Yeast Mon2p is a highly conserved protein that functions in the cytoplasm-to-vacuole transport pathway and is required for Golgi homeostasis

ATP9A deficiency causes ADHD and aberrant endosomal recycling via modulating RAB5 and RAB11 activity

ATP9A, a lipid flippase of the class II P4-ATPases, is involved in cellular vesicle trafficking. Its homozygous variants are linked to neurodevelopmental disorders in humans. However, its physiological function, the underlying mechanism as well as its pathophysiological relevance in humans and animals are still largely unknown. Here, we report two independent families in which the nonsense mutations c.433C>T/c.658C>T/c.983G>A (p. Arg145*/p. Arg220*/p. Trp328*) in ATP9A (NM_006045.3) cause autosomal recessive hypotonia, intellectual disability (ID) and attention deficit hyperactivity disorder (ADHD). Atp9a null mice show decreased muscle strength, memory deficits and hyperkinetic movement disorder, recapitulating the symptoms observed in patients. Abnormal neurite morphology and impaired synaptic transmission are found in the primary motor cortex and hippocampus of the Atp9a null mice. ATP9A is also required for maintaining neuronal neurite morphology and the viability of neural cells in vitro. It mainly localizes to endosomes and plays a pivotal role in endosomal recycling pathway by modulating small GTPase RAB5 and RAB11 activation. However, ATP9A pathogenic mutants have aberrant subcellular localization and cause abnormal endosomal recycling. These findings provide strong evidence that ATP9A deficiency leads to neurodevelopmental disorders and synaptic dysfunctions in both humans and mice, and establishes novel regulatory roles for ATP9A in RAB5 and RAB11 activity-dependent endosomal recycling pathway and neurological diseases.

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.

Genomic and Targeted Approaches Unveil the Cell Membrane as a Major Target of the Antifungal Cytotoxin Amantelide A

Amantelide A, a polyhydroxylated macrolide isolated from a marine cyanobacterium, displays broad-spectrum activity against mammalian cells, bacterial pathogens, and marine fungi. We conducted comprehensive mechanistic studies to identify the molecular targets and pathways affected by amantelide A. Our investigations relied on chemical structure similarities with compounds of known mechanisms, yeast knockout mutants, yeast chemogenomic profiling, and direct biochemical and biophysical methods. We established that amantelide A exerts its antifungal action by binding to ergosterol-containing membranes followed by pore formation and cell death, a mechanism partially shared with polyene antifungals. Binding assays demonstrated that amantelide A also binds to membranes containing epicholesterol or mammalian cholesterol, thus suggesting that the cytotoxicity to mammalian cells might be due to its affinity to cholesterol-containing membranes. However, membrane interactions were not completely dependent on sterols. Yeast chemogenomic profiling suggested additional direct or indirect effects on actin. Accordingly, we performed actin polymerization assays, which suggested that amantelide A also promotes actin polymerization in cell-free systems. However, the C-33 acetoxy derivative amantelide B showed a similar effect on actin dynamics in vitro but no significant activity against yeast. Overall, these studies suggest that the membrane effects are the most functionally relevant for amantelide A mechanism of action.

LC-MS analysis reveals biological and metabolic processes essential for Candida albicans biofilm growth

Candidiasis is the most common fungal infection associated with high morbidity and mortality among immunocompromised patients. The ability to form biofilm is essential for Candida albicans pathogenesis and drug resistance. In this study, the planktonic cell and biofilm proteomes of C. albicans SC5314 strain analyzed using Liquid Chromatography-Mass Spectrometry (LC-MS) were compared. In total, 280 and 449 proteins are annotated from the planktonic cell and biofilm proteomes, respectively. The biofilm proteome demonstrated significantly higher proportion of proteins associated with the endomembrane system, mitochondrion and cytoplasm than planktonic proteome. Among proteins detected, 143 and 207 biological processes are annotated, of which, 38 and 102 are specific to the planktonic cell and biofilm proteomes, respectively, while 105 are common biological processes. The specific biological processes of C. albicans planktonic cell proteome are associated with cell polarity, energy metabolism and nucleotide (purine) metabolism, oxido-reduction coenzyme metabolic process, monosaccharide and amino acid (methionine) biosynthesis, regulation of anatomical structure morphogenesis and cell cycling, and single organism reproduction. Meanwhile, regulation of cellular macromolecule biosynthesis and metabolism, transcription and gene expression are major biological processes specifically associated with C. albicans biofilm proteome. Biosynthesis of leucine, isoleucine, and thiocysteine are highlighted as planktonic-related pathways, whereas folate metabolism, fatty acid metabolism and biosynthesis of amino acids (lysine, serine and glycine) are highlighted as biofilm-related pathways. In summary, LC-MS-based proteomic analysis reveals different adaptative strategies of C. albicans via specific biological and metabolic processes for planktonic cell and biofilm lifestyles. The mass spectrometry data are available via ProteomeXchange with identifiers PXD007830 (for biofilm proteome) and PXD007831 (for planktonic cell proteome).

