Mon2, a relative of large Arf exchange factors, recruits Dop1 to the Golgi apparatus

Dopey proteins are essential but overlooked regulators of membrane trafficking

Dopey family proteins play crucial roles in diverse processes from morphogenesis to neural function and are conserved from yeast to mammals. Understanding the mechanisms behind these critical functions could have major clinical significance, as dysregulation of Dopey proteins has been linked to the cognitive defects in Down syndrome, as well as neurological diseases. Dopey proteins form a complex with the non-essential GEF-like protein Mon2 and an essential lipid flippase from the P4-ATPase family. Different combinations of Dopey, Mon2 and flippases have been linked to regulating membrane remodeling, from endosomal recycling to extracellular vesicle formation, through their interactions with lipids and other membrane trafficking regulators, such as ARL1, SNX3 and the kinesin-1 light chain KLC2. Despite these important functions and their likely clinical significance, Dopey proteins remain understudied and their roles elusive. Here, we review the major scientific discoveries relating to Dopey proteins and detail key open questions regarding their function to draw attention to these fascinating enigmas.

MON-2, a Golgi protein, mediates autophagy-dependent longevity in Caenorhabditis elegans

The Golgi apparatus plays a central role in trafficking cargoes such as proteins and lipids. Defects in the Golgi apparatus lead to various diseases, but its role in organismal longevity is largely unknown. Using a quantitative proteomic approach, we found that a Golgi protein, MON-2, was up-regulated in long-lived Caenorhabditis elegans mutants with mitochondrial respiration defects and was required for their longevity. Similarly, we showed that DOP1/PAD-1, which acts with MON-2 to traffic macromolecules between the Golgi and endosome, contributed to the longevity of respiration mutants. Furthermore, we demonstrated that MON-2 was required for up-regulation of autophagy, a longevity-associated recycling process, by activating the Atg8 ortholog GABARAP/LGG-1 in C. elegans. Consistently, we showed that mammalian MON2 activated GABARAPL2 through physical interaction, which increased autophagic flux in mammalian cells. Thus, the evolutionarily conserved role of MON2 in trafficking between the Golgi and endosome is an integral part of autophagy-mediated longevity.

Structural basis of the P4B ATPase lipid flippase activity

P4 ATPases are lipid flippases that are phylogenetically grouped into P4A, P4B and P4C clades. The P4A ATPases are heterodimers composed of a catalytic α-subunit and accessory β-subunit, and the structures of several heterodimeric flippases have been reported. The S. cerevisiae Neo1 and its orthologs represent the P4B ATPases, which function as monomeric flippases without a β-subunit. It has been unclear whether monomeric flippases retain the architecture and transport mechanism of the dimeric flippases. Here we report the structure of a P4B ATPase, Neo1, in its E1-ATP, E2P-transition, and E2P states. The structure reveals a conserved architecture as well as highly similar functional intermediate states relative to dimeric flippases. Consistently, structure-guided mutagenesis of residues in the proposed substrate translocation path disrupted Neo1’s ability to establish membrane asymmetry. These observations indicate that evolutionarily distant P4 ATPases use a structurally conserved mechanism for substrate transport.

The P4 ATPase lipid flippases play a crucial role in membrane biogenesis. Here the authors report the structure of the monomeric P4B ATPase Neo1 in several states, clarifying the mechanism of substrate transport.

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.

Conserved mechanism of phospholipid substrate recognition by the P4-ATPase Neo1 from Saccharomyces cerevisiae

The type IV P-type ATPases (P4-ATPases) thus far characterized are lipid flippases that transport specific substrates, such as phosphatidylserine (PS) and phosphatidylethanolamine (PE), from the exofacial leaflet to the cytofacial leaflet of membranes. This transport activity generates compositional asymmetry between the two leaflets important for signal transduction, cytokinesis, vesicular transport, and host-pathogen interactions. Most P4-ATPases function as a heterodimer with a β-subunit from the Cdc50 protein family, but Neo1 from Saccharomyces cerevisiae and its metazoan orthologs lack a β-subunit requirement and it is unclear how these proteins transport substrate. Here we tested if residues linked to lipid substrate recognition in other P4-ATPases also contribute to Neo1 function in budding yeast. Point mutations altering entry gate residues in the first (Q209A) and fourth (S457Q) transmembrane segments of Neo1, where phospholipid substrate would initially be selected, disrupt PS and PE membrane asymmetry, but do not perturb growth of cells. Mutation of both entry gate residues inactivates Neo1, and cells expressing this variant are inviable. We also identified a gain-of-function mutation in the second transmembrane segment of Neo1 (Neo1[Y222S]), predicted to help form the entry gate, that substantially enhances Neo1's ability to replace the function of a well characterized phospholipid flippase, Drs2, in establishing PS and PE asymmetry. These results suggest a common mechanism for substrate recognition in widely divergent P4-ATPases.

