KDEL receptor (Erd2p)-mediated retrograde transport of the cholera toxin A subunit from the Golgi involves COPI, p23, and the COOH terminus of Erd2p

p24-Tango1 interactions ensure ER-Golgi interface stability and efficient transport

The eukaryotic p24 family, consisting of α-, β-, γ- and δ-p24 subfamilies, has long been known to be involved in regulating secretion. Despite increasing interest in these proteins, fundamental questions remain about their role. Here, we systematically investigated Drosophila p24 proteins. We discovered that members of all four p24 subfamilies are required for general secretion and that their localizations between ER exit site (ERES) and Golgi are interdependent in an α→βδ→γ sequence. We also found that localization of p24 proteins and ERES determinant Tango1 requires interaction through their respective GOLD and SH3 lumenal domains, with Tango1 loss sending p24 proteins to the plasma membrane and vice versa. Finally, we show that p24 loss expands the COPII zone at ERES and increases the number of ER-Golgi vesicles, supporting a restrictive role of p24 proteins on vesicle budding for efficient transport. Our results reveal Tango1-p24 interplay as central to the generation of a stable ER-Golgi interface.

Sortase-Modified Cholera Toxoids Show Specific Golgi Localization

Cholera toxoid is an established tool for use in cellular tracing in neuroscience and cell biology. We use a sortase labeling approach to generate site-specific N-terminally modified variants of both the A2-B5 heterohexamer and B5 pentamer forms of the toxoid. Both forms of the toxoid are endocytosed by GM1-positive mammalian cells, and while the heterohexameric toxoid was principally localized in the ER, the B5 pentamer showed an unexpectedly specific localization in the medial/trans-Golgi. This study suggests a future role for specifically labeled cholera toxoids in live-cell imaging beyond their current applications in neuronal tracing and labeling of lipid rafts in fixed cells.

STED imaging of endogenously tagged ARF GTPases reveals their distinct nanoscale localizations

ADP-ribosylation factor (ARF) GTPases are major regulators of cellular membrane homeostasis. High sequence similarity and multiple, possibly redundant functions of the five human ARFs make investigating their function a challenging task. To shed light on the roles of the different Golgi-localized ARF members in membrane trafficking, we generated CRISPR-Cas9 knockins (KIs) of type I (ARF1 and ARF3) and type II ARFs (ARF4 and ARF5) and mapped their nanoscale localization with stimulated emission depletion (STED) super-resolution microscopy. We find ARF1, ARF4, and ARF5 on segregated nanodomains on the cis-Golgi and ER-Golgi intermediate compartments (ERGIC), revealing distinct roles in COPI recruitment on early secretory membranes. Interestingly, ARF4 and ARF5 define Golgi-tethered ERGIC elements decorated by COPI and devoid of ARF1. Differential localization of ARF1 and ARF4 on peripheral ERGICs suggests the presence of functionally different classes of intermediate compartments that could regulate bi-directional transport between the ER and the Golgi. Furthermore, ARF1 and ARF3 localize to segregated nanodomains on the trans-Golgi network (TGN) and are found on TGN-derived post-Golgi tubules, strengthening the idea of distinct roles in post-Golgi sorting. This work provides the first map of the nanoscale organization of human ARF GTPases on cellular membranes and sets the stage to dissect their numerous cellular roles.

The manipulation of cell signaling and host cell biology by cholera toxin

Vibrio cholerae colonizes the small intestine and releases cholera toxin into the extracellular space. The toxin binds to the apical surface of the epithelium, is internalized into the host endomembrane system, and escapes into the cytosol where it activates the stimulatory alpha subunit of the heterotrimeric G protein by ADP-ribosylation. This initiates a cAMP-dependent signaling pathway that stimulates chloride efflux into the gut, with diarrhea resulting from the accompanying osmotic movement of water into the intestinal lumen. G protein signaling is not the only host system manipulated by cholera toxin, however. Other cellular mechanisms and signaling pathways active in the intoxication process include endocytosis through lipid rafts, retrograde transport to the endoplasmic reticulum, the endoplasmic reticulum-associated degradation system for protein delivery to the cytosol, the unfolded protein response, and G protein de-activation through degradation or the function of ADP-ribosyl hydrolases. Although toxin-induced chloride efflux is thought to be an irreversible event, alterations to these processes could facilitate cellular recovery from intoxication. This review will highlight how cholera toxin exploits signaling pathways and other cell biology events to elicit a diarrheal response from the host.

Molecular mechanisms of endomembrane trafficking in plants

Endomembrane trafficking is essential for all eukaryotic cells. The best-characterized membrane trafficking organelles include the endoplasmic reticulum (ER), Golgi apparatus, early and recycling endosomes, multivesicular body, or late endosome, lysosome/vacuole, and plasma membrane. Although historically plants have given rise to cell biology, our understanding of membrane trafficking has mainly been shaped by the much more studied mammalian and yeast models. Whereas organelles and major protein families that regulate endomembrane trafficking are largely conserved across all eukaryotes, exciting variations are emerging from advances in plant cell biology research. In this review, we summarize the current state of knowledge on plant endomembrane trafficking, with a focus on four distinct trafficking pathways: ER-to-Golgi transport, endocytosis, trans-Golgi network-to-vacuole transport, and autophagy. We acknowledge the conservation and commonalities in the trafficking machinery across species, with emphasis on diversity and plant-specific features. Understanding the function of organelles and the trafficking machinery currently nonexistent in well-known model organisms will provide great opportunities to acquire new insights into the fundamental cellular process of membrane trafficking.

Mimicry of Short Linear Motifs by Bacterial Pathogens: A Drugging Opportunity

Bacterial pathogens have developed complex strategies to successfully survive and proliferate within their hosts. Throughout the infection cycle, direct interaction with host cells occurs. Many bacteria have been found to secrete proteins, such as effectors and toxins, directly into the host cell with the potential to interfere with cell regulatory processes, either enzymatically or through protein-protein interactions (PPIs). Short linear motifs (SLiMs) are abundant peptide modules in cell signaling proteins. Here, we cover the reported examples of eukaryotic-like SLiM mimicry being used by pathogenic bacteria to hijack host cell machinery and discuss how drugs targeting SLiM-regulated cell signaling networks are being evaluated for interference with bacterial infections. This emerging anti-infective opportunity may become an essential contributor to antibiotic replacement strategies.

