cyanobacteria.

Asymmetric Inheritance of Aggregated Proteins and Age Reset in Yeast Are Regulated by Vac17-Dependent Vacuolar Functions

Age can be reset during mitosis in both yeast and stem cells to generate a young daughter cell from an aged and deteriorated one. This phenomenon requires asymmetry-generating genes (AGGs) that govern the asymmetrical inheritance of aggregated proteins. Using a genome-wide imaging screen to identify AGGs in Saccharomyces cerevisiae, we discovered a previously unknown role for endocytosis, vacuole fusion, and the myosin-dependent adaptor protein Vac17 in asymmetrical inheritance of misfolded proteins. Overproduction of Vac17 increases deposition of aggregates into cytoprotective vacuole-associated sites, counteracts age-related breakdown of endocytosis and vacuole integrity, and extends replicative lifespan. The link between damage asymmetry and vesicle trafficking can be explained by a direct interaction between aggregates and vesicles. We also show that the protein disaggregase Hsp104 interacts physically with endocytic vesicle-associated proteins, such as the dynamin-like protein, Vps1, which was also shown to be required for Vac17-dependent sequestration of protein aggregates. These data demonstrate that two physiognomies of aging-reduced endocytosis and protein aggregation-are interconnected and regulated by Vac17.

Membrane scission driven by the PROPPIN Atg18

Sorting, transport, and autophagic degradation of proteins in endosomes and lysosomes, as well as the division of these organelles, depend on scission of membrane-bound tubulo-vesicular carriers. How scission occurs is poorly understood, but family proteins bind these membranes. Here, we show that the yeast PROPPIN Atg18 carries membrane scission activity. Purified Atg18 drives tubulation and scission of giant unilamellar vesicles. Upon membrane contact, Atg18 folds its unstructured CD loop into an amphipathic α-helix that inserts into the bilayer. This allows the protein to engage its two lipid binding sites for PI3P and PI(3,5)P₂ PI(3,5)P₂ induces Atg18 oligomerization, which should concentrate lipid-inserted α-helices in the outer membrane leaflet and drive membrane tubulation and scission. The scission activity of Atg18 is compatible with its known roles in endo-lysosomal protein trafficking, autophagosome biogenesis, and vacuole fission. Key features required for membrane tubulation and scission by Atg18 are shared by other PROPPINs, suggesting that membrane scission may be a generic function of this protein family.

The constriction and scission machineries involved in mitochondrial fission

A key event in the evolution of eukaryotic cells was the engulfment of an aerobic bacterium by a larger anaerobic archaebacterium, leading to a close relationship between the host and the newly formed endosymbiont. Mitochondria, originating from this event, have evolved to be the main place of cellular ATP production. Maintaining elements of their independence, mitochondria undergo growth and division in the cell, thereby ensuring that new daughter cells inherit a mitochondrial complement. Mitochondrial division is also important for other processes, including quality control, mitochondrial (mt)DNA inheritance, transport and cell death. However, unlike bacterial fission, which uses a dynamin-related protein to constrict the membrane at its inner face, mitochondria use dynamin and dynamin-related proteins to constrict the outer membrane from the cytosolic face. In this Review, we summarize the role of proteins from the dynamin superfamily in mitochondrial division. This includes recent findings highlighting that dynamin-2 (Dnm2) is involved in mitochondrial scission, which led to the reappraisal of the role of dynamin-related protein 1 (Drp1; also known as Dnm1l) and its outer membrane adaptors as components of the mitochondrial constriction machinery along with ER components and actin.

The Upsides and Downsides of Organelle Interconnectivity

Interconnectivity and feedback control are hallmarks of biological systems. This includes communication between organelles, which allows them to function and adapt to changing cellular environments. While the specific mechanisms for all communications remain opaque, unraveling the wiring of organelle networks is critical to understand how biological systems are built and why they might collapse, as occurs in aging. A comprehensive understanding of all the routes involved in inter-organelle communication is still lacking, but important themes are beginning to emerge, primarily in budding yeast. These routes are reviewed here in the context of sub-system proteostasis and complex adaptive systems theory.

Yeast dynamin interaction with ESCRT proteins at the endosome

The dynamin-like protein, Vps1, is a GTPase involved in cargo sorting and membrane remodeling in multiple cellular trafficking pathways. Recently, Vps1 has been shown to genetically interact with ESCRT subunits. We tested the hypothesis that the functional connection of Vps1 with some of these subunits of ESCRT complexes occurs via a physical interaction. By utilizing the yeast two-hybrid system, we revealed that Vps1 physically interacts with the ESCRT-II subunits, Vps22 and Vps36, and the ESCRT-III subunit Vps24. We found that Vps1 and ESCRT-II components colocalize with Pep12, an endosomal marker. Additionally, loss of Vps1 or depletion of the GTPase activity of Vps1 results in a moderate defect in Cps1 targeting to the vacuole. Here, we discussed the potential implications of Vps1 and ESCRT interaction and their roles in the endosome-to-vacuole traffic. In summary, yeast dynamin interacts with ESCRT II and III complexes, and it functions in Cps1 trafficking toward the vacuole.

