Fast-evolving cofactors regulate the role of HEATR5 complexes in intra-Golgi trafficking

The highly conserved HEATR5 proteins are best known for their roles in membrane traffic mediated by the adaptor protein complex-1 (AP1). HEATR5 proteins rely on fast-evolving cofactors to bind to AP1. However, how HEATR5 proteins interact with these cofactors is unknown. Here, we report that the budding yeast HEATR5 protein, Laa1, functions in two biochemically distinct complexes. These complexes are defined by a pair of mutually exclusive Laa1-binding proteins, Laa2 and the previously uncharacterized Lft1/Yml037c. Despite limited sequence similarity, biochemical analysis and structure predictions indicate that Lft1 and Laa2 bind Laa1 via structurally similar mechanisms. Both Laa1 complexes function in intra-Golgi recycling. However, only the Laa2-Laa1 complex binds to AP1 and contributes to its localization. Finally, structure predictions indicate that human HEATR5 proteins bind to a pair of fast-evolving interacting partners via a mechanism similar to that observed in yeast. These results reveal mechanistic insight into how HEATR5 proteins bind their cofactors and indicate that Laa1 performs functions besides recruiting AP1.

Two functionally distinct HEATR5 protein complexes are defined by fast-evolving co-factors in yeast

The highly conserved HEATR5 proteins are best known for their roles in membrane traffic mediated by the adaptor protein complex-1 (AP1). HEATR5 proteins rely on fast-evolving co-factors to bind to AP1. However, how HEATR5 proteins interact with these co-factors is unknown. Here, we report that the budding yeast HEATR5 protein, Laa1, functions in two biochemically distinct complexes. These complexes are defined by a pair of mutually exclusive Laa1-binding proteins, Laa2 and the previously uncharacterized Lft1/Yml037c. Despite limited sequence similarity, biochemical analysis and structure predictions indicate that Lft1 and Laa2 bind Laa1 via structurally similar mechanisms. Both Laa1 complexes function in intra-Golgi recycling. However, only the Laa2-Laa1 complex binds to AP1 and contributes to its localization. Finally, structure predictions indicate that human HEATR5 proteins bind to a pair of fast-evolving interacting partners via a mechanism similar to that observed in yeast. These results reveal mechanistic insight into how HEATR5 proteins bind their co-factors and indicate that Laa1 performs functions besides recruiting AP1.

A practical guide to expert learner skills in the research environment

The ability to complete an experiment successfully without mistakes is at the core of the scientific enterprise. Some scientists seem more able to avoid mistakes and missteps than others. In the field of cell biology, their success is often attributed to having naturally good hands. Here I propose that rather than possessing "good hands," an innate and unlearnable aptitude for science, such scientists possess expert learning skills that can be effectively mastered. I share a straightforward approach to gaining and teaching expert learning skills in the research lab environment. I also discuss ongoing efforts by others to develop curricula that teach expert learning skills in laboratory courses.

Monitoring Effects of Membrane Traffic Via Changes in Cell Polarity and Morphogenesis in Three-Dimensional Human Pluripotent Stem Cell Cysts

Membrane traffic at the Golgi and endosomes plays many critical roles in the polarization and the morphogenesis of epithelial tissues. Studies into the roles of traffic in morphogenesis in mammals are often complicated by early embryonic lethality of mutations in membrane traffic as well as the inherent difficulty in imaging developing embryos posed by their size and location. Increasingly, human pluripotent stem cell (hPSC)-derived embryo- and organ-like systems (e.g., embryoids, organoids) provide a useful platform to illuminate the requirements of traffic in human embryonic tissue morphogenesis because these in vitro models are highly amenable to fluorescence microscopy and provide the ability to examine the role of essential genes not possible with animal studies. Here, we present a method to generate hPSC-cysts, a 3-D hPSC-based model of human epiblast lumen formation. This system provides unique opportunities to examine the role of membrane traffic during epithelial morphogenesis. We also present methods to process hPSC-cysts for immunofluorescence and staining with commonly used fluorescence labels useful for detecting defects in polarization and morphogenesis caused by defects in membrane traffic.

Human epiblast lumenogenesis: From a cell aggregate to a lumenal cyst

The formation of a central lumen in the human epiblast is a critical step for development. However, because the lumen forms in the epiblast coincident with implantation, the molecular and cellular events of this early lumenogenesis process cannot be studied in vivo. Recent developments using new model systems have revealed insight into the underpinnings of epiblast formation. To provide an up-to-date comprehensive review of human epiblast lumenogenesis, we highlight recent findings from human and mouse models with an emphasis on new molecular understanding of a newly described apicosome compartment, a novel 'formative' state of pluripotency that coordinates with epiblast polarization, and new evidence about the physical and polarized trafficking mechanisms contributing to lumenogenesis.