Dopey1-Mon2 complex binds to dual-lipids and recruits kinesin-1 for membrane trafficking

Proteins are transported among eukaryotic organelles along the cytoskeleton in membrane carriers. The mechanism regarding the motility of carriers and the positioning of organelles is a fundamental question in cell biology that remains incompletely understood. Here, we find that Dopey1 and Mon2 assemble into a complex and localize to the Golgi, endolysosome and endoplasmic reticulum exit site. The Golgi localization of Dopey1 and Mon2 requires their binding to phosphatidylinositol-4-phosphate and phosphatidic acid, respectively, two lipids known for the biogenesis of membrane carriers and the specification of organelle identities. The N-terminus of Dopey1 further interacts with kinesin-1, a plus-end or centrifugal-direction microtubule motor. Dopey1-Mon2 complex functions as a dual-lipid-regulated cargo-adaptor to recruit kinesin-1 to secretory and endocytic organelles or membrane carriers for centrifugally biased bidirectional transport. Dopey1-Mon2 complex therefore provides an important missing link to coordinate the budding of a membrane carrier and subsequent bidirectional transport along the microtubule.

Proteins are transported among eukaryotic organelles along the cytoskeleton in membrane carriers. Here authors find that the Dopey1-Mon2 complex functions as a dual-lipid-regulated cargo-adaptor to recruit kinesin-1 to secretory and endocytic organelles or membrane carriers.

Yeast Dop1 is required for glycosyltransferase retrieval from the trans-Golgi network

BACKGROUND

Glycosyltransferases are type II membrane proteins that are responsible for glycan modification of proteins and lipids, and localize to distinct cisternae in the Golgi apparatus. During cisternal maturation, retrograde trafficking helps maintain the steady-state localization of these enzymes in the sub-compartments of the Golgi.

METHODS

To understand how glycosyltransferases are recycled in the late Golgi complex, we searched for genes that are essential for budding yeast cell growth and that encode proteins localized in endosomes and in the Golgi. We specifically analyzed the roles of Dop1 and its binding partner Neo1 in retaining Golgi-resident glycosyltransferases, in the late Golgi complex.

RESULTS

Dop1 primarily localized to younger compartments of the trans-Golgi network (TGN) and seemed to cycle within the TGN. In contrast, Neo1, a P4-ATPase that interacts with Dop1, localized to the TGN. Abolition of DOP1 expression led to defects in the FM4-64 endocytic pathway. Dop1 and Neo1 were required for correct glycosylation of invertase, a secretory protein, at the Golgi. In DOP1-shutdown cells, Och1, a mannosyltransferase that is typically located in the cis-Golgi, mislocalized to the TGN. In addition, the function of multiple glycosyltransferases required for N- and O-glycosylation were impaired in DOP1-shutdown cells.

CONCLUSIONS

Our results indicate that Dop1 is involved in vesicular transport at the TGN, and is critical for retrieving glycosyltransferases from the TGN to the Golgi in yeast.

GENERAL SIGNIFICANCE

Golgi-resident glycosyltransferases recycling from the TGN to the Golgi is dependent on Dop1 and the P4-ATPase Neo1.

Phospholipid flippases in membrane remodeling and transport carrier biogenesis

Molecular mechanisms underlying the formation of multiple classes of transport carriers or vesicles from Golgi and endosomal membranes remain poorly understood. However, one theme that has emerged over three decades is the dramatic influence of membrane lipid remodeling on transport mechanisms. A large cohort of lipid transfer proteins, lipid transporters, and lipid modifying enzymes are linked to protein sorting, carrier formation and SNARE-mediated fusion events. Here, we focus on one type of lipid transporter, phospholipid flippases in the type IV P-type ATPase (P4-ATPase) family, and discuss recent advances in defining P4-ATPase influences on membrane remodeling and vesicular transport.

SNX3-retromer requires an evolutionary conserved MON2:DOPEY2:ATP9A complex to mediate Wntless sorting and Wnt secretion

Wntless transports Wnt morphogens to the cell surface and is required for Wnt secretion and morphogenic gradients formation. Recycling of endocytosed Wntless requires the sorting nexin-3 (SNX3)-retromer-dependent endosome-to-Golgi transport pathway. Here we demonstrate the essential role of SNX3-retromer assembly for Wntless transport and report that SNX3 associates with an evolutionary conserved endosome-associated membrane re-modelling complex composed of MON2, DOPEY2 and the putative aminophospholipid translocase, ATP9A. In vivo suppression of Ce-mon-2, Ce-pad-1 or Ce-tat-5 (respective MON2, DOPEY2 and ATP9A orthologues) phenocopy a loss of SNX3-retromer function, leading to enhanced lysosomal degradation of Wntless and a Wnt phenotype. Perturbed Wnt signalling is also observed upon overexpression of an ATPase-inhibited TAT-5(E246Q) mutant, suggesting a role for phospholipid flippase activity during SNX3-retromer-mediated Wntless sorting. Together, these findings provide in vitro and in vivo mechanistic details to describe SNX3-retromer-mediated transport during Wnt secretion and the formation of Wnt-morphogenic gradients.

