Maintenance of Golgi structure and function depends on the integrity of ER export

Spatial-Temporal Study of Rab1b Dynamics and Function at the ER-Golgi Interface

The GTPase Rab1b is involved in ER to Golgi transport, with multiple Rab1b effectors (located at ERES, VTCs and the Golgi complex) being required for its function. In this study, we performed live-cell dual-expression studies to analyze the dynamics of Rab1b and some effectors located at the ERES-Golgi interface. Rab1b occupied widely distributed mobile punctate and tubular structures, displaying a transient overlaps with its effectors and showing that these overlaps occurred at the same time in spatially distinct steps of ER to Golgi transport. In addition, we assessed Rab1b dynamics during cargo sorting by analyzing the concentration at ERES of a Golgi protein (SialT2-CFP) during Brefeldin A washout (BFA WO). Rab1b was associated to most of the ERES structures, but at different times during BFA WO, and recurrently SialT2-CFP was sorted in the ERES-Rab1b positive structures. Furthermore, we reveal for first time that Rab1b localization time at ERES depended on GBF1, a Rab1b effector that acts as the guanine nucleotide exchange factor of Arf1, and that Rab1b membrane association/dissociation dynamics at ERES was dependent on the GBF1 membrane association and activity, which strongly suggests that GBF1 activity modulates Rab1b membrane cycling dynamic.

TECPR2 Cooperates with LC3C to Regulate COPII-Dependent ER Export

Hereditary spastic paraplegias (HSPs) are a diverse group of neurodegenerative diseases that are characterized by axonopathy of the corticospinal motor neurons. A mutation in the gene encoding for Tectonin β-propeller containing protein 2 (TECPR2) causes HSP that is complicated by neurological symptoms. While TECPR2 is a human ATG8 binding protein and positive regulator of autophagy, the exact function of TECPR2 is unknown. Here, we show that TECPR2 associates with several trafficking components, among them the COPII coat protein SEC24D. TECPR2 is required for stabilization of SEC24D protein levels, maintenance of functional ER exit sites (ERES), and efficient ER export in a manner dependent on binding to lipidated LC3C. TECPR2-deficient HSP patient cells display alterations in SEC24D abundance and ER export efficiency. Additionally, TECPR2 and LC3C are required for autophagosome formation, possibly through maintaining functional ERES. Collectively, these results reveal that TECPR2 functions as molecular scaffold linking early secretion pathway and autophagy.

Downregulation of the small GTPase SAR1A: a key event underlying alcohol-induced Golgi fragmentation in hepatocytes

The hepatic asialoglycoprotein receptor (ASGP-R) is posttranslationally modified in the Golgi en route to the plasma membrane, where it mediates clearance of desialylated serum glycoproteins. It is known that content of plasma membrane-associated ASGP-R is decreased after ethanol exposure, although the mechanisms remain elusive. Previously, we found that formation of compact Golgi requires dimerization of the largest Golgi matrix protein giantin. We hypothesize that ethanol-impaired giantin function may be related to altered trafficking of ASGP-R. Here we report that in HepG2 cells expressing alcohol dehydrogenase and hepatocytes of ethanol-fed rats, ethanol metabolism results in Golgi disorganization. This process is initiated by dysfunction of SAR1A GTPase followed by altered COPII vesicle formation and impaired Golgi delivery of the protein disulfide isomerase A3 (PDIA3), an enzyme that catalyzes giantin dimerization. Additionally, we show that SAR1A gene silencing in hepatocytes mimics the effect of ethanol: dedimerization of giantin, arresting PDIA3 in the endoplasmic reticulum (ER) and large-scale alterations in Golgi architecture. Ethanol-induced Golgi fission has no effect on ER-to-Golgi transportation of ASGP-R, however, it results in its deposition in cis-medial-, but not trans-Golgi. Thus, alcohol-induced deficiency in COPII vesicle formation predetermines Golgi fragmentation which, in turn, compromises the Golgi-to-plasma membrane transportation of ASGP-R.

ER trapping reveals Golgi enzymes continually revisit the ER through a recycling pathway that controls Golgi organization

Whether Golgi enzymes remain localized within the Golgi or constitutively cycle through the endoplasmic reticulum (ER) is unclear, yet is important for understanding Golgi dependence on the ER. Here, we demonstrate that the previously reported inefficient ER trapping of Golgi enzymes in a rapamycin-based assay results from an artifact involving an endogenous ER-localized 13-kD FK506 binding protein (FKBP13) competing with the FKBP12-tagged Golgi enzyme for binding to an FKBP-rapamycin binding domain (FRB)-tagged ER trap. When we express an FKBP12-tagged ER trap and FRB-tagged Golgi enzymes, conditions precluding such competition, the Golgi enzymes completely redistribute to the ER upon rapamycin treatment. A photoactivatable FRB-Golgi enzyme, highlighted only in the Golgi, likewise redistributes to the ER. These data establish Golgi enzymes constitutively cycle through the ER. Using our trapping scheme, we identify roles of rab6a and calcium-independent phospholipase A2 (iPLA2) in Golgi enzyme recycling, and show that retrograde transport of Golgi membrane underlies Golgi dispersal during microtubule depolymerization and mitosis.

Onco-Golgi: Is Fragmentation a Gate to Cancer Progression?

The Golgi apparatus-complex is a highly dynamic organelle which is considered the "heart" of intracellular transportation. Since its discovery by Camillo Golgi in 1873, who described it as the "black reaction," and despite the enormous volume of publications about Golgi, this apparatus remains one of the most enigmatic of the cytoplasmic organelles. A typical mammalian Golgi consists of a parallel series of flattened, disk-shaped cisternae which align into stacks. The tremendous volume of Golgi-related incoming and outgoing traffic is mediated by different motor proteins, including members of the dynein, kinesin, and myosin families. Yet in spite of the strenuous work it performs, Golgi contrives to maintain its monolithic morphology and orchestration of matrix and residential proteins. However, in response to stress, alcohol, and treatment with many pharmacological drugs over time, Golgi undergoes a kind of disorganization which ranges from mild enlargement to critical scattering. While fragmentation of the Golgi was confirmed in cancer by electron microscopy almost fifty years ago, it is only in recent years that we have begun to understand the significance of Golgi fragmentation in the biology of tumors. Below author would like to focus on how Golgi fragmentation opens the doors for cascades of fatal pathways which may facilitate cancer progression and metastasis. Among the issues addressed will be the most important cancer-specific hallmarks of Golgi fragmentation, including aberrant glycosylation, abnormal expression of the Ras GTPases, dysregulation of kinases, and hyperactivity of myosin motor proteins.

Protein kinase A activity is necessary for fission and fusion of Golgi to endoplasmic reticulum retrograde tubules

It is becoming increasingly accepted that together with vesicles, tubules play a major role in the transfer of cargo between different cellular compartments. In contrast to our understanding of the molecular mechanisms of vesicular transport, little is known about tubular transport. How signal transduction molecules regulate these two modes of membrane transport processes is also poorly understood. In this study we investigated whether protein kinase A (PKA) activity regulates the retrograde, tubular transport of Golgi matrix proteins from the Golgi to the endoplasmic reticulum (ER). We found that Golgi-to-ER retrograde transport of the Golgi matrix proteins giantin, GM130, GRASP55, GRASP65, and p115 was impaired in the presence of PKA inhibitors. In addition, we unexpectedly found accumulation of tubules containing both Golgi matrix proteins and resident Golgi transmembrane proteins. These tubules were still attached to the Golgi and were highly dynamic. Our data suggest that both fission and fusion of retrograde tubules are mechanisms regulated by PKA activity.

