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

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.

GBF1 (Gartenzwerg)-dependent secretion is required for Drosophila tubulogenesis

Here we report on the generation and in vivo analysis of a series of loss-of-function mutants for the Drosophila ArfGEF, Gartenzwerg. The Drosophila gene gartenzwerg (garz) encodes the orthologue of mammalian GBF1. garz is expressed ubiquitously in embryos with substantially higher abundance in cells forming diverse tubular structures such as salivary glands, trachea, proventriculus or hindgut. In the absence of functional Garz protein, the integrity of the Golgi complex is impaired. As a result, both vesicle transport of cargo proteins and directed apical membrane delivery are severely disrupted. Dysfunction of the Arf1-COPI machinery caused by a loss of Garz leads to perturbations in establishing a polarized epithelial architecture of tubular organs. Furthermore, insufficient apical transport of proteins and other membrane components causes incomplete luminal diameter expansion and deficiencies in extracellular matrix assembly. The fact that homologues of Garz are present in every annotated metazoan genome indicates that secretion processes mediated by the GBF-type ArfGEFs play a universal role in animal development.

AMPA receptors commandeer an ancient cargo exporter for use as an auxiliary subunit for signaling

Fast excitatory neurotransmission in the mammalian central nervous system is mainly mediated by ionotropic glutamate receptors of the AMPA subtype (AMPARs). AMPARs are protein complexes of the pore-lining α-subunits GluA1-4 and auxiliary β-subunits modulating their trafficking and gating. By a proteomic approach, two homologues of the cargo exporter cornichon, CNIH-2 and CNIH-3, have recently been identified as constituents of native AMPARs in mammalian brain. In heterologous reconstitution experiments, CNIH-2 promotes surface expression of GluAs and modulates their biophysical properties. However, its relevance in native AMPAR physiology remains controversial. Here, we have studied the role of CNIH-2 in GluA processing both in heterologous cells and primary rat neurons. Our data demonstrate that CNIH-2 serves an evolutionarily conserved role as a cargo exporter from the endoplasmic reticulum (ER). CNIH-2 cycles continuously between ER and Golgi complex to pick up cargo protein in the ER and then to mediate its preferential export in a coat protein complex (COP) II dependent manner. Interaction with GluA subunits breaks with this ancestral role of CNIH-2 confined to the early secretory pathway. While still taking advantage of being exported preferentially from the ER, GluAs recruit CNIH-2 to the cell surface. Thus, mammalian AMPARs commandeer CNIH-2 for use as a bona fide auxiliary subunit that is able to modify receptor signaling.

TRAPPC9 mediates the interaction between p150 and COPII vesicles at the target membrane

Background

The transport of endoplasmic reticulum (ER)-derived COPII vesicles toward the ER-Golgi intermediate compartment (ERGIC) requires cytoplasmic dynein and is dependent on microtubules. p150^Glued, a subunit of dynactin, has been implicated in the transport of COPII vesicles via its interaction with COPII coat components Sec23 and Sec24. However, whether and how COPII vesicle tether, TRAPP (Transport protein particle), plays a role in the interaction between COPII vesicles and microtubules is currently unknown.

Principle Findings

We address the functional relationship between COPII tether TRAPP and dynactin. Overexpressed TRAPP subunits interfered with microtubule architecture by competing p150^Glued away from the MTOC. TRAPP subunit TRAPPC9 bound directly to p150^Glued via the same carboxyl terminal domain of p150^Glued that binds Sec23 and Sec24. TRAPPC9 also inhibited the interaction between p150^Glued and Sec23/Sec24 both in vitro and in vivo, suggesting that TRAPPC9 serves to uncouple p150^Glued from the COPII coat, and to relay the vesicle-dynactin interaction at the target membrane.

Conclusions

These findings provide a new perspective on the function of TRAPP as an adaptor between the ERGIC membrane and dynactin. By preserving the connection between dynactin and the tethered and/or fused vesicles, TRAPP allows nascent ERGIC to continue the movement along the microtubules as they mature into the cis-Golgi.

Live-cell assays to identify regulators of ER-to-Golgi trafficking

We applied fluorescence microscopy-based quantitative assays to living cells to identify regulators of endoplasmic reticulum (ER)-to-Golgi trafficking and/or Golgi complex maintenance. We first validated an automated procedure to identify factors which influence Golgi-to-ER relocalization of GalT-CFP (β1,4-galactosyltransferase I-cyan fluorescent protein) after brefeldin A (BFA) addition and/or wash-out. We then tested 14 proteins that localize to the ER and/or Golgi complex when overexpressed for a role in ER-to-Golgi trafficking. Nine of them interfered with the rate of BFA-induced redistribution of GalT-CFP from the Golgi complex to the ER, six of them interfered with GalT-CFP redistribution from the ER to a juxtanuclear region (i.e. the Golgi complex) after BFA wash-out and six of them were positive effectors in both assays. Notably, our live-cell approach captures regulator function in ER-to-Golgi trafficking, which was missed in previous fixed cell assays, as well as assigns putative roles for other less characterized proteins. Moreover, we show that our assays can be extended to RNAi and chemical screens.

Regulation of post-Golgi traffic of G protein-coupled receptors

Anterograde trafficking of newly synthesized G protein-coupled -receptors (GPCRs) from the endoplasmic reticulum to the cell surface represents a crucial checkpoint in controlling the amount of the functional receptors at the cell surface and the strength of signaling initiated by the receptors. In contrast to the extensively studied, well-understood endocytic and recycling pathways, the molecular mechanisms underlying the cell-surface targeting of the receptors remain poorly defined. In this chapter, I will discuss current advances in understanding post-Golgi transport of GPCRs by focusing on specific motifs or sequences that may function as sorting signals regulating export from the Golgi and subsequent transport to the plasma membrane of GPCRs.

