Variations on the intracellular transport theme: maturing cisternae and trafficking tubules

The Diffusion Model of Intra-Golgi Transport Has Limited Power

The Golgi complex (GC) is the main station along the cell biosecretory pathway. Until now, mechanisms of intra-Golgi transport (IGT) have remained unclear. Herein, we confirm that the goodness-of-fit of the regression lines describing the exit of a cargo from the Golgi zone (GZ) corresponds to an exponential decay. When the GC was empty before the re-initiation of the intra-Golgi transport, this parameter of the curves describing the kinetics of different cargoes (which are deleted in Golgi vesicles) with different diffusional mobilities within the GZ as well as their exit from the GZ was maximal for the piecewise nonlinear regression, wherein the first segment was horizontal, while the second segment was similar to the exponential decay. The kinetic curve describing cargo exit from the GC per se resembled a linear decay. The Monte-Carlo simulation revealed that such curves reflect the role of microtubule growth in cells with a central GC or the random hovering of ministacks in cells lacking a microtubule. The synchronization of cargo exit from the GC already filled with a cargo using the wave synchronization protocol did not reveal the equilibration of cargo within a Golgi stack, which would be expected from the diffusion model (DM) of IGT. Moreover, not all cisternae are connected to each other in mini-stacks that are transporting membrane proteins. Finally, the kinetics of post-Golgi carriers and the important role of SNAREs for IGT at different level of IGT also argue against the DM of IGT.

Comparison of the Cisterna Maturation-Progression Model with the Kiss-and-Run Model of Intra-Golgi Transport: Role of Cisternal Pores and Cargo Domains

The Golgi complex is the central station of the secretory pathway. Knowledge about the mechanisms of intra-Golgi transport is inconsistent. Here, we compared the explanatory power of the cisterna maturation-progression model and the kiss-and-run model. During intra-Golgi transport, conventional cargoes undergo concentration and form cisternal distensions or distinct membrane domains that contain only one membrane cargo. These domains and distension are separated from the rest of the Golgi cisternae by rows of pores. After the arrival of any membrane cargo or a large cargo aggregate at the Golgi complex, the cis-Golgi SNAREs become enriched within the membrane of cargo-containing domains and then replaced by the trans-Golgi SNAREs. During the passage of these domains, the number of cisternal pores decreases. Restoration of the cisternal pores is COPI-dependent. Our observations are more in line with the kiss-and-run model.

COPII collar defines the boundary between ER and ER exit site and does not coat cargo containers

COPII and COPI mediate the formation of membrane vesicles translocating in opposite directions within the secretory pathway. Live-cell and electron microscopy revealed a novel mode of function for COPII during cargo export from the ER. COPII is recruited to membranes defining the boundary between the ER and ER exit sites, facilitating selective cargo concentration. Using direct observation of living cells, we monitored cargo selection processes, accumulation, and fission of COPII-free ERES membranes. CRISPR/Cas12a tagging, the RUSH system, and pharmaceutical and genetic perturbations of ER-Golgi transport demonstrated that the COPII coat remains bound to the ER–ERES boundary during protein export. Manipulation of the cargo-binding domain in COPII Sec24B prohibits cargo accumulation in ERES. These findings suggest a role for COPII in selecting and concentrating exported cargo rather than coating Golgi-bound carriers. These findings transform our understanding of coat proteins’ role in ER-to-Golgi transport.

Golgi maturation-dependent glycoenzyme recycling controls glycosphingolipid biosynthesis and cell growth via GOLPH3

Glycosphingolipids are important components of the plasma membrane where they modulate the activities of membrane proteins including signalling receptors. Glycosphingolipid synthesis relies on competing reactions catalysed by Golgi-resident enzymes during the passage of substrates through the Golgi cisternae. The glycosphingolipid metabolic output is determined by the position and levels of the enzymes within the Golgi stack, but the mechanisms that coordinate the intra-Golgi localisation of the enzymes are poorly understood. Here, we show that a group of sequentially-acting enzymes operating at the branchpoint among glycosphingolipid synthetic pathways binds the Golgi-localised oncoprotein GOLPH3. GOLPH3 sorts these enzymes into vesicles for intra-Golgi retro-transport, acting as a component of the cisternal maturation mechanism. Through these effects, GOLPH3 controls the sub-Golgi localisation and the lysosomal degradation rate of specific enzymes. Increased GOLPH3 levels, as those observed in tumours, alter glycosphingolipid synthesis and plasma membrane composition thereby promoting mitogenic signalling and cell proliferation. These data have medical implications as they outline a novel oncogenic mechanism of action for GOLPH3 based on glycosphingolipid metabolism.

Post-ER Stress Biogenesis of Golgi Is Governed by Giantin

BACKGROUND

The Golgi apparatus undergoes disorganization in response to stress, but it is able to restore compact and perinuclear structure under recovery. This self-organization mechanism is significant for cellular homeostasis, but remains mostly elusive, as does the role of giantin, the largest Golgi matrix dimeric protein.

METHODS

In HeLa and different prostate cancer cells, we used the model of cellular stress induced by Brefeldin A (BFA). The conformational structure of giantin was assessed by proximity ligation assay and atomic force microscopy. The post-BFA distribution of Golgi resident enzymes was examined by 3D SIM high-resolution microscopy.