A Novel S100A8/A9 Induced Fingerprint of Mesenchymal Stem Cells associated with Enhanced Wound Healing

We here investigated whether the unique capacity of mesenchymal stem cells (MSCs) to re-establish tissue homeostasis depends on their potential to sense danger associated molecular pattern (DAMP) and to mount an adaptive response in the interest of tissue repair. Unexpectedly, after injection of MSCs which had been pretreated with the calcium-binding DAMP protein S100A8/A9 into murine full-thickness wounds, we observed a significant acceleration of healing even exceeding that of non-treated MSCs. This correlates with a fundamental reprogramming of the transcriptome in S100A8/A9 treated MSCs as deduced from RNA-seq analysis and its validation. A network of genes involved in proteolysis, macrophage phagocytosis, and inflammation control profoundly contribute to the clean-up of the wound site. In parallel, miR582-5p and genes boosting energy and encoding specific extracellular matrix proteins are reminiscent of scar-reduced tissue repair. This unprecedented finding holds substantial promise to refine current MSC-based therapies for difficult-to-treat wounds and fibrotic conditions.

A germline-limited piggyBac transposase gene is required for precise excision in Tetrahymena genome rearrangement

Developmentally programmed genome rearrangement accompanies differentiation of the silent germline micronucleus into the transcriptionally active somatic macronucleus in the ciliated protozoan Tetrahymena thermophila. Internal eliminated sequences (IES) are excised, followed by rejoining of MAC-destined sequences, while fragmentation occurs at conserved chromosome breakage sequences, generating macronuclear chromosomes. Some macronuclear chromosomes, referred to as non-maintained chromosomes (NMC), are lost soon after differentiation. Large NMC contain genes implicated in development-specific roles. One such gene encodes the domesticated piggyBac transposase TPB6, required for heterochromatin-dependent precise excision of IES residing within exons of functionally important genes. These conserved exonic IES determine alternative transcription products in the developing macronucleus; some even contain free-standing genes. Examples of precise loss of some exonic IES in the micronucleus and retention of others in the macronucleus of related species suggest an evolutionary analogy to introns. Our results reveal that germline-limited sequences can encode genes with specific expression patterns and development-related functions, which may be a recurring theme in eukaryotic organisms experiencing programmed genome rearrangement during germline to soma differentiation.

Quantitative high-content imaging identifies novel regulators of Neo1 trafficking at endosomes

P4-ATPases are a family of putative phospholipid flippases that regulate lipid membrane asymmetry, which is important for vesicle formation. Two yeast flippases, Drs2 and Neo1, have nonredundant functions in the recycling of the synaptobrevin-like v-SNARE Snc1 from early endosomes. Drs2 activity is needed to form vesicles and regulate its own trafficking, suggesting that flippase activity and localization are linked. However, the role of Neo1 in endosomal recycling is not well characterized. To identify novel regulators of Neo1 trafficking and activity at endosomes, we first identified mutants with impaired recycling of a Snc1-based reporter and subsequently used high-content microscopy to classify these mutants based on the localization of Neo1 or its binding partners, Mon2 and Dop1. This analysis identified a role for Arl1 in stabilizing the Mon2/Dop1 complex and uncovered a new function for Vps13 in early endosome recycling and Neo1 localization. We further showed that the cargo-selective sorting nexin Snx3 is required for Neo1 trafficking and identified an Snx3 sorting motif in the Neo1 N-terminus. Of importance, the Snx3-dependent sorting of Neo1 was required for the correct sorting of another Snx3 cargo protein, suggesting that the incorporation of Neo1 into recycling tubules may influence their formation.

Multiple activities of Arl1 GTPase in the trans-Golgi network

ADP-ribosylation factors (Arfs) and ADP-ribosylation factor-like proteins (Arls) are highly conserved small GTPases that function as main regulators of vesicular trafficking and cytoskeletal reorganization. Arl1, the first identified member of the large Arl family, is an important regulator of Golgi complex structure and function in organisms ranging from yeast to mammals. Together with its effectors, Arl1 has been shown to be involved in several cellular processes, including endosomal trans-Golgi network and secretory trafficking, lipid droplet and salivary granule formation, innate immunity and neuronal development, stress tolerance, as well as the response of the unfolded protein. In this Commentary, we provide a comprehensive summary of the Arl1-dependent cellular functions and a detailed characterization of several Arl1 effectors. We propose that involvement of Arl1 in these diverse cellular functions reflects the fact that Arl1 is activated at several late-Golgi sites, corresponding to specific molecular complexes that respond to and integrate multiple signals. We also provide insight into how the GTP-GDP cycle of Arl1 is regulated, and highlight a newly discovered mechanism that controls the sophisticated regulation of Arl1 activity at the Golgi complex.