GM130 gain-of-function induces cell pathology in a model of lysosomal storage disease

Cell pathology in lysosomal storage diseases is characterized by the formation of distended vacuoles with characteristics of lysosomes. Our previous studies in mucopolysaccharidosis type IIIB (MPSIIIB), a disease in which a genetic defect induces the accumulation of undigested heparan sulfate (HS) fragments, led to the hypothesis that abnormal lysosome formation was related to events occurring at the Golgi level. We reproduced the enzyme defect of MPSIIIB in HeLa cells using tetracycline-inducible expression of shRNAs directed against α-N-acetylglucosaminidase (NAGLU) and addressed this hypothesis. HeLa cells deprived of NAGLU accumulated abnormal lysosomes. The Golgi matrix protein GM130 was over-expressed. The cis- and medial-Golgi compartments were distended, elongated and formed circularized ribbons. The Golgi microtubule network was enlarged with increased amounts of AKAP450, a partner of GM130 controlling this network. GM130 down-regulation prevented pathology in HeLa cells deprived of NAGLU, whereas GM130 over-expression in control HeLa cells mimicked the pathology of deprived cells. We concluded that abnormal lysosomes forming in cells accumulating HS fragments were the consequence of GM130 gain-of-function and subsequent alterations of the Golgi ribbon architecture. These results indicate that GM130 functions are modulated by HS glycosaminoglycans and therefore possibly controlled by extracellular cues.

Small GTPase regulation of GPCR anterograde trafficking

The physiological functions of heterotrimeric G protein-coupled receptors (GPCRs) are dictated by their intracellular trafficking and precise targeting to the functional destinations. Over the past decades, most studies on the trafficking of GPCRs have focused on the events involved in endocytosis and recycling. By contrast, the molecular mechanisms underlying anterograde transport of newly synthesized GPCRs from the endoplasmic reticulum (ER) to the cell surface have only now begun to be revealed. In this review we discuss current advances in understanding the role of Ras-like GTPases, specifically the Rab and Sar1/ARF subfamilies, in regulating cell-surface transport of GPCRs en route from the ER and the Golgi.

Entry and exit mechanisms at the cis-face of the Golgi complex

Vesicular transport of protein and lipid cargo from the endoplasmic reticulum (ER) to cis-Golgi compartments depends on coat protein complexes, Rab GTPases, tethering factors, and membrane fusion catalysts. ER-derived vesicles deliver cargo to an ER-Golgi intermediate compartment (ERGIC) that then fuses with and/or matures into cis-Golgi compartments. The forward transport pathway to cis-Golgi compartments is balanced by a retrograde directed pathway that recycles transport machinery back to the ER. How trafficking through the ERGIC and cis-Golgi is coordinated to maintain organelle structure and function is poorly understood and highlights central questions regarding trafficking routes and organization of the early secretory pathway.

Rab1b regulates COPI and COPII dynamics in mammalian cells

Rabs GTPases are key regulatory factors that specifically associate to organelles that integrate membrane transport pathways. Rabs, through their interactions with diverse effector proteins, regulate the formation, movement, tethering and fusion of transport carriers (vesicles and/or tubules). The mammalian Rab1b GTPase is required for ER to Golgi transport and interacts with multiple effectors localized at the ER-Golgi interface. Here, we focus on interactions between Rab1b and effectors that play essential roles in COPII and COPI vesicle formation/function. Based on evidence to date, we propose a model of Rab1b action at the ER exit sites.

Cell-free sorting of peroxisomal membrane proteins from the endoplasmic reticulum

Several yeast and mammalian peroxisomal membrane proteins (PMPs) are delivered to peroxisomes via the endoplasmic reticulum (ER). Fluorescence microscopy showed a focused assembly of PMPs in a specialized domain of the ER, referred to as the preperoxisomal ER. It is proposed that preperoxisomal vesicles containing PMPs bud from this domain to either fuse with preexisting peroxisomes or to mature into functional peroxisomes by uptake of peroxisomal membrane and matrix proteins. However, such vesicular entities are not identified nor are the biochemical requirements for the budding process known. We developed an in vitro cell-free ER-budding assay using Pichia pastoris and followed two endogenous PMPs, Pex11p and Pex3p during their ER exit. Both the PMPs were copackaged in the ER-budded vesicles that float on a Nycodenz gradient. PMP budding from the ER was dependent on ATP, temperature, cytosol, and Pex19p and generated preperoxisomal vesicles with an incomplete complement of PMPs. Surprisingly, Pex11p budding was independent of Pex3p; however, the budded vesicles were devoid of most of the PMPs otherwise present in the wild-type vesicles and might represent peroxisomal remnants. Our findings provide a biochemical platform to uncover the mechanism of PMP budding from the ER.

New components of the Golgi matrix

The eukaryotic Golgi apparatus is characterized by a stack of flattened cisternae that are surrounded by transport vesicles. The organization and function of the Golgi require Golgi matrix proteins, including GRASPs and golgins, which exist primarily as fiber-like bridges between Golgi cisternae or between cisternae and vesicles. In this review, we highlight recent findings on Golgi matrix proteins, including their roles in maintaining the Golgi structure, vesicle tethering, and novel, unexpected functions. These new discoveries further our understanding of the molecular mechanisms that maintain the structure and the function of the Golgi, as well as its relationship with other cellular organelles such as the centrosome.

Atg35, a micropexophagy-specific protein that regulates micropexophagic apparatus formation in Pichia pastoris

Autophagy-related (Atg) pathways deliver cytosol and organelles to the vacuole in double-membrane vesicles called autophagosomes, which are formed at the phagophore assembly site (PAS), where most of the core Atg proteins assemble. Atg28 is a component of the core autophagic machinery partially required for all Atg pathways in Pichia pastoris. This coiled-coil protein interacts with Atg17 and is essential for micropexophagy. However, the role of Atg28 in micropexophagy was unknown. We used the yeast two-hybrid system to search for Atg28 interaction partners from P. pastoris and identified a new Atg protein, named Atg35. The atg35∆ mutant was not affected in macropexophagy, cytoplasm-to-vacuole targeting or general autophagy. However, both Atg28 and Atg35 were required for micropexophagy and for the formation of the micropexophagic apparatus (MIPA). This requirement correlated with a stronger expression of both proteins on methanol and glucose. Atg28 mediated the interaction of Atg35 with Atg17. Trafficking of overexpressed Atg17 from the peripheral ER to the nuclear envelope was required to organize a peri-nuclear structure (PNS), the site of Atg35 colocalization during micropexophagy. In summary, Atg35 is a new Atg protein that relocates to the PNS and specifically regulates MIPA formation during micropexophagy.