RESULTS

We detected that giantin is rather flexible than an extended coiled-coil dimer and BFA-induced Golgi disassembly was associated with giantin monomerization. A fusion of the nascent Golgi membranes after BFA washout is forced by giantin re-dimerization via disulfide bond in its luminal domain and assisted by Rab6a GTPase. GM130-GRASP65-dependent enzymes are able to reach the nascent Golgi membranes, while giantin-sensitive enzymes appeared at the Golgi after its complete recovery via direct interaction of their cytoplasmic tail with N-terminus of giantin.

CONCLUSION

Post-stress recovery of Golgi is conducted by giantin dimer and Golgi proteins refill membranes according to their docking affiliation rather than their intra-Golgi location.

Models of Intracellular Transport: Pros and Cons

Intracellular transport is one of the most confusing issues in the field of cell biology. Many different models and their combinations have been proposed to explain the experimental data on intracellular transport. Here, we analyse the data related to the mechanisms of endoplasmic reticulum-to-Golgi and intra-Golgi transport from the point of view of the main models of intracellular transport; namely: the vesicular model, the diffusion model, the compartment maturation–progression model, and the kiss-and-run model. This review initially describes our current understanding of Golgi function, while highlighting the recent progress that has been made. It then continues to discuss the outstanding questions and potential avenues for future research with regard to the models of these transport steps. To compare the power of these models, we have applied the method proposed by K. Popper; namely, the formulation of prohibitive observations according to, and the consecutive evaluation of, previous data, on the basis on the new models. The levels to which the different models can explain the experimental observations are different, and to date, the most powerful has been the kiss-and-run model, whereas the least powerful has been the diffusion model.

Role of Intracellular Transport in the Centriole-Dependent Formation of Golgi Ribbon

The intracellular transport is the most confusing issue in the field of cell biology. The Golgi complex (GC) is the central station along the secretory pathway. It contains Golgi glycosylation enzymes, which are responsible for protein and lipid glycosylation, and in many cells, it is organized into a ribbon. Position and structure of the GC depend on the position and function of the centriole. Here, we analyze published data related to the role of centriole and intracellular transport (ICT) for the formation of Golgi ribbon and specifically stress the importance of the delivery of membranes containing cargo and membrane proteins to the cell centre where centriole/centrosome is localized. Additionally, we re-examined the formation of Golgi ribbon from the point of view of different models of ICT.

Membrane detachment is not essential for COG complex function

COG is a multisubunit vesicle tethering complex in the Golgi. We demonstrate that both COG subcomplexes can be permanently attached to Golgi membranes and that major COG functions do not require cycling between the membrane and cytosol.

The conserved oligomeric Golgi (COG) complex is a vesicle tether of the “complexes associated with tethering containing helical rods” family, which functions on the cytoplasmic side of Golgi. It is currently unknown whether COG function, or function of any multisubunit vesicular tether, depends on cycling between the membrane and cytosol. Therefore, we permanently anchored key subunits of COG subcomplexes (COG4, COG7, and COG8) to Golgi membranes using transmembrane protein TMEM115 (TMEM-COG). All TMEM-COG subunits tested were Golgi localized, integrated into the COG complex, and stabilized membrane association of endogenous subunits. Interestingly, TMEM-COG4 and TMEM-COG7 equally rescued COG function in organization of Golgi markers, glycosylation, and abundance of COG-sensitive proteins. In contrast, TMEM-COG8 was not as effective, indicating that N-terminal attachment of COG8 interfered with overall COG structure and function, and none of the TMEM-COG subunits rescued the abnormal Golgi architecture caused by COG knockout. Collectively, these data indicate that both subcomplexes of the COG complex can perform most of COG function when permanently attached to membranes and that the cytosolic pool of COG is not completely essential to COG function.

Conserved Oligomeric Golgi and Neuronal Vesicular Trafficking

The conserved oligomeric Golgi (COG) complex is an evolutionary conserved multi-subunit vesicle tethering complex essential for the majority of Golgi apparatus functions: protein and lipid glycosylation and protein sorting. COG is present in neuronal cells, but the repertoire of COG function in different Golgi-like compartments is an enigma. Defects in COG subunits cause alteration of Golgi morphology, protein trafficking, and glycosylation resulting in human congenital disorders of glycosylation (CDG) type II. In this review we summarize and critically analyze recent advances in the function of Golgi and Golgi-like compartments in neuronal cells and functions and dysfunctions of the COG complex and its partner proteins.

Study of Ethanol-Induced Golgi Disorganization Reveals the Potential Mechanism of Alcohol-Impaired N-Glycosylation

BACKGROUND

It is known that ethanol (EtOH) and its metabolites have a negative effect on protein glycosylation. The fragmentation of the Golgi apparatus induced by alteration of the structure of largest Golgi matrix protein, giantin, is the major consequence of damaging effects of EtOH-metabolism on the Golgi; however, the link between this and abnormal glycosylation remains unknown. Because previously we have shown that Golgi morphology dictates glycosylation, we examined the effect EtOH administration has on function of Golgi residential enzymes involved in N-glycosylation.

METHODS

HepG2 cells transfected with mouse ADH1 (VA-13 cells) were treated with 35 mM EtOH for 72 hours. Male Wistar rats were pair-fed Lieber-DeCarli diets for 5 to 8 weeks. Characterization of Golgi-associated mannosyl (α-1,3-)-glycoprotein beta-1,2-N-acetylglucosaminyltransferase (MGAT1), α-1,2-mannosidase (Man-I), and α-mannosidase II (Man-II) were performed in VA-13 cells and rat hepatocytes followed by three-dimensional structured illumination microscopy (3D SIM).