Exomer complex regulates protein traffic at the TGN through differential interactions with cargos and clathrin adaptor complexes

Protein sorting at the trans-Golgi network (TGN) usually requires the assistance of cargo adaptors. However, it remains to be examined how the same complex can mediate both the export and retention of different proteins or how sorting complexes interact among themselves. In Saccharomyces cerevisiae, the exomer complex is involved in the polarized transport of some proteins from the TGN to the plasma membrane (PM). Intriguingly, exomer and its cargos also show a sort of functional relationship with TGN clathrin adaptors that is still unsolved. Here, using a wide range of techniques, including time-lapse and BIFC microscopy, we describe new molecular implications of the exomer complex in protein sorting and address its different layers of functional interaction with clathrin adaptor complexes. Exomer mutants show impaired amino acid uptake because it facilitates not only the polarized delivery of amino acid permeases to the PM but also participates in their endosomal traffic. We propose a model for exomer where it modulates the recruitment of TGN clathrin adaptors directly or indirectly through the Arf1 function. Moreover, we describe an in vivo competitive relationship between the exomer and AP-1 complexes for the model cargo Chs3. These results highlight a broad role for exomer in regulating protein sorting at the TGN that is complementary to its role as cargo adaptor and present a model to understand the complexity of TGN protein sorting.

Spatially resolved cell polarity proteomics of a human epiblast model

Critical early steps in human embryonic development include polarization of the inner cell mass, followed by formation of an expanded lumen that will become the epiblast cavity. Recently described three-dimensional (3D) human pluripotent stem cell-derived cyst (hPSC-cyst) structures can replicate these processes. To gain mechanistic insights into the poorly understood machinery involved in epiblast cavity formation, we interrogated the proteomes of apical and basolateral membrane territories in 3D human hPSC-cysts. APEX2-based proximity bioinylation, followed by quantitative mass spectrometry, revealed a variety of proteins without previous annotation to specific membrane subdomains. Functional experiments validated the requirement for several apically enriched proteins in cyst morphogenesis. In particular, we found a key role for the AP-1 clathrin adaptor complex in expanding the apical membrane domains during lumen establishment. These findings highlight the robust power of this proximity labeling approach for discovering novel regulators of epithelial morphogenesis in 3D stem cell-based models.

Plasma membrane to vacuole traffic induced by glucose starvation requires Gga2-dependent sorting at the trans-Golgi network

BACKGROUND INFORMATION

In the yeast Saccharomyces cerevisiae, acute glucose starvation induces rapid endocytosis followed by vacuolar degradation of many plasma membrane proteins. This process is essential for cell viability, but the regulatory mechanisms that control it remain poorly understood. Under normal growth conditions, a major regulatory decision for endocytic cargo occurs at the trans-Golgi network (TGN) where proteins can recycle back to the plasma membrane or can be recognized by TGN-localised clathrin adaptors that direct them towards the vacuole. However, glucose starvation reduces recycling and alters the localization and post-translational modification of TGN-localised clathrin adaptors. This raises the possibility that during glucose starvation endocytosed proteins are routed to the vacuole by a novel mechanism that bypasses the TGN or does not require TGN-localised clathrin adaptors.

RESULTS

Here, we investigate the role of TGN-localised clathrin adaptors in the traffic of several amino acid permeases, including Can1, during glucose starvation. We find that Can1 transits through the TGN after endocytosis in both starved and normal conditions. Can1 and other amino acid permeases require TGN-localised clathrin adaptors for maximal delivery to the vacuole. Furthermore, these permeases are actively sorted to the vacuole, because ectopically forced de-ubiquitination at the TGN results in the recycling of the Tat1 permase in starved cells. Finally, we report that the Mup1 permease requires the clathrin adaptor Gga2 for vacuolar delivery. In contrast, the clathrin adaptor protein complex AP-1 plays a minor role, potentially in retaining permeases in the TGN, but it is otherwise dispensable for vacuolar delivery.

CONCLUSIONS AND SIGNIFICANCE

This work elucidates one membrane trafficking pathway needed for yeast to respond to acute glucose starvation. It also reveals the functions of TGNlocalised clathrin adaptors in this process. Our results indicate that the same machinery is needed for vacuolar protein sorting at the GN in glucose starved cells as is needed in the presence of glucose. In addition, our findings provide further support for the model that the TGN is a transit point for many endocytosed proteins, and that Gga2 and AP-1 function in distinct pathways at the TGN.

Plasma membrane tension regulates eisosome structure and function

Eisosomes are membrane furrows at the cell surface of yeast that have been shown to function in two seemingly distinct pathways, membrane stress response and regulation of nutrient transporters. We found that many stress conditions affect both of these pathways by changing plasma membrane tension and thus the morphology and composition of eisosomes. For example, alkaline stress causes swelling of the cell and an endocytic response, which together increase membrane tension, thereby flattening the eisosomes. The flattened eisosomes affect membrane stress pathways and release nutrient transporters, which aids in their down-regulation. In contrast, glucose starvation or hyperosmotic shock causes cell shrinking, which results in membrane slack and the deepening of eisosomes. Deepened eisosomes are able to trap nutrient transporters and protect them from rapid endocytosis. Therefore, eisosomes seem to coordinate the regulation of both membrane tension and nutrient transporter stability.

