Tubulation of Golgi membranes in vivo and in vitro in the absence of brefeldin A

Negative Staining Electron Microscopy for Morpho-Functional Characterization of Purified Golgi Membranes from Mammalian Cells

The Golgi apparatus is a highly dynamic organelle that controls lipid and protein sorting in the endocytic and exocytic cellular pathways. Perturbation of the lipid homeostasis or of the molecular machineries that regulate membrane remodeling/trafficking events on the Golgi membranes can dramatically change the morphology and functions of the Golgi apparatus. So far, several approaches have been described to characterize and define the Golgi morphology in intact cells and in vitro. Here, we describe the application of negative staining (NS) electron microscopy (EM) on purified Golgi membranes from HeLa cells. This approach allows to quantify and functionally characterize membrane remodeling events upon specific treatments that alter the Golgi morphology.

Oral absorption of peptides and nanoparticles across the human intestine: Opportunities, limitations and studies in human tissues

In this contribution, we review the molecular and physiological barriers to oral delivery of peptides and nanoparticles. We discuss the opportunities and predictivity of various in vitro systems with special emphasis on human intestine in Ussing chambers. First, the molecular constraints to peptide absorption are discussed. Then the physiological barriers to peptide delivery are examined. These include the gastric and intestinal environment, the mucus barrier, tight junctions between epithelial cells, the enterocytes of the intestinal epithelium, and the subepithelial tissue. Recent data from human proteome studies are used to provide information about the protein expression profiles of the different physiological barriers to peptide and nanoparticle absorption. Strategies that have been employed to increase peptide absorption across each of the barriers are discussed. Special consideration is given to attempts at utilizing endogenous transcytotic pathways. To reliably translate in vitro data on peptide or nanoparticle permeability to the in vivo situation in a human subject, the in vitro experimental system needs to realistically capture the central aspects of the mentioned barriers. Therefore, characteristics of common in vitro cell culture systems are discussed and compared to those of human intestinal tissues. Attempts to use the cell and tissue models for in vitro-in vivo extrapolation are reviewed.

Reconstitution of Phospholipase A2-Dependent Golgi Membrane Tubules

The Golgi complex is the Grand Central Station of intracellular membrane trafficking in the secretory and endocytic pathways. Anterograde and retrograde export of cargo from the Golgi complex involves a complex interplay between the formation of coated vesicles and membrane tubules, although much less is known about tubule-mediated trafficking. Recent advances using in vitro assays have identified several cytoplasmic phospholipase A2 (PLA2) enzymes that are required for the biogenesis of membrane tubules and their roles in the functional organization of the Golgi complex. In this chapter we describe methods for the cell-free reconstitution of PLA2-dependent Golgi membrane tubule formation. These methods should facilitate the identification of other proteins that regulate this process.

Pearling instability of membrane tubes driven by curved proteins and actin polymerization

Membrane deformation inside living cells is crucial for the proper shaping of various intracellular organelles and is necessary during the fission/fusion processes that allow membrane recycling and transport (e.g. endocytosis). Proteins that induce membrane curvature play a key role in such processes, mostly by adsorbing to the membrane and forming a scaffold that deforms the membrane according to the curvature of the proteins. In this paper we explore the possibility of membrane tube destabilization through a pearling mechanism enabled by the combined effects of the adsorbed curved proteins and the actin polymerization that they recruit. The pearling instability can serve as the initiation for fission of the tube into vesicles. We find that adsorbed curved proteins are more likely to stabilize the tubes, while the actin polymerization can provide the additional constrictive force needed for the robust instability. We discuss the relevance of the theoretical results to in vivo and in vitro experiments.

Methods for analyzing the role of phospholipase A₂ enzymes in endosome membrane tubule formation

Cargo export from mammalian endosomal compartments often involves membrane tubules, into which soluble and membrane-bound cargos are segregated for subsequent intracellular transport. These membrane tubules are highly dynamic and their formation is mediated by a variety of endosome-associated proteins. However, little is known about how these membrane tubules are temporally or spatially regulated, so other tubule-associated proteins are likely to be discovered and analyzed. Therefore, methods to examine the biogenesis and regulation of endosome membrane tubules will prove to be valuable for cell biologists. In this chapter, we describe methods for studying this process using both cell-free, in vitro reconstitution assays, and in vivo image analysis tools.

Phenomenology based multiscale models as tools to understand cell membrane and organelle morphologies

An intriguing question in cell biology is "how do cells regulate their shape?" It is commonly believed that the observed cellular morphologies are a result of the complex interaction among the lipid molecules (constituting the cell membrane), and with a number of other macromolecules, such as proteins. It is also believed that the common biophysical processes essential for the functioning of a cell also play an important role in cellular morphogenesis. At the cellular scale-where typical dimensions are in the order of micrometers-the effects arising from the molecular scale can either be modeled as equilibrium or non-equilibrium processes. In this chapter, we discuss the dynamically triangulated Monte Carlo technique to model and simulate membrane morphologies at the cellular scale, which in turn can be used to investigate several questions related to shape regulation in cells. In particular, we focus on two specific problems within the framework of isotropic and anisotropic elasticity theories: namely, (i) the origin of complex, physiologically relevant, membrane shapes due to the interaction of the membrane with curvature remodeling proteins, and (ii) the genesis of steady state cellular shapes due to the action of non-equilibrium forces that are generated by the fission and fusion of transport vesicles and by the binding and unbinding of proteins from the parent membrane.

A stress assembly that confers cell viability by preserving ERES components during amino-acid starvation

Nutritional restriction leads to protein translation attenuation that results in the storage and degradation of free mRNAs in cytoplasmic assemblies. In this study, we show in Drosophila S2 cells that amino-acid starvation also leads to the inhibition of another major anabolic pathway, the protein transport through the secretory pathway, and to the formation of a novel reversible non-membrane bound stress assembly, the Sec body that incorporates components of the ER exit sites. Sec body formation does not depend on membrane traffic in the early secretory pathway, yet requires both Sec23 and Sec24AB. Sec bodies have liquid droplet-like properties, and they act as a protective reservoir for ERES components to rebuild a functional secretory pathway after re-addition of amino-acids acting as a part of a survival mechanism. Taken together, we propose that the formation of these structures is a novel stress response mechanism to provide cell viability during and after nutrient stress.

DOI:http://dx.doi.org/10.7554/eLife.04132.001

Proteins are needed by living cells to perform vital tasks and are made from building blocks called amino-acids. However, if a cell is starved of amino-acids, protein assembly comes to a halt, and if cells are deprived of amino acids for a long time, the cell may die.

To survive short periods of amino-acid starvation, the cell has developed many protective mechanisms. For example, it can start to break down existing proteins, allowing the cell to scavenge and reuse the amino-acids to make other proteins that are more important for short-term survival. The cell may also temporarily halt certain processes: for example, newly constructed proteins may no longer be transported from the cell structure where they are made—called the endoplasmic reticulum—to their final destinations in the cell. However, the protein transport apparatus is also made of proteins and needs to be protected from being broken down so that once starvation ends, the cell can more quickly return to normal working order.