RESULTS

First, we detected that EtOH administration results in the loss of sialylated N-glycans on asialoglycoprotein receptor; however, the high-mannose-type N-glycans are increased. Further analysis by 3D SIM revealed that EtOH treatment despite Golgi disorganization does not change cis-Golgi localization for Man-I, but does induce medial-to-cis relocation of MGAT1 and Man-II. Using different approaches, including electron microscopy, we revealed that EtOH treatment results in dysfunction of ADP-ribosylation factor 1 (Arf1) GTPase followed by a deficiency in COPI vesicles at the Golgi. Silencing beta-COP or expression of GDP-bound mutant Arf1(T31N) mimics the EtOH effect on retaining MGAT1 and Man-II at the cis-Golgi, suggesting that (i) EtOH specifically blocks activation of Arf1, and (ii) EtOH alters the proper localization of Golgi enzymes through impairment of COPI. Importantly, the level of MGAT1 was reduced, because likely MGAT1, contrary to Man-I and Man-II, is giantin sensitive.

CONCLUSIONS

Thus, we provide the mechanism by which EtOH-induced Golgi remodeling may significantly modify formation of N-glycans.

COPI selectively drives maturation of the early Golgi

COPI coated vesicles carry material between Golgi compartments, but the role of COPI in the secretory pathway has been ambiguous. Previous studies of thermosensitive yeast COPI mutants yielded the surprising conclusion that COPI was dispensable both for the secretion of certain proteins and for Golgi cisternal maturation. To revisit these issues, we optimized the anchor-away method, which allows peripheral membrane proteins such as COPI to be sequestered rapidly by adding rapamycin. Video fluorescence microscopy revealed that COPI inactivation causes an early Golgi protein to remain in place while late Golgi proteins undergo cycles of arrival and departure. These dynamics generate partially functional hybrid Golgi structures that contain both early and late Golgi proteins, explaining how secretion can persist when COPI has been inactivated. Our findings suggest that cisternal maturation involves a COPI-dependent pathway that recycles early Golgi proteins, followed by multiple COPI-independent pathways that recycle late Golgi proteins.

DOI:http://dx.doi.org/10.7554/eLife.13232.001

Proteins play many important roles for cells, and these roles often require the proteins to be in particular locations in or around the cells. A set of cell compartments called the Golgi packages certain proteins into bubble-like structures called vesicles to enable the proteins to be used elsewhere in the cell or released to the outside of the cell, in a process called the secretory pathway. The operation of the secretory pathway requires the Golgi compartments to be continually remodeled.

Proteins and other materials can be ferried between the compartments of the Golgi by another type of vesicle. These vesicles are coated with a group, or complex, of proteins called COPI, which forms a curved lattice around the vesicles and helps them to capture the materials they will transport. However, it is not clear whether COPI is also involved in remodeling of the Golgi compartments.

Papanikou, Day et al. addressed this question using a technique called the “anchor-away method” combined with microscopy to study COPI in yeast cells. The yeast were genetically engineered so that COPI activity was effectively shut down in the presence of a drug called rapamycin. The experiments show that COPI is involved in the early stages of remodeling the Golgi compartments, but not the later stages. This finding supports the emerging view of the Golgi as a self-organizing cellular machine, and it provides a framework for uncovering the engineering principles that underlie the secretory pathway.

DOI:http://dx.doi.org/10.7554/eLife.13232.002

Transport of soluble proteins through the Golgi occurs by diffusion via continuities across cisternae

The mechanism of transport through the Golgi complex is not completely understood, insofar as no single transport mechanism appears to account for all of the observations. Here, we compare the transport of soluble secretory proteins (albumin and α1-antitrypsin) with that of supramolecular cargoes (e.g., procollagen) that are proposed to traverse the Golgi by compartment progression-maturation. We show that these soluble proteins traverse the Golgi much faster than procollagen while moving through the same stack. Moreover, we present kinetic and morphological observations that indicate that albumin transport occurs by diffusion via intercisternal continuities. These data provide evidence for a transport mechanism that applies to a major class of secretory proteins and indicate the co-existence of multiple intra-Golgi trafficking modes.

Golgi's way: a long path toward the new paradigm of the intra-Golgi transport

The transport of proteins and lipids is one of the main cellular functions. The vesicular model, compartment (or cisterna) maturation model, and the diffusion model compete with each other for the right to be the paradigm within the field of the intra-Golgi transport. These models have significant difficulties explaining the existing experimental data. Recently, we proposed the kiss-and-run (KAR) model of intra-Golgi transport (Mironov and Beznoussenko in Int J Mol Sci 13(6):6800-6819, 2012), which can be symmetric, when fusion and fission occur in the same location, and asymmetric, when fusion and fission take place at different sites. Here, we compare the ability of main models of the intra-Golgi transport to explain the existing results examining the evidence in favor and against each model. We propose that the KAR model has the highest potential for the explanation of the majority of experimental observations existing within the field of intracellular transport.