Cargo loading at the ER

Trafficking of newly synthesized cargo through the early secretory pathway defines and maintains the intracellular organization of eukaryotic cells as well as the organization of tissues and organs. The importance of this pathway is underlined by the increasing number of mutations in key components of the ER export machinery that are causative of a diversity of human diseases. Here we discuss the molecular mechanisms that dictate cargo selection during vesicle budding. While, in vitro reconstitution assays, unicellular organisms such as budding yeast, and mammalian cell culture still have much to offer in terms of gaining a full understanding of the molecular basis for secretory cargo export, such assays have to date been limited to analysis of smaller, freely diffusible cargoes. The export of large macromolecular complexes from the ER such as collagens (up to 300 nm) or lipoproteins (~500 nm) presents a clear problem in terms of maintaining both selectivity and efficiency of export. It has also become clear that in order to translate our knowledge of the molecular basis for ER export to a full understanding of the implications for normal development and disease progression, the use of metazoan models is essential. Combined, these approaches are now starting to shed light not only on the mechanisms of macromolecular cargo export from the ER but also reveal the implications of failure of this process to human development and disease.

Role of endosomes in simian virus 40 entry and infection

After binding to its cell surface receptor ganglioside GM1, simian virus 40 (SV40) is endocytosed by lipid raft-mediated endocytosis and slowly transported to the endoplasmic reticulum, where partial uncoating occurs. We analyzed the intracellular pathway taken by the virus in HeLa and CV-1 cells by using a targeted small interfering RNA (siRNA) silencing screen, electron microscopy, and live-cell imaging as well as by testing a variety of cellular inhibitors and other perturbants. We found that the virus entered early endosomes, late endosomes, and probably endolysosomes before reaching the endoplasmic reticulum and that this pathway was part of the infectious route. The virus was especially sensitive to a variety of perturbations that inhibited endosome acidification and maturation. Contrary to our previous models, which postulated the passage of the virus through caveolin-rich organelles that we called caveosomes, we conclude that SV40 depends on the classical endocytic pathway for infectious entry.

The transporter associated with antigen processing (TAP) is active in a post-ER compartment

The translocation of cytosolic peptides into the lumen of the endoplasmic reticulum (ER) is a crucial step in the presentation of intracellular antigen to T cells by major histocompatibility complex (MHC) class I molecules. It is mediated by the transporter associated with antigen processing (TAP) protein, which binds to peptide-receptive MHC class I molecules to form the MHC class I peptide-loading complex (PLC). We investigated whether TAP is present and active in compartments downstream of the ER. By fluorescence microscopy, we found that TAP is localized to the ERGIC (ER-Golgi intermediate compartment) and the Golgi of both fibroblasts and lymphocytes. Using an in vitro vesicle formation assay, we show that COPII vesicles, which carry secretory cargo out of the ER, contain functional TAP that is associated with MHC class I molecules. Together with our previous work on post-ER localization of peptide-receptive class I molecules, our results suggest that loading of peptides onto class I molecules in the context of the peptide-loading complex can occur outside the ER.

Role of Rab1b in COPII dynamics and function

In eukaryotic cells, proteins destined for secretion are translocated into the endoplasmic reticulum (ER) and packaged into so-called COPII-coated vesicles. In the ER exit sites (ERES), COPII has the capacity of deforming the lipid bilayer, where it modulates the selective sorting and concentration of cargo proteins. In this study, we analyze the involvement of Rab1b in COPII dynamics and function by expressing either the Rab1b negative-mutant (Rab1N121I) or the Rab1b GTP restricted mutant (Rab1Q67L), or performing short interference RNA-based knockdown. We show that Rab1b interacts with the COPII components Sec23, Sec24 and Sec31 and that Rab1b inhibition changes the COPII phenotype. FRAP assays reveal that Rab1b modulates COPII association/dissociation kinetics at the ERES interface. Furthermore, Rab1b inhibition delays cargo sorting at the ER exit sites. We postulate that Rab1b is a key regulatory component of COPII dynamics and function.

Sec16A defines the site for vesicle budding from the endoplasmic reticulum on exit from mitosis

Mitotic inhibition of COPII-dependent export of proteins from the endoplasmic reticulum results in disassembly of the Golgi complex. This ensures ordered inheritance of organelles by the two daughter cells. Reassembly of the Golgi is intimately linked to the re-initiation of ER export on exit from mitosis. Here, we show that unlike all other COPII components, which are cytosolic during metaphase, Sec16A remains associated with ER exit sites throughout mitosis, and thereby could provide a template for the rapid assembly of functional export domains in anaphase. Full assembly of COPII at exit sites precedes reassembly of the Golgi in telophase.

Sequential depletion and acquisition of proteins during Golgi stack disassembly and reformation

Herein, we report the stepwise transport of multiple plant Golgi membrane markers during disassembly of the Golgi apparatus in tobacco leaf epidermal cells in response to the induced expression of the GTP-locked Sar1p or Brefeldin A (BFA), and reassembly on BFA washout. The distribution of fluorescent Golgi-resident N-glycan processing enzymes and matrix proteins (golgins) with specific cis-trans-Golgi sub-locations was followed by confocal microscopy during disassembly and reassembly. The first event during Golgi disassembly was the loss of trans-Golgi enzymes and golgins from Golgi membranes, followed by a sequential redistribution of medial and cis-Golgi enzymes into the endoplasmic reticulum (ER), whilst golgins were relocated to the ER or cytoplasm. This event was confirmed by fractionation and immuno-blotting. The sequential redistribution of Golgi components in a trans-cis sequence may highlight a novel retrograde trafficking pathway between the trans-Golgi and the ER in plants. Release of Golgi markers from the ER upon BFA washout occurred in the opposite sequence, with cis-matrix proteins labelling Golgi-like structures before cis/medial enzymes. Trans-enzyme location was preceded by trans-matrix proteins being recruited back to Golgi membranes. Our results show that Golgi disassembly and reassembly occur in a highly ordered fashion in plants.