A Model for ERD2 Function in Higher Plants

ER lumenal proteins have a K(H)DEL motif at their C-terminus. This is recognized by the ERD2 receptor (KDEL receptor in animals), which localizes to the Golgi apparatus and serves to capture escaped ER lumenal proteins. ERD2-ligand complexes are then transported back to the ER via COPI coated vesicles. The neutral pH of the ER causes the ligands to dissociate with the receptor being returned to the Golgi. According to this generally accepted scenario, ERD2 cycles between the ER and the Golgi, although it has been found to have a predominant Golgi localization. In this short article, we present a model for the functioning of ERD2 receptors in higher plants that explains why it is difficult to detect fluorescently tagged ERD2 proteins in the ER. The model assumes that the residence time for ERD2 in the ER is very brief and restricted to a specific domain of the ER. This is the small disc of ER immediately subjacent to the first cis-cisterna of the Golgi stack, representing specialized ER export and import sites and therefore constituting part of what is known as the "secretory unit", a mobile aggregate of ER domain plus Golgi stack. ERD2 molecules in the ER domain of the secretory unit may be small in number, transient and optically difficult to differentiate from the larger population of ERD2 molecules in the overlying Golgi stack in the confocal microscope.

Coatopathies: Genetic Disorders of Protein Coats

Protein coats are supramolecular complexes that assemble on the cytosolic face of membranes to promote cargo sorting and transport carrier formation in the endomembrane system of eukaryotic cells. Several types of protein coats have been described, including COPI, COPII, AP-1, AP-2, AP-3, AP-4, AP-5, and retromer, which operate at different stages of the endomembrane system. Defects in these coats impair specific transport pathways, compromising the function and viability of the cells. In humans, mutations in subunits of these coats cause various congenital diseases that are collectively referred to as coatopathies. In this article, we review the fundamental properties of protein coats and the diseases that result from mutation of their constituent subunits.

Reovirus σNS and μNS Proteins Remodel the Endoplasmic Reticulum to Build Replication Neo-Organelles

Like most viruses that replicate in the cytoplasm, mammalian reoviruses assemble membranous neo-organelles called inclusions that serve as sites of viral genome replication and particle morphogenesis. Viral inclusion formation is essential for viral infection, but how these organelles form is not well understood. We investigated the biogenesis of reovirus inclusions. Correlative light and electron microscopy showed that endoplasmic reticulum (ER) membranes are in contact with nascent inclusions, which form by collections of membranous tubules and vesicles as revealed by electron tomography. ER markers and newly synthesized viral RNA are detected in inclusion internal membranes. Live-cell imaging showed that early in infection, the ER is transformed into thin cisternae that fragment into small tubules and vesicles. We discovered that ER tubulation and vesiculation are mediated by the reovirus σNS and μNS proteins, respectively. Our results enhance an understanding of how viruses remodel cellular compartments to build functional replication organelles.

Viruses modify cellular structures to build replication organelles. These organelles serve as sites of viral genome replication and particle morphogenesis and are essential for viral infection. However, how these organelles are constructed is not well understood. We found that the replication organelles of mammalian reoviruses are formed by collections of membranous tubules and vesicles derived from extensive remodeling of the peripheral endoplasmic reticulum (ER). We also observed that ER tubulation and vesiculation are triggered by the reovirus σNS and μNS proteins, respectively. Our results enhance an understanding of how viruses remodel cellular compartments to build functional replication organelles and provide functions for two enigmatic reovirus replication proteins. Most importantly, this research uncovers a new mechanism by which viruses form factories for particle assembly.

Loss of Arabidopsis p24 function affects ERD2 trafficking and Golgi structure, and activates the unfolded protein response

The p24 family of proteins (also known as the TMED family) are key regulators of protein trafficking along the secretory pathway, but very little is known about their functions in plants. A quadruple loss-of-function mutant affecting the p24 genes from the δ-1 subclass of the p24δ subfamily (p24δ3δ4δ5δ6) showed alterations in the Golgi, suggesting that these p24 proteins play a role in the organization of the compartments of the early secretory pathway in Arabidopsis Loss of p24δ-1 proteins also induced the accumulation of the K/HDEL receptor ERD2a (ER lumen protein-retaining receptor A) at the Golgi and increased secretion of BiP family proteins, ER chaperones containing an HDEL signal, probably due to an inhibition of COPI-dependent Golgi-to-ER transport of ERD2a and thus retrieval of K/HDEL ligands. Although the p24δ3δ4δ5δ6 mutant showed enhanced sensitivity to salt stress, it did not show obvious phenotypic alterations under standard growth conditions. Interestingly, this mutant showed a constitutive activation of the unfolded protein response (UPR) and the transcriptional upregulation of the COPII subunit gene SEC31A, which may help the plant to cope with the transport defects seen in the absence of p24 proteins.

N-Linked Glycosylation of the p24 Family Protein p24δ5 Modulates Retrograde Golgi-to-ER Transport of K/HDEL Ligands in Arabidopsis

The K/HDEL receptor ERD2 mediates the transport of soluble endoplasmic reticulum (ER)-resident proteins containing a C-terminal K/HDEL signal from the Golgi apparatus back to the ER via COPI (COat Protein I)-coated vesicles. Sorting of ERD2 within COPI vesicles is facilitated by p24 proteins. In Arabidopsis, p24δ5 has been shown to interact directly with ERD2 via its luminal GOLD (GOLgi Dynamics) domain and with COPI proteins via its cytoplasmic C-terminal tail at the acidic pH of the Golgi apparatus. Several members of the p24 family in mammals and yeast have been shown to be glycosylated, but whether Arabidopsis p24 proteins are glycosylated and the role of the sugar moiety in p24 function remain unclear. Here, we show that Arabidopsis p24δ5 protein is N-glycosylated in its GOLD domain. Furthermore, we demonstrate that this post-translational modification is important for its coupled transport with p24β2 at the ER-Golgi interface, for its interaction with the K/HDEL receptor ERD2, and for retrograde transport of ERD2 and K/HDEL ligands from the Golgi apparatus back to the ER.

T3SS-Independent Uptake of the Short-Trip Toxin-Related Recombinant NleC Effector of Enteropathogenic Escherichia coli Leads to NF-κB p65 Cleavage

Effector proteins secreted by the type 3 secretion system (T3SS) of pathogenic bacteria have been shown to precisely modulate important signaling cascades of the host for the benefit of the pathogens. Among others, the non-LEE encoded T3SS effector protein NleC of enteropathogenic Escherichia coli (EPEC) is a Zn-dependent metalloprotease and suppresses innate immune responses by directly targeting the NF-κB signaling pathway. Many pathogenic bacteria release potent bacterial toxins of the A-B type, which-in contrast to the direct cytoplasmic injection of T3SS effector proteins-are released first into the environment. In this study, we found that NleC displays characteristics of bacterial A-B toxins, when applied to eukaryotic cells as a recombinant protein. Although lacking a B subunit, that typically mediates the uptake of toxins, recombinant NleC (rNleC) induces endocytosis via lipid rafts and follows the endosomal-lysosomal pathway. The conformation of rNleC is altered by low pH to facilitate its escape from acidified endosomes. This is reminiscent of the homologous A-B toxin AIP56 of the fish pathogen Photobacterium damselae piscicida (Phdp). The recombinant protease NleC is functional inside eukaryotic cells and cleaves p65 of the NF-κB pathway. Here, we describe the endocytic uptake mechanism of rNleC, characterize its intracellular trafficking and demonstrate that its specific activity of cleaving p65 requires activation of host cells e.g., by IL1β. Further, we propose an evolutionary link between some T3SS effector proteins and bacterial toxins from apparently unrelated bacteria. In summary, these properties might suggest rNleC as an interesting candidate for future applications as a potential therapeutic against immune disorders.