Zacharogianni et al. identify a strategy cells use to store and protect part of their protein transport apparatus during times of stress. Starving fruit fly cells of amino-acids causes the cells to form protective stress assemblies incorporating the proteins associated with the ‘exit sites’ that release proteins from the endoplasmic reticulum. These assemblies are called Sec bodies, and when amino-acid starvation ends, these bodies release the exit site components unharmed. This allows the cell to quickly resume protein transport and so speeds the cell's recovery. If the Sec bodies do not form, the cells are more likely to die during amino-acid starvation.

The Sec bodies are distinct from previously identified stress assemblies that form in the cell during stress, but they share features with them, such as being liquid droplets. Some of these assemblies have been linked to degenerative diseases like amyotrophic lateral sclerosis (ALS). Further research will be necessary to determine if there are any similar harmful side effects associated with the formation of Sec bodies.

DOI:http://dx.doi.org/10.7554/eLife.04132.002

Gβ1γ2 activates phospholipase A2-dependent Golgi membrane tubule formation

Heterotrimeric G proteins transduce the ligand binding of transmembrane G protein coupled receptors into a variety of intracellular signaling pathways. Recently, heterotrimeric Gβγ subunit signaling at the Golgi complex has been shown to regulate the formation of vesicular transport carriers that deliver cargo from the Golgi to the plasma membrane. In addition to vesicles, membrane tubules have also been shown to mediate export from the Golgi complex, which requires the activity of cytoplasmic phospholipase A₂ (PLA₂) enzyme activity. Through the use of an in vitro reconstitution assay with isolated Golgi complexes, we provide evidence that Gβ1γ2 signaling also stimulates Golgi membrane tubule formation. In addition, we show that an inhibitor of Gβγ activation of PLA₂ enzymes inhibits in vitro Golgi membrane tubule formation. Additionally, purified Gβγ protein stimulates membrane tubules in the presence of low (sub-threshold) cytosol concentrations. Importantly, this Gβγ stimulation of Golgi membrane tubule formation was inhibited by treatment with the PLA₂ antagonist ONO-RS-082. These studies indicate that Gβ1γ2 signaling activates PLA₂ enzymes required for Golgi membrane tubule formation, thus establishing a new layer of regulation for this process.

The cytosolic chaperone α-crystallin B rescues folding and compartmentalization of misfolded multispan transmembrane proteins

The α-crystallin B chain (CRYAB or HspB5) is a cytosolic chaperone belonging to the small heat shock protein family, which is known to help in the folding of cytosolic proteins. Here we show that CRYAB binds the mutant form of at least two multispan transmembrane proteins (TMPs), exerting an anti-aggregation activity. It rescues the folding of mutant Frizzled4, which is responsible for a rare autosomal dominant form of familial exudative vitreoretinopathy (Fz4-FEVR), and the mutant ATP7B Cu transporter (ATP7B-H1069Q) associated with a common form of Wilson's disease. In the case of Fz4-FEVR, CRYAB prevents the formation of inter-chain disulfide bridges between the lumenal ectodomains of the aggregated mutant chains, which enables correct folding and promotes appropriate compartmentalization on the plasma membrane. ATP7B-H1069Q, with help from CRYAB, folds into the proper conformation, moves to the Golgi complex, and responds to copper overload in the same manner as wild-type ATP7B. These findings strongly suggest that CRYAB plays a pivotal role, previously undetected, in the folding of multispan TMPs and, from the cytosol, is able to orchestrate folding events that take place in the lumen of the ER. Our results contribute to the explanation of the complex scenario behind multispan TMP folding; additionally, they serve to expose interesting avenues for novel therapeutic approaches.

Golgi tubules: their structure, formation and role in intra-Golgi transport

Tubules are common Golgi elements that can form extensive networks associated with the cis-, lateral and trans-Golgi sides, but despite this, they have almost been forgotten for decades. The molecular mechanisms involved in their formation, elongation and fission are only just beginning to be understood. However, the role of these membranes is not well understood. In the present review, we analyze the mechanisms that induce Golgi tubulation or, conversely, disrupt tubules in order to throw some lights on the nature of these elements. The putative role of these elements in the framework of current models for intra-Golgi transport is also discussed.

Regulation of the Golgi complex by phospholipid remodeling enzymes

The mammalian Golgi complex is a highly dynamic organelle consisting of stacks of flattened cisternae with associated coated vesicles and membrane tubules that contribute to cargo import and export, intra-cisternal trafficking, and overall Golgi architecture. At the morphological level, all of these structures are continuously remodeled to carry out these trafficking functions. Recent advances have shown that continual phospholipid remodeling by phospholipase A (PLA) and lysophospholipid acyltransferase (LPAT) enzymes, which deacylate and reacylate Golgi phospholipids, respectively, contributes to this morphological remodeling. Here we review the identification and characterization of four cytoplasmic PLA enzymes and one integral membrane LPAT that participate in the dynamic functional organization of the Golgi complex, and how some of these enzymes are integrated to determine the relative abundance of COPI vesicle and membrane tubule formation. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Recent progress in histochemistry and cell biology

Studies published in Histochemistry and Cell Biology in the year 2011 represent once more a manifest of established and newly sophisticated techniques being exploited to put tissue- and cell type-specific molecules into a functional context. The review is therefore the Histochemistry and Cell Biology's yearly intention to provide interested readers appropriate summaries of investigations touching the areas of tissue biology, developmental biology, the biology of the immune system, stem cell research, the biology of subcellular compartments, in order to put the message of such studies into natural scientific-/human- and also pathological-relevant correlations.

The ceramide-enriched trans-Golgi compartments reorganize together with other parts of the Golgi apparatus in response to ATP-depletion

In this study, the ceramide-enriched trans-Golgi compartments representing sites of synthesis of sphingomyelin and higher organized lipids were visualized in control and ATP-depleted hepatoma and endothelial cells using internalization of BODIPY-ceramide and the diaminobenzidine photooxidation method for combined light-electron microscopical exploration. Metabolic stress induced by lowering the cellular ATP-levels leads to reorganizations of the Golgi apparatus and the appearance of tubulo-glomerular bodies and networks. The results obtained with three different protocols, in which BODIPY-ceramide either was applied prior to, concomitantly with, or after ATP-depletion, revealed that the ceramide-enriched compartments reorganize together with other parts of the Golgi apparatus under these conditions. They were found closely associated with and integrated in the tubulo-glomerular bodies formed in response to ATP-depletion. This is in line with the changes of the staining patterns obtained with the Helix pomatia lectin and the GM130 and TGN46 immuno-reactions occurring in response to ATP-depletion and is confirmed by 3D electron tomography. The 3D reconstructions underlined the glomerular character of the reorganized Golgi apparatus and demonstrated continuities of ceramide positive and negative parts. Most interestingly, BODIPY-ceramide becomes concentrated in compartments of the tubulo-glomerular Golgi bodies, even though the reorganization took place before BODIPY-ceramide administration. This indicates maintained functionalities although the regular Golgi stack organization is abolished; the results provide novel insights into Golgi structure-function relationships, which might be relevant for cells affected by metabolic stress.