Storage vesicles in neurons are related to Golgi complex alterations in mucopolysaccharidosis IIIB

The accumulation of intracellular storage vesicles is a hallmark of lysosomal storage diseases. Neither the identity nor origin of these implicated storage vesicles have yet been established. The vesicles are often considered as lysosomes, endosomes, and/or autophagosomes that are engorged with undigested materials. Our studies in the mouse model of mucopolysaccharidosis type IIIB, a lysosomal storage disease that induces neurodegeneration, showed that large storage vesicles in cortical neurons did not receive material from either the endocytic or autophagy pathway, which functioned normally. Storage vesicles expressed GM130, a Golgi matrix protein, which mediates vesicle tethering in both pre- and cis-Golgi compartments. However, other components of the tethering/fusion complex were not associated with GM130 on storage vesicles, likely accounting for both the resistance of the vesicles to brefeldin A and the alteration of Golgi ribbon architecture, which comprised distended cisterna connected to LAMP1-positive storage vesicles. We propose that alteration in the GM130-mediated control of vesicle trafficking in pre-Golgi and Golgi compartments affects Golgi biogenesis and gives rise to a dead-end storage compartment. Vesicle accumulation, Golgi disorganization, and alterations of other GM130 functions may account for neuron dysfunction and death.

Conserved molecular mechanisms underlying homeostasis of the Golgi complex

The Golgi complex performs a central function in the secretory pathway in the sorting and sequential processing of a large number of proteins destined for other endomembrane organelles, the plasma membrane, or secretion from the cell, in addition to lipid metabolism and signaling. The Golgi apparatus can be regarded as a self-organizing system that maintains a relatively stable morphofunctional organization in the face of an enormous flux of lipids and proteins. A large number of the molecular players that operate in these processes have been identified, their functions and interactions defined, but there is still debate about many aspects that regulate protein trafficking and, in particular, the maintenance of these highly dynamic structures and processes. Here, we consider how an evolutionarily conserved underlying mechanism based on retrograde trafficking that uses lipids, COPI, SNAREs, and tethers could maintain such a homeodynamic system.

A testis-specific regulator of complex and hybrid N-glycan synthesis

GnT1IP inhibits GlcNAcT-1 to change N-glycosylation patterns on secretory proteins, potentially regulating germ cell adhesion to Sertoli cells during spermatogenesis.

Database analyses identified 4933434I20Rik as a glycosyltransferase-like gene expressed mainly in testicular germ cells and regulated during spermatogenesis. Expression of a membrane-bound form of the protein resulted in a marked and specific reduction in N-acetylglucosaminyltransferase I (GlcNAcT-I) activity and complex and hybrid N-glycan synthesis. Thus, the novel activity was termed GlcNAcT-I inhibitory protein (GnT1IP). Membrane-bound GnT1IP localizes to the ER, the ER-Golgi intermediate compartment (ERGIC), and the cis-Golgi. Coexpression of membrane-anchored GnT1IP with GlcNAcT-I causes association of the two proteins, inactivation of GlcNAcT-I, and mislocalization of GlcNAcT-I from the medial-Golgi to earlier compartments. Therefore, GnT1IP is a regulator of GlcNAcT-I and complex and hybrid N-glycan production. Importantly, the formation of high mannose N-glycans resulting from inhibition of GlcNAcT-I by GnT1IP markedly increases the adhesion of CHO cells to TM4 Sertoli cells. Testicular germ cells might use GnT1IP to induce the expression of high mannose N-glycans on glycoproteins, thereby facilitating Sertoli–germ cell attachment at a particular stage of spermatogenesis.

Cloning and characterisation of GIRK1 variants resulting from alternative RNA editing of the KCNJ3 gene transcript in a human breast cancer cell line

The aim of this study was to investigate the impact of increased mRNA levels encoding GIRK1 in breast tumours on GIRK protein expression. mRNA levels encoding hGIRK1 and hGIRK4 in the MCF7, MCF10A and MDA-MB-453 breast cancer cell lines were assessed and the corresponding proteins detected using Western blots. cDNAs encoding for four hGIRK1 splice variants (hGIRK1a, 1c, 1d and 1e) were cloned from the MCF7 cell line. Subcellular localisation of fluorescence labelled hGIRK1a-e and hGIRK4 and of endogenous GIRK1 and GIRK4 subunits was monitored in the MCF7 cell line. All hGIRK1 splice variants and hGIRK4 were predominantly located within the endoplasmic reticulum. Heterologous expression in Xenopus laevis oocytes and two electrode voltage clamp experiments together with confocal microscopy were performed. Only the hGIRK1a subunit was able to form functional GIRK channels in connection with hGIRK4. The other splice variants are expressed, but exert a dominant negative effect on heterooligomeric channel function. Hence, alternative splicing of the KCNJ3 gene transcript in the MCF7 cell line leads to a family of mRNA's, encoding truncated versions of the hGIRK1 protein. The very high abundance of mRNA's encoding GIRK1 together with the presence of GIRK1 protein suggests a pathophysiological role in breast cancer.

PERK (EIF2AK3) regulates proinsulin trafficking and quality control in the secretory pathway

OBJECTIVE

Loss-of-function mutations in Perk (EIF2AK3) result in permanent neonatal diabetes in humans (Wolcott-Rallison Syndrome) and mice. Previously, we found that diabetes associated with Perk deficiency resulted from insufficient proliferation of beta-cells and from defects in insulin secretion. A substantial fraction of PERK-deficient beta-cells display a highly abnormal cellular phenotype characterized by grossly distended endoplasmic reticulum (ER) and retention of proinsulin. We investigated over synthesis, lack of ER-associated degradation (ERAD), and defects in ER to Golgi trafficking as possible causes.