Receptor-mediated sorting of soluble vacuolar proteins: myths, facts, and a new model

To prevent their being released to the cell exterior, acid hydrolases are recognized by receptors at some point in the secretory pathway and diverted towards the lytic compartment of the cell (lysosome or vacuole). In animal cells, the receptor is called the mannosyl 6-phosphate receptor (MPR) and it binds hydrolase ligands in the trans-Golgi network (TGN). These ligands are then sequestered into clathrin-coated vesicles (CCVs) because of motifs in the cytosolic tail of the MPR which interact first with monomeric adaptors (Golgi-localized, Gamma-ear-containing, ARF-binding proteins, GGAs) and then with tetrameric (adaptin) adaptor complexes. The CCVs then fuse with an early endosome, whose more acidic lumen causes the ligands to dissociate. The MPRs are then recycled back to the TGN via retromer-coated carriers. Plants have vacuolar sorting receptors (VSRs) which were originally identified in CCVs isolated from pea (Pisum sativum L.) cotyledons. It was therefore assumed that VSRs would have an analogous function in plants to MPRs in animals. Although this dogma has enjoyed wide support over the last 20 years there are many inconsistencies. Recently, results have been published which are quite contrary to it. It now emerges that VSRs and their ligands can interact very early in the secretory pathway, and dissociate in the TGN, which, in contrast to its mammalian counterpart, has a pH of 5.5. Multivesicular endosomes in plants lack proton pump complexes and consequently have an almost neutral internal pH, which discounts them as organelles of pH-dependent receptor-ligand dissociation. These data force a critical re-evaluation of the role of CCVs at the TGN, especially considering that vacuolar cargo ligands have never been identified in them. We propose that one population of TGN-derived CCVs participate in retrograde transport of VSRs from the TGN. We also present a new model to explain how secretory and vacuolar cargo proteins are effectively separated after entering the late Golgi/TGN compartments.

Exploring the genetics of irritable bowel syndrome: a GWA study in the general population and replication in multinational case-control cohorts

OBJECTIVE

IBS shows genetic predisposition, but adequately powered gene-hunting efforts have been scarce so far. We sought to identify true IBS genetic risk factors by means of genome-wide association (GWA) and independent replication studies.

DESIGN

We conducted a GWA study (GWAS) of IBS in a general population sample of 11,326 Swedish twins. IBS cases (N=534) and asymptomatic controls (N=4932) were identified based on questionnaire data. Suggestive association signals were followed-up in 3511 individuals from six case-control cohorts. We sought genotype-gene expression correlations through single nucleotide polymorphism (SNP)-expression quantitative trait loci interactions testing, and performed in silico prediction of gene function. We compared candidate gene expression by real-time qPCR in rectal mucosal biopsies of patients with IBS and controls.

RESULTS

One locus at 7p22.1, which includes the genes KDELR2 (KDEL endoplasmic reticulum protein retention receptor 2) and GRID2IP (glutamate receptor, ionotropic, delta 2 (Grid2) interacting protein), showed consistent IBS risk effects in the index GWAS and all replication cohorts and reached p=9.31×10(-6) in a meta-analysis of all datasets. Several SNPs in this region are associated with cis effects on KDELR2 expression, and a trend for increased mucosal KDLER2 mRNA expression was observed in IBS cases compared with controls.

CONCLUSIONS

Our results demonstrate that general population-based studies combined with analyses of patient cohorts provide good opportunities for gene discovery in IBS. The 7p22.1 and other risk signals detected in this study constitute a good starting platform for hypothesis testing in future functional investigations.

Heat-Labile Enterotoxins

Heat-labile enterotoxins (LTs) of Escherichia coli are closely related to cholera toxin (CT), which was originally discovered in 1959 in culture filtrates of the gram-negative bacterium Vibrio cholerae. Several other gram-negative bacteria also produce enterotoxins related to CT and LTs, and together these toxins form the V. cholerae-E. coli family of LTs. Strains of E. coli causing a cholera-like disease were designated enterotoxigenic E. coli (ETEC) strains. The majority of LTI genes (elt) are located on large, self-transmissible or mobilizable plasmids, although there are instances of LTI genes being located on chromosomes or carried by a lysogenic phage. The stoichiometry of A and B subunits in holotoxin requires the production of five B monomers for every A subunit. One proposed mechanism is a more efficient ribosome binding site for the B gene than for the A gene, increasing the rate of initiation of translation of the B gene independently from A gene translation. The three-dimensional crystal structures of representative members of the LT family (CT, LTpI, and LTIIb) have all been determined by X-ray crystallography and found to be highly similar. Site-directed mutagenesis has identified many residues in the CT and LT A subunits, including His44, Val53, Ser63, Val97, Glu110, and Glu112, that are critical for the structures and enzymatic activities of these enterotoxins. For the enzymatically active A1 fragment to reach its substrate, receptor-bound holotoxin must gain access to the cytosol of target cells.

Vesicles versus Tubes: Is Endoplasmic Reticulum-Golgi Transport in Plants Fundamentally Different from Other Eukaryotes?

The endoplasmic reticulum (ER) is the gateway to the secretory pathway in all eukaryotic cells. Its products subsequently pass through the Golgi apparatus on the way to the cell surface (true secretion) or to the lytic compartment of the cell (vacuolar protein transport). In animal cells, the Golgi apparatus is present as a stationary larger order complex near the nucleus, and transport between the cortical ER and the Golgi complex occurs via an intermediate compartment which is transported on microtubules. By contrast, higher plant cells have discrete mobile Golgi stacks that move along the cortical ER, and the intermediate compartment is absent. Although many of the major molecular players involved in ER-Golgi trafficking in mammalian and yeast (Saccharomyces cerevisiae) cells have homologs in higher plants, the narrow interface (less than 500 nm) between the Golgi and the ER, together with the motility factor, makes the identification of the transport vectors responsible for bidirectional traffic between these two organelles much more difficult. Over the years, a controversy has arisen over the two major possibilities by which transfer can occur: through vesicles or direct tubular connections. In this article, four leading plant cell biologists attempted to resolve this issue. Unfortunately, their opinions are so divergent and often opposing that it was not possible to reach a consensus. Thus, we decided to let each tell his or her version individually. The review begins with an article by Federica Brandizzi that provides the necessary molecular background on coat protein complexes in relation to the so-called secretory units model for ER-Golgi transport in highly vacuolated plant cells. The second article, written by Chris Hawes, presents the evidence in favor of tubules. It is followed by an article from David Robinson defending the classical notion that transport occurs via vesicles. The last article, by Akihiko Nakano, introduces the reader to possible alternatives to vesicles or tubules, which are now emerging as a result of exciting new developments in high-resolution light microscopy in yeast.