Lysophosphatidic acid acyltransferase 3 regulates Golgi complex structure and function

Recent studies have suggested that the functional organization of the Golgi complex is dependent on phospholipid remodeling enzymes. Here, we report the identification of an integral membrane lysophosphatidic acid–specific acyltransferase, LPAAT3, which regulates Golgi membrane tubule formation, trafficking, and structure by altering phospholipids and lysophospholipids. Overexpression of LPAAT3 significantly inhibited the formation of Golgi membrane tubules in vivo and in vitro. Anterograde and retrograde protein trafficking was slower in cells overexpressing LPAAT3 and accelerated in cells with reduced expression (by siRNA). Golgi morphology was also dependent on LPAAT3 because its knockdown caused the Golgi to become fragmented. These data are the first to show a direct role for a specific phospholipid acyltransferase in regulating membrane trafficking and organelle structure.

Differential sorting and Golgi export requirements for raft-associated and raft-independent apical proteins along the biosynthetic pathway

Sorting signals for apically destined proteins are highly diverse and can be present within the luminal, membrane-associated, and cytoplasmic domains of these proteins. A subset of apical proteins partition into detergent-resistant membranes, and the association of these proteins with glycolipid-enriched microdomains or lipid rafts may be important for their proper targeting. Recently, we observed that raft-associated and raft-independent apical proteins take different routes to the apical surface of polarized Madin-Darby canine kidney cells (Cresawn, K. O., Potter, B. A., Oztan, A., Guerriero, C. J., Ihrke, G., Goldenring, J. R., Apodaca, G., and Weisz, O. A. (2007) EMBO J. 26, 3737-3748). Here we reconstituted in vitro the export of raft-associated and raft-independent markers staged intracellularly at 19 degrees C. Surprisingly, whereas release of the raft-associated protein influenza hemagglutinin was dependent on the addition of an ATP-regenerating system and cytosol, release of a yellow fluorescent protein (YFP)-tagged raft-independent protein (the 75-kDa neurotrophin receptor; YFP-p75) was efficient even in the absence of these constituents. Subsequent studies suggested that YFP-p75 is released from the trans-Golgi network in fragile tubules that do not withstand isolation procedures. Moreover, immunofluorescence analysis revealed that hemagglutinin and YFP-p75 segregate into distinct subdomains of the Golgi complex at 19 degrees C. Our data suggest that raft-associated and raft-independent proteins accumulate at distinct intracellular sites upon low temperature staging, and that upon warming, they exit these compartments in transport carriers that have very different membrane characteristics and morphologies.

AMP-activated protein kinase phosphorylates Golgi-specific brefeldin A resistance factor 1 at Thr1337 to induce disassembly of Golgi apparatus

Sufficiency and depletion of nutrients regulate the cellular activities through the protein phosphorylation reaction; however, many protein substrates remain to be clarified. GBF1 (Golgi-specific brefeldin A resistance factor 1), a guanine nucleotide exchange factor for the ADP-ribosylation factor family associated with the Golgi apparatus, was isolated as a phosphoprotein from the glucose-depleted cells by using the phospho-Akt-substrate antibody, which recognizes the substrate proteins of several protein kinases. The phosphorylation of GBF1 was induced by 2-deoxyglucose (2-DG), which blocks glucose utilization and increases the intracellular AMP concentration, and by AICAR, an AMP-activated protein kinase (AMPK) activator. This phosphorylation was observed in the cells expressing the constitutively active AMPK. The 2-DG-induced phosphorylation of GBF1 was suppressed by Compound C, an AMPK inhibitor, and by the overexpression of the kinase-negative AMPK. Analysis using the deletion and point mutants identified Thr(1337) as the 2-DG-induced phosphorylation site in GBF1, which is phosphorylated by AMPK in vitro. ATP depletion is known to provoke the Golgi apparatus disassembly. Immunofluorescent microscopic analysis with the Golgi markers indicated that GBF1 associates with the fragmented Golgi apparatus in the cells treated with 2-DG and AICAR. The expression of the kinase-negative AMPK and the GBF1 mutant replacing Thr(1337) by Ala prevented the 2-DG-induced Golgi disassembly. These results indicate that GBF1 is a novel AMPK substrate and that the AMPK-mediated phosphorylation of GBF1 at Thr(1337) has a critical role, presumably by attenuating the function of GBF1, in the disassembly of the Golgi apparatus induced under stress conditions that lower the intracellular ATP concentration.

Formation, stability, and mobility of one-dimensional lipid bilayers on polysilicon nanowires

Curved lipid membranes are ubiquitous in living systems and play an important role in many biological processes. To understand how curvature and lipid composition affect membrane formation and fluidity, we have assembled and studied mixed 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) supported lipid bilayers on amorphous silicon nanowires grown around carbon nanotube cores with controlled wire diameters ranging from 20 to 200 nm. We found that lipid vesicles fused onto nanowire substrates and formed continuous bilayers for all DOPC-DOPE mixtures tested (with the DOPE content of up to 30%). Our measurements demonstrate that nanowire-supported bilayers are mobile, exhibit fast recovery after photobleaching, and have a low concentration of defects. Lipid diffusion coefficients in these high-curvature tubular membranes are comparable to the values reported for flat supported bilayers and increase slightly with decreasing nanowire diameter. A free space diffusion model adequately describes the effect of bilayer curvature on the lipid mobility for nanowire substrates with diameters greater than 50 nm, but shows significant deviations from the experimental values for smaller diameter nanowires.

Biogenesis of tubular ER-to-Golgi transport intermediates

Tubular transport intermediates (TTIs) have been described as one class of transport carriers in endoplasmic reticulum (ER)-to-Golgi transport. In contrast to vesicle budding and fusion, little is known about the molecular regulation of TTI synthesis, transport and fusion with target membranes. Here we have used in vivo imaging of various kinds of GFP-tagged proteins to start to address these questions. We demonstrate that under steady-state conditions TTIs represent approximately 20% of all moving transport carriers. They increase in number and length when more transport cargo becomes available at the donor membrane, which we induced by either temperature-related transport blocks or increased expression of the respective GFP-tagged transport markers. The formation and motility of TTIs is strongly dependent on the presence of intact microtubules. Microinjection of GTPgammaS increases the frequency of TTI synthesis and the length of these carriers. When Rab proteins are removed from membranes by microinjection of recombinant Rab-GDI, the synthesis of TTIs is completely blocked. Microinjection of the cytoplasmic tails of the p23 and p24 membrane proteins also abolishes formation of p24-containing TTIs. Our data suggest that TTIs are ER-to-Golgi transport intermediates that form preferentially when transport-competent cargo exists in excess at the donor membrane. We propose a model where the interaction of the cytoplasmic tails of membrane proteins with microtubules are key determinants for TTI synthesis and may also serve as a so far unappreciated model for aspects of transport carrier formation.

Functional One-Dimensional Lipid Bilayers on Carbon Nanotube Templates

Use of biological machines and environments in novel bioinorganic nanostructures is critical for development of new types of biosensors, bio-NEMS devices, and functional materials. Lipid bilayers that mimic a cell membrane have already played an important role in such applications. We present supported lipid bilayers that spontaneously assemble in a continuous nanoshell around a template of a carbon nanotube wrapped with hydrophilic polymer cushion layers. We demonstrate that such 1-D lipid membranes are fluid and can heal defects, even over repeated damage-recovery cycles. A simple diffusion model can describe mobility of lipid molecules in these 1-D nanoshells. These structures could lead to the development of new classes of biosensors and bioelectronic devices.