Exploring genetic suppression interactions on a global scale

Genetic suppression occurs when the phenotypic defects caused by a mutation in a particular gene are rescued by a mutation in a second gene. To explore the principles of genetic suppression, we examined both literature-curated and unbiased experimental data, involving systematic genetic mapping and whole-genome sequencing, to generate a large-scale suppression network among yeast genes. Most suppression pairs identified novel relationships among functionally related genes, providing new insights into the functional wiring diagram of the cell. In addition to suppressor mutations, we identified frequent secondary mutations,in a subset of genes, that likely cause a delay in the onset of stationary phase, which appears to promote their enrichment within a propagating population. These findings allow us to formulate and quantify general mechanisms of genetic suppression.

Structural Insights into Arl1-Mediated Targeting of the Arf-GEF BIG1 to the trans-Golgi

The GTPase Arf1 is the major regulator of vesicle traffic at both the cis- and trans-Golgi. Arf1 is activated at the cis-Golgi by the guanine nucleotide exchange factor (GEF) GBF1 and at the trans-Golgi by the related GEF BIG1 or its paralog, BIG2. The trans-Golgi-specific targeting of BIG1 and BIG2 depends on the Arf-like GTPase Arl1. We find that Arl1 binds to the dimerization and cyclophilin binding (DCB) domain in BIG1 and report a crystal structure of human Arl1 bound to this domain. Residues in the DCB domain that bind Arl1 are required for BIG1 to locate to the Golgi in vivo. DCB domain-binding residues in Arl1 have a distinct conformation from those in known Arl1-effector complexes, and this plasticity allows Arl1 to interact with different effectors of unrelated structure. The findings provide structural insight into how Arf1 GEFs, and hence active Arf1, achieve their correct subcellular distribution.

Oskar-induced endocytic activation and actin remodeling for anchorage of the Drosophila germ plasm

In many animals, germ-cell fate is specified by inheritance of the germ plasm, which is enriched in maternal RNAs and proteins. Assembly of the Drosophila germ (pole) plasm begins with the localization and translation of oskar (osk) RNA at the oocyte posterior pole. osk RNA produces two isoforms, long and short Osk. Short Osk recruits other pole plasm components, and long Osk restricts them to the oocyte cortex. Although molecular functions of long Osk remain mysterious, it is known to be involved in endocytic activation and actin cytoskeletal remodeling. We identified several vesicular trafficking machinery components that act downstream of long Osk in pole plasm assembly. These included the Rab5 effector protein Rabenosyn-5 (Rbsn-5) and the Golgi/endosomal protein Mon2, both of which were crucial for Osk-induced actin remodeling and the anchoring of pole plasm components. We propose that, in response to long Osk, the Rab5/Rbsn-5-dependent endocytic pathway promotes the formation of specialized vesicles, and Mon2 acts on these vesicles as a scaffold to instruct actin nucleators like Cappuccino and Spire to remodel the actin cytoskeleton, which anchors pole plasm components to the cortex. This mechanism may be applicable to the asymmetric localization of macromolecular structures such as protein-RNA complexes in other systems.

The VF rat with abnormal myelinogenesis has a mutation in Dopey1

The vacuole formation (VF) rat is an autosomal recessive myelin mutant characterized by generalized tremor, hypomyelination, and periaxonal vacuole formation of the central nervous system (CNS). Here, we report the most likely causative gene for neurological disease in the VF rat and pursue its roles in the development and maintenance of the CNS myelin. We identified a nonsense mutation in the dopey family member 1 (Dopey1) located on rat chromosome 8. Expression level of Dopey1 mRNA was decreased and DOPEY1 protein was undetectable both in the white and gray matter of the spinal cords in the VF rats. Double immunohistochemistry demonstrated that DOPEY1 was mainly expressed in neurons and oligodendrocytes in the wild-type rats, whereas no positive cells were detected in the VF rats. We also demonstrated a marked reduction in myelin components both at mRNA and protein levels during myelinogenesis in the VF rats. In addition, proteolipid protein and myelin-associated glycoprotein accumulated in oligodendrocyte cell body, suggesting that Dopey1 is likely to be involved in the traffic of myelin components. Our results highlighted the importance of Dopey1 for the development and maintenance of the CNS myelin.