Adaptor protein complex-1 (AP-1) is recruited by the HEATR5 protein Laa1 and its co-factor Laa2 in yeast

Cellular membrane trafficking mediated by the clathrin adaptor protein complex-1 (AP-1) is important for the proper composition and function of organelles of the endolysosomal system. Normal AP-1 function requires proteins of the HEAT repeat-containing 5 (HEATR5) family. Although HEATR5 proteins were first identified based on their ability to interact with AP-1, the functional significance of this interaction was unknown. We used bioinformatics-based phenotypic profiling and information from genome-wide fluorescence microscopy studies in the budding yeast Saccharomyces cerevisiae to identify a protein, Laa2, that mediates the interaction between AP-1 and the yeast HEATR5 protein Laa1. Further characterization of Laa2 revealed that it binds to both Laa1 and AP-1. Laa2 contains a motif similar to the characterized γ-ear-binding sites found in other AP-1-binding proteins. This motif in Laa2 is essential for the Laa1-AP-1 interaction. Moreover, mutation of this motif disrupted AP-1 localization and function and caused effects similar to mutations that remove the γ-ear of AP-1. These results indicate that Laa2 mediates the interaction between Laa1 and AP-1 and reveal that this interaction promotes the stable association of AP-1 with membranes in yeast.

A simple and inexpensive quantitative technique for determining chemical sensitivity in Saccharomyces cerevisiae

Chemical sensitivity, growth inhibition in response to a chemical, is a powerful phenotype that can reveal insight into diverse cellular processes. Chemical sensitivity assays are used in nearly every model system, however the yeast Saccharomyces cerevisiae provides a particularly powerful platform for discovery and mechanistic insight from chemical sensitivity assays. Here we describe a simple and inexpensive approach to determine chemical sensitivity quantitatively in yeast in the form of half maximal inhibitory concentration (IC50) using common laboratory equipment. We demonstrate the utility of this method using chemicals commonly used to monitor changes in membrane traffic. When compared to traditional agar-based plating methods, this method is more sensitive and can detect defects not apparent using other protocols. Additionally, this method reduces the experimental protocol from five days to 18 hours for the toxic amino acid canavanine. Furthermore, this method provides reliable results using lower amounts of chemicals. Finally, this method is easily adapted to additional chemicals as demonstrated with an engineered system that activates the spindle assembly checkpoint in response to rapamycin with differing efficiencies. This approach provides researchers with a cost-effective method to perform chemical genetic profiling without specialized equipment.

Clathrin binding by the adaptor Ent5 promotes late stages of clathrin coat maturation

Clathrin adaptors link cargo to the clathrin coat. The clathrin adaptor Ent5 is also required for the maturation of clathrin coats at the trans-Golgi network or endosome, suggesting that it plays a key mechanistic role in coat formation. This function requires only the Ent5 clathrin-binding sites and not its interaction with other endosomal adaptors.

Clathrin is a ubiquitous protein that mediates membrane traffic at many locations. To function, clathrin requires clathrin adaptors that link it to transmembrane protein cargo. In addition to this cargo selection function, many adaptors also play mechanistic roles in the formation of the transport carrier. However, the full spectrum of these mechanistic roles is poorly understood. Here we report that Ent5, an endosomal clathrin adaptor in Saccharomyces cerevisiae, regulates the behavior of clathrin coats after the recruitment of clathrin. We show that loss of Ent5 disrupts clathrin-dependent traffic and prolongs the lifespan of endosomal structures that contain clathrin and other adaptors, suggesting a defect in coat maturation at a late stage. We find that the direct binding of Ent5 with clathrin is required for its role in coat behavior and cargo traffic. Surprisingly, the interaction of Ent5 with other adaptors is dispensable for coat behavior but not cargo traffic. These findings support a model in which Ent5 clathrin binding performs a mechanistic role in coat maturation, whereas Ent5 adaptor binding promotes cargo incorporation.

Transcription errors induce proteotoxic stress and shorten cellular lifespan

Transcription errors occur in all living cells; however, it is unknown how these errors affect cellular health. To answer this question, we monitor yeast cells that are genetically engineered to display error-prone transcription. We discover that these cells suffer from a profound loss in proteostasis, which sensitizes them to the expression of genes that are associated with protein-folding diseases in humans; thus, transcription errors represent a new molecular mechanism by which cells can acquire disease phenotypes. We further find that the error rate of transcription increases as cells age, suggesting that transcription errors affect proteostasis particularly in aging cells. Accordingly, transcription errors accelerate the aggregation of a peptide that is implicated in Alzheimer's disease, and shorten the lifespan of cells. These experiments reveal a previously unappreciated role for transcriptional fidelity in cellular health and aging.