GAIP participates in budding of membrane carriers at the trans-Golgi network

Galpha interacting protein (GAIP) is a regulator of G protein signaling protein that associates dynamically with vesicles and has been implicated in membrane trafficking, although its specific role is not yet known. Using an in vitro budding assay, we show that GAIP is recruited to a specific population of trans-Golgi network-derived vesicles and that these are distinct from coatomer or clathrin-coated vesicles. A truncation mutant (NT-GAIP) encoding only the N-terminal half of GAIP is recruited to trans-Golgi network membranes during the formation of vesicle carriers. Overexpression of NT-GAIP induces the formation of long, coated tubules, which are stabilized by microtubules. Results from the budding assay and from imaging in live cells show that these tubules remain attached to the Golgi stack rather than being released as carrier vesicles. NT-GAIP expression blocks membrane budding and results in the accumulation of tubular carrier intermediates. NT-GAIP-decorated tubules are competent to load vesicular stomatitis virus protein G-green fluorescent protein as post-Golgi, exocytic cargo and in cells expressing NT-GAIP there is reduced surface delivery of vesicular stomatitis virus protein G-green fluorescent protein. We conclude that GAIP functions as an essential part of the membrane budding machinery for a subset of post-Golgi exocytic carriers derived from the trans-Golgi network.

Role of Rab Proteins in Epithelial Membrane Traffic

Small GTPase rab proteins play an important role in various aspects of membrane traffic, including cargo selection, vesicle budding, vesicle motility, tethering, docking, and fusion. Recent data suggest also that rabs, and their divalent effector proteins, organize organelle subdomains and as such may define functional organelle identity. Most rabs are ubiquitously expressed. However, some rabs are preferentially expressed in epithelial cells where they appear intimately associated with the epithelial-specific transcytotic pathway and/or tight junctions. This review discusses the role of rabs in epithelial membrane transport.

GBF1, a guanine nucleotide exchange factor for ADP-ribosylation factors, is localized to the cis-Golgi and involved in membrane association of the COPI coat

Formation of coated carrier vesicles, such as COPI-coated vesicles from the cis-Golgi, is triggered by membrane binding of the GTP-bound form of ADP-ribosylation factors. This process is blocked by brefeldin A, which is an inhibitor of guanine nucleotide exchange factors for ADP-ribosylation factor. GBF1 is one of the guanine nucleotide-exchange factors for ADP-ribosylation factor and is localized in the Golgi region. In the present study, we have determined the detailed subcellular localization of GBF1. Immunofluorescence microscopy of cells treated with nocodazole or incubated at 15 degrees C has suggested that GBF1 behaves similarly to proteins recycling between the cis-Golgi and the endoplasmic reticulum. Immunoelectron microscopy has revealed that GBF1 localizes primarily to vesicular and tubular structures apposed to the cis-face of Golgi stacks and minor fractions to the Golgi stacks. GBF1 overexpressed in cells causes recruitment of class I and class II ADP-ribosylation factors onto Golgi membranes. Furthermore, overexpressed GBF1 antagonizes various effects of brefeldin A, such as inhibition of membrane recruitment of ADP-ribosylation factors and the COPI coat, and redistribution of Golgi-resident and itinerant proteins. These observations indicate that GBF1 is involved in the formation of COPI-coated vesicles from the cis-Golgi or the pre-Golgi intermediate compartment through activating ADP-ribosylation factors.

Overexpression of an ADP-ribosylation factor-guanine nucleotide exchange factor, BIG2, uncouples brefeldin A-induced adaptor protein-1 coat dissociation and membrane tubulation

BIG2 is a guanine nucleotide exchange factor (GEF) for the ADP-ribosylation factor (ARF) family of small GTPases, which regulate membrane association of COPI and adaptor protein (AP)-1 coat protein complexes. A fungal metabolite, brefeldin A (BFA), inhibits ARF-GEFs and leads to redistribution of coat proteins from membranes to the cytoplasm and membrane tubulation of the Golgi complex and the trans-Golgi network (TGN). To investigate the function of BIG2, we examined the effects of BIG2-overexpression on the BFA-induced redistribution of ARF, coat proteins, and organelle markers. The BIG2 overexpression blocked BFA-induced redistribution from membranes of ARF1 and the AP-1 complex but not that of the COPI complex. These observations indicate that BIG2 is implicated in membrane association of AP-1, but not that of COPI, through activating ARF. Furthermore, not only BIG2 but also ARF1 and AP-1 were found as queues of spherical swellings along the BFA-induced membrane tubules emanating from the TGN. These observations indicate that BFA-induced AP-1 dissociation from TGN membranes and tubulation of TGN membranes are not coupled events and suggest that a BFA target other than ARF-GEFs exists in the cell.

Role of rRAB22b, an oligodendrocyte protein, in regulation of transport of vesicles from trans Golgi to endocytic compartments

Intracellular membrane trafficking plays an essential role in the biogenesis and maintenance of myelin. Members of the Rab protein family are important components of the systems that regulate intracellular vesicle transport. We examine the function of rRab22b, a novel rat Rab protein cloned from an oligodendrocyte cDNA library, by visualizing and identifying in living Hela cells the organelles that contain rRab22b. Our results show that rRab22b is present in the trans Golgi/TGN and endocytic compartments. Trafficking of membranes from trans Golgi to endocytic compartments takes place via small tubulo vesicular organelles containing rRab22b. The formation of vesicles in the trans Golgi also appears to be regulated by rRab22b. Additionally, our results suggest that rRab22b controls the transport of vesicles from the trans Golgi to endocytic compartments that localize in oligodendrocyte processes. That rRab22b is involved in the transport of certain proteins from trans Golgi to myelin is suggested by the evidence that certain proteins being targeted to the plasma membrane are first transported from trans Golgi to endocytic compartments.

Inhibition of transferrin recycling and endosome tubulation by phospholipase A2 antagonists

We report here that a broad spectrum of phospholipase A(2) (PLA(2)) antagonists produce a concentration-dependent, differential block in the endocytic recycling pathway of transferrin (Tf) and Tf receptors (TfRs) but have no acute affect on Tf uptake from the cell surface. At low concentrations of antagonists (approximately 1 microm), Tf and TfR accumulated in centrally located recycling endosomes, whereas at higher concentrations (approximately 10 microm), Tf-TfR accumulated in peripheral sorting endosomes. Several independent lines of evidence suggest that this inhibition of recycling may result from the inhibition of tubule formation. First, BFA-stimulated endosome tubule formation was similarly inhibited by PLA(2) antagonists. Second, endocytosed tracers were found in larger spherical endosomes in the presence of PLA(2) antagonists. And third, endosome tubule formation in a cell-free, cytosol-dependent reconstitution system was equally sensitive PLA(2) antagonists. These results are consistent with the conclusion that endosome membrane tubules are formed by the action of a cytoplasmic PLA(2) and that PLA(2)-dependent tubules are involved in intracellular recycling of Tf and TfR. When taken together with previous studies on the Golgi complex, these results also indicate that an intracellular PLA(2) activity provides a novel molecular mechanism for inducing tubule formation from multiple organelles.