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.

MON2 Guides Wntless Transport to the Golgi through Recycling Endosomes

Endocytic cargos are transported to recycling endosomes (RE) but how these sorting platforms are generated is not well understood. Here we describe our biochemical and live imaging studies of the conserved MON2-DOPEY complex in RE formation. MON2 mainly co-localized with RE marker RAB4B in peripheral dots and perinuclear region. The peripheral RE approached, interacted with, and separated from sorting nexin 3 (SNX3)-positive early endosomes (EE). Membrane-bound DOPEY2 was recruited to RE dependent upon MON2 expression, and showed binding abilities to kinesin and dynein/dynactin motor proteins. MON2-knockout impaired segregation of RE from EE and led to a decreased tubular recycling endosomal network, whereas RE was accumulated at perinuclear regions in DOPEY2-knockout cells. MON2 depletion also impaired intracellular transferrin receptor recycling, as well as retrograde transport of Wntless during its passage through RE before delivery from EE to the Golgi. Together, these data suggest that the MON2 drives separation of RE from EE and is required for efficient transport of endocytic cargo molecules.Key words: membrane trafficking, MON2, recycling endosomes, Wntless.

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.

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.

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.

Serum- and Glucocorticoid-Inducible Kinase-1 (SGK-1) Plays a Role in Membrane Trafficking in Caenorhabditis elegans

The mammalian serum- and glucocorticoid-inducible kinase SGK1 regulates the endocytosis of ion channels. Here we report that in C. elegans sgk-1 null mutants, GFP-tagged MIG-14/Wntless, the sorting receptor of Wnt, failed to localize to the basolateral membrane of intestinal cells; instead, it was mis-sorted to lysosomes. This effect can be explained in part by altered sphingolipid levels, because reducing glucosylceramide biosynthesis restored the localization of MIG-14::GFP. Membrane traffic was not perturbed in general, as no obvious morphological defects were detected for early endosomes, the Golgi apparatus, and the endoplasmic reticulum (ER) in sgk-1 null animals. The recycling of MIG-14/Wntless through the Golgi might be partially responsible for the observed phenotype because the subcellular distribution of two plasma membrane cargoes that do not recycle through the trans-Golgi network (TGN) was affected to a lesser degree. Consistently, knockdown of the ArfGEF gbf-1 altered the distribution of SGK-1 at the basolateral membrane of intestinal cells. In addition, we found that sgk-1(RNAi) induced unfolded protein response in the ER, suggesting at least an indirect role of SGK-1 early in the secretory pathway. We propose that SGK-1 function is required for lipid homeostasis and that it acts at different intracellular trafficking steps.

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.

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.

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.

A plasma-membrane E-MAP reveals links of the eisosome with sphingolipid metabolism and endosomal trafficking

The plasma membrane delimits the cell and controls material and information exchange between itself and the environment. How different plasma-membrane processes are coordinated and how the relative abundance of plasma-membrane lipids and proteins is homeostatically maintained are not yet understood. Here, we used a quantitative genetic interaction map, or E-MAP, to functionally interrogate a set of approximately 400 genes involved in various aspects of plasma-membrane biology, including endocytosis, signaling, lipid metabolism and eisosome function. From this E-MAP, we derived a set of 57,799 individual interactions between genes functioning in these various processes. Using triplet genetic motif analysis, we identified a new component of the eisosome, Eis1, and linked the poorly characterized gene EMP70 to endocytic and eisosome function. Finally, we implicated Rom2, a GDP/GTP exchange factor for Rho1 and Rho2, in the regulation of sphingolipid metabolism.

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.