Segregation of the Qb-SNAREs GS27 and GS28 into Golgi vesicles regulates intra-Golgi transport

The Golgi apparatus is the main glycosylation and sorting station along the secretory pathway. Its structure includes the Golgi vesicles, which are depleted of anterograde cargo, and also of at least some Golgi-resident proteins. The role of Golgi vesicles remains unclear. Here, we show that Golgi vesicles are enriched in the Qb-SNAREs GS27 (membrin) and GS28 (GOS-28), and depleted of nucleotide sugar transporters. A block of intra-Golgi transport leads to accumulation of Golgi vesicles and partitioning of GS27 and GS28 into these vesicles. Conversely, active intra-Golgi transport induces fusion of these vesicles with the Golgi cisternae, delivering GS27 and GS28 to these cisternae. In an in vitro assay based on a donor compartment that lacks UDP-galactose translocase (a sugar transporter), the segregation of Golgi vesicles from isolated Golgi membranes inhibits intra-Golgi transport; re-addition of isolated Golgi vesicles devoid of UDP-galactose translocase obtained from normal cells restores intra-Golgi transport. We conclude that this activity is due to the presence of GS27 and GS28 in the Golgi vesicles, rather than the sugar transporter. Furthermore, there is an inverse correlation between the number of Golgi vesicles and the number of inter-cisternal connections under different experimental conditions. Finally, a rapid block of the formation of vesicles via COPI through degradation of ϵCOP accelerates the cis-to-trans delivery of VSVG. These data suggest that Golgi vesicles, presumably with COPI, serve to inhibit intra-Golgi transport by the extraction of GS27 and GS28 from the Golgi cisternae, which blocks the formation of inter-cisternal connections.

Golgi pH, its regulation and roles in human disease

Most organelles within the exocytic and endocytic pathways typically acidify their interiors, a phenomenon that is known to be crucial for their optimal functioning in eukaryotic cells. This review highlights recent advances in our understanding of how Golgi acidity is maintained and regulated, and how its misregulation contributes to organelle dysfunction and disease. Both its biosynthetic products (glycans) and protein-sorting events are highly sensitive to changes in Golgi luminal pH and are affected in certain human disease states such as cancers and cutis laxa. Other potential disease states that are caused by, or are associated with, Golgi pH misregulation will also be discussed.

The kiss-and-run model of intra-Golgi transport

The Golgi apparatus (GA) is the main station along the secretory pathway. Mechanisms of intra-Golgi transport remain unresolved. Three models compete with each other for the right to be defined as the paradigm. The vesicular model cannot explain the following: (1) lipid droplets and aggregates of procollagen that are larger than coatomer I (COPI)-dependent vesicles are transported across the GA; and (2) most anterograde cargoes are depleted in COPI vesicles. The compartment progression/maturation model has the following problems: (1) most Golgi-resident proteins are depleted in COPI vesicles; (2) there are no COPI vesicles for the recycling of the resident proteins in the trans-most-Golgi cisterna; and (3) different proteins have different rates of intra-Golgi transport. The diffusion model based on permanent inter-cisternal connections cannot explain the existence of lipid, ionic and protein gradients across the Golgi stacks. In contrast, the kiss-and-run model has the potential to explain most of the experimental observations. The kiss-and-run model can be symmetric when fusion and then fission occurs in the same place, and asymmetric when fusion takes place in one location, whereas fission takes place in another. The asymmetric kiss-and-run model resembles the carrier maturation mechanism, and it can be used to explain the transport of large cargo aggregates.

Models for Golgi traffic: a critical assessment

A variety of secretory cargoes move through the Golgi, but the pathways and mechanisms of this traffic are still being debated. Here, we evaluate the strengths and weaknesses of five current models for Golgi traffic: (1) anterograde vesicular transport between stable compartments, (2) cisternal progression/maturation, (3) cisternal progression/maturation with heterotypic tubular transport, (4) rapid partitioning in a mixed Golgi, and (5) stable compartments as cisternal progenitors. Each model is assessed for its ability to explain a set of key observations encompassing multiple cell types. No single model can easily explain all of the observations from diverse organisms. However, we propose that cisternal progression/maturation is the best candidate for a conserved core mechanism of Golgi traffic, and that some cells elaborate this core mechanism by means of heterotypic tubular transport between cisternae.

Neuronal protein trafficking: emerging consequences of endoplasmic reticulum dynamics

The highly polarized morphology and complex geometry of neurons is determined to a great extent by the structural and functional organization of the secretory pathway. It is intuitive to propose that the spatial arrangement of secretory organelles and their dynamic behavior impinge on protein trafficking and neuronal function, but these phenomena and their consequences are not well delineated. Here we analyze the architecture and motility of the archetypal endoplasmic reticulum (ER), and their relationship to the microtubule cytoskeleton and post-translational modifications of tubulin. We also review the dynamics of the ER in axons, dendrites and spines, and discuss the role of ER dynamics on protein mobility and trafficking in neurons.

Rab GTPase function in Golgi trafficking

The Rab, ARF, and Arl members of the Ras superfamily of small GTPases work together to control specific intracellular trafficking pathways. Here we focus on their roles in protein transport to and within the Golgi apparatus.