RESEARCH DESIGN AND METHODS

ER functions of PERK were investigated in cell culture and mice in which Perk was impaired or gene dosage modulated. The Ins2(+/Akita) mutant mice were used as a model system to test the role of PERK in ERAD.

RESULTS

We report that loss of Perk function does not lead to uncontrolled protein synthesis but impaired ER-to-Golgi anterograde trafficking, retrotranslocation from the ER to the cytoplasm, and proteasomal degradation. PERK was also shown to be required to maintain the integrity of the ER and Golgi and processing of ATF6. Moreover, decreasing Perk dosage surprisingly ameliorates the progression of the Akita mutants toward diabetes.

CONCLUSIONS

PERK is a positive regulator of ERAD and proteasomal activity. Reducing PERK activity ameliorates the progression of diabetes in the Akita mouse, whereas increasing PERK dosage hastens its progression. We speculate that PERK acts as a metabolic sensor in the insulin-secreting beta-cells to modulate the trafficking and quality control of proinsulin in the ER relative to the physiological demands for circulating insulin.

Identification of a site in Sar1 involved in the interaction with the cytoplasmic tail of glycolipid glycosyltransferases

Glycolipid glycosyltransferases (GGT) are transported from the endoplasmic reticulum (ER) to the Golgi, their site of residence, via COPII vesicles. An interaction of a (R/K)X(R/K) motif at their cytoplasmic tail (CT) with Sar1 is critical for the selective concentration in the transport vesicles. In this work using computational docking, we identify three putative binding pockets in Sar1 (sites A, B, and C) involved in the interaction with the (R/K)X(R/K) motif. Sar1 mutants with alanine replacement of amino acids in site A were tested in vitro and in cells. In vitro, mutant versions showed a reduced ability to bind immobilized peptides with the CT sequence of GalT2. In cells, Sar1 mutants (Sar1(D198A)) specifically affect the exiting of GGT from the ER, resulting in an ER/Golgi concentration ratio favoring the ER. Neither the typical Golgi localization of GM130 nor the exiting and transport of the G protein of the vesicular stomatitis virus were affected. The protein kinase inhibitor H89 produced accumulation of Sec23, Sar1, and GalT2 at the ER exit sites; Sar1(D189A) also accumulated at these sites, but in this case GalT2 remained disperse along ER membranes. The results indicate that amino acids in site A of Sar1 are involved in the interaction with the CT of GGT for concentration at ER exiting sites.

Spatial regulation of Golgi phosphatidylinositol-4-phosphate is required for enzyme localization and glycosylation fidelity

The enrichment of phosphatidylinositol-4-phosphate (PI(4)P) at the trans Golgi network (TGN) is instrumental for proper protein and lipid sorting, yet how the restricted distribution of PI(4)P is achieved remains unknown. Here, we show that lipid phosphatase Suppressor of actin mutations 1 (SAC1) is crucial for the spatial regulation of Golgi PI(4)P. Ultrastructural analysis revealed that SAC1 is predominantly located at cisternal Golgi membranes but is absent from the TGN, thus confining PI(4)P to the TGN. RNAi-mediated knockdown of SAC1 caused changes in Golgi morphology and mislocalization of Golgi enzymes. Enzymes involved in glycan processing such as mannosidase-II (Man-II) and N-acetylglucosamine transferase-I (GnT-I) redistributed to aberrant intracellular structures and to the cell surface in SAC1 knockdown cells. SAC1 depletion also induced a unique pattern of Golgi-specific defects in N-and O-linked glycosylation. These results indicate that SAC1 organizes PI(4)P distribution between the Golgi complex and the TGN, which is instrumental for resident enzyme partitioning and Golgi morphology.

Protein sorting receptors in the early secretory pathway

Estimates based on proteomic analyses indicate that a third of translated proteins in eukaryotic genomes enter the secretory pathway. After folding and assembly of nascent secretory proteins in the endoplasmic reticulum (ER), the coat protein complex II (COPII) selects folded cargo for export in membrane-bound vesicles. To accommodate the great diversity in secretory cargo, protein sorting receptors are required in a number of instances for efficient ER export. These transmembrane sorting receptors couple specific secretory cargo to COPII through interactions with both cargo and coat subunits. After incorporation into COPII transport vesicles, protein sorting receptors release bound cargo in pre-Golgi or Golgi compartments, and receptors are then recycled back to the ER for additional rounds of cargo export. Distinct types of protein sorting receptors that recognize carbohydrate and/or polypeptide signals in secretory cargo have been characterized. Our current understanding of the molecular mechanisms underlying cargo receptor function are described.

Autophagosome formation depends on the small GTPase Rab1 and functional ER exit sites

Autophagy is an important cellular degradation pathway present in all eukaryotic cells. Via this pathway, portions of the cytoplasm and/or organelles are sequestered in double-membrane structures called autophagosomes. In spite of the significant advance achieved in autophagy, the long-standing question about the source of the autophagic membrane remains unsolved. We have investigated the role of the secretory pathway in autophagosome biogenesis. Sar1 and Rab1b are monomeric GTPases that control traffic from the endoplasmic reticulum (ER) to the Golgi. We present evidence indicating that the activity of both proteins is required for autophagosome formation. Overexpression of dominant-negative mutants and the use of siRNAs impaired autophagosome generation as determined by LC3 puncta formation and light chain 3 (LC3)-II processing. In addition, our results indicate that the autophagic and secretory pathways intersect at a level preceding the brefeldin A blockage, suggesting that the transport from the cis/medial Golgi is not necessary for autophagosome biogenesis. Our present results highlight the role of transport from the ER in the initial events of the autophagic vacuole development.

Biogenesis of the plant Golgi apparatus

It has long been assumed that the individual cisternal stacks that comprise the plant Golgi apparatus multiply by some kind of fission process. However, more recently, it has been demonstrated that the Golgi apparatus can be experimentally disassembled and the reformation process from the ER (endoplasmic reticulum) monitored sequentially using confocal fluorescence and electron microscopy. Some other evidence suggests that Golgi stacks may arise de novo in cells. In the present paper, we review some of the more recent findings on plant Golgi stack biogenesis and propose a new model for their growth de novo from ER exit sites.