Glucose starvation inhibits autophagy via vacuolar hydrolysis and induces plasma membrane internalization by down-regulating recycling

Cellular energy influences all aspects of cellular function. Although cells can adapt to a gradual reduction in energy, acute energy depletion poses a unique challenge. Because acute depletion hampers the transport of new energy sources into the cell, the cell must use endogenous substrates to replenish energy after acute depletion. In the yeast Saccharomyces cerevisiae, glucose starvation causes an acute depletion of intracellular energy that recovers during continued glucose starvation. However, how the cell replenishes energy during the early phase of glucose starvation is unknown. In this study, we investigated the role of pathways that deliver proteins and lipids to the vacuole during glucose starvation. We report that in response to glucose starvation, plasma membrane proteins are directed to the vacuole through reduced recycling at the endosomes. Furthermore, we found that vacuolar hydrolysis inhibits macroautophagy in a target of rapamycin complex 1-dependent manner. Accordingly, we found that endocytosis and hydrolysis are required for survival in glucose starvation, whereas macroautophagy is dispensable. Together, these results suggest that hydrolysis of components delivered to the vacuole independent of autophagy is the cell survival mechanism used by S. cerevisiae in response to glucose starvation.

A phosphatidylinositol transfer protein integrates phosphoinositide signaling with lipid droplet metabolism to regulate a developmental program of nutrient stress-induced membrane biogenesis

Lipid droplet (LD) utilization is an important cellular activity that regulates energy balance and release of lipid second messengers. Because fatty acids exhibit both beneficial and toxic properties, their release from LDs must be controlled. Here we demonstrate that yeast Sfh3, an unusual Sec14-like phosphatidylinositol transfer protein, is an LD-associated protein that inhibits lipid mobilization from these particles. We further document a complex biochemical diversification of LDs during sporulation in which Sfh3 and select other LD proteins redistribute into discrete LD subpopulations. The data show that Sfh3 modulates the efficiency with which a neutral lipid hydrolase-rich LD subclass is consumed during biogenesis of specialized membrane envelopes that package replicated haploid meiotic genomes. These results present novel insights into the interface between phosphoinositide signaling and developmental regulation of LD metabolism and unveil meiosis-specific aspects of Sfh3 (and phosphoinositide) biology that are invisible to contemporary haploid-centric cell biological, proteomic, and functional genomics approaches.

The actin-microtubule cross-linking activity of Drosophila Short stop is regulated by intramolecular inhibition

The authors investigated the regulation of the Drosophila actin-microtubule cross-linker Short stop (Shot) and found that Shot undergoes an intramolecular conformational change that regulates its cross-linking activity. This intramolecular interaction depends on Shot's NH2-terminal actin-binding domain and EF-hand-GAS2 domain.

Actin and microtubule dynamics must be precisely coordinated during cell migration, mitosis, and morphogenesis—much of this coordination is mediated by proteins that physically bridge the two cytoskeletal networks. We have investigated the regulation of the Drosophila actin-microtubule cross-linker Short stop (Shot), a member of the spectraplakin family. Our data suggest that Shot's cytoskeletal cross-linking activity is regulated by an intramolecular inhibitory mechanism. In its inactive conformation, Shot adopts a “closed” conformation through interactions between its NH₂-terminal actin-binding domain and COOH-terminal EF-hand-GAS2 domain. This inactive conformation is targeted to the growing microtubule plus end by EB1. On activation, Shot binds along the microtubule through its COOH-terminal GAS2 domain and binds to actin with its NH₂-terminal tandem CH domains. We propose that this mechanism allows Shot to rapidly cross-link dynamic microtubules in response to localized activating signals at the cell cortex.

Energy metabolism regulates clathrin adaptors at the trans-Golgi network and endosomes

Glucose is a master regulator of cell behavior in the yeast Saccharomyces cerevisiae. It acts as both a metabolic substrate and a potent regulator of intracellular signaling cascades. Glucose starvation induces the transient delocalization and then partial relocalization of clathrin adaptors at the trans-Golgi network and endosomes. Although these localization responses are known to depend on the protein kinase A (PKA) signaling pathway, the molecular mechanism of this regulation is unknown. Here we demonstrate that PKA and the AMP-regulated kinase regulate adaptor localization through changes in energy metabolism. We show that genetic and chemical manipulation of intracellular ATP levels cause corresponding changes in adaptor localization. In permeabilized cells, exogenous ATP is sufficient to induce adaptor localization. Furthermore, we reveal distinct energy-dependent steps in adaptor localization: a step that requires the ADP-ribosylation factor ARF, an ATP-dependent step that requires the phosphatidyl-inositol-4 kinase Pik1, and third ATP-dependent step for which we provide evidence but for which the mechanism is unknown. We propose that these energy-dependent mechanisms precisely synchronize membrane traffic with overall proliferation rates and contribute a crucial aspect of energy conservation during acute glucose starvation.

Adaptor autoregulation promotes coordinated binding within clathrin coats

Membrane traffic is an essential process that allows protein and lipid exchange between the endocytic, lysosomal, and secretory compartments. Clathrin-mediated traffic between the trans-Golgi network and endosomes mediates responses to the environment through the sorting of biosynthetic and endocytic protein cargo. Traffic through this pathway is initiated by the controlled assembly of a clathrin-adaptor protein coat on the cytosolic surface of the originating organelle. In this process, clathrin is recruited by different adaptor proteins that act as a bridge between clathrin and the transmembrane cargo proteins to be transported. Interactions between adaptors and clathrin and between different types of adaptors lead to the formation of a densely packed protein network within the coat. A key unresolved issue is how the highly complex adaptor-clathrin interaction and adaptor-adaptor interaction landscape lead to the correct spatiotemporal assembly of the clathrin coat. Here we report the discovery of a new autoregulatory motif within the clathrin adaptor Gga2 that drives synergistic binding of Gga2 to clathrin and the adaptor Ent5. This autoregulation influences the temporal and/or spatial location of the Gga2-Ent5 interaction. We propose that this synergistic binding provides built-in regulation to ensure the correct assembly of clathrin coats.

