The ER to Golgi interface is the major concentration site of secretory proteins in the exocrine pancreatic cell

The Regulated Secretion and Models of Intracellular Transport: The Goblet Cell as an Example

Transport models are extremely important to map thousands of proteins and their interactions inside a cell. The transport pathways of luminal and at least initially soluble secretory proteins synthesized in the endoplasmic reticulum can be divided into two groups: the so-called constitutive secretory pathway and regulated secretion (RS) pathway, in which the RS proteins pass through the Golgi complex and are accumulated into storage/secretion granules (SGs). Their contents are released when stimuli trigger the fusion of SGs with the plasma membrane (PM). In specialized exocrine, endocrine, and nerve cells, the RS proteins pass through the baso-lateral plasmalemma. In polarized cells, the RS proteins secrete through the apical PM. This exocytosis of the RS proteins increases in response to external stimuli. Here, we analyze RS in goblet cells to try to understand the transport model that can be used for the explanation of the literature data related to the intracellular transport of their mucins.

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.

LMAN1-MCFD2 complex is a cargo receptor for the ER-Golgi transport of α1-antitrypsin

α1-antitrypsin (AAT) is a serine protease inhibitor synthesized in hepatocytes and protects the lung from damage by neutrophil elastase. AAT gene mutations result in AAT deficiency (AATD), which leads to lung and liver diseases. The AAT Z variant forms polymer within the endoplasmic reticulum (ER) of hepatocytes and results in reduction of AAT secretion and severe disease. Previous studies demonstrated a secretion defect of AAT in LMAN1 deficient cells, and mild decreases in AAT levels in male LMAN1 and MCFD2 deficient mice. LMAN1 is a transmembrane lectin that forms a complex with a small soluble protein MCFD2. The LMAN1-MCFD2 protein complex cycles between the ER and the Golgi. Here we report that LMAN1 and MCFD2 knockout (KO) HepG2 and HEK293T cells display reduced AAT secretion and elevated intracellular AAT levels due to a delayed ER-to-Golgi transport of AAT. Secretion defects in KO cells were rescued by wild-type LMAN1 or MCFD2, but not by mutant proteins. Elimination of the second glycosylation site of AAT abolished LMAN1 dependent secretion. Co-immunoprecipitation experiment in MCFD2 KO cells suggested that AAT interaction with LMAN1 is independent of MCFD2. Furthermore, our results suggest that secretion of the Z variant, both monomers and polymers, is also LMAN1-dependent. Results provide direct evidence supporting that the LMAN1-MCFD2 complex is a cargo receptor for the ER-to-Golgi transport of AAT and that interactions of LMAN1 with an N-glycan of AAT is critical for this process. These results have implications in production of recombinant AAT and in developing treatments for AATD patients.

Differential sorting behavior for soluble and transmembrane cargoes at the trans-Golgi network in endocrine cells

Regulated secretion of neuropeptides and peptide hormones by secretory granules (SGs) is central to physiology. Formation of SGs occurs at the trans-Golgi network (TGN) where their soluble cargo aggregates to form a dense core, but the mechanisms controlling the sorting of regulated secretory cargoes (soluble and transmembrane) away from constitutively secreted proteins remain unclear. Optimizing the use of the retention using selective hooks method in (neuro-)endocrine cells, we now quantify TGN budding kinetics of constitutive and regulated secretory cargoes. We further show that, by monitoring two cargoes simultaneously, it becomes possible to visualize sorting to the constitutive and regulated secretory pathways in real time. Further analysis of the localization of SG cargoes immediately after budding from the TGN revealed that, surprisingly, the bulk of two studied transmembrane SG cargoes (phogrin and VMAT2) does not sort directly onto SGs during budding, but rather exit the TGN into nonregulated vesicles to get incorporated to SGs at a later step. This differential behavior of soluble and transmembrane cargoes suggests a more complex model of SG biogenesis than anticipated.

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.

Recent Insights Into the Pathogenic Mechanism of Pancreatitis: Role of Acinar Cell Organelle Disorders

Acute pancreatitis (AP) is a potentially lethal inflammatory disease that lacks specific therapy. Damaged pancreatic acinar cells are believed to be the site of AP initiation. The primary function of these cells is the synthesis, storage, and export of digestive enzymes. Beginning in the endoplasmic reticulum and ending with secretion of proteins stored in zymogen granules, distinct pancreatic organelles use ATP produced by mitochondria to move and modify nascent proteins through sequential vesicular compartments. Compartment-specific accessory proteins concentrate cargo and promote vesicular budding, targeting, and fusion. The autophagy-lysosomal-endosomal pathways maintain acinar cell homeostasis by removing damaged/dysfunctional organelles and recycling cell constituents for substrate and energy. Here, we discuss studies in experimental and genetic AP models, primarily from our groups, which show that acinar cell injury is mediated by distinct mechanisms of organelle dysfunction involved in protein synthesis and trafficking, secretion, energy generation, and autophagy. These early AP events (often first manifest by abnormal cytosolic Ca signaling) in the acinar cell trigger the inflammatory and cell death responses of pancreatitis. Manifestations of acinar cell organelle disorders are also prominent in human pancreatitis. Our findings suggest that targeting specific mediators of organelle dysfunction could reduce disease severity.

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.