How pathogens use linear motifs to perturb host cell networks

Molecular mimicry is one of the powerful stratagems that pathogens employ to colonise their hosts and take advantage of host cell functions to guarantee their replication and dissemination. In particular, several viruses have evolved the ability to interact with host cell components through protein short linear motifs (SLiMs) that mimic host SLiMs, thus facilitating their internalisation and the manipulation of a wide range of cellular networks. Here we present convincing evidence from the literature that motif mimicry also represents an effective, widespread hijacking strategy in prokaryotic and eukaryotic parasites. Further insights into host motif mimicry would be of great help in the elucidation of the molecular mechanisms behind host cell invasion and the development of anti-infective therapeutic strategies.

Arabidopsis p24δ5 and p24δ9 facilitate Coat Protein I-dependent transport of the K/HDEL receptor ERD2 from the Golgi to the endoplasmic reticulum

The p24 proteins belong to a family of type I membrane proteins which cycle between the endoplasmic reticulum (ER) and Golgi via coat protein I (COPI) and COPII vesicles. Current nomenclature classifies them into four subfamilies, although plant p24 proteins belong to either the p24β or the p24δ subfamilies. Here, we show that Arabidopsis p24δ5/δ9 and HDEL ligands shift the steady-state distribution of the K/HDEL receptor ERD2 from the Golgi to the ER. We also show that p24δ5/δ9 interact directly with ERD2. This interaction requires the Golgi dynamics (GOLD) domain in p24δ5 and is much higher at acidic than at neutral pH, consistent with both proteins interacting at the cis-Golgi. In addition, p24δ5 also inhibits the secretion of HDEL ligands, but not constitutive secretion, showing a role for p24δ5 in retrograde Golgi-to-ER transport. Both p24δ5 and ERD2 interact with ADP-ribosylation factor 1 (ARF1) and COPI subunits, mostly at acidic pH, consistent with COPI vesicles being involved in retrograde transport of both proteins. In contrast, both proteins interact with the COPII subunit Sec23, mostly at neutral pH, consistent with this interaction taking place at the ER for anterograde transport to the Golgi apparatus.

Mobile effector proteins on phage genomes

Bacteriophage genomes found in a range of bacterial pathogens encode a diverse array of virulence factors ranging from superantigens or pore forming lysins to numerous exotoxins. Recent studies have uncovered an entirely new class of bacterial virulence factors, called effector proteins or effector toxins, which are encoded within phage genomes that reside among several pathovars of Escherichia coli and Salmonella enterica. These effector proteins have multiple domains resulting in proteins that can be multifunctional. The effector proteins encoded within phage genomes are translocated directly from the bacterial cytosol into their eukaryotic target cells by specialized bacterial type three secretion systems (T3SSs). In this review, we will give an overview of the different types of effector proteins encoded within phage genomes and examine their roles in bacterial pathogenesis.

Fast structural responses of gap junction membrane domains to AB5 toxins

Gap junctions (GJs) represent connexin-rich membrane domains that connect interiors of adjoining cells in mammalian tissues. How fast GJs can respond to bacterial pathogens has not been known previously. Using Bessel beam plane illumination and confocal spinning disk microscopy, we found fast (~500 ms) formation of connexin-depleted regions (CDRs) inside GJ plaques between cells exposed to AB5 toxins. CDR formation appears as a fast redistribution of connexin channels within GJ plaques with minor changes in outline or geometry. CDR formation does not depend on membrane trafficking or submembrane cytoskeleton and has no effect on GJ conductance. However, CDR responses depend on membrane lipids, can be modified by cholesterol-clustering agents and extracellular K(+) ion concentration, and influence cAMP signaling. The CDR response of GJ plaques to bacterial toxins is a phenomenon observed for all tested connexin isoforms. Through signaling, the CDR response may enable cells to sense exposure to AB5 toxins. CDR formation may reflect lipid-phase separation events in the biological membrane of the GJ plaque, leading to increased connexin packing and lipid reorganization. Our data demonstrate very fast dynamics (in the millisecond-to-second range) within GJ plaques, which previously were considered to be relatively stable, long-lived structures.

Putative p24 complexes in Arabidopsis contain members of the delta and beta subfamilies and cycle in the early secretory pathway

p24 proteins are a family of type I membrane proteins localized to compartments of the early secretory pathway and to coat protein I (COPI)- and COPII-coated vesicles. They can be classified, by sequence homology, into four subfamilies, named p24α, p24β, p24γ, and p24δ. In contrast to animals and fungi, plants contain only members of the p24β and p24δ subfamilies, the latter probably including two different subclasses. It has previously been shown that transiently expressed red fluorescent protein (RFP)-p24δ5 (p24δ1 subclass) localizes to the endoplasmic reticulum (ER) at steady state as a consequence of highly efficient COPI-based recycling from the Golgi apparatus. It is now shown that transiently expressed RFP-p24δ9 (p24δ2 subclass) also localizes to the ER. In contrast, transiently expressed green fluorescent protein (GFP)-p24β3 mainly localizes to the Golgi apparatus (as p24β2) and exits the ER in a COPII-dependent manner. Immunogold electron microscopy in Arabidopsis root tip cells using specific antibodies shows that endogenous p24δ9 localizes mainly to the ER but also partially to the cis-Golgi. In contrast, endogenous p24β3 mainly localizes to the Golgi apparatus. By a combination of experiments using transient expression, knock-out mutants, and co-immunoprecipitation, it is proposed that Arabidopsis p24 proteins form different heteromeric complexes (including members of the β and δ subfamilies) which are important for their stability and their coupled trafficking at the ER-Golgi interface. Evidence is also provided for a role for p24δ5 in retrograde Golgi-ER transport of the KDEL-receptor ERD2.

Retrograde traffic from the Golgi to the endoplasmic reticulum

Proteins to be secreted are transported from the endoplasmic reticulum (ER) to the Golgi apparatus. The transport of these proteins requires the localization and activity of proteins that create ER exit sites, coat proteins to collect cargo and to reshape the membrane into a transport container, and address labels--SNARE proteins--to target the vesicles specifically to the Golgi apparatus. In addition some proteins may need export chaperones or export receptors to enable their exit into transport vesicles. ER export factors, SNAREs, and misfolded Golgi-resident proteins must all be retrieved from the Golgi to the ER again. This retrieval is also part of the organellar homeostasis pathway essential to maintaining the identity of the ER and of the Golgi apparatus. In this review, I will discuss the different processes in retrograde transport from the Golgi to the ER and highlight the mechanistic insights we have obtained in the last couple of years.