Mammalian Mon2/Ysl2 regulates endosome-to-Golgi trafficking but possesses no guanine nucleotide exchange activity toward Arl1 GTPase

Arl1 is a member of Arf family small GTPases that is essential for the organization and function of Golgi complex. Mon2/Ysl2, which shares significant homology with Sec7 family Arf guanine nucleotide exchange factors, was poorly characterized in mammalian cells. Here, we report the first in depth characterization of mammalian Mon2. We found that Mon2 localized to trans-Golgi network which was dependent on both its N and C termini. The depletion of Mon2 did not affect the Golgi localized or cellular active form of Arl1. Furthermore, our in vitro assay demonstrated that recombinant Mon2 did not promote guanine nucleotide exchange of Arl1. Therefore, our results suggest that Mon2 could be neither necessary nor sufficient for the guanine nucleotide exchange of Arl1. We demonstrated that Mon2 was involved in endosome-to-Golgi trafficking as its depletion accelerated the delivery of furin and CI-M6PR to Golgi after endocytosis.

Arl1p regulates spatial membrane organization at the trans-Golgi network through interaction with Arf-GEF Gea2p and flippase Drs2p

ADP ribosylation factors (Arfs) are the central regulators of vesicle trafficking from the Golgi complex. Activated Arfs facilitate vesicle formation through stimulating coat assembly, activating lipid-modifying enzymes and recruiting tethers and other effectors. Lipid translocases (flippases) have been implicated in vesicle formation through the generation of membrane curvature. Although there is no evidence that Arfs directly regulate flippase activity, an Arf-guanine-nucleotide-exchange factor (GEF) Gea2p has been shown to bind to and stimulate the activity of the flippase Drs2p. Here, we provide evidence for the interaction and activation of Drs2p by Arf-like protein Arl1p in yeast. We observed that Arl1p, Drs2p and Gea2p form a complex through direct interaction with each other, and each interaction is necessary for the stability of the complex and is indispensable for flippase activity. Furthermore, we show that this Arl1p-Drs2p-Gea2p complex is specifically required for recruiting golgin Imh1p to the Golgi. Our results demonstrate that activated Arl1p can promote the spatial modulation of membrane organization at the trans-Golgi network through interacting with the effectors Gea2p and Drs2p.

Cdk1-dependent control of membrane-trafficking dynamics

Cyclin-dependent kinase 1 (Cdk1) is required for initiation and maintenance of polarized cell growth in budding yeast. Cdk1 activates Rho-family GTPases, which trigger polarization of the actin cytoskeleton for delivery of membrane to growth sites. It is found that Cdk1's function in polarized growth extends beyond that of actin organization.

Cyclin-dependent kinase 1 (Cdk1) is required for initiation and maintenance of polarized cell growth in budding yeast. Cdk1 activates Rho-family GTPases, which polarize the actin cytoskeleton for delivery of membrane to growth sites via the secretory pathway. Here we investigate whether Cdk1 plays additional roles in the initiation and maintenance of polarized cell growth. We find that inhibition of Cdk1 causes a cell surface growth defect that is as severe as that caused by actin depolymerization. However, unlike actin depolymerization, Cdk1 inhibition does not result in a massive accumulation of intracellular secretory vesicles or their cargoes. Analysis of post-Golgi vesicle dynamics after Cdk1 inhibition demonstrates that exocytic vesicles are rapidly mistargeted away from the growing bud, possibly to the endomembrane/vacuolar system. Inhibition of Cdk1 also causes defects in the organization of endocytic and exocytic zones at the site of growth. Cdk1 thus modulates membrane-trafficking dynamics, which is likely to play an important role in coordinating cell surface growth with cell cycle progression.

Mon2 is a negative regulator of the monomeric G protein, Arl1

Using site-directed mutants of ARL1 predicted to alter nucleotide binding, we examined phenotypes associated with the loss of ARL1 , including effects on membrane traffic and K (+) homeostasis. The GTP-restricted allele, ARL[Q72L] , complemented the membrane traffic phenotype (CPY secretion), but not the K (+) homeostasis phenotypes (sensitivity to hygromycin B, steady-state levels of K (+) , and accumulation of (86) Rb (+) ), while the XTP-restricted mutant, ARL1[D130N] , complemented the ion phenotypes, but not the membrane traffic phenotype. A GDP-restricted allele, ARL1[T32N] , did not effectively complement either phenotype. These results are consistent with a model in which Arl1 has three different conformations in vivo. We also explored the relationship between ARL1 and MON2 using the synthetic lethal phenotype exhibited by these two genes and demonstrated that MON2 is a negative regulator of the GTP-restricted allele of ARL1 , ARL1[Q72L] . Finally, we constructed several new alleles predicted to alter binding of Arl1 to the sole GRIP domain containing protein in yeast, Imh1, and found that ARL1[F52G] and ARL1[Y82G] were unable to complement the loss of ARL1 with respect to either the membrane traffic or K (+) homeostasis phenotypes. Our study expands understanding of the roles of Arl1 in vivo.