L1 syndrome mutations impair neuronal L1 function at different levels by divergent mechanisms

Mutations in the human L1CAM gene cause neurodevelopmental disorders collectively referred to as L1 syndrome. Here, we investigated cellular pathomechanisms underlying two L1 syndrome mutations, R184Q and W1036L. We demonstrate that these mutations cause partial endoplasmic reticulum (ER) retention of L1, reduce L1 cell surface expression, but do not induce ER stress in neuronal NSC-34 cells. We provide evidence that surface trafficking of mutated L1 is affected by defective sorting to ER exit sites and attenuated ER export. However, in differentiated neuronal cultures and long-term cultured hippocampal slices, the L1-R184Q protein is restricted to cell bodies, whereas L1-W1036L also aberrantly localizes to dendrites. These trafficking defects preclude axonal targeting of L1, thereby affecting L1-mediated axon growth and arborization. Our results indicate that L1 syndrome mutations impair neuronal L1 function at different levels, firstly by attenuating ER export and secondly by interfering with polarized neuronal trafficking.

p28, a novel ERGIC/cis Golgi protein, required for Golgi ribbon formation

The mammalian Golgi apparatus consists of individual cisternae that are stacked in a polarized manner to form the compact zones of the Golgi. Several stacks are linked to form a ribbon via dynamic lateral bridges. The determinants required for maintaining the characteristic Golgi structure are incompletely understood. Here, we have characterized p28, a new gamma-subfamily member of p24 membrane proteins. p28 localized to endoplasmic reticulum-Golgi intermediate compartment (ERGIC) and cis Golgi and accumulated in the ERGIC upon Brefeldin A treatment, typical for a protein cycling in the early secretory pathway. p28 interacted with a subset of p24 proteins. Its depletion by small interfering RNA (siRNA) led to fragmentation of the Golgi without affecting the overall organization of microtubules but considerably reducing the amount of acetylated tubulin. The distribution of COPI and tethers, including GM130, was not affected. At the ultrastructural level, the Golgi fragments appeared as mini-stacks with apparently unchanged cis-trans topology. Golgi fragmentation did not impair anterograde or retrograde traffic. Fluorescence recovery after photobleaching (FRAP) experiments revealed that silencing p28 prevents protein exchange between Golgi stacks during reassembly after Brefeldin A-induced Golgi breakdown. These results show that the formation of a Golgi ribbon requires the structural membrane protein p28 in addition to previously identified SNAREs, coat proteins and tethers.

Rab7b controls trafficking from endosomes to the TGN

Rab7b is a recently identified member of the Rab GTPase protein family and has high similarity to Rab7. It has been reported that Rab7b is lysosome associated, that it is involved in monocytic differentiation and that it promotes lysosomal degradation of TLR4 and TLR9. Here we investigated further the localization and function of this GTPase. We found that wild-type Rab7b is lysosome associated whereas an activated, GTP-bound form of Rab7b localizes to the Golgi apparatus. In contrast to Rab7, Rab7b is not involved in EGF and EGFR degradation. Depletion of Rab7b or expression of Rab7b T22N, a Rab7b dominant-negative mutant, impairs cathepsin-D maturation and causes increased secretion of hexosaminidase. Moreover, expression of Rab7b T22N or depletion of Rab7b alters TGN46 distribution, cation-independent mannose-6-phosphate receptor (CI-MPR) trafficking, and causes an increase in the levels of the late endosomal markers CI-MPR and cathepsin D. Vesicular stomatitis virus G protein (VSV-G) trafficking, by contrast, is normal in Rab7b-depleted or Rab7b-T22N-expressing cells. In addition, depletion of Rab7b prevents cholera toxin B-subunit from reaching the Golgi. Altogether, these data indicate that Rab7b is required for normal lysosome function, and, in particular, that it is an essential factor for retrograde transport from endosomes to the trans-Golgi network (TGN).

Requirements for transitional endoplasmic reticulum site structure and function in Saccharomyces cerevisiae

Secretory proteins are exported from the ER at specialized regions known as transitional ER (tER). COPII proteins are enriched at tER sites, but mechanisms underlying assembly and maintenance are unclear. This study characterizes tER sites in Saccharomyces cerevisiae and probes protein and lipid requirements for tER site structure and function.

Secretory proteins are exported from the endoplasmic reticulum (ER) at specialized regions known as the transitional ER (tER). Coat protein complex II (COPII) proteins are enriched at tER sites, although the mechanisms underlying tER site assembly and maintenance are not understood. Here, we investigated the dynamic properties of tER sites in Saccharomyces cerevisiae and probed protein and lipid requirements for tER site structure and function. Thermosensitive sec12 and sec16 mutations caused a collapse of tER sites in a manner that depended on nascent secretory cargo. Continual fatty acid synthesis was required for ER export and for normal tER site structure, whereas inhibition of sterol and ceramide synthesis produced minor effects. An in vitro assay to monitor assembly of Sec23p-green fluorescent protein at tER sites was established to directly test requirements. tER sites remained active for ∼10 min in vitro and depended on Sec12p function. Bulk phospholipids were also required for tER site structure and function in vitro, whereas depletion of phophatidylinositol selectively inhibited coat protein complex II (COPII) budding but not assembly of tER site structures. These results indicate that tER sites persist through relatively stringent treatments in which COPII budding was strongly inhibited. We propose that tER site structures are stable elements that are assembled on an underlying protein and lipid scaffold.