Group IV phospholipase A(2)alpha controls the formation of inter-cisternal continuities involved in intra-Golgi transport

The organization of intra-Golgi trafficking and the nature of the transport intermediates involved (e.g., vesicles, tubules, or tubular continuities) remain incompletely understood. It was recently shown that successive cisternae in the Golgi stack are interconnected by membrane tubules that form during the arrival of transport carriers from the endoplasmic reticulum. Here, we examine the mechanisms of generation and the function of these tubules. In principle, tubule formation might depend on several protein- and/or lipid-based mechanisms. Among the latter, we have studied the phospholipase A(2) (PLA(2))-mediated generation of wedge-shaped lysolipids, with the resulting local positive membrane curvature. We show that the arrival of cargo at the Golgi complex induces the recruitment of Group IVA Ca(2+)-dependent, cytosolic PLA(2) (cPLA(2)alpha) onto the Golgi complex itself, and that this cPLA(2)alpha is required for the formation of the traffic-dependent intercisternal tubules and for intra-Golgi transport. In contrast, silencing of cPLA(2)alpha has no inhibitory effects on peri-Golgi vesicles. These findings identify cPLA(2)alpha as the first component of the machinery that is responsible for the formation of intercisternal tubular continuities and support a role for these continuities in transport through the Golgi complex.

Functional analysis of GS28, an intra-Golgi SNARE, in Caenorhabditis elegans

Intra-Golgi retrograde transport is assumed to maintain Golgi function by recycling Golgi-resident proteins to younger cisternae in the progression of entire Golgi stack from cis to trans. GS28 (Golgi SNARE of 28 kDa, also known as GOS28) is a Golgi-localized SNARE protein and has been implicated in intra-Golgi retrograde transport. However, the in vivo functions of GS28, and consequently, the roles of the intra-Golgi retrograde transport in animal development are largely unknown. In this study, we generated deletion mutants of Caenorhabditis elegans GS28 and performed a synthetic lethal RNAi screen using GS28 mutants. We found that another Golgi-localized SNARE, Ykt6, functions cooperatively with GS28 in embryonic development. During post-embryonic development, GS28 mutants exhibited reduced seam cell numbers and a missing ray phenotype under Ykt6 knockdown conditions, suggesting that cell proliferation and/or differentiation of stem cell-like seam cells are impaired in GS28- and Ykt6-depleted worms. We also demonstrated that GS28 and Ykt6 act redundantly for the proper expression of Golgi-resident proteins in adult intestinal cells. This study reveals the in vivo importance of the Golgi-localized SNAREs GS28 and Ykt6.

Heterotypic tubular connections at the endoplasmic reticulum-Golgi complex interface

Electron microscopy and cryoimmunocytochemistry were used to characterize tubular connections in the secretory pathway using rat spermatids as model. Our results support the existence of a complex tubular network enriched in the Golgi matrix protein GM130 that transiently joins the cis-Golgi side and the endoplasmic reticulum. These tubules occasionally contain the endoplasmic reticulum resident protein PDI but not COPII complexes or KDEL receptor. At the lateral edges of the stacks tubules were seen to connect cisternae belonging to the same or adjacent stacks. These connections were observed in all cisternae but preferentially on the cis side. Giantin, Gos28 and Rab6 were detected in the tubules; importantly, we reported the presence of cis-trans heterotypic connections between cisternae. On the trans-Golgi side, we occasionally observed tubules highly immunoreactive for Rab6 connecting the stack with the forming acrosome. Together, our results support the existence of transient continuities throughout the secretory pathways.

Overview of protein trafficking in the secretory and endocytic pathways

Proteins migrate from the site of their synthesis to intracellular, surface, or extracellular sites depending on their function. This well-illustrated overview provides an introduction to the pathways of protein trafficking. It includes descriptions of the relevant cellular compartments, modifications associated with the movement of proteins through these compartments, signals used for targeting and translocation, mechanisms for transit between compartments, and methods for sorting proteins.

Morphogenesis of post-Golgi transport carriers

The trans-Golgi network (TGN) is one of the main, if not the main, sorting stations in the process of intracellular protein trafficking. It is therefore of central importance to understand how the key players in the TGN-based sorting and delivery process, the post-Golgi carriers (PGCs), form and function. Over the last few years, modern morphological approaches have generated new insights into the questions of PGC biogenesis, structure and dynamics. Here, we present a view by which the “lifecycle” of a PGC consists of several distinct stages: the formation of TGN tubular export domains (where different cargoes are segregated from each other and from the Golgi enzymes); the docking of these tubular domains onto molecular motors and their extrusion towards the cell periphery along microtubules; the fission of the forming PGC from the donor membrane; and the delivery of the newly formed PGC to its specific acceptor organelle. It is now important to add the many molecular machineries that have been described as operating at the TGN to this “morphofunctional map” of the TGN export process.

The Golgi apparatus and main discoveries in the field of intracellular transport

In this chapter, we summarize important findings in the field of intracellular transport, which have considerably contributed to the understanding of the function and organization of the Golgi apparatus (GA). It is not possible to mention all authors in this huge field. We apologize for gaps and incompleteness, and are thankful for suggestions and corrections.