Drosophila Mon2 couples Oskar-induced endocytosis with actin remodeling for cortical anchorage of the germ plasm

Drosophila pole (germ) plasm contains germline and abdominal determinants. Its assembly begins with the localization and translation of oskar (osk) RNA at the oocyte posterior, to which the pole plasm must be restricted for proper embryonic development. Osk stimulates endocytosis, which in turn promotes actin remodeling to form long F-actin projections at the oocyte posterior pole. Although the endocytosis-coupled actin remodeling appears to be crucial for the pole plasm anchoring, the mechanism linking Osk-induced endocytic activity and actin remodeling is unknown. Here, we report that a Golgi-endosomal protein, Mon2, acts downstream of Osk to remodel cortical actin and to anchor the pole plasm. Mon2 interacts with two actin nucleators known to be involved in osk RNA localization in the oocyte, Cappuccino (Capu) and Spire (Spir), and promotes the accumulation of the small GTPase Rho1 at the oocyte posterior. We also found that these actin regulators are required for Osk-dependent formation of long F-actin projections and cortical anchoring of pole plasm components. We propose that, in response to the Osk-mediated endocytic activation, vesicle-localized Mon2 acts as a scaffold that instructs the actin-remodeling complex to form long F-actin projections. This Mon2-mediated coupling event is crucial to restrict the pole plasm to the oocyte posterior cortex.

The cellular factors Vps18 and Mon2 are required for efficient production of infectious HIV-1 particles

Like all viruses, HIV-1 requires cellular host factors for replication. The mechanisms for production of progeny virions involving these host factors, however, are not fully understood. To better understand these mechanisms, we used a yeast (Saccharomyces cerevisiae) genetic screen to identify mutant strains in which HIV-1 Gag targeting to the plasma membrane was aberrant. Of the 917 mutants identified, we selected 14 mutants whose missing genes had single orthologous counterparts in human and tested them for Gag-induced viruslike particle (VLP) release in yeast cells. We found that the Vps18 and Mon2 proteins were important for HIV-1 Gag-induced VLP release in yeast. In eukaryote cells, these host proteins are highly conserved and function in protein trafficking. Depletion of hVps18 or hMon2 reduced the efficient production of infectious HIV-1 virions in human cells. Our data suggest that these cellular factors play an important role in the efficient production of infectious HIV-1 virion particles.

Syt1p promotes activation of Arl1p at the late Golgi to recruit Imh1p

In yeast, Arl3p recruits Arl1p GTPase to regulate Golgi function and structure. However, the molecular mechanism involved in regulating activation of Arl1p at the Golgi is unknown. Here, we show that Syt1p promoted activation of Arl1p and recruitment of a golgin protein, Imh1p, to the Golgi. Deletion of SYT1 resulted in the majority of Arl1p being distributed diffusely throughout the cytosol. Overexpression of Syt1p increased Arl1p-GTP production in vivo and the Syt1-Sec7 domain promoted nucleotide exchange on Arl1p in vitro. Syt1p function required the N-terminal region, Sec7 and PH domains. Arl1p, but not Arl3p, interacted with Syt1p. Localization of Syt1p to the Golgi did not require Arl3p. Unlike arl1Δ or arl3Δ mutants, syt1Δ did not show defects in Gas1p transport, cell wall integrity or vacuolar structure. These findings reveal that activation of Arl1p is regulated in part by Syt1p, and imply that Arl1p activation, by using more than one GEF, exerts distinct biological activities at the Golgi compartment.

Intracellular phospholipase A1 and acyltransferase, which are involved in Caenorhabditis elegans stem cell divisions, determine the sn-1 fatty acyl chain of phosphatidylinositol

Phosphatidylinositol (PI), an important constituent of membranes, contains stearic acid as the major fatty acid at the sn-1 position. This fatty acid is thought to be incorporated into PI through fatty acid remodeling by sequential deacylation and reacylation. However, the genes responsible for the reaction are unknown, and consequently, the physiological significance of the sn-1 fatty acid remains to be elucidated. Here, we identified acl-8, -9, and -10, which are closely related to each other, and ipla-1 as strong candidates for genes involved in fatty acid remodeling at the sn-1 position of PI. In both ipla-1 mutants and acl-8 acl-9 acl-10 triple mutants of Caenorhabditis elegans, the stearic acid content of PI is reduced, and asymmetric division of stem cell-like epithelial cells is defective. The defects in asymmetric division of these mutants are suppressed by a mutation of the same genes involved in intracellular retrograde transport, suggesting that ipla-1 and acl genes act in the same pathway. IPLA-1 and ACL-10 have phospholipase A(1) and acyltransferase activity, respectively, both of which recognize the sn-1 position of PI as their substrate. We propose that the sn-1 fatty acid of PI is determined by ipla-1 and acl-8, -9, -10 and crucial for asymmetric divisions.