Glucose regulates clathrin adaptors at the trans-Golgi network and endosomes

Traffic at the trans-Golgi network (TGN) and endosomes is regulated by glucose via an unknown mechanism that depends on protein kinase A (PKA). TGN–endosomal clathrin adaptors exhibit specific responses to glucose starvation that likely are coordinated with other cell behaviors regulated by PKA.

Glucose is a rich source of energy and the raw material for biomass increase. Many eukaryotic cells remodel their physiology in the presence and absence of glucose. The yeast Saccharomyces cerevisiae undergoes changes in transcription, translation, metabolism, and cell polarity in response to glucose availability. Upon glucose starvation, translation initiation and cell polarity are immediately inhibited, and then gradually recover. In this paper, we provide evidence that, as in cell polarity and translation, traffic at the trans-Golgi network (TGN) and endosomes is regulated by glucose via an unknown mechanism that depends on protein kinase A (PKA). Upon glucose withdrawal, clathrin adaptors exhibit a biphasic change in localization: they initially delocalize from the membrane within minutes and later partially recover onto membranes. Additionally, the removal of glucose induces changes in posttranslational modifications of adaptors. Ras and Gpr1 signaling pathways, which converge on PKA, are required for changes in adaptor localization and changes in posttranslational modifications. Acute inhibition of PKA demonstrates that inhibition of PKA prior to glucose withdrawal prevents several adaptor responses to starvation. This study demonstrates that PKA activity prior to glucose starvation primes membrane traffic at the TGN and endosomes in response to glucose starvation.

Regulating polarity by directing traffic: Cdc42 prevents adherens junctions from crumblin' aPart

The GTPase Cdc42 was among the original genes identified with roles in cell polarity, and interest in its cellular roles from yeast to humans remains high. Cdc42 is a well-known regulator of the actin cytoskeleton, but also plays important roles in vesicular trafficking. In this issue, Harris and Tepass (Harris, K.P, and U. Tepass. 2008. J. Cell. Biol. 183:1129–1143) provide new insights into how Cdc42 and Par proteins work together to modulate cell adhesion and polarity during embryonic morphogenesis by regulating the traffic of key cell junction proteins.

Composite synthetic lethal identification of membrane traffic inhibitors

Small molecule inhibitors provide powerful tools to characterize highly dynamic and complex eukaryotic cell pathways such as those mediating membrane traffic. However, a lack of easy and generalizable assays has constrained identification of novel inhibitors despite availability of diverse chemical libraries. Here, we report a facile growth-based strategy in yeast to screen for pathway-specific inhibitors. The approach uses well characterized synthetic genetic growth defects to guide design of cells genetically sensitized for inhibition of chosen pathways. With this strategy, we identified a family of piperazinyl phenylethanone compounds as inhibitors of traffic between the trans-Golgi network (TGN) and endosomes that depends on the clathrin adaptor complex AP-1. The compounds did not significantly alter other trafficking pathways involving the TGN or endosomes, indicating specificity. Compound treatment also altered localization of AP-1 in mammalian cells. These previously uncharacterized inhibitors will be useful for future studies of clathrin-mediated transport in yeast, and potentially in other organisms. Furthermore, the easily automated technology should be adaptable for identification of inhibitors of other cellular processes.

Negative regulation of yeast Eps15-like Arp2/3 complex activator, Pan1p, by the Hip1R-related protein, Sla2p, during endocytosis

Control of actin assembly nucleated by the Arp2/3 complex plays a crucial role during budding yeast endocytosis. The yeast Eps15-related Arp2/3 complex activator, Pan1p, is essential for endocytic internalization and proper actin organization. Pan1p activity is negatively regulated by Prk1 kinase phosphorylation after endocytic internalization. Phosphorylated Pan1p is probably then dephosphorylated in the cytosol. Pan1p is recruited to endocytic sites approximately 25 s before initiation of actin polymerization, suggesting that its Arp2/3 complex activation activity is kept inactive during early stages of endocytosis by a yet-to-be-identified mechanism. However, how Pan1p is maintained in an inactive state is not clear. Using tandem affinity purification-tagged Pan1p, we identified End3p as a stoichiometric component of the Pan1p complex, and Sla2p, a yeast Hip1R-related protein, as a novel binding partner of Pan1p. Interestingly, Sla2p specifically inhibited Pan1p Arp2/3 complex activation activity in vitro. The coiled-coil region of Sla2p was important for Pan1p inhibition, and a pan1 partial loss-of-function mutant suppressed the temperature sensitivity, endocytic phenotypes, and actin phenotypes observed in sla2DeltaCC mutant cells that lack the coiled-coil region. Overall, our results establish that Sla2p's regulation of Pan1p plays an important role in controlling Pan1p-stimulated actin polymerization during endocytosis.