The COPII complex and lysosomal VAMP7 determine intracellular Salmonella localization and growth

Salmonella invades epithelial cells and survives within a membrane-bound compartment, the Salmonella-containing vacuole (SCV). We isolated and determined the host protein composition of the SCV at 30 min and 3 h of infection to identify and characterize novel regulators of intracellular bacterial localization and growth. Quantitation of the SCV protein content revealed 392 host proteins specifically enriched at SCVs, out of which 173 associated exclusively with early SCVs, 124 with maturing SCV and 95 proteins during both time-points. Vacuole interactions with endoplasmic reticulum-derived coat protein complex II vesicles modulate early steps of SCV maturation, promoting SCV rupture and bacterial hyper-replication within the host cytosol. On the other hand, SCV interactions with VAMP7-positive lysosome-like vesicles promote Salmonella-induced filament formation and bacterial growth within the late SCV. Our results reveal that the dynamic communication between the SCV and distinct host organelles affects both intracellular Salmonella localization and growth at successive steps of host cell invasion.

The Golgi apparatus regulates cGMP-dependent protein kinase I compartmentation and proteolysis

cGMP-dependent protein kinase I (PKGI) is an important effector of cGMP signaling that regulates vascular smooth muscle cell (SMC) phenotype and proliferation. PKGI has been detected in the perinuclear region of cells, and recent data indicate that proprotein convertases (PCs) typically resident in the Golgi apparatus (GA) can stimulate PKGI proteolysis and generate a kinase fragment that localizes to the nucleus and regulates gene expression. However, the role of the endomembrane system in PKGI compartmentation and processing is unknown. Here, we demonstrate that PKGI colocalizes with endoplasmic reticulum (ER), ER-Golgi intermediate compartment, GA cisterna, and trans-Golgi network proteins in pulmonary artery SMC and cell lines. Moreover, PKGI localizes with furin, a trans-Golgi network-resident PC known to cleave PKGI. ER protein transport influences PKGI localization because overexpression of a constitutively inactive Sar1 transgene caused PKGI retention in the ER. Additionally, PKGI appears to reside within the GA because PKGI immunoreactivity was determined to be resistant to cytosolic proteinase K treatment in live cells. The GA appears to play a role in PKGI proteolysis because overexpression of inositol 1,4,5-trisphosphate receptor-associated cGMP kinase substrate, not only tethered heterologous PKGI-β to the ER and decreased its localization to the GA, but also diminished PKGI proteolysis and nuclear translocation. Also, inhibiting intra-GA protein transport with monensin was observed to decrease PKGI cleavage. These studies detail a role for the endomembrane system in regulating PKGI compartmentation and proteolysis. Moreover, they support the investigation of mechanisms regulating PKGI-dependent nuclear cGMP signaling in the pulmonary vasculature with Golgi dysfunction.

Self-organization of core Golgi material is independent of COPII-mediated endoplasmic reticulum export

The Golgi is a highly organized and dynamic organelle that receives and distributes material from and to the endoplasmic reticulum (ER) and the endocytic pathway. One open question about Golgi organization is whether it is solely based on ER-to-Golgi transport. Here, we analyzed the kinetics of Golgi breakdown in the absence of COPII-dependent ER export with high temporal and spatial resolution using quantitative fluorescence microscopy. We found that Golgi breakdown occurred in two phases. While Golgi enzymes continuously redistributed to the ER, we consistently observed extensive Golgi fragmentation at the beginning of the breakdown, followed by microtubule-dependent formation of a Golgi remnant structure (phase 1). Further Golgi disintegration occurred less uniformly (phase 2). Remarkably, cisternal Golgi morphology was lost early in phase 1 and Golgi fragments instead corresponded to variably sized vesicle clusters. These breakdown intermediates were devoid of COPI-dependent recycling material, but contained typical 'core' Golgi components. Furthermore, Golgi breakdown intermediates were able to disassemble and reassemble following cell division, indicating that they retained important regulatory capabilities. Taken together, these findings support the view that Golgi self-organization exists independently of ER-to-Golgi transport.

ER stress-induced clearance of misfolded GPI-anchored proteins via the secretory pathway

Proteins destined for the cell surface are first assessed in the endoplasmic reticulum (ER) for proper folding before release into the secretory pathway. This ensures that defective proteins are normally prevented from entering the extracellular environment, where they could be disruptive. Here, we report that, when ER folding capacity is saturated during stress, misfolded glycosylphosphatidylinositol-anchored proteins dissociate from resident ER chaperones, engage export receptors, and quantitatively leave the ER via vesicular transport to the Golgi. Clearance from the ER commences within minutes of acute ER stress, before the transcriptional component of the unfolded protein response is activated. These aberrant proteins then access the cell surface transiently before destruction in lysosomes. Inhibiting this stress-induced pathway by depleting the ER-export receptors leads to aggregation of the ER-retained misfolded protein. Thus, this rapid response alleviates the elevated burden of misfolded proteins in the ER at the onset of ER stress, promoting protein homeostasis in the ER.

Host endoplasmic reticulum COPII proteins control cell-to-cell spread of the bacterial pathogen Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen that uses actin-dependent motility to spread between human cells. Cell-to-cell spread involves the formation by motile bacteria of plasma membrane-derived structures termed 'protrusions'. In cultured enterocytes, the secreted Listeria protein InlC promotes protrusion formation by binding and inhibiting the human scaffolding protein Tuba. Here we demonstrate that protrusions are controlled by human COPII components that direct trafficking from the endoplasmic reticulum. Co-precipitation experiments indicated that the COPII proteins Sec31A and Sec13 interact directly with a Src homology 3 domain in Tuba. This interaction was antagonized by InlC. Depletion of Sec31A or Sec13 restored normal protrusion formation to a Listeria mutant lacking inlC, without affecting spread of wild-type bacteria. Genetic impairment of the COPII component Sar1 or treatment of cells with brefeldin A affected protrusions similarly to Sec31A or Sec13 depletion. These findings indicated that InlC relieves a host-mediated restriction of Listeria spread otherwise imposed by COPII. Inhibition of Sec31A, Sec13 or Sar1 or brefeldin A treatment also perturbed the structure of cell-cell junctions. Collectively, these findings demonstrate an important role for COPII in controlling Listeria spread. We propose that COPII may act by delivering host proteins that generate tension at cell junctions.