A quantitative assessment of gene expression (QAGE) reveals differential overexpression of DOPEY2, a candidate gene for mental retardation, in Down syndrome brain regions

The brain alterations and mental retardation in Down syndrome are associated with overdosage of chromosome 21 genes. To shed light on the understanding of the molecular effect of this genetic overdosage, gene expression studies have crucial importance to quantify expression variations in Down syndrome tissues compared to normal ones. Herein, an in situ Quantitative Assessment of Gene Expression (QAGE) was used to quantify and statistically analyze, for the first time, DOPEY2 expression variations in different regions of the Down syndrome human fetal brains and to compare them to corresponding normal brains. DOPEY2, which is localized in the Down Syndrome Critical Region (DSCR) and is a candidate gene for neurological alterations in Down syndrome, showed a delimited regional and cellular expression pattern in the cortex, hippocampus and cerebellum, characterized by different transcriptional intensities in both normal and trisomic brains. DOPEY2 is overexpressed more than 50% (1.79-, 1.97- and 2.12-folds in the cortex, cerebellum and hippocampus, respectively), and showed statistically significant differences in the overexpression ratios in the three brain regions expressing DOPEY2. The demonstration of differential DOPEY2 expression and overexpression in human fetal brains suggests that this gene is submitted to a complex transcriptional control and could depend from other human chromosome 21 genes. Moreover, DOPEY2 overexpression in the brain regions, that are altered in Down syndrome patients and involved in learning and memory processes, is in agreement to the hypothesis that this gene plays a potential role in functional brain alterations and in the pathogenesis of mental retardation in Down syndrome. This new in situ QAGE approach allowed quantitative measurements of transcriptional changes and statistical evaluations of the expression and overexpression patterns of DOPEY2 at specific regions of the brain, which is a complementary approach to qRT-PCR and microarray for transcriptome study. Moreover, this approach could be a powerful tool to study the candidate chromosome 21 genes for Down syndrome and other pathologies caused by regionalized quantitative transcriptional alterations, for greater interpretation of functional processes driving gene expression.

Beta-catenin asymmetry is regulated by PLA1 and retrograde traffic in C. elegans stem cell divisions

Asymmetric division is an important property of stem cells. In Caenorhabditis elegans, the Wnt/beta-catenin asymmetry pathway determines the polarity of most asymmetric divisions. The Wnt signalling components such as beta-catenin localize asymmetrically to the cortex of mother cells to produce two distinct daughter cells. However, the molecular mechanism to polarize them remains to be elucidated. Here, we demonstrate that intracellular phospholipase A(1) (PLA(1)), a poorly characterized lipid-metabolizing enzyme, controls the subcellular localizations of beta-catenin in the terminal asymmetric divisions of epithelial stem cells (seam cells). In mutants of ipla-1, a single C. elegans PLA(1) gene, cortical beta-catenin is delocalized and the asymmetry of cell-fate specification is disrupted in the asymmetric divisions. ipla-1 mutant phenotypes are rescued by expression of ipla-1 in seam cells in a catalytic activity-dependent manner. Furthermore, our genetic screen utilizing ipla-1 mutants reveals that reduction of endosome-to-Golgi retrograde transport in seam cells restores normal subcellular localization of beta-catenin to ipla-1 mutants. We propose that membrane trafficking regulated by ipla-1 provides a mechanism to control the cortical asymmetry of beta-catenin.

Yeast and human Ysl2p/hMon2 interact with Gga adaptors and mediate their subcellular distribution

The Gga proteins represent a family of ubiquitously expressed clathrin adaptors engaged in vesicle budding at the tubular endosomal network/trans Golgi network. Their membrane recruitment is commonly thought to involve interactions with Arf and signals in cargo through the so-called VHS domain. For yeast Gga proteins, however, partners binding to its VHS domain have remained elusive and Gga localization does not absolutely depend on Arf. Here, we demonstrate that yeast Gga recruitment relies on a network of interactions between the scaffold Ysl2p/Mon2p, the small GTPase Arl1p, and the flippase Neo1p. Deletion of either YSL2 or ARL1 causes mislocalization of Gga2p, whereas a neo1-69 mutant accumulates Gga2p on aberrant structures. Remarkably, Ysl2p directly interacts with human and yeast Ggas through the VHS domain, and binding to Gga proteins is also found for the human Ysl2p orthologue hMon2. Thus, Ysl2p represents an essential, evolutionarily conserved member of a network controlling direct binding and membrane docking of Ggas. Because activated Arl1p is part of the network that binds Gga2p, Arf and Arf-like GTPases may interact in a regulatory cascade.