Distinct complexes of yeast Snx4 family SNX-BARs mediate retrograde trafficking of Snc1 and Atg27

The yeast SNX4 sub-family of sorting nexin containing a Bin-Amphiphysin-Rvs domain (SNX-BAR) proteins, Snx4/Atg24, Snx41 and Atg20/Snx42, are required for endocytic recycling and selective autophagy. Here, we show that Snx4 forms 2 functionally distinct heterodimers: Snx4-Atg20 and Snx4-Snx41. Each heterodimer coats an endosome-derived tubule that mediates retrograde sorting of distinct cargo; the v-SNARE, Snc1, is a cargo of the Snx4-Atg20 pathway, and Snx4-Snx41 mediates retrograde sorting of Atg27, an integral membrane protein implicated in selective autophagy. Live cell imaging of individual endosomes shows that Snx4 and the Vps5-Vps17 retromer SNX-BAR heterodimer operate concurrently on a maturing endosome. Consistent with this, the yeast dynamin family protein, Vps1, which was previously shown to promote fission of retromer-coated tubules, promotes fission of Snx4-Atg20 coated tubules. The results indicate that the yeast SNX-BAR proteins coat 3 distinct types of endosome-derived carriers that mediate endosome-to-Golgi retrograde trafficking.

Clathrin-mediated endocytosis in budding yeast at a glance

Clathrin-mediated endocytosis is an essential cellular process that involves the concerted assembly and disassembly of many different proteins at the plasma membrane. In yeast, live-cell imaging has shown that the spatiotemporal dynamics of these proteins is highly stereotypical. Recent work has focused on determining how the timing and functions of endocytic proteins are regulated. In this Cell Science at a Glance article and accompanying poster, we review our current knowledge of the timeline of endocytic site maturation and discuss recent works focusing on how phosphorylation, ubiquitylation and lipids regulate various aspects of the process.

Septin Organization and Functions in Budding Yeast

The septins are a conserved family of GTP-binding proteins present in all eukaryotic cells except plants. They were originally discovered in the baker's yeast Saccharomyces cerevisiae that serves until today as an important model organism for septin research. In yeast, the septins assemble into a highly ordered array of filaments at the mother bud neck. The septins are regulators of spatial compartmentalization in yeast and act as key players in cytokinesis. This minireview summarizes the recent findings about structural features and cell biology of the yeast septins.

Membrane fission by dynamin: what we know and what we need to know

The large GTPase dynamin is the first protein shown to catalyze membrane fission. Dynamin and its related proteins are essential to many cell functions, from endocytosis to organelle division and fusion, and it plays a critical role in many physiological functions such as synaptic transmission and muscle contraction. Research of the past three decades has focused on understanding how dynamin works. In this review, we present the basis for an emerging consensus on how dynamin functions. Three properties of dynamin are strongly supported by experimental data: first, dynamin oligomerizes into a helical polymer; second, dynamin oligomer constricts in the presence of GTP; and third, dynamin catalyzes membrane fission upon GTP hydrolysis. We present the two current models for fission, essentially diverging in how GTP energy is spent. We further discuss how future research might solve the remaining open questions presently under discussion.

Elucidating Key Motifs Required for Arp2/3-Dependent and Independent Actin Nucleation by Las17/WASP

Actin nucleation is the key rate limiting step in the process of actin polymerization, and tight regulation of this process is critical to ensure actin filaments form only at specific times and at defined regions of the cell. Arp2/3 is a well-characterised protein complex that can promote nucleation of new filaments, though its activity requires additional nucleation promotion factors (NPFs). The best recognized of these factors are the WASP family of proteins that contain binding motifs for both monomeric actin and for Arp2/3. Previously we demonstrated that the yeast WASP homologue, Las17, in addition to activating Arp2/3 can also nucleate actin filaments de novo, independently of Arp2/3. This activity is dependent on its polyproline rich region. Through biochemical and in vivo analysis we have now identified key motifs within the polyproline region that are required for nucleation and elongation of actin filaments, and have addressed the role of the WH2 domain in the context of actin nucleation without Arp2/3. We have also demonstrated that full length Las17 is able to bind liposomes giving rise to the possibility of direct linkage of nascent actin filaments to specific membrane sites to which Las17 has been recruited. Overall, we propose that Las17 functions as the key initiator of de novo actin filament formation at endocytic sites by nucleating, elongating and tethering nascent filaments which then serve as a platform for Arp2/3 recruitment and function.

Golgi membrane-associated degradation pathway in yeast and mammals

Autophagy is a cellular process that degrades subcellular constituents, and is conserved from yeast to mammals. Although autophagy is believed to be essential for living cells, cells lacking Atg5 or Atg7 are healthy, suggesting that a non-canonical degradation pathway exists to compensate for the lack of autophagy. In this study, we show that the budding yeast Saccharomyces cerevisiae, which lacks Atg5, undergoes bulk protein degradation using Golgi-mediated structures to compensate for autophagy when treated with amphotericin B1, a polyene antifungal drug. We named this mechanism Golgi membrane-associated degradation (GOMED) pathway. This process is driven by the disruption of PI(4)P-dependent anterograde trafficking from the Golgi, and it also exists in Atg5-deficient mammalian cells. Biologically, when an Atg5-deficient β-cell line and Atg7-deficient β-cells were cultured in glucose-deprived medium, a disruption in the secretion of insulin granules from the Golgi occurred, and GOMED was induced to digest these (pro)insulin granules. In conclusion, GOMED is activated by the disruption of PI(4)P-dependent anterograde trafficking in autophagy-deficient yeast and mammalian cells.