ADP-ribosylation factors modulate the cell surface transport of G protein-coupled receptors

ADP-ribosylation factors (ARFs) regulate vesicular traffic through recruiting coat proteins. However, their functions in the anterograde transport of nascent G protein-coupled receptors (GPCRs) from the endoplasmic reticulum to the plasma membrane remain poorly explored. Here we show that treatment with brefeldin A, an inhibitor of guanine nucleotide exchange on ARFs, markedly attenuated the cell surface numbers of alpha(2B)-adrenergic receptor (AR), beta(2)-AR, angiotensin II type 1 receptor, and chemokine (CXC motif) receptor 4. Functional inhibition of individual ARF GTPases by transient expression of the GDP-bound, GTP-bound, and guanine nucleotide-deficient mutants showed that the five human ARFs differentially modulated receptor cell surface expression and that the ARF1 mutants produced the most profound inhibitory effect. Furthermore, expression of the ARF1 GTPase-activating protein (GAP) ARFGAP1 significantly blocked receptor transport. Interestingly, the GDP- and GTP-bound ARF1 mutants arrested the receptors in distinct intracellular compartments. Consistent with the reduced receptor cell surface expression, extracellular signal-regulated kinase 1 and 2 activation by receptor agonists was significantly attenuated by the GDP-bound mutant ARF1T31N. Moreover, coimmunoprecipitation showed that alpha(2B)-AR associated with ARF1 and glutathione transferase pull-down assay indicated that the alpha(2B)-AR C terminus directly interacted with ARF1. These data show that ARF1 GTPase is involved in the regulation of cell surface expression of GPCRs at multiple transport steps.

Membrane traffic within the Golgi apparatus

Newly synthesized secretory cargo molecules pass through the Golgi apparatus while resident Golgi proteins remain in the organelle. However, the pathways of membrane traffic within the Golgi are still uncertain. Most of the available data can be accommodated by the cisternal maturation model, which postulates that Golgi cisternae form de novo, carry secretory cargoes forward and ultimately disappear. The entry face of the Golgi receives material that has been exported from transitional endoplasmic reticulum sites, and the exit face of the Golgi is intimately connected with endocytic compartments. These conserved features are enhanced by cell-type-specific elaborations such as tubular connections between mammalian Golgi cisternae. Key mechanistic questions remain about the formation and maturation of Golgi cisternae, the recycling of resident Golgi proteins, the origins of Golgi compartmental identity, the establishment of Golgi architecture, and the roles of Golgi structural elements in membrane traffic.

Discrete, continuous, and stochastic models of protein sorting in the Golgi apparatus

The Golgi apparatus plays a central role in processing and sorting proteins and lipids in eukaryotic cells. Golgi compartments constantly exchange material with each other and with other cellular components, allowing them to maintain and reform distinct identities despite dramatic changes in structure and size during cell division, development, and osmotic stress. We have developed three minimal models of membrane and protein exchange in the Golgi-a discrete, stochastic model, a continuous ordinary differential equation model, and a continuous stochastic differential equation model-each based on two fundamental mechanisms: vesicle-coat-mediated selective concentration of cargoes and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins during vesicle formation and SNARE-mediated selective fusion of vesicles. By exploring where the models differ, we hope to discover whether the discrete, stochastic nature of vesicle-mediated transport is likely to have appreciable functional consequences for the Golgi. All three models show similar ability to restore and maintain distinct identities over broad parameter ranges. They diverge, however, in conditions corresponding to collapse and reassembly of the Golgi. The results suggest that a continuum model provides a good description of Golgi maintenance but that considering the discrete nature of vesicle-based traffic is important to understanding assembly and disassembly of the Golgi. Experimental analysis validates a prediction of the models that altering guanine nucleotide exchange factor expression levels will modulate Golgi size.

Lipids and cholesterol as regulators of traffic in the endomembrane system

The endomembrane system of eukaryotic cells uses membrane-enclosed carriers to move diverse macromolecules among different membrane-bound compartments, a requirement for cells to secrete and take up molecules from their environment. Two recycling pathways-biosynthetic and endocytic, each with specific lipid components-make up this system, with the Golgi apparatus mediating transport between the two. Here, we integrate lipid-based mechanisms into the description of this system. A partitioning model of the Golgi apparatus is discussed as a working hypothesis to explain how membrane lipids and proteins that are segregated based on lateral lipid partitioning support the unique composition of the biosynthetic and endocytic recycling pathways in the face of constant trafficking of molecular constituents. We further discuss how computational modeling can allow for interpretation of experimental findings and provide mechanistic insight into these important cellular pathways.

Requirements for ER-arrest and sequential exit to the golgi of Tomato spotted wilt virus glycoproteins

The envelope glycoproteins Gn and Gc are major determinants in the assembly of Tomato spotted wilt virus (TSWV) particles at the Golgi complex. In this article, the ER-arrest of singly expressed Gc and the transport of both glycoproteins to the Golgi upon coexpression have been analyzed.While preliminary results suggest that the arrest of Gc at the ER (endoplasmic reticulum) did not appear to result from improper folding, transient expression of chimeric Gc, in which the transmembrane domain (TMD) and/or cytoplasmic tail (CT) were swapped for those from Gn, showed that the TMD of Gn was sufficient to allow ER exit and transport to the Golgi. Expression of both glycoproteins in the presence of overexpressed Sar1p specific guanosine nucleotide exchange factor Sec12p, resulted in ER-retention demonstrating that the viral glycoproteins are transported to the Golgi in a COPII (coat protein II)-dependent manner. Inhibition of ER Golgi transport by brefeldin A (BFA) had a similar effect on the localization of Gn. However, inhibition of ER (endoplasmic reticulum) to Golgi transport of coexpressed Gc and Gn by overexpression of Sec12p or by BFA revealed distinct localization patterns, i.e. diffuse ER localization versus concentration at specific spots.

Golgins and GRASPs: holding the Golgi together

The GRASP and golgin families of proteins have emerged as key components of the Golgi apparatus, with major roles in both the structural organisation of this organelle and the trafficking that occurs there. Both types of protein participate in membrane tethering events that occur upstream of membrane fusion as well as contributing to the structural scaffold that defines Golgi architecture, referred to as the Golgi matrix. The importance of these proteins is highlighted by their targeting in mitosis, apoptosis, and pathogenic infections that cause dramatic structural and functional reorganisation of the Golgi apparatus. In this review we will discuss our current understanding of GRASP and golgin function, highlighting some of the common themes that have emerged as well as describing previously unsuspected roles for these proteins in various cellular processes.