Phosphatidylinositol 3-kinase and COPII generate LC3 lipidation vesicles from the ER-Golgi intermediate compartment

Formation of the autophagosome requires significant membrane input from cellular organelles. However, no direct evidence has been developed to link autophagic factors and the mobilization of membranes to generate the phagophore. Previously, we established a cell-free LC3 lipidation reaction to identify the ER-Golgi intermediate compartment (ERGIC) as a membrane source for LC3 lipidation, a key step of autophagosome biogenesis (Ge et al., eLife 2013; 2:e00947). We now report that starvation activation of autophagic phosphotidylinositol-3 kinase (PI3K) induces the generation of small vesicles active in LC3 lipidation. Subcellular fractionation studies identified the ERGIC as the donor membrane in the generation of small lipidation-active vesicles. COPII proteins are recruited to the ERGIC membrane in starved cells, dependent on active PI3K. We conclude that starvation activates the autophagic PI3K, which in turn induces the recruitment of COPII to the ERGIC to bud LC3 lipidation-active vesicles as one potential membrane source of the autophagosome.

Vesicles bearing Toxoplasma apicoplast membrane proteins persist following loss of the relict plastid or Golgi body disruption

Toxoplasma gondii and malaria parasites contain a unique and essential relict plastid called the apicoplast. Most apicoplast proteins are encoded in the nucleus and are transported to the organelle via the endoplasmic reticulum (ER). Three trafficking routes have been proposed for apicoplast membrane proteins: (i) vesicular trafficking from the ER to the Golgi and then to the apicoplast, (ii) contiguity between the ER membrane and the apicoplast allowing direct flow of proteins, and (iii) vesicular transport directly from the ER to the apicoplast. Previously, we identified a set of membrane proteins of the T. gondii apicoplast which were also detected in large vesicles near the organelle. Data presented here show that the large vesicles bearing apicoplast membrane proteins are not the major carriers of luminal proteins. The vesicles continue to appear in parasites which have lost their plastid due to mis-segregation, indicating that the vesicles are not derived from the apicoplast. To test for a role of the Golgi body in vesicle formation, parasites were treated with brefeldin A or transiently transfected with a dominant-negative mutant of Sar1, a GTPase required for ER to Golgi trafficking. The immunofluorescence patterns showed little change. These findings were confirmed using stable transfectants, which expressed the toxic dominant-negative sar1 following Cre-loxP mediated promoter juxtaposition. Our data support the hypothesis that the large vesicles do not mediate the trafficking of luminal proteins to the apicoplast. The results further show that the large vesicles bearing apicoplast membrane proteins continue to be observed in the absence of Golgi and plastid function. These data raise the possibility that the apicoplast proteome is generated by two novel ER to plastid trafficking pathways, plus the small set of proteins encoded by the apicoplast genome.

Low cytoplasmic pH reduces ER-Golgi trafficking and induces disassembly of the Golgi apparatus

The Golgi apparatus was dramatically disassembled when cells were incubated in a low pH medium. The cis-Golgi disassembled quickly, extended tubules and spread to the periphery of cells within 30 min. In contrast, medial- and trans-Golgi were fragmented in significantly larger structures of smaller numbers at a slower rate and remained largely in structures distinct from the cis-Golgi. Electron microscopy revealed the complete disassembly of the Golgi stack in low pH treated cells. The effect of low pH was reversible; the Golgi apparatus reassembled to form a normal ribbon-like structure within 1-2h after the addition of a control medium. The anterograde ER to Golgi transport and retrograde Golgi to ER transport were both reduced under low pH. Phospholipase A2 inhibitors (ONO, BEL) effectively suppressed the Golgi disassembly, suggesting that the phospholipase A2 was involved in the Golgi disassembly. Over-expression of Rab1, 2, 30, 33 and 41 also suppressed the Golgi disassembly under low pH, suggesting that they have protective role against Golgi disassembly. Low pH treatment reduced cytoplasmic pH, but not the luminal pH of the Golgi apparatus, strongly suggesting that reduction of the cytoplasmic pH triggered the Golgi disassembly. Because a lower cytoplasmic pH is induced in physiological or pathological conditions, disassembly of the Golgi apparatus and reduction of vesicular transport through the Golgi apparatus may play important roles in cell physiology and pathology. Furthermore, our findings indicated that low pH treatment can serve as an important tool to analyze the molecular mechanisms that support the structure and function of the Golgi apparatus.

Positive feedback between Golgi membranes, microtubules and ER exit sites directs de novo biogenesis of the Golgi

The Golgi complex is the central organelle of the secretory pathway. It undergoes dynamic changes during the cell cycle, but how it acquires and maintains its complex structure is unclear. To address this question, we have used laser nanosurgery to deplete BSC1 cells of the Golgi complex and have monitored its biogenesis by quantitative time-lapse microscopy and correlative electron microscopy. After Golgi depletion, endoplasmic reticulum (ER) export is inhibited and the number of ER exit sites (ERES) is reduced and does not increase for several hours. Occasional fusion of small post-ER carriers to form the first larger structures triggers a rapid and drastic growth of Golgi precursors, due to the capacity of these structures to attract more carriers by microtubule nucleation and to stimulate ERES biogenesis. Increasing the chances of post-ER carrier fusion close to ERES by depolymerizing microtubules results in the acceleration of Golgi and ERES biogenesis. Taken together, on the basis of our results, we propose a self-organizing principle of the early secretory pathway that integrates Golgi biogenesis, ERES biogenesis and the organization of the microtubule network by positive-feedback loops.

Imaging ER-to-Golgi transport: towards a systems view

Proteins synthesised at the endoplasmic reticulum (ER) have to undergo a number of consecutive and coordinated steps to reach the Golgi complex. To understand the dynamic complexity of ER-to-Golgi transport at the structural and molecular level, light microscopy approaches are fundamental tools that allow in vivo observations of protein dynamics and interactions of fluorescent proteins in living cells. Imaging protein and organelle dynamics close to the ultra-structural level became possible by combining light microscopy with electron microscopy analyses or super-resolution light microscopy methods. Besides, increasing evidence suggests that the early secretory pathway is tightly connected to other cellular processes, such as signal transduction, and quantitative information at the systems level is fundamental to achieve a comprehensive molecular understanding of these connections. High-throughput microscopy in fixed and living cells in combination with systematic perturbation of gene expression by, e.g. RNA interference, will open new avenues to gain such an understanding of the early secretory pathway at the systems level. In this Commentary, we first outline examples that revealed the dynamic organisation of ER-to-Golgi transport in living cells. Next, we discuss the use of advanced imaging methods in studying ER-to-Golgi transport and, finally, delineate the efforts in understanding ER-to-Golgi transport at the systems level.

Transport along the dendritic endoplasmic reticulum mediates the trafficking of GABAB receptors

In neurons, secretory organelles within the cell body are complemented by the dendritic endoplasmic reticulum (ER) and Golgi outposts (GOPs), whose role in neurotransmitter receptor trafficking is poorly understood. γ-aminobutyric acid (GABA) type B metabotropic receptors (GABABRs) regulate the efficacy of synaptic transmission throughout the brain. Their plasma membrane availability is controlled by mechanisms involving an ER retention motif and assembly-dependent ER export. Thus, they constitute an ideal molecular model to study ER trafficking, but the extent to which the dendritic ER participates in GABABR biosynthesis has not been thoroughly explored. Here, we show that GABAB1 localizes preferentially to the ER in dendrites and moves long distances within this compartment. Not only diffusion but also microtubule and dynein-dependent mechanisms control dendritic ER transport. GABABRs insert throughout the somatodendritic plasma membrane but dendritic post-ER carriers containing GABABRs do not fuse selectively with GOPs. This study furthers our understanding of the spatial selectivity of neurotransmitter receptors for dendritic organelles.