Novel Ist1-Did2 complex functions at a late step in multivesicular body sorting

In Saccharomyces cerevisiae, integral plasma membrane proteins destined for degradation and certain vacuolar membrane proteins are sorted into the lumen of the vacuole via the multivesicular body (MVB) sorting pathway, which depends on the sequential action of three endosomal sorting complexes required for transport. Here, we report the characterization of a new positive modulator of MVB sorting, Ist1. We show that endosomal recruitment of Ist1 depends on ESCRT-III. Deletion of IST1 alone does not cause cargo-sorting defects. However, synthetic genetic analysis of double mutants of IST1 and positive modulators of MVB sorting showed that ist1Delta is synthetic with vta1Delta and vps60Delta, indicating that Ist1 is also a positive component of the MVB-sorting pathway. Moreover, this approach revealed that Ist1-Did2 and Vta1-Vps60 compose two functional units. Ist1-Did2 and Vta1-Vps60 form specific physical complexes, and, like Did2 and Vta1, Ist1 binds to the AAA-ATPase Vps4. We provide evidence that the ist1Delta mutation exhibits a synthetic interaction with mutations in VPS2 (DID4) that compromise the Vps2-Vps4 interaction. We propose a model in which the Ist1-Did2 and Vta1-Vps60 complexes independently modulate late steps in the MVB-sorting pathway.

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.

Membrane stress is coupled to a rapid translational control of gene expression in chlorpromazine-treated cells

Chlorpromazine (CPZ) is a small permeable cationic amphiphilic molecule that inserts into membrane bilayers and binds to anionic lipids such as poly-phosphoinositides (PIs). Since PIs play important roles in many cellular processes, including signaling and membrane trafficking pathways, it has been proposed that CPZ affects cellular growth functions by preventing the recruitment of proteins with specific PI-binding domains. In this study, we have investigated the biological effects of CPZ in the yeast Saccharomyces cerevisiae. We screened a collection of approximately 4,800 gene knockout mutants, and found that mutants defective in membrane trafficking between the late-Golgi and endosomal compartments are highly sensitive to CPZ. Microscopy and transport analyses revealed that CPZ affects membrane structure of organelles, blocks membrane transport and activates the unfolded protein response (UPR). In addition, CPZ-treatment induces phosphorylation of the translation initiation factor (eIF2alpha), which reduces the general rate of protein synthesis and stimulates the production of Gcn4p, a major transcription factor that is activated in response to environmental stresses. Altogether, our results reveal that membrane stress within the cells rapidly activates an important gene expression program, which is followed by a general inhibition of protein synthesis. Remarkably, the increase of phosphorylated eIF2alpha and protein synthesis inhibition were also detected in CPZ-treated NIH-3T3 fibroblasts, suggesting the existence of a conserved mechanism of translational regulation that operates during a membrane stress.

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.

Phosphatidylinositol 4-Phosphate Is Required for Translation Initiation in Saccharomyces cerevisiae

The small natural product wortmannin inhibits protein synthesis by modulating several phosphatidylinositol (PI) metabolic pathways. A primary target of wortmannin in yeast is the plasma membrane-associated PI 4-kinase (PI4K) Stt4p, which is required for actin cytoskeleton organization. Here we show that wortmannin treatment or inactivation of Stt4p, but not disorganization of the actin cytoskeleton per se, leads to a rapid attenuation of translation initiation. Interestingly, inactivation of Pik1p, a wortmannin-insensitive, functionally distinct PI4K, implicated in the regulation of Golgi functions and secretion, also results in severe translation initiation defects with a marked increase of the phosphorylation of the translation initiation factor eIF2alpha. Because wortmannin largely phenocopies the effects of rapamycin (e.g. it triggers nuclear accumulation of Gln3p), it likely also inhibits the PI kinase-related, target of rapamycin (TOR) kinases. Importantly, however, neither inactivation of Stt4p nor Pik1p significantly affects TOR-controlled readouts other than translation initiation, indicating that these PI4Ks do not simply function upstream of TOR. Together, our results reveal the existence of a novel translation initiation control mechanism in yeast that is tightly coupled to the synthesis of distinct PI4P pools.

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

The protein Mon2 is distantly related to the guanine nucleotide exchange factors (GEFs) that activate Arf1 on Golgi membranes. However, unlike these "large" Arf GEFs, Mon2 lacks the Sec7 domain that catalyzes nucleotide exchange on Arf1. Here we report that yeast Mon2 shares extensive homology with the noncatalytic parts of both the BIG and Golgi brefeldin A resistance factor subfamilies of Arf GEFs and is located to the trans-Golgi. Moreover, we find that Mon2 forms a complex with Dop1, a large cytoplasmic protein conserved in evolution from humans to protozoa. Deletion of Mon2 results in mislocalization of Dop1 from the Golgi and defects in cycling between endosomes and the Golgi. However, unlike Mon2, Dop1 is essential for yeast viability. A conditional allele of Dop1 shows that loss of Dop1 activity not only affects endosome to Golgi transport but also causes a severe perturbation of the organization of the endoplasmic reticulum. Thus, it appears that Dop1 plays a widespread role in membrane organization, and Mon2 acts as a scaffold to recruit the Golgi-localized pool of Dop1.