Cab45 is required for Ca(2+)-dependent secretory cargo sorting at the trans-Golgi network

The Golgi resident protein Cab45 is required for trans-Golgi network Ca²⁺ homeostasis and sorting of cargos that are destined for secretion.

Ca²⁺ import into the lumen of the trans-Golgi network (TGN) by the secretory pathway calcium ATPase1 (SPCA1) is required for the sorting of secretory cargo. How is Ca²⁺ retained in the lumen of the Golgi, and what is its role in cargo sorting? We show here that a soluble, lumenal Golgi resident protein, Cab45, is required for SPCA1-dependent Ca²⁺ import into the TGN; it binds secretory cargo in a Ca²⁺-dependent reaction and is required for its sorting at the TGN.

Bacterial protein toxins and lipids: role in toxin targeting and activity

All bacterial toxins, which globally are hydrophilic proteins, interact first with their target cells by recognizing a surface receptor, which is either a lipid or a lipid derivative, or another compound but in a lipid environment. Intracellular active toxins follow various trafficking pathways, the sorting of which is greatly dependent on the nature of the receptor, notably lipidic receptor or receptor embedded into a distinct environment such as lipid microdomains. Numerous other toxins act locally on cell membrane. Indeed, phospholipase activity is a common mechanism shared by several membrane-damaging toxins. In addition, many toxins active intracellularly or on cell membrane modulate host cell phospholipid pathways. Unusually, a few bacterial toxins require a lipid post-translational modification to be active. Thereby, lipids are obligate partners of bacterial toxins.

COPI budding within the Golgi stack

The Golgi serves as a hub for intracellular membrane traffic in the eukaryotic cell. Transport within the early secretory pathway, that is within the Golgi and from the Golgi to the endoplasmic reticulum, is mediated by COPI-coated vesicles. The COPI coat shares structural features with the clathrin coat, but differs in the mechanisms of cargo sorting and vesicle formation. The small GTPase Arf1 initiates coating on activation and recruits en bloc the stable heptameric protein complex coatomer that resembles the inner and the outer shells of clathrin-coated vesicles. Different binding sites exist in coatomer for membrane machinery and for the sorting of various classes of cargo proteins. During the budding of a COPI vesicle, lipids are sorted to give a liquid-disordered phase composition. For the release of a COPI-coated vesicle, coatomer and Arf cooperate to mediate membrane separation.

Ricinus communis intoxications in human and veterinary medicine-a summary of real cases

Accidental and intended Ricinus communis intoxications in humans and animals have been known for centuries but the causative agent remained elusive until 1888 when Stillmark attributed the toxicity to the lectin ricin. Ricinus communis is grown worldwide on an industrial scale for the production of castor oil. As by-product in castor oil production ricin is mass produced above 1 million tons per year. On the basis of its availability, toxicity, ease of preparation and the current lack of medical countermeasures, ricin has gained attention as potential biological warfare agent. The seeds also contain the less toxic, but highly homologous Ricinus communis agglutinin and the alkaloid ricinine, and especially the latter can be used to track intoxications. After oil extraction and detoxification, the defatted press cake is used as organic fertilizer and as low-value feed. In this context there have been sporadic reports from different countries describing animal intoxications after uptake of obviously insufficiently detoxified fertilizer. Observations in Germany over several years, however, have led us to speculate that the detoxification process is not always performed thoroughly and controlled, calling for international regulations which clearly state a ricin threshold in fertilizer. In this review we summarize knowledge on intended and unintended poisoning with ricin or castor seeds both in humans and animals, with a particular emphasis on intoxications due to improperly detoxified castor bean meal and forensic analysis.

Several ADP-ribosylation factor (Arf) isoforms support COPI vesicle formation

Newly synthesized proteins and lipids are transported in vesicular carriers along the secretory pathway. Arfs (ADP-ribosylation factors), a family of highly conserved GTPases within the Ras superfamily, control recruitment of molecular coats to membranes, the initial step of coated vesicle biogenesis. Arf1 and coatomer constitute the minimal cytosolic machinery leading to COPI vesicle formation from Golgi membranes. Although some functional redundancies have been suggested, other Arf isoforms have been poorly analyzed in this context. In this study, we found that Arf1, Arf4, and Arf5, but not Arf3 and Arf6, associate with COPI vesicles generated in vitro from Golgi membranes and purified cytosol. Using recombinant myristoylated proteins, we show that Arf1, Arf4, and Arf5 each support COPI vesicle formation individually. Unexpectedly, we found that Arf3 could also mediate vesicle biogenesis. However, Arf3 was excluded from the vesicle fraction in the presence of the other isoforms, highlighting a functional competition between the different Arf members.

How viruses hijack cell regulation

Viruses, as obligate intracellular parasites, are the pathogens that have the most intimate relationship with their host, and as such, their genomes have been shaped directly by interactions with the host proteome. Every step of the viral life cycle, from entry to budding, is orchestrated through interactions with cellular proteins. Accordingly, viruses will hijack and manipulate these proteins utilising any achievable mechanism. Yet, the extensive interactions of viral proteomes has yielded a conundrum: how do viruses commandeer so many diverse pathways and processes, given the obvious spatial constraints imposed by their compact genomes? One important approach is slowly being revealed, the extensive mimicry of host protein short linear motifs (SLiMs).

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.

Bioengineering a unique deimmunized bispecific targeted toxin that simultaneously recognizes human CD22 and CD19 receptors in a mouse model of B-cell metastases

A drug of high potency and reduced immunogenicity is needed to develop a targeted biological drug that when injected systemically can penetrate to malignant B cells. Therefore, a novel deimmunized bispecific ligand-directed toxin targeted by dual high-affinity single-chain Fvs (scFv) spliced to PE38 with a KDEL COOH-terminus was genetically engineered. The aims were to reduce toxin immunogenicity using mutagenesis, measure the ability of mutated drug to elicit antitoxin antibody responses, and show that mutated drug was effective against systemic B-cell lymphoma in vivo. Both human anti-CD22 scFv and anti-CD19 scFv were cloned onto the same single-chain molecule with truncated pseudomonas exotoxin (PE38) to create the drug. Site-specific mutagenesis was used to mutate amino acids in seven key epitopic toxin regions that dictate B-cell generation of neutralizing antitoxin antibodies. Bioassays were used to determine whether mutation reduced potency, and ELISAs were done to determine whether antitoxin antibodies were reduced. Finally, a powerful genetically altered luciferase xenograft model was used that could be imaged in real time to determine the effect on systemic malignant human B-cell lymphoma, Raji-luc. Patient B-lineage acute lymphoblastic leukemia, B-cell chronic lymphocytic leukemia, and B lymphoma were high in CD22 and CD19 expression. 2219KDEL7mut was significantly effective against systemic Raji-luc in mice and prevented metastatic spread. Mutagenesis reduced neutralizing antitoxin antibodies by approximately 80% with no apparent loss in in vitro or in vivo activity. Because 2219KDEL7mut immunogenicity was significantly reduced and the drug was highly effective in vivo, we can now give multiple drug treatments with targeted toxins in future clinical trials.