New cerebellar phenotypes in YAC transgenic mouse in vivo library of human Down syndrome critical region-1

Down syndrome (DS) is the most frequent genetic cause of mental retardation (MR) associated with neurological alterations. To allow a genetic dissection of DS phenotype, we studied eight transgenic mouse lines carrying YACs containing human DNA fragments covering DS critical region (DCR-1), as an in vivo library. Herein, we found an increased brain size in the 152F7-mice containing DYRK1A gene. We also identified a new cerebellar alteration in two independent lines carrying 230E8-YAC. These mice showed significant elongation of the cerebellar antero-posterior axis (p<0.001), determined by increased length of rostral folia of the vermis (lobule II-V, p<0.0001; lobule VI, p<0.001). In addition, we identified a major neurological defect in culmen and declivus lobules in the 230E8-mice. We analyzed P30, P12, and P9 stages and detected high significant increased lengths of anterior lobules (II-VI) of 230E8-mice at P30 and P12 (lobule II-V, p<0.0001; lobule VI, p<0.05), but not at P9, indicating that this new phenotype appears between P9 and P12. Interestingly, 230E8-mice also present increased cortical cell density and mild learning defects. 230E8-YAC contains seven genes, some of which could be potentially responsible for this phenotype. Between them, we proposed DOPEY2 as potential candidate gene for these cerebellar alterations considering its high expression in the brain and that its homologous genes in yeast, Caenorhabditis elegans and Drosophila are involved in morphogenesis, suggesting a conserved role of DOPEY2 as a patterning gene.

Interactions between Conserved Domains within Homodimers in the BIG1, BIG2, and GBF1 Arf Guanine Nucleotide Exchange Factors

Guanine nucleotide exchange factors carrying a Sec7 domain (ArfGEFs) activate the small GTP-binding protein Arf, a major regulator of membrane remodeling and protein trafficking in eukaryotic cells. Only two of the seven subfamilies of ArfGEFs (GBF and BIG) are found in all eukaryotes. In addition to the Sec7 domain, which catalyzes GDP/GTP exchange on Arf, the GBF and BIG ArfGEFs have five common homology domains. Very little is known about the functions of these noncatalytic domains, but it is likely that they serve to integrate upstream signals that define the conditions of Arf activation. Here we describe interactions between two conserved domains upstream of the Sec7 domain (DCB and HUS) that determine the architecture of the N-terminal regions of the GBF and BIG ArfGEFs using a combination of biochemical, yeast two-hybrid, and cellular assays. Our data demonstrate a strong interaction between DCB domains within GBF1, BIG1, and BIG2 to maintain homodimers and an interaction between DCB and HUS domains within each homodimer. The DCB/HUS interaction is mediated by the HUS box, the most conserved motif in large ArfGEFs after the Sec7 domain. In support of the in vitro data, we show that both the DCB and the HUS domains are necessary for GBF1 dimerization in mammalian cells and that the DCB domain is essential for yeast viability. We propose that the dimeric DCB-HUS structural unit exists in all members of the GBF and BIG ArfGEF groups and in the related Mon2p family and probably serves an important regulatory role in Arf activation.

Yeast mutant with efficient secretion identified by a novel secretory reporter, Cluc

Yeast is an important host for the production of pharmaceutical or industrial proteins by virtue of its genetic information and easy handling. A number of heterologous proteins have been produced and purified from yeast cell cultures as secreted forms. Here, we describe a novel screening system of Saccharomyces cerevisiae and its application to improve the secretion efficiency of yeast. In our system, a natural secretory luciferase from Cypridina noctiluca is used as a reporter enzyme. The accumulation of enzymatically active luciferase in culture medium makes it possible to screen many samples simultaneously in a simple and sensitive assay. Using this system, we have discovered that the deletion mutant of MON2, which encoded a scaffold protein for vesicle formation located at the late Golgi, secreted luciferase highly efficiently to the extracellular space. Thus, we conclude that this new reporter assay is useful for the improvement and screening of yeast secretory strains.

Nucleocytosolic acetyl-coenzyme a synthetase is required for histone acetylation and global transcription

Metabolic enzymes rarely regulate informational processes like gene expression. Yeast acetyl-CoA synthetases (Acs1p and 2p) are exceptional, as they are important not only for carbon metabolism but also are shown here to supply the acetyl-CoA for histone acetylation by histone acetyltransferases (HATs). acs2-Ts mutants exhibit global histone deacetylation, transcriptional defects, and synthetic growth defects with HAT mutants at high temperatures. In glycerol with ethanol, Acs1p is an alternate acetyl-CoA source for HATs. Rapid deacetylation after Acs2p inactivation suggests nuclear acetyl-CoA synthesis is rate limiting for histone acetylation. Different histone lysines exhibit distinct deacetylation rates, with N-terminal tail lysines deacetylated rapidly and H3 lysine 56 slowly. Yeast mitochondrial and nucleocytosolic acetyl-CoA pools are biochemically isolated. Thus, acetyl-CoA metabolism is directly linked to chromatin regulation and may affect diverse cellular processes in which acetylation and metabolism intersect, such as disease states and aging.