Interactions between adaptor protein-1 of the clathrin coat and microtubules via type 1a microtubule-associated proteins

The classical view suggests that adaptor proteins of the clathrin coat mediate the sorting of cargo protein passengers into clathrin-coated pits and the recruitment of clathrin into budding areas in the donor membrane. In the present study, we provide biochemical and morphological evidence that the adaptor protein 1 (AP-1) adaptor of the trans-Golgi network clathrin interacts with microtubules. AP-1 in cytosolic extracts interacted with in vitro assembled microtubules, and these interactions were inhibited by ATP depletion of the extracts or in the presence of 5'-adenylylimidodiphosphate. An overexpressed gamma-subunit of the AP-1 complex associated with microtubules, suggesting that this subunit may mediate the interaction of AP-1 with the cytoskeleton. Purified AP-1 did not interact with purified microtubules, but interaction occurred when an isolated microtubule-associated protein fraction was added to the reaction mix. The gamma-adaptin subunit of AP-1 specifically co-immunoprecipitated with a microtubule-associated protein of type 1a from rat brain cytosol. This suggests that type 1a microtubule-associated protein may mediate the association of AP-1 with microtubules in the cytoplasm. The microtubule binding activity of AP-1 was markedly inhibited in cytosol of mitotic cells. By means of its interaction with microtubule-associated proteins, we propose novel roles for AP-1 adaptors in modulating the dynamics of the cytoskeleton, the stability and shape of coated organelles, and the loading of nascent AP-1-coated vesicles onto appropriate microtubular tracks.

Traffic pattern of cystic fibrosis transmembrane regulator through the early exocytic pathway

The pathway of transport of the cystic fibrosis transmembrane regulator (CFTR) through the early exocytic pathway has not been examined. In contrast to most membrane proteins that are concentrated during export from the ER and therefore readily detectable at elevated levels in pre-Golgi intermediates and Golgi compartments, wild-type CFTR could not be detected in these compartments using deconvolution immunofluorescence microscopy. To determine the basis for this unusual feature, we analyzed CFTR localization using quantitative immunoelectron microscopy (IEM). We found that wild-type CFTR is present in pre-Golgi compartments and peripheral tubular elements associated with the cis and trans faces of the Golgi stack, albeit at a concentration 2-fold lower than that found in the endoplasmic reticulum (ER). delta F508 CFTR, a mutant form that is not efficiently delivered to the cell surface and the most common mutation in cystic fibrosis, could also be detected at a reduced concentration in pre-Golgi intermediates and peripheral cis Golgi elements, but not in post-Golgi compartments. Our results suggest that the low level of wild-type CFTR in the Golgi region reflects a limiting step in selective recruitment by the ER export machinery, an event that is largely deficient in delta F508. We raise the possibility that novel modes of selective anterograde and retrograde traffic between the ER and the Golgi may serve to regulate CFTR function in the early secretory compartments.

Lumenal targeted GFP, used as a marker of soluble cargo, visualises rapid ERGIC to Golgi traffic by a tubulo-vesicular network

The mechanism by which soluble proteins without sorting motifs are transported to the cell surface is not clear. Here we show that soluble green fluorescent protein (GFP) targeted to the lumen of the endoplasmic reticulum but lacking any known retrieval, retention or targeting motifs, was accumulated in the lumen of the ERGIC if cells were kept at reduced temperature. Upon activation of anterograde transport by rewarming of cells, lumenal GFP stained a microtubule-dependent, pre-Golgi tubulo-vesicular network that served as transport structure between peripheral ERGIC-elements and the perinuclear Golgi complex. Individual examples of these tubular elements up to 20 microm in length were observed. Time lapse imaging indicated rapid anterograde flow of soluble lumenal GFP through this network. Transport tubules, stained by lumenal GFP, segregated rapidly from COPI-positive membranes after transport activation. A transmembrane cargo marker, the temperature sensitive glycoprotein of the vesicular stomatitis virus, ts-045 G, is also not present in tubules which contained the soluble cargo marker lum-GFP. These results suggest a role for pre-Golgi vesicular tubular membranes in long distance anterograde transport of soluble cargo. http://www.biologists.com/JCS/movies/jcs1334.html

Reconstitution of brefeldin A-induced golgi tubulation and fusion with the endoplasmic reticulum in semi-intact chinese hamster ovary cells

The fungal metabolite brefeldin A (BFA) induces the disassembly of the Golgi complex in mammalian cells. The drug seems to accentuate tubule formation and causes the subsequent fusion with the endoplasmic reticulum (ER). To investigate the biochemical requirements and kinetics of BFA-induced Golgi disassembly, we have reconstituted the process of green fluorescent protein-tagged Golgi complex disassembly in streptolysin O-permeabilized semi-intact Chinese hamster ovary cells. For quantitative analysis of the morphological changes to the Golgi complex in semi-intact cells, we developed a novel morphometric analysis. Based on this analysis, we have dissected the BFA-induced Golgi disassembly process biochemically into two processes, Golgi tubule formation and fusion with the ER, and found that the formation is induced by only ATP and the residual factors in the cells and that the subsequent fusion is mediated in an N-ethylmaleimide-sensitive factor-dependent manner via Golgi tubules. Tubulation occurs by two pathways that depend on either microtubule integrity or exogenously added cytosol. In the presence of GTPgammaS, coat protein I inhibited the Golgi tubule fusion with the ER but showed no apparent effect on tubulation. Additionally, we analyzed the kinetics of tubulation and fusion independently in nocodazole-treated and -untreated semi-intact cells and found that tubulation is a rate-limiting step of the Golgi disassembly.

In vitro formation of the endoplasmic reticulum occurs independently of microtubules by a controlled fusion reaction

We have established an in vitro system for the formation of the endoplasmic reticulum (ER). Starting from small membrane vesicles prepared from Xenopus laevis eggs, an elaborate network of membrane tubules is formed in the presence of cytosol. In the absence of cytosol, the vesicles only fuse to form large spheres. Network formation requires a ubiquitous cytosolic protein and nucleoside triphosphates, is sensitive to N-ethylmaleimide and high cytosolic Ca(2+) concentrations, and proceeds via an intermediate stage in which vesicles appear to be clustered. Microtubules are not required for membrane tubule and network formation. Formation of the ER network shares significant similarities with formation of the nuclear envelope. Our results suggest that the ER network forms in a process in which cytosolic factors modify and regulate a basic reaction of membrane vesicle fusion.

Coat proteins regulating membrane traffic

This review focuses on the roles of coat proteins in regulating the membrane traffic of eukaryotic cells. Coat proteins are recruited to the donor organelle membrane from a cytosolic pool by specific small GTP-binding proteins and are required for the budding of coated vesicles. This review first describes the four types of coat complexes that have been characterized so far: clathrin and its adaptors, the adaptor-related AP-3 complex, COPI, and COPII. It then discusses the ascribed functions of coat proteins in vesicular transport, including the physical deformation of the membrane into a bud, the selection of cargo, and the targeting of the budded vesicle. It also mentions how the coat proteins may function in an alternative model for transport, namely via tubular connections, and how traffic is regulated. Finally, this review outlines the evidence that related coat proteins may regulate other steps of membrane traffic.