Golgi fragmentation induced by heat shock or inhibition of heat shock proteins is mediated by non-muscle myosin IIA via its interaction with glycosyltransferases

The Golgi apparatus is a highly dynamic organelle which frequently undergoes morphological changes in certain normal physiological processes or in response to stress. The mechanisms are largely not known. We have found that heat shock of Panc1 cells expressing core 2 N-acetylglucosaminyltransferase-M (Panc1-C2GnT-M) induces Golgi disorganization by increasing non-muscle myosin IIA (NMIIA)-C2GnT-M complexes and polyubiquitination and proteasomal degradation of C2GnT-M. These effects are prevented by inhibition or knockdown of NMIIA. Also, the speed of Golgi fragmentation induced by heat shock is found to be positively correlated with the levels of C2GnT-M in the Golgi. The results are reproduced in LNCaP cells expressing high levels of two endogenous glycosyltransferases-core 2 N-acetylglucosaminyltransferase-L:1 and β-galactoside:α2-3 sialyltransferase 1. Further, during recovery after heat shock, Golgi reassembly as monitored by a Golgi matrix protein giantin precedes the return of C2GnT-M to the Golgi. The results are consistent with the roles of giantin as a building block of the Golgi architecture and a docking site for transport vesicles carrying glycosyltransferases. In addition, inhibition/depletion of HSP70 or HSP90 in Panc1-C2GnT-M cells also causes an increase of NMIIA-C2GnT-M complexes and NMIIA-mediated Golgi fragmentation but results in accumulation or degradation of C2GnT-M, respectively. These results can be explained by the known functions of these two HSP: participation of HSP90 in protein folding and HSP70 in protein folding and degradation. We conclude that NMIIA is the master regulator of Golgi fragmentation induced by heat shock or inhibition/depletion of HSP70/90.

Clathrin terminal domain-ligand interactions regulate sorting of mannose 6-phosphate receptors mediated by AP-1 and GGA adaptors

Clathrin plays important roles in intracellular membrane traffic including endocytosis of plasma membrane proteins and receptors and protein sorting between the trans-Golgi network (TGN) and endosomes. Whether clathrin serves additional roles in receptor recycling, degradative sorting, or constitutive secretion has remained somewhat controversial. Here we have used acute pharmacological perturbation of clathrin terminal domain (TD) function to dissect the role of clathrin in intracellular membrane traffic. We report that internalization of major histocompatibility complex I (MHCI) is inhibited in cells depleted of clathrin or its major clathrin adaptor complex 2 (AP-2), a phenotype mimicked by application of Pitstop® inhibitors of clathrin TD function. Hence, MHCI endocytosis occurs via a clathrin/AP-2-dependent pathway. Acute perturbation of clathrin also impairs the dynamics of intracellular clathrin/adaptor complex 1 (AP-1)- or GGA (Golgi-localized, γ-ear-containing, Arf-binding protein)-coated structures at the TGN/endosomal interface, resulting in the peripheral dispersion of mannose 6-phosphate receptors. By contrast, secretory traffic of vesicular stomatitis virus G protein, recycling of internalized transferrin from endosomes, or degradation of EGF receptor proceeds unperturbed in cells with impaired clathrin TD function. These data indicate that clathrin is required for the function of AP-1- and GGA-coated carriers at the TGN but may be dispensable for outward traffic en route to the plasma membrane.

Decoupling polarization of the Golgi apparatus and GM1 in the plasma membrane

Cell polarization is a process of coordinated cellular rearrangements that prepare the cell for migration. GM1 is synthesized in the Golgi apparatus and localized in membrane microdomains that appear at the leading edge of polarized cells, but the mechanism by which GM1 accumulates asymmetrically is unknown. The Golgi apparatus itself becomes oriented toward the leading edge during cell polarization, which is thought to contribute to plasma membrane asymmetry. Using quantitative image analysis techniques, we measure the extent of polarization of the Golgi apparatus and GM1 in the plasma membrane simultaneously in individual cells subject to a wound assay. We find that GM1 polarization starts just 10 min after stimulation with growth factors, while Golgi apparatus polarization takes 30 min. Drugs that block Golgi polarization or function have no effect on GM1 polarization, and, conversely, inhibiting GM1 polarization does not affect Golgi apparatus polarization. Evaluation of Golgi apparatus and GM1 polarization in single cells reveals no correlation between the two events. Our results indicate that Golgi apparatus and GM1 polarization are controlled by distinct intracellular cascades involving the Ras/Raf/MEK/ERK and the PI3K/Akt/mTOR pathways, respectively. Analysis of cell migration and invasion suggest that MEK/ERK activation is crucial for two dimensional migration, while PI3K activation drives three dimensional invasion, and no cumulative effect is observed from blocking both simultaneously. The independent biochemical control of GM1 polarity by PI3K and Golgi apparatus polarity by MEK/ERK may act synergistically to regulate and reinforce directional selection in cell migration.

Integrated self-organization of transitional ER and early Golgi compartments

COPII coated vesicles bud from an ER domain termed the transitional ER (tER), but the mechanism that clusters COPII vesicles at tER sites is unknown. tER sites are closely associated with early Golgi or pre-Golgi structures, suggesting that the clustering of nascent COPII vesicles could be achieved by tethering to adjacent membranes. This model challenges the prevailing view that COPII vesicles are clustered by a scaffolding protein at the ER surface. Although Sec16 was proposed to serve as such a scaffolding protein, recent data suggest that rather than organizing COPII into higher-order structures, Sec16 acts at the level of individual COPII vesicles to regulate COPII turnover. A plausible synthesis is that tER sites are created by tethering to Golgi membranes and are regulated by Sec16. Meanwhile, the COPII vesicles that bud from tER sites are thought to nucleate new Golgi cisternae. Thus, an integrated self-organization process may generate tER-Golgi units.

MAIGO5 functions in protein export from Golgi-associated endoplasmic reticulum exit sites in Arabidopsis

Plant cells face unique challenges to efficiently export cargo from the endoplasmic reticulum (ER) to mobile Golgi stacks. Coat protein complex II (COPII) components, which include two heterodimers of Secretory23/24 (Sec23/24) and Sec13/31, facilitate selective cargo export from the ER; however, little is known about the mechanisms that regulate their recruitment to the ER membrane, especially in plants. Here, we report a protein transport mutant of Arabidopsis thaliana, named maigo5 (mag5), which abnormally accumulates precursor forms of storage proteins in seeds. mag5-1 has a deletion in the putative ortholog of the Saccharomyces cerevisiae and Homo sapiens Sec16, which encodes a critical component of ER exit sites (ERESs). mag mutants developed abnormal structures (MAG bodies) within the ER and exhibited compromised ER export. A functional MAG5/SEC16A-green fluorescent protein fusion localized at Golgi-associated cup-shaped ERESs and cycled on and off these sites at a slower rate than the COPII coat. MAG5/SEC16A interacted with SEC13 and SEC31; however, in the absence of MAG5/SEC16A, recruitment of the COPII coat to ERESs was accelerated. Our results identify a key component of ER export in plants by demonstrating that MAG5/SEC16A is required for protein export at ERESs that are associated with mobile Golgi stacks, where it regulates COPII coat turnover.