Biogenesis of secretory organelles during B cell differentiation

The differentiation of B cells into Ig-secreting plasma cells requires the expansion of secretory organelles to cope with the increased cargo load. To evaluate the timeline of this process, we have quantitated the kinetics of secretory organelle expansion relative to Ig secretion and examined regulatory components of secretory transport following in vitro activation of human B lymphocytes. Unstimulated B cells contain minimal endomembranes. After activation, ER membrane induction appears as tightly packed spherical structures of 0.5-1 mum diameter concentrated in a juxtanuclear position. When the cells differentiate into plasmablasts, there is dramatic cell-size increase, but the ER remains concentrated close to the nucleus and only later fills the entire cell. In sharp contrast, previous studies in other cell types have found that the ER expands in synchrony with increasing cell size during interphase, by extension of ER tubules under the PM. In this study, the Golgi remains consistently as a single juxtanuclear structure but linearly expands sixfold in volume during B cell activation. Furthermore, following active cell proliferation, ER exit sites proliferate rapidly, increasing almost fourfold in number, in parallel with a sharp increase in Ig secretion. These findings demonstrate that the control of organelle biogenesis and expansion in primary human B cells are differentially regulated by cargo flux caused by Ig synthesis.

Role of vesicle tethering factors in the ER-Golgi membrane traffic

Tethers are a diverse group of loosely related proteins and protein complexes grouped into three families based on structural and functional similarities. A well-accepted role for tethering factors is the initial attachment of transport carriers to acceptor membranes prior to fusion. However, accumulating evidence indicates that tethers are more than static bridges. Tethers have been shown to interact with components of the fusion machinery and with components involved in vesicle formation. Tethers belonging to the three families act at the same stage of traffic, suggesting that they mediate distinct events during vesicle tethering. Thus, multiple tether-facilitated events are required to provide selectivity to vesicle fusion. In this review, we highlight findings that support this model.

Golgi membrane dynamics after induction of a dominant-negative mutant Sar1 GTPase in tobacco

An inducible system has been established in Nicotiana tabacum plants allowing controlled expression of Sar1-GTP and thus the investigation of protein dynamics after inhibition of endoplasmic reticulum (ER) to Golgi transport. Complete Golgi disassembly and redistribution of Golgi markers into the ER was observed within 18-24h after induction. At the ultrastructural level Sar1-GTP expression led to a decrease in Golgi stack size followed by Golgi fragmentation and accumulation of vesicle remnants. Induction of Sar1-GTP resulted in redistribution of the green fluorescent protein (GFP)-tagged Arabidopsis golgins AtCASP and GC1 (golgin candidate 1, an Arabidopsis golgin 84 isoform) into the ER or cytoplasm, respectively. Additionally, both fusion proteins were observed in punctate structures, which co-located with a yellow fluorescent protein (YFP)-tagged version of Sar1-GTP. The Sar1-GTP-inducible system is compared with constitutive Sar1-GTP expression and brefeldin A treatment, and its potential for the study of the composition of ER exit sites and early cis-Golgi structures is discussed.

GRASP65 and GRASP55 sequentially promote the transport of C-terminal valine-bearing cargos to and through the Golgi complex

The Golgi matrix proteins GRASP65 and GRASP55 have recognized roles in maintaining the architecture of the Golgi complex, in mitotic progression and in unconventional protein secretion whereas, surprisingly, they have been shown to be dispensable for the transport of commonly used reporter cargo proteins along the secretory pathway. However, it is becoming increasingly clear that many trafficking machineries operate in a cargo-specific manner, thus we have investigated whether GRASPs may control the trafficking of selected classes of cargo. We have taken into consideration the C-terminal valine-bearing receptors CD8alpha and Frizzled4 that we show bind directly to the PSD95-DlgA-zo-1 (PDZ) domains of GRASP65 and GRASP55. We demonstrate that both GRASPs are needed sequentially for the efficient transport to and through the Golgi complex of these receptors, thus highlighting a novel role for the GRASPs in membrane trafficking. Our results open new perspectives for our understanding of the regulation of surface expression of a class of membrane proteins, and suggests the causal mechanisms of a dominant form of autosomal human familial exudative vitreoretinopathy that arises from the Frizzled4 mutation involving its C-terminal valine.

Rab6 and Rab11 regulate Chlamydia trachomatis development and golgin-84-dependent Golgi fragmentation

Many intracellular pathogens that replicate in special membrane bound compartments exploit cellular trafficking pathways by targeting small GTPases, including Rab proteins. Members of the Chlamydiaceae recruit a subset of Rab proteins to their inclusions, but the significance of these interactions is uncertain. Using RNA interference, we identified Rab6 and Rab11 as important regulators of Chlamydia infections. Depletion of either Rab6 or Rab11, but not the other Rab proteins tested, decreased the formation of infectious particles. We further examined the interplay between these Rab proteins and the Golgi matrix components golgin-84 and p115 with regard to Chlamydia-induced Golgi fragmentation. Silencing of the Rab proteins blocked Chlamydia-induced and golgin-84 knockdown-stimulated Golgi disruption, whereas Golgi fragmentation was unaffected in p115 depleted cells. Interestingly, p115-induced Golgi fragmentation could rescue Chlamydia propagation in Rab6 and Rab11 knockdown cells. Furthermore, transport of nutrients to Chlamydia, as monitored by BODIPY-Ceramide, was inhibited by Rab6 and Rab11 knockdown. Taken together, our results demonstrate that Rab6 and Rab11 are key regulators of Golgi stability and further support the notion that Chlamydia subverts Golgi structure to enhance its intracellular development.

The Golgi apparatus: lessons from Drosophila

Historically, Drosophila has been a model organism for studying molecular and developmental biology leading to many important discoveries in this field. More recently, the fruit fly has started to be used to address cell biology issues including studies of the secretory pathway, and more specifically on the functional integrity of the Golgi apparatus. A number of advances have been made that are reviewed below. Furthermore, with the development of RNAi technology, Drosophila tissue culture cells have been used to perform genome-wide screens addressing similar issues. Last, the Golgi function has been involved in specific developmental processes, thus shedding new light on the functions of a number of Golgi proteins.