Membrane flow through the Golgi apparatus: specific disassembly of the cis-Golgi network by ATP depletion

Incubation of NRK cells for 30 to 45 minutes with 50 mM 2-deoxy-D-glucose (DOG) in glucose and pyruvate-free medium results in depletion of the cellular ATP pool and in specific disassembly of the cis-Golgi network (CGN), with the stack of Golgi cisternae (SGC) and the trans-Golgi network (TGN) remaining intact and sensitive to BFA. The disassembly of the CGN is mediated by long tubular structures extending outwards from the Golgi complex and involves microtubules. Upon removal of DOG and addition of glucose and pyruvate to the culture medium, the morphology of the CGN is slowly reestablished. Reconstruction of the CGN involves COPI/COPII-positive vesicles that resume the transport of proteins and in particular of CGN membrane proteins out of the ER. Exit of CGN membrane proteins from the ER is insensitive to BFA. In cells pretreated with nocodazole, the CGN membrane proteins are transported to the vicinity of the SGC fragments dispersed throughout the cytoplasm. Ultrastructural studies of cells engaged in the reconstruction of the CGN revealed that the CGN cisterna emerge as tubular structures extending from 0.2-0.3 microm uncoated vesicles prior to their organization on the cis-side of the SGC.

Membrane tubule-mediated reassembly and maintenance of the Golgi complex is disrupted by phospholipase A2 antagonists

Although membrane tubules can be found extending from, and associated with, the Golgi complex of eukaryotic cells, their physiological function has remained unclear. To gain insight into the biological significance of membrane tubules, we have developed methods for selectively preventing their formation. We show here that a broad range of phospholipase A2 (PLA2) antagonists not only arrest membrane tubule-mediated events that occur late in the assembly of the Golgi complex but also perturb its normal steady-state tubulovesicular architecture by inducing a reversible fragmentation into separate "mini-stacks." In addition, we show that these same compounds prevent the formation of membrane tubules from Golgi stacks in an in vitro reconstitution system. This in vitro assay was further used to demonstrate that the relevant PLA2 activity originates from the cytoplasm. Taken together, these results demonstrate that Golgi membrane tubules, sensitive to potent and selective PLA2 antagonists, mediate both late events in the reassembly of the Golgi complex and the dynamic maintenance of its steady-state architecture. In addition, they implicate a role for cytoplasmic PLA2 enzymes in mediating these membrane trafficking events.

Osmotically induced cell volume changes alter anterograde and retrograde transport, Golgi structure, and COPI dissociation

Physiological conditions that impinge on constitutive traffic and affect organelle structure are not known. We report that osmotically induced cell volume changes, which are known to occur under a variety of conditions, rapidly inhibited endoplasmic reticulum (ER)-to-Golgi transport in mammalian cells. Both ER export and ER Golgi intermediate compartment (ERGIC)-to-Golgi trafficking steps were blocked, but retrograde transport was active, and it mediated ERGIC and Golgi collapse into the ER. Extensive tubulation and relatively rapid Golgi resident redistribution were observed under hypo-osmotic conditions, whereas a slower redistribution of the same markers, without apparent tubulation, was observed under hyperosmotic conditions. The osmotic stress response correlated with the perturbation of COPI function, because both hypo- and hyperosmotic conditions slowed brefeldin A-induced dissociation of betaCOP from Golgi membranes. Remarkably, Golgi residents reemerged after several hours of sustained incubation in hypotonic or hypertonic medium. Reemergence was independent of new protein synthesis but required PKC, an activity known to mediate cell volume recovery. Taken together these results indicate the existence of a coupling between cell volume and constitutive traffic that impacts organelle structure through independent effects on anterograde and retrograde flow and that involves, in part, modulation of COPI function.

Golgi structure in three dimensions: functional insights from the normal rat kidney cell

Three-dimensional reconstructions of portions of the Golgi complex from cryofixed, freeze-substituted normal rat kidney cells have been made by dual-axis, high-voltage EM tomography at approximately 7-nm resolution. The reconstruction shown here ( approximately 1 x 1 x 4 microm3) contains two stacks of seven cisternae separated by a noncompact region across which bridges connect some cisternae at equivalent levels, but none at nonequivalent levels. The rest of the noncompact region is filled with both vesicles and polymorphic membranous elements. All cisternae are fenestrated and display coated buds. They all have about the same surface area, but they differ in volume by as much as 50%. The trans-most cisterna produces exclusively clathrin-coated buds, whereas the others display only nonclathrin coated buds. This finding challenges traditional views of where sorting occurs within the Golgi complex. Tubules with budding profiles extend from the margins of both cis and trans cisternae. They pass beyond neighboring cisternae, suggesting that these tubules contribute to traffic to and/or from the Golgi. Vesicle-filled "wells" open to both the cis and lateral sides of the stacks. The stacks of cisternae are positioned between two types of ER, cis and trans. The cis ER lies adjacent to the ER-Golgi intermediate compartment, which consists of discrete polymorphic membranous elements layered in front of the cis-most Golgi cisterna. The extensive trans ER forms close contacts with the two trans-most cisternae; this apposition may permit direct transfer of lipids between ER and Golgi membranes. Within 0.2 microm of the cisternae studied, there are 394 vesicles (8 clathrin coated, 190 nonclathrin coated, and 196 noncoated), indicating considerable vesicular traffic in this Golgi region. Our data place structural constraints on models of trafficking to, through, and from the Golgi complex.

Coalescence of Golgi fragments in microtubule-deprived living cells

The process of stack coalescence, an important mechanism of Golgi recovery from mitosis, was examined using novel experimental paradigms. In living cells with disrupted (by nocodazole) microtubules, galactosyl transferase-GFP-labelled Golgi fragments constantly appeared, grew, sometimes moved with a speed of 1-2 microns/min, coalesced or gradually diminished and disappeared. The rate of Golgi fragment turnover and coalescence was highly balanced to maintain a constant number of Golgi units per cell. Moreover some Golgi islands appear and some received new GalTase-GFP after photobleaching of cell cytoplasm. Short tubules extending from the rims of scattered Golgi fragments frequently formed bridges between ministacks, inducing their coalescence. The frequency of coalescence could also be inhibited by disruption of actin microfilaments. After the Golgi redistribution into endoplasmic reticulum induced by brefeldin A, either the growth of small Golgi fragments or their coalescence leads to compartmentalized stack formation without the participation of microtubules. These results demonstrate that this coalescence between isolated Golgi stacks is microtubule-independent and could thus be mediated by membranous tubules.

Molecular cloning, characterization, and dynamics of rat formiminotransferase cyclodeaminase, a Golgi-associated 58-kDa protein

A peripherally associated 58-kDa Golgi protein (58K) of unknown function has been previously described (Bloom, G. S., and Brashear, T. A. (1989) J. Biol. Chem. 264, 16083-16092). To molecularly characterize 58K, we used a monoclonal anti-58K antibody (monoclonal antibody 58K-9) to screen a rat liver cDNA expression library. Positive clones were isolated, characterized, and partially sequenced. The obtained sequences show a high level of identity with sequences of porcine formiminotransferase cyclodeaminase (FTCD), suggesting that 58K is rat FTCD. Rat FTCD is structurally similar to porcine FTCD, a metabolic enzyme involved in conversion of histidine to glutamic acid, and exists in dimeric, tetrameric, and octameric complexes resistant to proteolysis. To define parameters of FTCD association with the Golgi, comparison of its behavior with various Golgi and ER-to-Golgi intermediate compartment marker proteins was examined under specific conditions. The results show that extraction parameters of FTCD are similar to those of GM130, a tightly associated Golgi matrix protein. FTCD appears to be a dynamic component of the Golgi, and a proportion of FTCD molecules cycle between the Golgi and earlier compartments of the secretory pathway. FTCD remains associated with Golgi fragments during microtubule disruption and is not released into cytosol during brefeldin A treatment. Instead, FTCD relocates from the Golgi, but the time course of its redistribution is distinct from that of mannosidase II relocation. FTCD is already dispersed into small punctate structures at a time when mannosidase II is still largely localized to Golgi structures. FTCD is not observed in tubules originating from the Golgi and containing mannosidase II. Instead, it appears to redistribute in small vesicles arranged in a linear "pearls on a string" pattern. These results suggest that FTCD relocation is temporally and spatially distinct from mannosidase II relocation and that FTCD provides a novel marker to study Golgi dynamics.