A non-canonical di-acidic signal at the C-terminus of Kv1.3 determines anterograde trafficking and surface expression

Impairment of Kv1.3 expression at the cell membrane in leukocytes and sensory neuron contributes to the pathophysiology of autoimmune diseases and sensory syndromes. Molecular mechanisms underlying Kv1.3 channel trafficking to the plasma membrane remain elusive. We report a novel non-canonical di-acidic signal (E483/484) at the C-terminus of Kv1.3 essential for anterograde transport and surface expression. Notably, homologous motifs are conserved in neuronal Kv1 and Shaker channels. Biochemical analysis revealed interactions with the Sec24 subunit of the coat protein complex II. Disruption of this complex retains the channel at the endoplasmic reticulum. A molecular model of the Kv1.3-Sec24a complex suggests salt-bridges between the di-acidic E483/484 motif in Kv1.3 and the di-basic R750/752 sequence in Sec24. These findings identify a previously unrecognized motif of Kv channels essential for their expression on the cell surface. Our results contribute to our understanding of how Kv1 channels target to the cell membrane, and provide new therapeutic strategies for the treatment of pathological conditions.

Inter-Golgi transport mediated by COPI-containing vesicles carrying small cargoes

A core prediction of the vesicular transport model is that COPI vesicles are responsible for trafficking anterograde cargoes forward. In this study, we test this prediction by examining the properties and requirements of inter-Golgi transport within fused cells, which requires mobile carriers in order for exchange of constituents to occur. We report that both small soluble and membrane-bound secretory cargo and exogenous Golgi resident glycosyl-transferases are exchanged between separated Golgi. Large soluble aggregates, which traverse individual stacks, do not transfer between Golgi, implying that small cargoes (which can fit in a typical transport vesicle) are transported by a different mechanism. Super-resolution microscopy reveals that the carriers of both anterograde and retrograde cargoes are the size of COPI vesicles, contain coatomer, and functionally require ARF1 and coatomer for transport. The data suggest that COPI vesicles traffic both small secretory cargo and steady-state Golgi resident enzymes among stacked cisternae that are stationary.

DOI:http://dx.doi.org/10.7554/eLife.01296.001

All eukaryotic cells contain an organelle called the Golgi apparatus, which consists of a series of four to six flattened structures called cisternae. Proteins that are intended for secretion from the cell, or proteins that go on to become part of the cell membrane, must pass through the Golgi, where they undergo modifications that ensure they are targeted to the correct place.

There are two main models for how proteins are transported from the entry side of the Golgi, known as the cis face, to the exit side (trans face), through a process known as anterograde transport. One possibility is that the cargo protein matures within a single cisterna, which gradually moves from the cis to the trans face without the protein ever leaving it. Alternatively, the cisternae may remain fixed in position, while individual proteins are carried between them by specialized transport vesicles called COPI vesicles.

Now, Pellett et al. have used modern molecular biology techniques to revisit this question, more than 25 years after members of the same group first obtained evidence suggesting the involvement of COPI vesicles. To do this, they labelled the proteins that reside within the Golgi of one cell green, and those within the Golgi of another cell, red. They then fused the two cells together, and traced the movement of labelled proteins between the two organelles.

Proteins that are known to undergo anterograde transport were also transported between the two Golgi, whereas large protein aggregates were not. Super-resolution microscopy revealed that the transported proteins were carried in vesicles the size of COPI vesicles and surrounded by a coat protein that resembles COPI. Moreover, transport involved the adaptor protein ARF, which helps to load cargo into COPI vesicles.

By providing evidence that Golgi resident proteins and proteins that normally undergo anterograde transport can be carried by COPI vesicles between two physically separate Golgi, Pellett et al. increase the weight of evidence that COPI vesicles may also be responsible for both retrograde and anterograde transport within the Golgi itself.

DOI:http://dx.doi.org/10.7554/eLife.01296.002

The B7-1 cytoplasmic tail enhances intracellular transport and mammalian cell surface display of chimeric proteins in the absence of a linear ER export motif

Membrane-tethered proteins (mammalian surface display) are increasingly being used for novel therapeutic and biotechnology applications. Maximizing surface expression of chimeric proteins on mammalian cells is important for these applications. We show that the cytoplasmic domain from the B7-1 antigen, a commonly used element for mammalian surface display, can enhance the intracellular transport and surface display of chimeric proteins in a Sar1 and Rab1 dependent fashion. However, mutational, alanine scanning and deletion analysis demonstrate the absence of linear ER export motifs in the B7 cytoplasmic domain. Rather, efficient intracellular transport correlated with the presence of predicted secondary structure in the cytoplasmic tail. Examination of the cytoplasmic domains of 984 human and 782 mouse type I transmembrane proteins revealed that many previously identified ER export motifs are rarely found in the cytoplasmic tail of type I transmembrane proteins. Our results suggest that efficient intracellular transport of B7 chimeric proteins is associated with the structure rather than to the presence of a linear ER export motif in the cytoplasmic tail, and indicate that short (less than ~ 10-20 amino acids) and unstructured cytoplasmic tails should be avoided to express high levels of chimeric proteins on mammalian cells.

A role for endoplasmic reticulum exit sites in foot-and-mouth disease virus infection

Picornaviruses replicate their genomes in association with cellular membranes. While enteroviruses are believed to utilize membranes of the early secretory pathway, the origin of the membranes used by foot-and-mouth disease virus (FMDV) for replication are unknown. Secretory-vesicle traffic through the early secretory pathway is mediated by the sequential acquisition of two distinct membrane coat complexes, COPII and COPI, and requires the coordinated actions of Sar1, Arf1 and Rab proteins. Sar1 is essential for generating COPII vesicles at endoplasmic reticulum (ER) exit sites (ERESs), while Arf1 and Rab1 are required for subsequent vesicle transport by COPI vesicles. In the present study, we have provided evidence that FMDV requires pre-Golgi membranes of the early secretory pathway for infection. Small interfering RNA depletion of Sar1 or expression of a dominant-negative (DN) mutant of Sar1a inhibited FMDV infection. In contrast, a dominant-active mutant of Sar1a, which allowed COPII vesicle formation but inhibited the secretory pathway by stabilizing COPII coats, caused major disruption to the ER–Golgi intermediate compartment (ERGIC) but did not inhibit infection. Treatment of cells with brefeldin A, or expression of DN mutants of Arf1 and Rab1a, disrupted the Golgi and enhanced FMDV infection. These results show that reagents that block the early secretory pathway at ERESs have an inhibitory effect on FMDV infection, while reagents that block the early secretory pathway immediately after ER exit but before the ERGIC and Golgi make infection more favourable. Together, these observations argue for a role for Sar1 in FMDV infection and that initial virus replication takes place on membranes that are formed at ERESs.