Clostridial glucosylating toxins enter cells via clathrin-mediated endocytosis

Clostridium difficile toxin A (TcdA) and toxin B (TcdB), C. sordellii lethal toxin (TcsL) and C. novyi α-toxin (TcnA) are important pathogenicity factors, which represent the family of the clostridial glucosylating toxins (CGTs). Toxin A and B are associated with antibiotic-associated diarrhea and pseudomembraneous colitis. Lethal toxin is involved in toxic shock syndrome after abortion and α-toxin in gas gangrene development. CGTs enter cells via receptor-mediated endocytosis and require an acidified endosome for translocation of the catalytic domain into the cytosol. Here we studied the endocytic processes that mediate cell internalization of the CGTs. Intoxication of cells was monitored by analyzing cell morphology, status of Rac glucosylation in cell lysates and transepithelial resistance of cell monolayers. We found that the intoxication of cultured cells by CGTs was strongly delayed when cells were preincubated with dynasore, a cell-permeable inhibitor of dynamin, or chlorpromazine, an inhibitor of the clathrin-dependent endocytic pathway. Additional evidence about the role of clathrin in the uptake of the prototypical CGT family member toxin B was achieved by expression of a dominant-negative inhibitor of the clathrin-mediated endocytosis (Eps15 DN) or by siRNA against the clathrin heavy chain. Accordingly, cells that expressed dominant-negative caveolin-1 were not protected from toxin B-induced cell rounding. In addition, lipid rafts impairment by exogenous depletion of sphingomyelin did not decelerate intoxication of HeLa cells by CGTs. Taken together, our data indicate that the endocytic uptake of the CGTs involves a dynamin-dependent process that is mainly governed by clathrin.

Identification of gp96 as a novel target for treatment of autoimmune disease in mice

Heat shock proteins have been implicated as endogenous activators for dendritic cells (DCs). Chronic expression of heat shock protein gp96 on cell surfaces induces significant DC activations and systemic lupus erythematosus (SLE)-like phenotypes in mice. However, its potential as a therapeutic target against SLE remains to be evaluated. In this work, we conducted chemical approach to determine whether SLE-like phenotypes can be compromised by controlling surface translocation of gp96. From screening of chemical library, we identified a compound that binds and suppresses surface presentation of gp96 by facilitating its oligomerization and retrograde transport to endoplasmic reticulum. In vivo administration of this compound reduced maturation of DCs, populations of antigen presenting cells, and activated B and T cells. The chemical treatment also alleviated the SLE-associated symptoms such as glomerulonephritis, proteinuria, and accumulation of anti-nuclear and -DNA antibodies in the SLE model mice resulting from chronic surface exposure of gp96. These results suggest that surface translocation of gp96 can be chemically controlled and gp96 as a potential therapeutic target to treat autoimmune disease like SLE.

Scyl1 regulates Golgi morphology

Background

Membrane trafficking is a defining feature of eukaryotic cells, and is essential for the maintenance of organelle homeostasis and identity. We previously identified Scy1-like 1 (Scyl1), a member of the Scy1-like family of catalytically inactive protein kinases, as a high-affinity binding partner of COPI coats. COPI-coated vesicles control Golgi to endoplasmic reticulum trafficking and we observed that disruption of Scyl1 function leads to a decrease in trafficking of the KDEL receptor via the COPI pathway. We reasoned that if Scyl1 plays a major role in COPI trafficking its disruption could influence Golgi homeostasis.

Methodology/Principal Findings

We performed Scyl1 knock down in cultured cells using previously established methods and observed an alteration in Golgi morphology. Both the surface area and volume of the Golgi is increased in Scyl1-depleted cells, but the continuity and polarity of the organelle is unperturbed. At the ultrastructural level we observe a decrease in the orderly structure of the Golgi with an increase in cisternal luminal width, while the number of Golgi cisternae remains unchanged. The golgin family of proteins forms a detergent resistant network that controls Golgi homeostasis. Disruption of this protein network by knock down of the golgin p115 disrupts the Golgi localization of Scyl1. Moreover, we find that Scyl1 interacts with 58K/formiminotransferase cyclodeaminase (FTCD), a protein that is tightly associated with the cis face of the Golgi.

Conclusions/Significance

Our results place Scyl1 at an interface between the golgin network and COPI trafficking and demonstrate that Scyl1 is required for the maintenance of Golgi morphology. Coupled with the observation from others that Scyl1 is the gene product responsible for the neurodegenerative mouse model mdf, our results additionally implicate the regulation of COPI trafficking and Golgi homeostasis in neurodegeneration.

Calreticulin-dependent recycling in the early secretory pathway mediates optimal peptide loading of MHC class I molecules

Calreticulin is a lectin chaperone of the endoplasmic reticulum (ER). In calreticulin-deficient cells, major histocompatibility complex (MHC) class I molecules travel to the cell surface in association with a sub-optimal peptide load. Here, we show that calreticulin exits the ER to accumulate in the ER-Golgi intermediate compartment (ERGIC) and the cis-Golgi, together with sub-optimally loaded class I molecules. Calreticulin that lacks its C-terminal KDEL retrieval sequence assembles with the peptide-loading complex but neither retrieves sub-optimally loaded class I molecules from the cis-Golgi to the ER, nor supports optimal peptide loading. Our study, to the best of our knowledge, demonstrates for the first time a functional role of intracellular transport in the optimal loading of MHC class I molecules with antigenic peptide.