Retrograde transport from the pre-Golgi intermediate compartment and the Golgi complex is affected by the vacuolar H+-ATPase inhibitor bafilomycin A1

The effect of the vacuolar H+-ATPase inhibitor bafilomycin A1 (Baf A1) on the localization of pre-Golgi intermediate compartment (IC) and Golgi marker proteins was used to study the role of acidification in the function of early secretory compartments. Baf A1 inhibited both brefeldin A- and nocodazole-induced retrograde transport of Golgi proteins to the endoplasmic reticulum (ER), whereas anterograde ER-to-Golgi transport remained largely unaffected. Furthermore, p58/ERGIC-53, which normally cycles between the ER, IC, and cis-Golgi, was arrested in pre-Golgi tubules and vacuoles, and the number of p58-positive approximately 80-nm Golgi (coatomer protein I) vesicles was reduced, suggesting that the drug inhibits the retrieval of the protein from post-ER compartments. In parallel, redistribution of beta-coatomer protein from the Golgi to peripheral pre-Golgi structures took place. The small GTPase rab1p was detected in short pre-Golgi tubules in control cells and was efficiently recruited to the tubules accumulating in the presence of Baf A1. In contrast, these tubules showed no enrichment of newly synthesized, anterogradely transported proteins, indicating that they participate in retrograde transport. These results suggest that the pre-Golgi structures contain an active H+-ATPase that regulates retrograde transport at the ER-Golgi boundary. Interestingly, although Baf A1 had distinct effects on peripheral pre-Golgi structures, only more central, p58-containing elements accumulated detectable amounts of 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine (DAMP), a marker for acidic compartments, raising the possibility that the lumenal pH of the pre-Golgi structures gradually changes in parallel with their translocation to the Golgi region.

An essential role for the phosphatidylinositol transfer protein in the scission of coatomer-coated vesicles from the trans-Golgi network

We identified the phosphatidylinositol transfer protein (PITP) as being responsible for a powerful latent, nucleotide-independent, Golgi-vesiculating activity that is present in the cytosol but is only manifested as an uncontrolled activity in a cytosolic protein subfraction, in which it is separated from regulatory components that appear to normally limit its action to the scission of COPI-coated buds from trans-Golgi network membranes. A specific anti-PITP antibody that recognizes the two mammalian PITP isoforms fully inhibited the capacity of the cytosol to support normal vesicle generation as well as the uncontrolled vesiculating activity manifested by the cytosolic protein subfraction. The phosphatidylinositol- (PI) loaded form of the yeast PITP, Sec14p, but not the phosphatidylcholine- (PC) loaded form of the protein, was capable of substituting for the cytosolic subfraction in promoting the scission of coated buds from the trans-Golgi network. At higher concentration, however, Sec14p, when loaded with PI, but not with PC or phosphatidylglycerol, caused by itself an indiscriminate vesiculation of uncoated Golgi membranes that could be suppressed by PC-Sec14p, which also suppresses the uncontrolled vesiculation caused by the cytosolic subfraction. We propose that, by delivering PI to specific sites in the Golgi membrane near the necks of coated buds, PITP induces local changes in the organization of the lipid bilayer, possibly involving PI metabolites, that triggers the fusion of the ectoplasmic faces of the Golgi membrane necessary for the scission of COPI-coated vesicles.

Evidence that phospholipase A2 activity is required for Golgi complex and trans Golgi network membrane tubulation

Membrane tubules of uniform diameter (60-80 nm) and various lengths (up to several micrometers) emanate from elements of the Golgi stack and trans Golgi network (TGN). These organelle membrane tubules are thought to be involved in membrane trafficking and maintenance of Golgi/TGN architecture. The number of these tubules, and their frequency of formation, can be greatly enhanced by the fungal metabolite brefeldin A (BFA), an inhibitor of Golgi/TGN-associated coated vesicle formation. We show here that BFA stimulation of Golgi and TGN membrane tubulation, and the resultant retrograde transport of resident Golgi enzymes to the endoplasmic reticulum, was potently inhibited by a number of membrane-permeant antagonists of phospholipase A2 (PLA2; EC 3.1.1.4) activity. In addition, PLA2 inhibitors on their own caused a reversible fragmentation of the Golgi complex into juxtanuclear, stacked cisternal elements. We conclude from these observations that tubulation of Golgi complex and TGN membranes requires a PLA2 activity, and that this activity may participate not only in Golgi tubule-mediated retrograde trafficking to the endoplasmic reticulum, but also in the maintenance of Golgi complex architecture.

The Carboxyl-Terminal Region of Protein C Is Essential for Its Secretion

We have previously reported a mutated protein C, designated protein C Nagoya (PCN), characterized by the deletion of a single guanine residue (8857G). This frameshift mutation results in the replacement of the carboxyl-terminal 39 amino acids of wild-type protein C (G381-P419) by 81 abnormal amino acids. This elongated mutant was not effectively secreted, and was retained in the endoplasmic reticulum. To determine why PCN is not secreted, we constructed a series of mutants from which some or all of the 81 amino acids were deleted. None of these shortened proteins were secreted from producing cells, indicating that the carboxyl-terminal extension is not mainly responsible for the intracellular retention of PCN, and that the 39 carboxyl-terminal amino acids of wild-type protein C are required for secretion. To determine which residues are essential for the secretion of protein C, deletion mutants of the carboxyl-terminal region (D401-P419) were prepared. Metabolic labeling showed that mutants of protein C truncated before W417, Q414, E411, or K410 were efficiently secreted. On the other hand, the mutants truncated before D409 were retained and degraded intracellularly. Immunofluorescence and immunoelectron microscopy showed that truncation before D409 blocks the movement from rough endoplasmic reticulum to the Golgi apparatus. To understand the conformational change in the carboxyl-terminal region, two models of truncated activated protein C were constructed using energy optimization and molecular dynamics with water molecules.