The ER-Golgi intermediate compartment is a key membrane source for the LC3 lipidation step of autophagosome biogenesis

Autophagy is a catabolic process for bulk degradation of cytosolic materials mediated by double-membraned autophagosomes. The membrane determinant to initiate the formation of autophagosomes remains elusive. Here, we establish a cell-free assay based on LC3 lipidation to define the organelle membrane supporting early autophagosome formation. In vitro LC3 lipidation requires energy and is subject to regulation by the pathways modulating autophagy in vivo. We developed a systematic membrane isolation scheme to identify the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) as a primary membrane source both necessary and sufficient to trigger LC3 lipidation in vitro. Functional studies demonstrate that the ERGIC is required for autophagosome biogenesis in vivo. Moreover, we find that the ERGIC acts by recruiting the early autophagosome marker ATG14, a critical step for the generation of preautophagosomal membranes. DOI:http://dx.doi.org/10.7554/eLife.00947.001.

Organization of the ER-Golgi interface for membrane traffic control

Coat protein complex I (COPI) and COPII are required for bidirectional membrane trafficking between the endoplasmic reticulum (ER) and the Golgi. While these core coat machineries and other transport factors are highly conserved across species, high-resolution imaging studies indicate that the organization of the ER-Golgi interface is varied in eukaryotic cells. Regulation of COPII assembly, in some cases to manage distinct cellular cargo, is emerging as one important component in determining this structure. Comparison of the ER-Golgi interface across different systems, particularly mammalian and plant cells, reveals fundamental elements and distinct organization of this interface. A better understanding of how these interfaces are regulated to meet varying cellular secretory demands should provide key insights into the mechanisms that control efficient trafficking of proteins and lipids through the secretory pathway.

The ALS8 protein VAPB interacts with the ER-Golgi recycling protein YIF1A and regulates membrane delivery into dendrites

The vesicle-associated membrane protein (VAMP) associated protein B (VAPB) is an integral membrane protein localized to the endoplasmic reticulum (ER). The P56S mutation in VAPB has been linked to motor neuron degeneration in amyotrophic lateral sclerosis type 8 (ALS8) and forms ER-like inclusions in various model systems. However, the role of wild-type and mutant VAPB in neurons is poorly understood. Here, we identified Yip1-interacting factor homologue A (YIF1A) as a new VAPB binding partner and important component in the early secretory pathway. YIF1A interacts with VAPB via its transmembrane regions, recycles between the ER and Golgi and is mainly localized to the ER-Golgi intermediate compartments (ERGICs) in rat hippocampal neurons. VAPB strongly affects the distribution of YIF1A and is required for intracellular membrane trafficking into dendrites and normal dendritic morphology. When VAPB-P56S is present, YIF1A is recruited to the VAPB-P56S clusters and loses its ERGIC localization. These data suggest that both VAPB and YIF1A are important for ER-to-Golgi transport and that missorting of YIF1A may contribute to VAPB-associated motor neuron disease.

Silencing of mammalian Sar1 isoforms reveals COPII-independent protein sorting and transport

The Sar1 GTPase coordinates the assembly of coat protein complex-II (COPII) at specific sites of the endoplasmic reticulum (ER). COPII is required for ER-to-Golgi transport, as it provides a structural and functional framework to ship out protein cargoes produced in the ER. To investigate the requirement of COPII-mediated transport in mammalian cells, we used small interfering RNA (siRNA)-mediated depletion of Sar1A and Sar1B. We report that depletion of these two mammalian forms of Sar1 disrupts COPII assembly and the cells fail to organize transitional elements that coordinate classical ER-to-Golgi protein transfer. Under these conditions, minimal Golgi stacks are seen in proximity to juxtanuclear ER membranes that contain elements of the intermediate compartment, and from which these stacks coordinate biosynthetic transport of protein cargo, such as the vesicular stomatitis virus G protein and albumin. Here, transport of procollagen-I is inhibited. These data provide proof-of-principle for the contribution of alternative mechanisms that support biosynthetic trafficking in mammalian cells, providing evidence of a functional boundary associated with a bypass of COPII.

AMPK phosphorylates GBF1 for mitotic Golgi disassembly

In mammalian cells, the Golgi apparatus undergoes extensive fragmentation during mitosis; this is required not only for the partitioning of the complex but also for the process of mitosis. However, the molecular mechanism underlying the mitotic fragmentation of the Golgi is far from clear. Here, we show that AMP-activated protein kinase (AMPK) is phosphorylated and activated when cells enter mitosis. Activated AMPK phosphorylates GBF1, a guanine nucleotide exchange factor (GEF) for Arf-GTPases, disassociating GBF1 from the Golgi membrane and abolishing the action of GBF1 as an Arf1-GEF. We further demonstrate that the phosphorylation of AMPK and GBF1 is essential for Golgi disassembly and subsequent mitosis entry. These data suggest that AMPK-GBF1-Arf1 signaling is involved in the regulation of Golgi fragmentation during mitosis.

A non-enzymatic function of Golgi glycosyltransferases: mediation of Golgi fragmentation by interaction with non-muscle myosin IIA

The Golgi apparatus undergoes morphological changes under stress or malignant transformation, but the precise mechanisms are not known. We recently showed that non-muscle myosin IIA (NMIIA) binds to the cytoplasmic tail of Core 2 N-acetylglucosaminyltransferase mucus-type (C2GnT-M) and transports it to the endoplasmic reticulum for recycling. Here, we report that Golgi fragmentation induced by brefeldin A (BFA) or coatomer protein (β-COP) knockdown (KD) in Panc1-bC2GnT-M (c-Myc) cells is accompanied by the increased association of NMIIA with C2GnT-M and its degradation by proteasomes. Golgi fragmentation is prevented by inhibition or KD of NMIIA. Using multiple approaches, we have shown that the speed of BFA-induced Golgi fragmentation is positively correlated with the levels of this enzyme in the Golgi. The observation is reproduced in LNCaP cells which express high levels of two endogenous glycosyltransferases--C2GnT-L and β-galactoside α2,3 sialyltransferase 1. NMIIA is found to form complexes with these two enzymes but not Golgi matrix proteins. The KD of both enzymes or the prevention of Golgi glycosyltransferases from exiting endoplasmic reticulum reduced Golgi-associated NMIIA and decreased the BFA-induced fragmentation. Interestingly, the fragmented Golgi detected in colon cancer HT-29 cells can be restored to a compact morphology after inhibition or KD of NMIIA. The Golgi disorganization induced by the microtubule or actin destructive agent is NMIIA-independent and does not affect the levels of glycosyltransferases. We conclude that NMIIA interacts with Golgi residential but not matrix proteins, and this interaction is responsible for Golgi fragmentation induced by β-COP KD or BFA treatment. This is a novel non-enzymatic function of Golgi glycosyltransferases.

Photohighlighting approaches to access membrane dynamics of the Golgi apparatus

By providing quantitative, visual data of live cells, fluorescent protein-based microscopy techniques are furnishing novel insights into the complexities of membrane trafficking pathways and organelle dynamics. In this chapter, we describe experimental protocols employing fluorescent protein-based photohighlighting techniques to quantify protein movement into and out of the Golgi apparatus, an organelle that serves as the central sorting and processing station of the secretory pathway. The methods allow kinetic characteristics of Golgi-associated protein trafficking to be deciphered, which can help clarify how the Golgi maintains itself as a steady-state structure despite a continuous flux of secretory cargo passing into and out of this organelle. The guidelines presented in this chapter can also be applied to examine the dynamics of other intracellular organelle systems, elucidating mechanisms for how proteins are maintained in specific organelles and/or circulated to other destinations within the cell.