The COG complex, Rab6 and COPI define a novel Golgi retrograde trafficking pathway that is exploited by SubAB toxin

Toxin trafficking studies provide valuable information about endogenous pathways of intracellular transport. Subtilase cytotoxin (SubAB) is transported in a retrograde manner through the endosome to the Golgi and then to the endoplasmic reticulum (ER), where it specifically cleaves the ER chaperone BiP/GRP78 (Binding immunoglobin protein/Glucose-Regulated Protein of 78 kDa). To identify the SubAB Golgi trafficking route, we have used siRNA-mediated silencing and immunofluorescence microscopy in HeLa and Vero cells. Knockdown (KD) of subunits of the conserved oligomeric Golgi (COG) complex significantly delays SubAB cytotoxicity and blocks SubAB trafficking to the cis Golgi. Depletion of Rab6 and beta-COP proteins causes a similar delay in SubAB-mediated GRP78 cleavage and did not augment the trafficking block observed in COG KD cells, indicating that all three Golgi factors operate on the same 'fast' retrograde trafficking pathway. SubAB trafficking is completely blocked in cells deficient in the Golgi SNARE Syntaxin 5 and does not require the activity of endosomal sorting nexins SNX1 and SNX2. Surprisingly, depletion of Golgi tethers p115 and golgin-84 that regulates two previously described coat protein I (COPI) vesicle-mediated pathways did not interfere with SubAB trafficking, indicating that SubAB is exploiting a novel COG/Rab6/COPI-dependent retrograde trafficking pathway.

Cholera toxin: A paradigm for multi-functional engagement of cellular mechanisms (Review)

Cholera toxin (Ctx) from Vibrio cholerae and its closely related homologue, heat-labile enterotoxin (Etx) from Escherichia coli have become superb tools for illuminating pathways of cellular trafficking and immune cell function. These bacterial protein toxins should be viewed as conglomerates of highly evolved, multi-functional elements equipped to engage the trafficking and signalling machineries of cells. Ctx and Etx are members of a larger family of A-B toxins of bacterial (and plant) origin that are comprised of structurally and functionally distinct enzymatically active A and receptor-binding B sub-units or domains. Intoxication of mammalian cells by Ctx and Etx involves B pentamer-mediated receptor binding and entry into a vesicular pathway, followed by translocation of the enzymatic A1 domain of the A sub-unit into the target cell cytosol, where covalent modification of intracellular targets leads to activation of adenylate cyclase and a sequence of events culminating in life-threatening diarrhoeal disease. Importantly, Ctx and Etx also have the capacity to induce a wide spectrum of remarkable immunological processes. With respect to the latter, it has been found that these toxins activate signalling pathways that modulate the immune system. This review explores the complexities of the cellular interactions that are engaged by these bacterial protein toxins, and highlights some of the new insights to have recently emerged.

Selection and characterization of KDEL-specific VHH antibody fragments and their application in the study of ER resident protein expression

Several diseases are caused by defects in the protein secretory pathway of the cell, particularly in the endoplasmic reticulum (ER). These defects are manifested by the activation of the unfolded protein response (UPR) that involves the transcriptional up-regulation of several ER resident proteins, the down-regulation of protein translation and up-regulation of ER associated degradation (ERAD). Although this transcriptional up-regulation of ER resident proteins during ER stress has been well described, data on differential protein expression of these same proteins are hardly available. Tools that would enable the simultaneous analysis of this set of proteins would be of high importance. Since the C-terminal KDEL sequence is a conserved epitope present in a large set of ER resident proteins, an antibody directed against this sequence would be such a tool. Using a carefully designed selection strategy, VHH antibody fragments from a non-immune phage display library were isolated that recognize the KDEL sequence at the C-terminus of proteins, irrespective of the protein context. In an accepted in vitro model for ER stress, this antibody was shown to be an excellent tool to study differences in ER resident protein expression. Furthermore, the application of this antibody showed differences in ER resident protein levels during replicative senescence of human umbilical vein endothelial cells (HUVECs), underlining its significance in biological research. The selection strategy used to obtain these KDEL-specific antibodies opens up ways to select antibodies to other conserved epitopes, such as the nuclear localization signal (NLS) or the peroxisomal targeting sequence, permitting the simultaneous analysis of specific groups of proteins.

A traffic-activated Golgi-based signalling circuit coordinates the secretory pathway

As with other complex cellular functions, intracellular membrane transport involves the coordinated engagement of a series of organelles and machineries; however, the molecular basis of this coordination is unknown. Here we describe a Golgi-based signalling system that is activated by traffic and is involved in monitoring and balancing trafficking rates into and out of the Golgi complex. We provide evidence that the traffic signal is due to protein chaperones that leave the endoplasmic reticulum and reach the Golgi complex where they bind to the KDEL receptor. This initiates a signalling reaction that includes the activation of a Golgi pool of Src kinases and a phosphorylation cascade that in turn activates intra-Golgi trafficking, thereby maintaining the dynamic equilibrium of the Golgi complex. The concepts emerging from this study should help to understand the control circuits that coordinate high-order cellular functions.

Cholera toxin up-regulates endoplasmic reticulum proteins that correlate with sensitivity to the toxin

Cholera toxin (CT) contains one A chain and five B chains. The A chain is an enzyme that covalently modifies a trimeric G protein in the cytoplasm, resulting in the overproduction of cAMP. The B chain binds the glycosphingolipid G(M1), the cell surface receptor for CT, which initiates receptor-mediated endocytosis of the toxin. After endocytosis, CT enters the endoplasmic reticulum (ER) via retrograde vesicular traffic where the A chain retro-translocates through the ER membrane to reach the cytoplasm. The retro-translocation mechanism is poorly understood, but may involve proteins of the ER stress response, including the ER associated degradation (ERAD) pathway. We report here that treating cells with CT or CTB quickly up-regulates the levels of BiP, Derlin-1, and Derlin-2, known participants in the ER stress response and ERAD. CT did not induce calnexin, another known responder to ER stress, indicating that the CT-mediated induction of ER proteins is selective in this time frame. These data suggest that CT may promote retro-translocation of the A chain to the cytoplasm by rapidly up-regulating a set of ER proteins involved in the retro-translocation process. In support of this idea, a variety of conditions that induced BiP, Derlin-1, and Derlin-2 sensitized cells to CT and conditions that inhibited their induction de-sensitized cells to CT. Moreover, specifically suppressing Derlin-1 with siRNA protected cells from CT. In addition, Derlin-1 co-immunoprecipitated with CTA or CTB from CT-treated cells using anti-CTA or anti-CTB antibodies. Altogether, the results are consistent with the hypothesis that the B chain of CT up-regulates ER proteins that may assist in the retro-translocation of the A chain across the ER membrane.

The cargo receptors Surf4, endoplasmic reticulum-Golgi intermediate compartment (ERGIC)-53, and p25 are required to maintain the architecture of ERGIC and Golgi

Rapidly cycling proteins of the early secretory pathway can operate as cargo receptors. Known cargo receptors are abundant proteins, but it remains mysterious why their inactivation leads to rather limited secretion phenotypes. Studies of Surf4, the human orthologue of the yeast cargo receptor Erv29p, now reveal a novel function of cargo receptors. Surf4 was found to interact with endoplasmic reticulum-Golgi intermediate compartment (ERGIC)-53 and p24 proteins. Silencing Surf4 together with ERGIC-53 or silencing the p24 family member p25 induced an identical phenotype characterized by a reduced number of ERGIC clusters and fragmentation of the Golgi apparatus without effect on anterograde transport. Live imaging showed decreased stability of ERGIC clusters after knockdown of p25. Silencing of Surf4/ERGIC-53 or p25 resulted in partial redistribution of coat protein (COP) I but not Golgi matrix proteins to the cytosol and partial resistance of the cis-Golgi to brefeldin A. These findings imply that cargo receptors are essential for maintaining the architecture of ERGIC and Golgi by controlling COP I recruitment.