Analysis of GTPase-activating proteins: Rab1 and Rab43 are key Rabs required to maintain a functional Golgi complex in human cells

Rab GTPases control vesicle movement and tethering membrane events in membrane trafficking. We used the 38 human Rab GTPase activating proteins (GAPs) to identify which of the 60 Rabs encoded in the human genome function at the Golgi complex. Surprisingly, this screen identified only two GAPs, RN-tre and TBC1D20, disrupting both Golgi organization and protein transport. RN-tre is the GAP for Rab43, and controls retrograde transport into the Golgi from the endocytic pathway. TBC1D20 is the ER-localized GAP for Rab1, and is the only GAP blocking the delivery of secretory cargo from the ER to the cell surface. Strikingly, its expression causes the loss of the Golgi complex, highlighting the importance of Rab1 for Golgi biogenesis. These effects can be antagonized by reticulon, a binding partner for TBC1D20 in the ER. Together, these findings indicate that Rab1 and Rab43 are key Rabs required for the biogenesis and maintenance of a functional Golgi structure, and suggest that other Rabs acting at the Golgi complex are likely to be functionally redundant.

Identification and characterization of small molecules that inhibit intracellular toxin transport

Shiga toxin (Stx), cholera toxin (Ctx), and the plant toxin ricin are among several toxins that reach their intracellular destinations via a complex route. Following endocytosis, these toxins travel in a retrograde direction through the endosomal system to the trans-Golgi network, Golgi apparatus, and endoplasmic reticulum (ER). There the toxins are transported across the ER membrane to the cytosol, where they carry out their toxic effects. Transport via the ER from the cell surface to the cytosol is apparently unique to pathogenic toxins, raising the possibility that various stages in the transport pathway can be therapeutically targeted. We have applied a luciferase-based high-throughput screen to a chemical library of small-molecule compounds in order to identify inhibitors of Stx. We report two novel compounds that protect against Stx and ricin inhibition of protein synthesis, and we demonstrate that these compounds reversibly inhibit bacterial transport at various stages in the endocytic pathway. One compound (compound 75) inhibited transport at an early stage of Stx and Ctx transport and also provided protection against diphtheria toxin, which enters the cytosol from early endosomes. In contrast, compound 134 inhibited transport from recycling endosomes through the Golgi apparatus and protected only against toxins that access the ER. Small-molecule compounds such as these will provide insight into the mechanism of toxin transport and lead to the identification of compounds with therapeutic potential against toxins routed through the ER.

Exocytotic process as a novel model for mineralization by osteoblasts in vitro and in vivo determined by electron microscopic analysis

The process of biomineralization has been examined during osteoblastic differentiation of bone marrow stroma cells (BMSCs) from embryonic chick in culture and in periosteum itself by a number of different techniques including transmission and scanning electron microscopy. In cell culture of BMSCs at days 20-25, crystals were accumulated extracellularly in the collagen matrix, resulting in large plate-like crystallites and noncollagen associated on the culture disk surface. In contrast, up to days 10-18, mainly intracellular mineralization was visible by numerous needle-like crystal structures in the cell cytoplasm and in vacuoles. After 20-30 days, the crystal content of these vacuoles is released, most probably by membrane fusion to the outside of the cells. Energy-dispersive X-ray analysis (EDX), electron spectroscopic imaging, and electron energy loss spectroscopy demonstrated that Ca, O, and P are located in the intra- and extracellular needle-like crystals. From EDX spectra a Ca/P ratio of 1.3 was estimated for the intracellular structures and a Ca/P ratio of 1.5, for the extracellular material (for comparison, the Ca/P ratio in tibiae is 1.6). X-ray diffraction and quantitative infrared spectral analyses also demonstrated an increase of crystalline bone apatite along the mineralization process. In addition to the finding in vitro, the presence of intracellular needle-like crystals in vacuoles could be demonstrated in vivo in osteoblastic cells of the periosteum in tibia of day 11. The results are in favor of a novel model for mineralization by osteoblasts, in which amorphous Ca/P material is directly secreted via an exocytotic process from vacuoles of the osteoblast, deposited extracellularly, propagated into the collagen fibril matrix, and matured to hydroxyapatite.

Opinion: The maturing role of COPI vesicles in intra-Golgi transport

COPI vesicles that surround the Golgi stack were first implicated in the anterograde movement of cargo, and then in the retrograde movement of Golgi enzymes. Recently, their role has been challenged again, and we discuss new data that both confirm and modify our view of these carriers.

Key Golgi factors for structural and functional maturation of bunyamwera virus

Several complex enveloped viruses assemble in the membranes of the secretory pathway, such as the Golgi apparatus. Among them, bunyaviruses form immature viral particles that change their structure in a trans-Golgi-dependent manner. To identify key Golgi factors for viral structural maturation, we have purified and characterized the three viral forms assembled in infected cells, two intracellular intermediates and the extracellular mature virion. The first viral form is a pleomorphic structure with fully endo-beta-N-acetylglucosaminidase H (Endo-H)-sensitive, nonsialylated glycoproteins. The second viral intermediate is a structure with hexagonal and pentagonal contours and partially Endo-H-resistant glycoproteins. Sialic acid is incorporated into the small glycoprotein of this second viral form. Growing the virus in glycosylation-deficient cells confirmed that acquisition of Endo-H resistance but not sialylation is critical for the trans-Golgi-dependent structural maturation and release of mature viruses. Conformational changes in viral glycoproteins triggered by changes in sugar composition would then induce the assembly of a compact viral particle of angular contours. These structures would be competent for the second maturation step, taking place during exit from cells, that originates fully infectious virions.