Distinct roles for TGN/endosome epsin-like adaptors Ent3p and Ent5p

Clathrin adaptors are key factors in clathrin-coated vesicle formation, coupling clathrin to cargo and/or the lipid bilayer. A physically interacting network of three classes of adaptors participate in clathrin-mediated traffic between the trans-Golgi network (TGN) and endosomes: AP-1, Gga proteins, and epsin-like proteins. Here we investigate functional relationships within this network through transport assays and protein localization analysis in living yeast cells. We observed that epsin-like protein Ent3p preferentially localized with Gga2p, whereas Ent5p distributed equally between AP-1 and Gga2p. Ent3p was mislocalized in Gga-deficient but not in AP-1-deficient cells. In contrast, Ent5p retained localization in cells lacking either or both AP-1 and Gga proteins. The Ent proteins were dispensable for AP-1 or Gga localization. Synthetic genetic growth and alpha-factor maturation defects were observed when ent5Delta but not ent3Delta was introduced together with deletions of the GGA genes. In AP-1-deficient cells, ent3Delta and to a lesser extent ent5Delta caused minor alpha-factor maturation defects, but together resulted in a near-lethal phenotype. Deletions of ENT3 and ENT5 also displayed synthetic defects similar to, but less severe than, synthetic effects of AP-1 and Gga inactivation. These results differentiate Ent3p and Ent5p function in vivo, suggesting that Ent3p acts primarily with Gga proteins, whereas Ent5p acts with both AP-1 and Gga proteins but is more critical for AP-1-mediated transport. The data also support a model in which the Ent adaptors provide important accessory functions to AP-1 and Gga proteins in TGN/endosome traffic.

Insights into TOR function and rapamycin response: chemical genomic profiling by using a high-density cell array method

With the advent of complete genome sequences, large-scale functional analyses are generating new excitement in biology and medicine. To facilitate genomewide functional analyses, we developed a high-density cell array with quantitative and automated readout of cell fitness. Able to print at > x 10 higher density on a standard microtiter plate area than currently possible, our cell array allows single-plate screening of the complete set of Saccharomyces cerevisiae gene-deletion library and significantly reduces the amount of small molecules and other materials needed for the study. We used this method to map the relation between genes and cell fitness in response to rapamycin, a medically important natural product that targets the eukaryotic kinase Tor. We discuss the implications for pharmacogenomics and the uncharted complexity in genotype-dependent drug response in molecularly targeted therapies. Our analysis leads to several basic findings, including a class of gene deletions that confer better fitness in the presence of rapamycin. This result provides insights into possible therapeutic uses of rapamycin/CCI-779 in the treatment of neurodegenerative diseases (including Alzheimer's, Parkinson's, and Huntington's diseases), and cautions the possible existence of similar rapamycin-enhanceable mutations in cancer. It is well established in yeast that although TOR2 has a unique rapamycin-insensitive function, TOR1 and TOR2 are interchangeable in the rapamycin-sensitive functions. We show that even the rapamycin-sensitive functions are distinct between TOR1 and TOR2 and map the functional difference to a approximately 120-aa region at the N termini of the proteins. Finally, we discuss using cell-based genomic pattern recognition in designing electronic or optical biosensors.

ENTH/ANTH domains expand to the Golgi

Clathrin-coated vesicles (CCVs) play important roles in nutrient uptake, downregulation of signaling receptors, pathogen invasion and biogenesis of endosomes and lysosomes. Although detailed models for endocytic CCV formation have emerged, the process of CCV formation at the Golgi and endosomes has been less clear. Key to endocytic CCV formation are proteins containing related phosphoinositide-binding ENTH and ANTH domains. Now, recent studies have identified novel ENTH/ANTH proteins that participate in CCV-mediated traffic between the trans-Golgi Network (TGN) and endosomes and have defined a molecular basis for interaction with AP-1 and GGA adaptors in clathrin coats of the TGN/endosomes. Thus, ENTH/ANTH domain proteins appear to be universal elements in nucleation of clathrin coats.

Yeast epsin-related proteins required for Golgi-endosome traffic define a gamma-adaptin ear-binding motif

Clathrin-coated vesicles (CCVs) are a central component of endocytosis and traffic between the trans-Golgi network (TGN) and endosomes. Although endocytic CCV formation is well characterized, much less is known about CCV formation at internal membranes. Here we describe two epsin amino-terminal homology (ENTH) domain-containing proteins, Ent3p and Ent5p, that are intimately involved in clathrin function at the Golgi. Both proteins associate with the clathrin adaptor Gga2p in vivo; Ent5p also interacts with the clathrin adaptor complex AP-1 and clathrin. A novel, conserved motif that mediates the interaction of Ent3p and Ent5p with gamma-ear domains of Gga2p and AP-1 is defined. Ent3p and Ent5p colocalize with clathrin, and cells lacking both Ent proteins exhibit defects in clathrin localization and traffic between the Golgi and endosomes. The findings suggest that Ent3p and Ent5p constitute a functionally related pair that co-operate with Gga proteins and AP-1 to recruit clathrin and promote formation of clathrin coats at the Golgi/endosomes. On the basis of our results and the established roles of epsin and epsin-related proteins in endocytosis, we propose that ENTH-domain-containing proteins are a universal component of CCV formation.