Multiple and stepwise interactions between coatomer and ADP-ribosylation factor-1 (Arf1)-GTP

The small GTPase ADP-ribosylation factor-1 (Arf1) plays a key role in the formation of coat protein I (COP I)-coated vesicles. Upon recruitment to the donor Golgi membrane by interaction with dimeric p24 proteins, Arf1's GDP is exchanged for GTP. Arf1-GTP then dissociates from p24, and together with other Golgi membrane proteins, it recruits coatomer, the heptameric coat protein complex of COP I vesicles, from the cytosol. In this process, Arf1 was shown to specifically interact with the coatomer beta and gamma-COP subunits through its switch I region, and with epsilon-COP. Here, we mapped the interaction of the Arf1-GTP switch I region to the trunk domains of beta and gamma-COP. Site-directed photolabeling at position 167 in the C-terminal helix of Arf1 revealed a novel interaction with coatomer via a putative longin domain of delta-COP. Thus, coatomer is linked to the Golgi through multiple interfaces with membrane-bound Arf1-GTP. These interactions are located within the core, adaptor-like domain of coatomer, indicating an organizational similarity between the COP I coat and clathrin adaptor complexes.

Regulation of virulence in Vibrio cholerae: the ToxR regulon

Vibrio cholerae is a gram-negative bacterium that is the causative agent of cholera. This disease consists of enormous fluid loss through stools, which can be fatal. Cholera epidemics appear in explosive outbreaks that have occurred repeatedly throughout history. The virulence factors toxin coregulated pilus (TCP) and cholera toxin (CT) are essential for colonization of the host and enterotoxicity, respectively. These virulence factors are under the control of ToxT, an AraC/XylS family protein that activates transcription of the genes encoding TCP and CT. ToxT is under the control of a virulence regulatory cascade known as the ToxR regulon, which responds to environmental stimuli to ensure maximal virulence-factor induction within the human intestine. An understanding of this intricate signaling pathway is essential for the development of methods to treat and prevent this devastating disease.

Two human ARFGAPs associated with COP-I-coated vesicles

ADP-ribosylation factors (ARFs) are critical regulators of vesicular trafficking pathways and act at multiple intracellular sites. ADP-ribosylation factor-GTPase-activating proteins (ARFGAPs) are proposed to contribute to site-specific regulation. In yeast, two distinct proteins, Glo3p and Gcs1p, together provide overlapping, essential ARFGAP function required for coat protein (COP)-I-dependent trafficking. In mammalian cells, only the Gcs1p orthologue, named ARFGAP1, has been characterized in detail. However, Glo3p is known to make the stronger contribution to COP I traffic in yeast. Here, based on a conserved signature motif close to the carboxy terminus, we identify ARFGAP2 and ARFGAP3 as the human orthologues of yeast Glo3p. By immunofluorescence (IF), ARFGAP2 and ARFGAP3 are closely colocalized with coatomer subunits in NRK cells in the Golgi complex and peripheral punctate structures. In contrast to ARFGAP1, both ARFGAP2 and ARFGAP3 are associated with COP-I-coated vesicles generated from Golgi membranes in the presence of GTP-γ-S in vitro. ARFGAP2 lacking its zinc finger domain directly binds to coatomer. Expression of this truncated mutant (ΔN-ARFGAP2) inhibits COP-I-dependent Golgi-to-endoplasmic reticulum transport of cholera toxin (CTX-K63) in vivo. Silencing of ARFGAP1 or a combination of ARFGAP2 and ARFGAP3 in HeLa cells does not decrease cell viability. However, silencing all three ARFGAPs causes cell death. Our data provide strong evidence that ARFGAP2 and ARFGAP3 function in COP I traffic.

Unsolved mysteries in membrane traffic

Remarkable strides have been made over the past 20 years in elucidating the molecular basis of membrane trafficking. Indeed, a combination of biochemical and genetic approaches have determined the identity and function of many of the core constituents needed for protein secretion and endocytosis. But much remains to be learned. This review highlights underlying themes in membrane traffic to help us refocus and solve many remaining and newly emerging issues that are fundamental to mammalian cell biology and human physiology.

Disparate effects of p24alpha and p24delta on secretory protein transport and processing

Background

The p24 family is thought to be somehow involved in endoplasmic reticulum (ER)-to-Golgi protein transport. A subset of the p24 proteins (p24α₃, -β₁, -γ₃ and -δ₂) is upregulated when Xenopus laevis intermediate pituitary melanotrope cells are physiologically activated to produce vast amounts of their major secretory cargo, the prohormone proopiomelanocortin (POMC).

Methodology/Principal Findings

Here we find that transgene expression of p24α₃ or p24δ₂ specifically in the Xenopus melanotrope cells in both cases causes an effective displacement of the endogenous p24 proteins, resulting in severely distorted p24 systems and disparate melanotrope cell phenotypes. Transgene expression of p24α₃ greatly reduces POMC transport and leads to accumulation of the prohormone in large, ER-localized electron-dense structures, whereas p24δ₂-transgenesis does not influence the overall ultrastructure of the cells nor POMC transport and cleavage, but affects the Golgi-based processes of POMC glycomaturation and sulfation.

Conclusions/Significance

Transgenic expression of two distinct p24 family members has disparate effects on secretory pathway functioning, illustrating the specificity and non-redundancy of our transgenic approach. We conclude that members of the p24 family furnish subcompartments of the secretory pathway with specific sets of machinery cargo to provide the proper microenvironments for efficient and correct secretory protein transport and processing.

Impact of live cell imaging on coated vesicle research

The role of membrane traffic is to transfer cargo between distinct subcellular compartments. Each individual trafficking event involves the creation, transport and fusion of vesicular and tubular carriers that are formed and regulated via cytoplasmic coat protein complexes. The dynamic nature of this process is therefore highly suitable for studying using live cell imaging techniques. Although these approaches have raised further questions for the field, they have also been instrumental in providing essential new information, in particular relating to the morphology of transport carriers and the exchange kinetics of coat proteins and their regulators on membranes. Here, we present an overview of live cell-imaging experiments that have been used in the study of coated-vesicle transport, and provide specific examples of their impact on our understanding of coat function.