Lessons from tomographic studies of the mammalian Golgi

Basic structure studies of the biosynthetic machinery of the cell by electron microscopy (EM) have underpinned much of our fundamental knowledge in the areas of molecular cell biology and membrane traffic. Driven by our collective desire to understand how changes in the complex and dynamic structure of this enigmatic organelle relate to its pivotal roles in the cell, the comparatively high-resolution glimpses of the Golgi and other compartments of the secretory pathway offered to us through EM have helped to inspire the development and application of some of our most informative, complimentary (molecular, biochemical and genetic) approaches. Even so, no one has yet even come close to relating the basic molecular mechanisms of transport, through and from the Golgi, to its ultrastructure, to everybody's satisfaction. Over the past decade, EM tomography has afforded new insights into structure-function relationships of the Golgi and provoked a re-evaluation of older paradigms. By providing a set of tools for structurally dissecting cells at high-resolution in three-dimensions (3D), EM tomography has emerged as a method for studying molecular cell biology in situ. As we move rapidly toward the establishment of molecular atlases of organelles through advances in proteomics and genomics, tomographic studies of the Golgi offer the tantalizing possibility that one day, we will be able to map the spatio-temporal coordinates of Golgi-related proteins and lipids accurately in the context of 4D cellular space.

Intra-Golgi transport: A way to a new paradigm?

The morpho-functional principles of intra-Golgi transport are, surprisingly, still not clear, which is in marked contrast to our advanced knowledge of the underlying molecular machineries. Recently, the conceptual and technological hindrances that had delayed progress in this area have been disappearing, and a cluster of powerful morphological techniques has been revealing new glimpses of the organization of traffic in intact cells. Here, we discuss the new concepts around the present models of intra-Golgi transport.

Structure and Dynamics of the Golgi Complex at 15 oC: Low Temperature Induces the Formation of Golgi-Derived Tubules

Immunofluorescence and cryoimmunoelectron microscopy were used to examine the morphologic and functional effects on the Golgi complex when protein transport is blocked at the ERGIC (endoplasmic reticulum-Golgi intermediate compartment) in HeLa cells incubated at low temperature (15 degrees C). At this temperature, the Golgi complex showed long tubules containing resident glycosylation enzymes but not matrix proteins. These Golgi-derived tubules also lacked anterograde (VSV-G) or retrograde (Shiga toxin) cargo. The formation of tubules was dependent on both energy and intact microtubule and actin cytoskeletons. Conversely, brefeldin A or cycloheximide treatments did not modify the appearance. When examined at the electron microscope, Golgi stacks were long and curved and appeared connected to tubules immunoreactive to galactosyltransferase antibodies but devoid of Golgi matrix proteins. Strikingly, COPI proteins moved from membranes to the cytosol at 15 degrees C, which could explain the formation of tubules.

Secretory traffic triggers the formation of tubular continuities across Golgi sub-compartments

The organization of secretory traffic remains unclear, mainly because of the complex structure and dynamics of the secretory pathway. We have thus studied a simplified system, a single synchronized traffic wave crossing an individual Golgi stack, using electron tomography. Endoplasmic-reticulum-to-Golgi carriers join the stack by fusing with cis cisternae and induce the formation of intercisternal tubules, through which they redistribute their contents throughout the stack. These tubules seem to be pervious to Golgi enzymes, whereas Golgi vesicles are depleted of both enzymes and cargo. Cargo then traverses the stack without leaving the cisternal lumen. When cargo exits the stack, intercisternal connections disappear. These findings provide a new view of secretory traffic that includes dynamic intercompartment continuities as key players.

Direct continuities between cisternae at different levels of the Golgi complex in glucose-stimulated mouse islet beta cells

Direct continuity between the membranes of cisternae in the Golgi complex in mammalian cells rarely has been observed; when seen, its documentation has been equivocal. Here we have used dual-axis electron microscope tomography to examine the architecture of the Golgi in three dimensions at approximately 6-nm resolution in rapidly frozen, freeze-substituted murine cells that make and secrete insulin in response to glucose challenge. Our data show three types of direct connections between Golgi cisternae that are normally distinct from one another. These connections all "bypass" interceding cisternae. We propose that when pancreatic beta cells are stimulated to synthesize and secrete insulin rapidly in vivo, such connections provide a continuous lumen that facilitates the rapid transit of large amounts of newly made protein for secretion. The heterotypic fusion of cisternae, even transiently, raises important questions about the molecular mechanisms that (i) facilitate the fusion/fission of cisternal membranes and control the directionality and specificity of such events, and (ii) retain Golgi processing enzymes at specific places within individual cisternae when two cisternae at different levels in the Golgi have fused, maintaining the sequential processing hierarchy that is a hallmark of Golgi organization.