Yeast Eps15-like endocytic protein, Pan1p, activates the Arp2/3 complex

Longstanding evidence supports a role for actin in endocytosis; an intact actin cytoskeleton is required for endocytosis in yeast, and drugs that inhibit actin polymerization inhibit endocytosis in both yeast and mammalian cells. The yeast Arp2/3 complex is required for the internalization step of endocytosis. In addition, some early endocytic events in mammalian cells are associated with the formation of actin tails similar to those generated by activated Arp2/3 complex. However, until now no Arp2/3 complex activator has been identified among proteins known to mediate early steps in endocytosis. Here we show that the yeast endocytic protein Pan1p binds to and activates the Arp2/3 complex. Genetic interactions between PAN1 and mutants of Arp2/3 subunits, or of the Arp2/3 activator LAS17, provide evidence for this activity in vivo. We suggest that Pan1p forms the core of an endocytic complex and physically couples actin polymerization nucleated by the Arp2/3 complex to the endocytic machinery, thus providing the forces necessary for endocytosis.

Clinical presentation and antiviral therapy for poxvirus infection in pudu (Pudu puda)

A severe poxvirus infection occurred in three pudu (Pudu puda), resulting in two fatalities. Cutaneous ulcers with mucopurulent exudate were present around the eyes and nose, at the lip margins, coronary bands, and teats. Mucosal ulcers were present in the oral cavity, esophagus, and forestomachs. In the two fatalities, a secondary disseminated fungal infection also occurred. Affected animals were leukopenic, hypocalcemic, and hyperphosphatemic and had elevated serum alkaline phosphatase, alanine aminotransferase, and aspartate aminotransferase levels. Electron microscopic examination of affected skin confirmed the presence of a poxvirus. Neutralizing antibody titers to this virus were present in the two pudu tested. One case was treated with cidofovir, 5 mg/kg i.v. q7d for four treatments. Complete recovery occurred in the treated animal. This is the second report of poxvirus infection in pudu and the first report describing clinical presentation, presence of secondary disseminated fungal infection, and successful treatment.

Identical twins with mental retardation, dysarthria, progressive spastic paraplegia, and brachydactyly type E: a new syndrome or variant of Fitzsimmons-Guilbert syndrome?

We report on concordantly affected female identical twins with mental retardation, dysarthria, progressive spastic paraplegia, and brachydactyly type E. The most similar condition reported is the syndrome described by Fitzsimmons and Guilbert in uniovular twins characterized by progressive spastic paraplegia, dysarthria, brachydactyly type E, and cone-shaped epiphyses. During the last 11 years a report of only one other patient with this syndrome has been published; hence, its phenotypic delineation may be only partial. Although our patients might expand the phenotypic spectrum of this syndrome, they may represent a new disorder.

Familial dystonia and choreoathetosis in three generations associated with bilateral striatal necrosis

Nine cases of dystonia and choreoathetosis (six females and three males) have developed in three generations of a single family. There has been one death. Neuropathologic examination disclosed bilateral striatal necrosis. In this family, the neurologic disorder has evolved gradually or in association with a febrile illness. There has been no neurologic recovery. The disease is worse in females, has been transmitted only through females, and shows incomplete penetrance and anticipation. The maternal inheritance pattern suggests either an autosomal dominant trait also affecting male reproductive ability or a defect involving the mitochondrial genome.

Periarticular hyperostosis and renal disease in six black lemurs of two family groups

Proliferative periosteal disease was identified in 6 black lemurs (Eulemur macaco macaco) of 2 family groups. Bilaterally symmetric formation of periosteal new bone at the metaphyseal regions of major long bones was first detected at the stifle and tarsal areas and was detected later at the carpal areas. Bony changes were accompanied by progressive renal disease. The syndrome progressed for 6 to 16 months before the lemurs were euthanatized because of debility. Necropsy revealed changes confined to the skeleton and kidneys. Formation of new bone was detected at all affected joints, and chronic renal disease was evident in each lemur. A specific cause was not identified. Although indistinguishable histologically from hypertrophic osteoarthropathy, several important differences were apparent. Distribution of the periosteal new bone was in the metaphyseal rather than diaphyseal areas. Thoracic or gastrointestinal lesions, typically seen with hypertrophic osteoarthropathy, were not detected, and substantial renal disease was evident. A genetic component may be involved in the development of this condition.