The Carboxyl-Terminal Region of Protein C Is Essential for Its Secretion

We have previously reported a mutated protein C, designated protein C Nagoya (PCN), characterized by the deletion of a single guanine residue (8857G). This frameshift mutation results in the replacement of the carboxyl-terminal 39 amino acids of wild-type protein C (G381-P419) by 81 abnormal amino acids. This elongated mutant was not effectively secreted, and was retained in the endoplasmic reticulum. To determine why PCN is not secreted, we constructed a series of mutants from which some or all of the 81 amino acids were deleted. None of these shortened proteins were secreted from producing cells, indicating that the carboxyl-terminal extension is not mainly responsible for the intracellular retention of PCN, and that the 39 carboxyl-terminal amino acids of wild-type protein C are required for secretion. To determine which residues are essential for the secretion of protein C, deletion mutants of the carboxyl-terminal region (D401-P419) were prepared. Metabolic labeling showed that mutants of protein C truncated before W417, Q414, E411, or K410 were efficiently secreted. On the other hand, the mutants truncated before D409 were retained and degraded intracellularly. Immunofluorescence and immunoelectron microscopy showed that truncation before D409 blocks the movement from rough endoplasmic reticulum to the Golgi apparatus. To understand the conformational change in the carboxyl-terminal region, two models of truncated activated protein C were constructed using energy optimization and molecular dynamics with water molecules.

The mammalian Golgi apparatus during M-phase

The Golgi apparatus in mammalian cells disassembles into several thousand vesicles as cells enter M-phase. Disassembly is dependent on the action of cdc2-kinase and at least two pathways contribute to the fragmentation: One involves the budding of COP-coated vesicles from Golgi cisternae with concomitant inhibition of fusion with their target membranes, the other is a less well characterised COP-independent pathway. During telophase, the Golgi fragments reassemble and fuse into a fully functional Golgi stack, using at least two distinct fusion pathways. The morphological changes of the Golgi apparatus during M-phase offer an ideal system to study how cellular organelles are generated and how their structure is maintained during interphase.

Retrograde trafficking of both Golgi complex and TGN markers to the ER induced by nordihydroguaiaretic acid and cyclofenil diphenol

Previous studies have shown that the Golgi stack and the trans-Golgi network (TGN) may play a role in capturing escaped resident endoplasmic reticulum (ER) proteins, and directing their retrograde transport back to that organelle. Whether this retrograde movement represents a highly specific or more generalized membrane trafficking pathway is unclear. To better understand both the retrograde and anterograde trafficking pathways of the secretory apparatus, we examined more closely the in vivo effects of two structurally unrelated compounds, the potent lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA), and the non-steroidal estrogen cyclofenil diphenol (CFD), both of which are known to inhibit secretion. In the presence of these compounds, transport of vesicular stomatitis virus G membrane glycoprotein from the ER to the Golgi complex, and from the TGN to the cell surface, was inhibited potently and rapidly. Surprisingly, we found that NDGA and CFD stimulated the rapid, but not concomitant, retrograde movement of both Golgi stack and TGN membrane proteins back to the ER until both organelles were morphologically absent from cells. Both NDGA- and CFD-stimulated TGN and Golgi retrograde membrane trafficking were inhibited by microtubule depolymerizing agents and energy poisons. Removal of NDGA and CFD resulted in the complete, but not concomitant, reformation of both Golgi stacks and their closely associated TGN compartments. These studies suggest that NDGA and CFD unmask a generalized bulk recycling pathway to the ER for both Golgi and TGN membranes and, further, that NDGA and CFD are useful for investigating the molecular mechanisms that control the formation and maintenance of both the Golgi stack proper and the TGN.

3D topography of noncompact zone Golgi tubules in rat spermatids: a computer-assisted serial section reconstruction study

BACKGROUND

In the Golgi apparatus, the 3D topography of saccules in the compact zones (CZs) is better understood than that of tubules in the noncompact zones (NCZs). The positioning of NCZ tubules relative to each other and to CZ saccules was studied in rat spermatids by computer-assisted serial section microscopy.

METHODS

Twenty-four (semi) serials (3-6 consecutive sections each) in total were collected from untreated tissues and from tissues treated for glucose-6-phosphatase (G6P) cytochemistry as an alignment tool. The serials proceeded along either the cis-trans or the medial-lateral axes of the Golgi and collectively sampled all portions of this organelle. Selected serials were computer reconstructed and the final models displayed in red-green/red-blue stereo.

RESULTS

In single thin sections, NCZ tubules typically appeared randomly oriented; however, in serial sections a high degree of organization was evident. Most tubules were traceable to the type of tubular networks (TNs) that interconnect equivalent CZ saccules (see review Rambourg and Clermont, 1990) Such TNs were present at consecutive saccular levels through each NCZ, were stacked like the saccules from which they originate, and in many regions were aligned from cis-trans. The cis-most of the TNs projected above the cis-pole of the stacked saccules and were penetrated by coated and uncoated ER buds.

CONCLUSIONS

The function of the extensive NCZ tubular domain, consisting of the stacked and aligned TNs, will have to be addressed in future studies. However, the specific topography of the cis-most TNs make them candidates to serve as acceptor membranes in ER-Golgi transport.

Dymple, a novel dynamin-like high molecular weight GTPase lacking a proline-rich carboxyl-terminal domain in mammalian cells

We have cloned human dymple, a novel dynamin family member. The full-length cDNA sequence encodes a protein composed of 736 amino acids with a molecular mass of 80 kDa. This amino acid sequence most resembles yeast DNM1P and VPS1P. Dymple lacks a proline-rich carboxyl-terminal domain through which dynamin binds to SH3 domains to be activated. Northern blot analysis revealed two transcript sizes of 2.5 and 4.2 kilobases with alternative polyadenylation at the highest levels in brain, skeletal muscle, and testis. It was further established that there are three patterns of alternative splicing producing in-frame deletions in the coding sequence of dymple in a tissue-specific manner. When overexpressed, wild-type dymple exhibited a punctate perinuclear cytoplasmic pattern, whereas an amino-terminal deletion mutant formed large aggregates bounded by a trans-Golgi network marker. Since dynamin participates in clathrin-mediated endocytosis through a well-characterized mechanism, the existence of a dynamin-like molecule in each specific vesicle transport pathway has been predicted. Our findings suggest that dymple may be the first example of such a subfamily in mammalian cells other than dynamin itself, although its precise role and membrane localization remain to be resolved.

Golgi tubule traffic and the effects of brefeldin A visualized in living cells

The Golgi complex is a dynamic organelle engaged in both secretory and retrograde membrane traffic. Here, we use green fluorescent protein-Golgi protein chimeras to study Golgi morphology in vivo. In untreated cells, membrane tubules were a ubiquitous, prominent feature of the Golgi complex, serving both to interconnect adjacent Golgi elements and to carry membrane outward along microtubules after detaching from stable Golgi structures. Brefeldin A treatment, which reversibly disassembles the Golgi complex, accentuated tubule formation without tubule detachment. A tubule network extending throughout the cytoplasm was quickly generated and persisted for 5-10 min until rapidly emptying Golgi contents into the ER within 15-30 s. Both lipid and protein emptied from the Golgi at similar rapid rates, leaving no Golgi structure behind, indicating that Golgi membranes do not simply mix but are absorbed into the ER in BFA-treated cells. The directionality of redistribution implied Golgi membranes are at a higher free energy state than ER membranes. Analysis of its kinetics suggested a mechanism that is analogous to wetting or adsorptive phenomena in which a tension-driven membrane flow supplements diffusive transfer of Golgi membrane into the ER. Such nonselective, flow-assisted transport of Golgi membranes into ER suggests that mechanisms that regulate retrograde tubule formation and detachment from the Golgi complex are integral to the existence and maintenance of this organelle.