Export of cyst wall material and Golgi organelle neogenesis in Giardia lamblia depend on endoplasmic reticulum exit sites

Giardia lamblia parasitism accounts for the majority of cases of parasitic diarrheal disease, making this flagellated eukaryote the most successful intestinal parasite worldwide. This organism has undergone secondary reduction/elimination of entire organelle systems such as mitochondria and Golgi. However, trophozoite to cyst differentiation (encystation) requires neogenesis of Golgi-like secretory organelles named encystation-specific vesicles (ESVs), which traffic, modify and partition cyst wall proteins produced exclusively during encystation. In this work we ask whether neogenesis of Golgi-related ESVs during G. lamblia differentiation, similarly to Golgi biogenesis in more complex eukaryotes, requires the maintenance of distinct COPII-associated endoplasmic reticulum (ER) subdomains in the form of ER exit sites (ERES) and whether ERES are also present in non-differentiating trophozoites. To address this question, we identified conserved COPII components in G. lamblia cells and determined their localization, quantity and dynamics at distinct ERES domains in vegetative and differentiating trophozoites. Analogous to ERES and Golgi biogenesis, these domains were closely associated to early stages of newly generated ESV. Ectopic expression of non-functional Sar1 GTPase variants caused ERES collapse and, consequently, ESV ablation, leading to impaired parasite differentiation. Thus, our data show how ERES domains remain conserved in G. lamblia despite elimination of steady-state Golgi. Furthermore, the fundamental eukaryotic principle of ERES to Golgi/Golgi-like compartment correspondence holds true in differentiating Giardia presenting streamlined machinery for secretory organelle biogenesis and protein trafficking. However, in the Golgi-less trophozoites ERES exist as stable ER subdomains, likely as the sole sorting centres for secretory traffic.

Transport of the GlcNAc-1-phosphotransferase α/β-subunit precursor protein to the Golgi apparatus requires a combinatorial sorting motif

The Golgi-resident N-acetylglucosamine-1-phosphotransferase (PT) complex is composed of two α-, β-, and γ-subunits and represents the key enzyme for the biosynthesis of mannose 6-phosphate recognition marker on soluble lysosomal proteins. Mutations in the PT complex cause the lysosomal storage diseases mucolipidosis II and III. A prerequisite for the enzymatic activity is the site-1 protease-mediated cleavage of the PT α/β-subunit precursor protein in the Golgi apparatus. Here, we have investigated structural requirements of the PT α/β-subunit precursor protein for its efficient export from the endoplasmic reticulum (ER). Both wild-type and a cleavage-resistant type III membrane PT α/β-subunit precursor protein are exported whereas coexpressed separate α- and β-subunits failed to reach the cis-Golgi compartment. Mutational analyses revealed combinatorial, non-exchangeable dileucine and dibasic motifs located in a defined sequence context in the cytosolic N- and C-terminal domains that are required for efficient ER exit and subsequent proteolytic activation of the α/β-subunit precursor protein in the Golgi. In the presence of a dominant negative Sar1 mutant the ER exit of the PT α/β-subunit precursor protein is inhibited indicating its transport in coat protein complex II-coated vesicles. Expression studies of missense mutations identified in mucolipidosis III patients that alter amino acids in the N- and C-terminal domains demonstrated that the substitution of a lysine residue in close proximity to the dileucine sorting motif impaired ER-Golgi transport and subsequent activation of the PT α/β-subunit precursor protein. The data suggest that the oligomeric type III membrane protein PT complex requires a combinatorial sorting motif that forms a tertiary epitope to be recognized by distinct sites within the coat protein complex II machinery.

Endoplasmic reticulum sorting and kinesin-1 command the targeting of axonal GABAB receptors

In neuronal cells the intracellular trafficking machinery controls the availability of neurotransmitter receptors at the plasma membrane, which is a critical determinant of synaptic strength. Metabotropic γ amino-butyric acid (GABA) type B receptors (GABA_BRs) are neurotransmitter receptors that modulate synaptic transmission by mediating the slow and prolonged responses to GABA. GABA_BRs are obligatory heteromers constituted by two subunits, GABA_BR1 and GABA_BR2. GABA_BR1a and GABA_BR1b are the most abundant subunit variants. GABA_BR1b is located in the somatodendritic domain whereas GABA_BR1a is additionally targeted to the axon. Sushi domains located at the N-terminus of GABA_BR1a constitute the only difference between both variants and are necessary and sufficient for axonal targeting. The precise targeting machinery and the organelles involved in sorting and transport have not been described. Here we demonstrate that GABA_BRs require the Golgi apparatus for plasma membrane delivery but that axonal sorting and targeting of GABA_BR1a operate in a pre-Golgi compartment. In the axon GABA_BR1a subunits are enriched in the endoplasmic reticulum (ER), and their dynamic behavior and colocalization with other secretory organelles like the ER-to-Golgi intermediate compartment (ERGIC) suggest that they employ a local secretory route. The transport of axonal GABA_BR1a is microtubule-dependent and kinesin-1, a molecular motor of the kinesin family, determines axonal localization. Considering that progression of GABA_BRs through the secretory pathway is regulated by an ER retention motif our data contribute to understand the role of the axonal ER in non-canonical sorting and targeting of neurotransmitter receptors.

Trafficking of an endogenous potassium channel in adult ventricular myocytes

The roles of several small GTPases in the expression of an endogenous potassium current, I(to,f), in adult rat ventricular myocytes have been investigated. The results indicate that forward trafficking of newly synthesized Kv4.2, which underlies I(to,f) in these cells, requires both Rab1 and Sar1 function. Expression of a Rab1 dominant negative (DN) reduced I(to,f) current density by roughly one-half relative to control, mCherry-transfected myocytes. Similarly, expression of a Sar1DN nearly halved I(to,f) current density. Rab11 is not essential to trafficking of Kv4.2, as expression of a Rab11DN had no effect on I(to,f) over the time frames investigated here. In a process dependent on intact endoplasmic reticulum (ER)-to-Golgi transport, however, overexpression of wild-type Rab11 resulted in a doubling of I(to,f) density; block of ER-to-Golgi traffic by Brefeldin A completely abrogated the effect. Also implicated in the trafficking of Kv4.2 are Rab5 and Rab4. Rab5DN expression increased endogenous I(to,f) by two- to threefold, nonadditively with inhibition of dynamin-dependent endocytosis. And, in a phenomenon similar to that previously reported for myoblast-expressed Kv1.5, Rab4DN expression roughly doubled endogenous peak transient currents. Colocalization experiments confirmed the involvement of Rab4 in postinternalization trafficking of Kv4.2. There was little role evident for the lysosome in the degradation of internalized Kv4.2, as overexpression of neither wild-type nor DN isoforms of Rab7 had any effect on I(to,f). Instead, degradation may depend largely on the proteasome; the proteasome inhibitor MG132 significantly increased I(to,f) density.

cis-Golgi proteins accumulate near the ER exit sites and act as the scaffold for Golgi regeneration after brefeldin A treatment in tobacco BY-2 cells

The Golgi apparatus forms stacks of cisternae in many eukaryotic cells. However, little is known about how such a stacked structure is formed and maintained. To address this question, plant cells provide a system suitable for live-imaging approaches because individual Golgi stacks are well separated in the cytoplasm. We established tobacco BY-2 cell lines expressing multiple Golgi markers tagged by different fluorescent proteins and observed their responses to brefeldin A (BFA) treatment and BFA removal. BFA treatment disrupted cis, medial, and trans cisternae but caused distinct relocalization patterns depending on the proteins examined. Medial- and trans-Golgi proteins, as well as one cis-Golgi protein, were absorbed into the endoplasmic reticulum (ER), but two other cis-Golgi proteins formed small punctate structures. After BFA removal, these puncta coalesced first, and then the Golgi stacks regenerated from them in the cis-to-trans order. We suggest that these structures have a property similar to the ER-Golgi intermediate compartment and function as the scaffold of Golgi regeneration.