Exclusive expression of the Rab11 effector SH3TC2 in Schwann cells links integrin-α6 and myelin maintenance to Charcot-Marie-Tooth disease type 4C

Charcot-Marie-Tooth disease type 4C (CMT4C) is one of the commonest autosomal recessive inherited peripheral neuropathies and is associated with mutations in the Rab11 effector, SH3TC2. Disruption of the SH3TC2-Rab11 interaction is the molecular abnormality underlying this disease. However, why SH3TC2 mutations cause an isolated demyelinating neuropathy remains unanswered. Here we show that SH3TC2 is an exclusive Schwann cell protein expressed late in myelination and is downregulated following denervation suggesting a functional role in myelin sheath maintenance. We support our data with an evolutionary cell biological analysis showing that the SH3TC2 gene, and its paralogue SH3TC1, are derived from an ancestral homologue, the duplication of which occurred in the common ancestor of jawed vertebrates, coincident with the appearance of Schwann cells and peripheral axon myelination. Furthermore, we report that SH3TC2 associates with integrin-α6, suggesting that aberrant Rab11-dependent endocytic trafficking of this critical laminin receptor in myelinated Schwann cells is connected to the demyelination seen in affected nerves. Our study therefore highlights the inherent evolutionary link between SH3TC2 and peripheral nerve myelination, pointing also towards a molecular mechanism underlying the specific demyelinating neuropathy that characterizes CMT4C.

Yeast lacking the amphiphysin family protein Rvs167 is sensitive to disruptions in sphingolipid levels

Rvs167 and Rvs161 in Saccharomyces cerevisiae are amphiphysin family proteins, which are involved in several important cellular events, such as invagination and scission of endocytic vesicles, and actin cytoskeleton organization. It has been reported that cellular dysfunctions caused by deletion of RVS167 or RVS161 are rescued by deletion of specific nonessential sphingolipid-metabolizing enzyme genes. Here, we found that yeast cells lacking RVS167 or RVS161 exhibit a decrease in sphingolipid levels. In rvs167∆ cells, the expression level of Orm2, a negative regulator of serine palmitoyltransferase (SPT) catalyzing the initial step of sphingolipid biosynthesis, was increased in a calcineurin-dependent manner, and the decrease in sphingolipid levels in rvs167∆ cells was reversed on deletion of ORM2. Moreover, repression of both ORM1 and ORM2 expression or overexpression of SPT caused a strong growth defect of rvs167∆ cells, indicating that enhancement of de novo sphingolipid biosynthesis is detrimental to rvs167∆ cells. In contrast, partial repression of LCB1-encoding SPT suppressed abnormal phenotypes caused by the deletion of RVS167, including supersensitivity to high temperature and salt stress, and impairment of endocytosis and actin cytoskeleton organization. In addition, the partial repression of SPT activity suppressed the temperature supersensitivity and abnormal vacuolar morphology caused by deletion of VPS1 encoding a dynamin-like GTPase, which is required for vesicle scission and is functionally closely related to Rvs167/Rvs161, whereas repression of both ORM1 and ORM2 expression in vps1∆ cells caused a growth defect. Thus, it was suggested that proper regulation of SPT activity is indispensable for amphiphysin-deficient cells.

Insights into dynamin-associated disorders through analysis of equivalent mutations in the yeast dynamin Vps1

The dynamins represent a superfamily of proteins that have been shown to function in a wide range of membrane fusion and fission events. An increasing number of mutations in the human classical dynamins, Dyn-1 and Dyn-2 has been reported, with diseases caused by these changes ranging from Charcot-Marie-Tooth disorder to epileptic encephalopathies. The budding yeast, Saccharomyces cerevisiae expresses a single dynamin-related protein that functions in membrane trafficking, and is considered to play a similar role to Dyn-1 and Dyn-2 during scission of endocytic vesicles at the plasma membrane. Large parts of the dynamin protein are highly conserved across species and this has enabled us in this study to select a number of disease causing mutations and to generate equivalent mutations in Vps1. We have then studied these mutants using both cellular and biochemical assays to ascertain functions of the protein that have been affected by the changes. Specifically, we demonstrate that the Vps1-G397R mutation (Dyn-2 G358R) disrupts protein oligomerization, Vps1-A447T (Dyn-1 A408T) affects the scission stage of endocytosis, while Vps1-R298L (Dyn-1 R256L) affects lipid binding specificity and possibly an early stage in endocytosis. Overall, we consider that the yeast model will potentially provide an avenue for rapid analysis of new dynamin mutations in order to understand the underlying mechanisms that they disrupt