Systematic Analysis of SNARE Molecules in Arabidopsis: Dissection of the post-Golgi Network in Plant Cells

In all eucaryotic cells, specific vesicle fusion during vesicular transport is mediated by membrane-associated proteins called SNAREs (soluble N-ethyl-maleimide sensitive factor attachment protein receptors). Sequence analysis identified a total of 54 SNARE genes (18 Qa-SNAREs/Syntaxins, 11 Qb-SNAREs, 8 Qc-SNAREs, 14 R-SNAREs/VAMPs and 3 SNAP-25) in the Arabidopsis genome. Almost all of them were ubiquitously expressed through out all tissues examined. A series of transient expression assays using green fluorescent protein (GFP) fused proteins revealed that most of the SNARE proteins were located on specific intracellular compartments: 6 in the endoplasmic reticulum, 9 in the Golgi apparatus, 4 in the trans-Golgi network (TGN), 2 in endosomes, 17 on the plasma membrane, 7 in both the prevacuolar compartment (PVC) and vacuoles, 2 in TGN/PVC/vacuoles, and 1 in TGN/PVC/plasma membrane. Some SNARE proteins showed multiple localization patterns in two or more different organelles, suggesting that these SNAREs shuttle between the organelles. Furthermore, the SYP41/SYP61-residing compartment, which was defined as the TGN, was not always located along with the Golgi apparatus, suggesting that this compartment is an independent organelle distinct from the Golgi apparatus. We propose possible combinations of SNARE proteins on all subcellular compartments, and suggest the complexity of the post-Golgi membrane traffic in higher plant cells.

Morphodynamics of the secretory pathway

A careful scrutiny of the dynamics of secretory compartments in the entire eukaryotic world reveals many common themes. The most fundamental theme is that the Golgi apparatus and related structures appear as compartments formed by the act of transporting cargo. The second common theme is the pivotal importance for endomembrane dynamics of shifting back and forth the equilibrium between full and perforated cisternae along the pathway. The third theme is the role of a continuous membrane flow in anterograde transfer of molecules from the endoplasmic reticulum through the Golgi apparatus. The last common theme is the self-regulatory balance between anatomical continuities and discontinuities of the endomembrane system. As this balance depends on secretory activity, it provides a source of morphological variability among cell types or, for a given cell type, according to environmental conditions. Beyond this first source of variability, it appears that divergent strategies pave the evolutionary routes in different eukaryotic kingdoms. These divergent strategies primarily affect the levels of stacking, of stabilization, and of clustering of the Golgi apparatus. They presumably underscore a trade-off between versatility and stability to adapt the secretory function to the degree of environmental variability. Nonequilibrium secretory structures would provide yeasts, and plants to a lesser extent, with the required versatility to cope with ever changing environments, by contrast to the stabler milieu intérieur of homeothermic animals.

Membrane fission: model for intermediate structures

Membrane budding-fission is a fundamental process generating intracellular carriers of proteins. Earlier works were focused only on formation of coated buds connected to the initial membrane by narrow membrane necks. We present the theoretical analysis of the whole pathway of budding-fission, including the crucial stage where the membrane neck undergoes fission and the carrier separates from the donor membrane. We consider two successive intermediates of the reaction: 1), a constricted membrane neck coming out of aperture of the assembling protein coat, and 2), hemifission intermediate resulting from self-fusion of the inner monolayer of the neck, while its outer monolayer remains continuous. Transformation of the constricted neck into the hemifission intermediate is driven by the membrane stress produced in the neck by the protein coat. Although apparently similar to hemifusion, the fission is predicted to have an opposite dependence on the monolayer spontaneous curvature. Analysis of the further stages of the process demonstrates that in all practically important cases the hemifission intermediate decays spontaneously into two separate membranes, thereby completing the fission process. We formulate the "job description" for fission proteins by calculating the energy they have to deliver and the radii of the protein coat aperture which have to be reached to drive the fission process.

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.

Ultrastructural characterization of endoplasmic reticulum--Golgi transport containers (EGTC)

Recent observations made in live cells expressing green fluorescent protein (GFP)-tagged cargo markers have demonstrated the existence of large, mobile transport intermediates linking peripheral ER exit sites (ERES) to the perinuclear Golgi. Using a procedure of rapid ethane freezing, we examined ultrastructurally the intermediates involved in ER-Golgi transport of the vesicular stomatitis virus (VSV) G protein. When released at the permissive temperature of 32 degrees C, VSVG is first found to be concentrated in pleiomorphic, membrane-bound structures (of about 0.4 to 1 microm in diameter) with extensive budding profiles. These structures are devoid of COPII components and Golgi markers, but are enriched in COPI, the retrograde cargo ERGIC53, and the tethering protein p115. The structures appear to be able to undergo fusion with the Golgi stack and are tentatively referred to as ER-Golgi transport containers, or EGTCs. VSVG protein exiting the ERES at 15 degrees C is first found in clusters or strings of COPII-containing small vesicles, and morphological analysis indicates that these clusters and strings of COPII vesicles may coalesce by homotypic fusion to form the EGTCs. Together with the large transport containers mediating transport from the trans-Golgi network to the plasma membrane, EGTCs represents an emerging class of large membranous structures mediating anterograde transport between the major stations of the exocytic pathway.

Models of intracellular transport and evolution of the Golgi complex

We have performed a systematic analysis of models explaining the mechanisms of the intracellular biosecretory transport. The models assessed include not only those based on one mechanism (the dissociation model (and its individual case, the vesicular model), the progression model (and its individual cases, the cisterna maturation/progression and the carrier maturation models), and the lateral diffusion model (and its individual case, the bolus model), but also combined models of transport (the percolating-vesicles model and the synthetic model), including several transport mechanisms. Most of these models are not able to explain recent data on the evolution of genes involved in intracellular transport and Golgi evolution. The carrier maturation model proposing that fusion of the large cargo domain with the distal (closer to the plasmalemma) compartment precedes fission of the domain from the proximal compartment exhibits the best performance in correlation with the available information on evolution of the biosecretory pathway.