COPI is required for enterovirus 71 replication

Enterovirus 71 (EV71), a member of the Picornaviridae family, is found in Asian countries where it causes a wide range of human diseases. No effective therapy is available for the treatment of these infections. Picornaviruses undergo RNA replication in association with membranes of infected cells. COPI and COPII have been shown to be involved in the formation of picornavirus-induced vesicles. Replication of several picornaviruses, including poliovirus and Echovirus 11 (EV11), is dependent on COPI or COPII. Here, we report that COPI, but not COPII, is required for EV71 replication. Replication of EV71 was inhibited by brefeldin A and golgicide A, inhibitors of COPI activity. Furthermore, we found EV71 2C protein interacted with COPI subunits by co-immunoprecipitation and GST pull-down assay, indicating that COPI coatomer might be directed to the viral replication complex through viral 2C protein. Additionally, because the pathway is conserved among different species of enteroviruses, it may represent a novel target for antiviral therapies.

The MARVEL transmembrane motif of occludin mediates oligomerization and targeting to the basolateral surface in epithelia

Occludin (Ocln), a MARVEL-motif-containing protein, is found in all tight junctions. MARVEL motifs are comprised of four transmembrane helices associated with the localization to or formation of diverse membrane subdomains by interacting with the proximal lipid environment. The functions of the Ocln MARVEL motif are unknown. Bioinformatics sequence- and structure-based analyses demonstrated that the MARVEL domain of Ocln family proteins has distinct evolutionarily conserved sequence features that are consistent with its basolateral membrane localization. Live-cell microscopy, fluorescence resonance energy transfer (FRET) and bimolecular fluorescence complementation (BiFC) were used to analyze the intracellular distribution and self-association of fluorescent-protein-tagged full-length human Ocln or the Ocln MARVEL motif excluding the cytosolic C- and N-termini (amino acids 60-269, FP-MARVEL-Ocln). FP-MARVEL-Ocln efficiently arrived at the plasma membrane (PM) and was sorted to the basolateral PM in filter-grown polarized MDCK cells. A series of conserved aromatic amino acids within the MARVEL domain were found to be associated with Ocln dimerization using BiFC. FP-MARVEL-Ocln inhibited membrane pore growth during Triton-X-100-induced solubilization and was shown to increase the membrane-ordered state using Laurdan, a lipid dye. These data demonstrate that the Ocln MARVEL domain mediates self-association and correct sorting to the basolateral membrane.

Irradiation-induced protein inactivation reveals Golgi enzyme cycling to cell periphery

Acute inhibition is a powerful technique to test proteins for direct roles and order their activities in a pathway, but as a general gene-based strategy, it is mostly unavailable in mammalian systems. As a consequence, the precise roles of proteins in membrane trafficking have been difficult to assess in vivo. Here we used a strategy based on a genetically encoded fluorescent protein that generates highly localized and damaging reactive oxygen species to rapidly inactivate exit from the endoplasmic reticulum (ER) during live-cell imaging and address the long-standing question of whether the integrity of the Golgi complex depends on constant input from the ER. Light-induced blockade of ER exit immediately perturbed Golgi membranes, and surprisingly, revealed that cis-Golgi-resident proteins continuously cycle to peripheral ER-Golgi intermediate compartment (ERGIC) membranes and depend on ER exit for their return to the Golgi. These experiments demonstrate that ER exit and extensive cycling of cis-Golgi components to the cell periphery sustain the mammalian Golgi complex.

Six-transmembrane topology for Golgi anti-apoptotic protein (GAAP) and Bax inhibitor 1 (BI-1) provides model for the transmembrane Bax inhibitor-containing motif (TMBIM) family

The Golgi anti-apoptotic protein (GAAP) is a hydrophobic Golgi protein that regulates intracellular calcium fluxes and apoptosis. GAAP is highly conserved throughout eukaryotes and some strains of vaccinia virus (VACV) and camelpox virus. Based on sequence, phylogeny, and hydrophobicity, GAAPs were classified within the transmembrane Bax inhibitor-containing motif (TMBIM) family. TMBIM members are anti-apoptotic and were predicted to have seven-transmembrane domains (TMDs). However, topology prediction programs are inconsistent and predicted that GAAP and other TMBIM members have six or seven TMDs. To address this discrepancy, we mapped the transmembrane topology of viral (vGAAP) and human (hGAAP), as well as Bax inhibitor (BI-1). Data presented show a six-, not seven-, transmembrane topology for vGAAP with a putative reentrant loop at the C terminus and both termini located in the cytosol. We find that this topology is also conserved in hGAAP and BI-1. This places the charged C terminus in the cytosol, and mutation of these charged residues in hGAAP ablated its anti-apoptotic function. Given the highly conserved hydrophobicity profile within the TMBIM family and recent phylogenetic data indicating that a GAAP-like protein may have been the ancestral progenitor of a subset of the TMBIM family, we propose that this vGAAP topology may be used as a model for the remainder of the TMBIM family of proteins. The topology described provides valuable information on the structure and function of an important but poorly understood family of proteins.

An evolving paradigm for the secretory pathway?

The paradigm that the secretory pathway consists of a stable endoplasmic reticulum and Golgi apparatus, using discrete transport vesicles to exchange their contents, gained important support from groundbreaking biochemical and genetic studies during the 1980s. However, the subsequent development of new imaging technologies with green fluorescent protein introduced data on dynamic processes not fully accounted for by the paradigm. As a result, we may be seeing an example of how a paradigm is evolving to account for the results of new technologies and their new ways of describing cellular processes.

AP1 is essential for generation of autophagosomes from the trans-Golgi network

Despite recent advances in understanding the functions of autophagy in developmental and pathological conditions, the underlying mechanism of where and how autophagosomal structures acquire membrane remains enigmatic. Here, we provide evidence that post-Golgi membrane traffic plays a crucial role in autophagosome formation. Increased secretion of constitutive cargo from the trans-Golgi network (TGN) to the plasma membrane induced the formation of microtubule-associated protein light chain 3 (LC3)-positive structures. At the early phase of autophagy, LC3 associated with and then budded off from a distinct TGN domain without constitutive TGN-to-plasma cargo and TGN-to-endosome proteins. The clathrin adaptor protein AP1 and clathrin localized to starvation- and rapamycin-induced autophagosomes. Dysfunction of the AP1-dependent clathrin coating at the TGN but not at the plasma membrane prevented autophagosome formation. Our results thus suggest an essential role of the TGN in autophagosome biogenesis, providing membrane to autophagosomes through an AP1-dependent pathway.