Stage-specific cuticular proteins of Ostertagia circumcincta and Ostertagia ostertagi

In this study we have shown that NHS-biotin and I125-streptavidin can detect cuticular polypeptides of Ostertagia spp. The labelled polypeptide profile of intact nematodes is simple compared to the profile obtained by labelling homogenates. None of the major internal polypeptides are labelled and the subset of proteins labelled in intact nematodes appears to be mainly surface associated. The results presented here demonstrate that NHS-biotin may be used as a reagent for the analysis of surface polypeptides. The surface polypeptide profiles of the five major developmental stages (L1, L2, L3, L4 and adult) of Ostertagia circumcincta show a series of stage-specific molecules with no polypeptides common to all stages, indicating that the cuticle is a dynamic structure which changes throughout the life cycle. Similarily comparison of Ostertagia ostertagi L3 and L4 stage surface profiles showed that each stage is clearly distinct; comparison of these stages between the two species shows an overall similarity.

Somatosensory evoked potentials predict neuromotor outcome after periventricular hemorrhage

Thirty-nine very low-birthweight (VLBW) preterm infants with periventricular hemorrhage (PVH) were studied with short-latency median nerve somatosensory evoked potentials (SEP) at two, four and/or six months corrected age, and subsequently were followed to a mean age of 22 months. All 12 infants with a single SEP showing unilateral absence or prolonged latency of the early cerebral (N1) response had motor abnormalities at follow-up. A single normal SEP predicted normal motor development in 19 of 36 infants; two normal SEPs did so in 15 of 26 infants, and three normal SEPs in 12 of 14 infants. These results demonstrate that SEPs play a useful rôle in predicting neuromotor outcome for VLBW preterm infants with PVH.

Angelman's (happy puppet) syndrome: clinical, CT scan and serial electroencephalographic study

Of four patients having Angelman's syndrome admitted to a state mental facility who were clinically and electroencephalographically evaluated, 2 patients had CT scan studies of the brain. The most impressive and striking features that help in the diagnosis are the mental and physical retardation, nondevelopment of speech despite adequate visual and auditory function, various types of seizures, and episodic uncontrollable laughter. The CT scans of the brain did not offer any clue as to the pathogenesis. The EEGs appeared to fall into two groups: in one an arrest of electrical maturation occurred between ages 1 and 3 and in the other a slow but progressive maturation was evident.

Sydenham's chorea: clinical, EEG, CT scan, and evoked potential studies

An 8-year-old child developed acute mental confusion, and choreoathetosis without a preceding history of acute rheumatic fever. Serial EEGs showed focal suppression of sleep spindle activity over the right cerebral hemisphere along with high-amplitude polymorphic slow-wave complexes. CT scan studies on 4 occasions and MRI scan failed to show focal lesions corresponding to the clinical manifestations. Results of brainstem auditory evoked potential studies were normal. The short- and long-latency somatosensory evoked potential patterns were abnormal during the acute phase of illness as well as during the follow-up, when the patient was clinically intact. The persistent EEG and evoked potential abnormalities in the presence of clinical recovery from chorea raised questions as to the existence of a subclinical disease process.

Intragenic reversion mutations that improve export of maltose-binding protein in Escherichia coli malE signal sequence mutants

Escherichia coli strains harboring malE signal sequence point mutations accumulate export-defective precursor maltose-binding protein (MBP) in the cytoplasm. Beginning with these mutants, a number of spontaneous intragenic revertants have been obtained in which export of the MBP to the periplasm is either partially or totally restored. With a single exception, each of the reversion mutations resulted in an increase in the overall hydrophobicity of the signal peptide hydrophobic core by one of five different mechanisms. In some revertants, MBP export was achieved at a rate comparable to the wild type MBP; in other cases, the rate of MBP export was significantly slower than wild type. The results indicate that the overall hydrophobicity of the signal peptide, rather than the absolute length of its uninterrupted hydrophobic core, is a major determinant of MBP export competency. An alteration at residue 19 of the mature MBP also has been identified that provides fairly efficient suppression of the export defect in the adjacent signal peptide, further suggesting that important export information may reside in this region of the precursor protein.

Cerebral cortical isolation in infantile neuroaxonal dystrophy

Three patients with infantile neuroaxonal dystrophy (INAD, Seitelberger's disease) studied between ages 2 and 5 years, with the characteristic electroencephalographic pattern of high-voltage, fast (16-24 c/sec) rhythms and absence of reactivity on eye-opening or closure (Radermecker and Dumon-Radermecker 1972), also showed no changes in response to intermittent photic stimulation and absence of evoked potentials to flashes, clicks or median nerve stimuli. Although some theta rhythms and delta activity appeared during drowsiness and sleep, the fast rhythms persisted as the dominant feature. There were no central transients or K-complexes. When the patients cried, with hyperventilation, and also during breath-holding spells, slow rhythms appeared and the fast rhythm was reduced. These findings are interpreted as evidence of cerebral cortex isolation, the fast rhythm representing the spontaneous ("idling") activity of the cortex largely disconnected from subcortical or remote cortical influences by the slowly progressive, selective degeneration of axons, characteristic of the pathology of INAD, but the cortex remains responsive to chemical influences. These electrophysiological features become established furing the third year of age, prior EEGs being normal and later ones showing paroxysmal and other abnormal features in addition to the fast rhythm.