Intermediate compartment (IC): from pre-Golgi vacuoles to a semi-autonomous membrane system

Despite its discovery more than three decades ago and well-established role in protein sorting and trafficking in the early secretory pathway, the intermediate compartment (IC) has remained enigmatic. The prevailing view is that the IC evolved as a specialized organelle to mediate long-distance endoplasmic reticulum (ER)–Golgi communication in metazoan cells, but is lacking in other eukaryotes, such as plants and fungi. However, this distinction is difficult to reconcile with the high conservation of the core machineries that regulate early secretory trafficking from yeast to man. Also, it has remained unclear whether the pleiomorphic IC components—vacuoles, tubules and vesicles—represent transient transport carriers or building blocks of a permanent pre-Golgi organelle. Interestingly, recent studies have revealed that the IC maintains its compositional, structural and spatial properties throughout the cell cycle, supporting a model that combines the dynamic and stable aspects of the organelle. Moreover, the IC has been assigned novel functions, such as cell signaling, Golgi-independent trafficking and autophagy. The emerging permanent nature of the IC and its connections with the centrosome and the endocytic recycling system encourage reconsideration of its relationship with the Golgi ribbon, role in Golgi biogenesis and ubiquitous presence in eukaryotic cells.

SEC23B is required for pancreatic acinar cell function in adult mice

Inactivation of Sec23b exclusively in the pancreatic acinar cells of adult mice results in loss of pancreatic mass, with evidence of cell loss, degeneration of exocrine cells (with smaller-than-normal zymogen granules and ER dilation), ER stress, and increased pancreatic cell apoptosis.

Mice with germline absence of SEC23B die perinatally, exhibiting massive pancreatic degeneration. We generated mice with tamoxifen-inducible, pancreatic acinar cell–specific Sec23b deletion. Inactivation of Sec23b exclusively in the pancreatic acinar cells of adult mice results in decreased overall pancreatic weights from pancreatic cell loss (decreased pancreatic DNA, RNA, and total protein content), as well as degeneration of exocrine cells, decreased zymogen granules, and alterations in the endoplasmic reticulum (ER), ranging from vesicular ER to markedly expanded cisternae with accumulation of moderate-density content or intracisternal granules. Acinar Sec23b deletion results in induction of ER stress and increased apoptosis in the pancreas, potentially explaining the loss of pancreatic cells and decreased pancreatic weight. These findings demonstrate that SEC23B is required for normal function of pancreatic acinar cells in adult mice.

A common genetic variation of melanoma inhibitory activity-2 labels a subtype of pancreatic adenocarcinoma with high endoplasmic reticulum stress levels

HNF1 homeobox A (HNF1A)-mediated gene expression constitutes an essential component of the secretory pathway in the exocrine pancreas. Melanoma inhibitory activity 2 (MIA2), a protein facilitating protein secretion, is an HNF1A target. Protein secretion is precisely coordinated by the endoplasmic reticulum (ER) stress/unfolded protein response (UPR) system. Here, we demonstrate that HNFA and MIA2 are expressed in a subset of human PDAC tissues and that HNF1A induced MIA2 in vitro. We identified a common germline variant of MIA2 (c.A617G: p.I141M) associated with a secretory defect of the MIA2 protein in PDAC cells. Patients carrying MIA2^I141M survived longer after tumor resection but the survival benefit was restricted to those patients who received adjuvant chemotherapy. The MIA2^I141M variant was associated with high expression of ER stress/UPR genes – in particular those of the ERN1/XBP arm – in human PDAC samples. Accordingly, PDAC cell lines expressing the MIA2^I141M variant expressed high levels of ERN1 and were more sensitive to gemcitabine. These findings define an interaction between the common MIA2^I141M variant and the ER stress/UPR system and specify a subgroup of PDAC patients who are more likely to benefit from adjuvant chemotherapy.

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.

Cell biology of the endoplasmic reticulum and the Golgi apparatus through proteomics

Enriched endoplasmic reticulum (ER) and Golgi membranes subjected to mass spectrometry have uncovered over a thousand different proteins assigned to the ER and Golgi apparatus of rat liver. This, in turn, led to the uncovering of several hundred proteins of poorly understood function and, through hierarchical clustering, showed that proteins distributed in patterns suggestive of microdomains in cognate organelles. This has led to new insights with respect to their intracellular localization and function. Another outcome has been the critical testing of the cisternal maturation hypothesis showing overwhelming support for a predominant role of COPI vesicles in the transport of resident proteins of the ER and Golgi apparatus (as opposed to biosynthetic cargo). Here we will discuss new insights gained and also highlight new avenues undertaken to further explore the cell biology of the ER and the Golgi apparatus through tandem mass spectrometry.

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.

The COPII pathway and hematologic disease

Multiple diseases, hematologic and nonhematologic, result from defects in the early secretory pathway. Congenital dyserythropoietic anemia type II (CDAII) and combined deficiency of coagulation factors V and VIII (F5F8D) are the 2 known hematologic diseases that result from defects in the endoplasmic reticulum (ER)-to-Golgi transport system. CDAII is caused by mutations in the SEC23B gene, which encodes a core component of the coat protein complex II (COPII). F5F8D results from mutations in either LMAN1 (lectin mannose-binding protein 1) or MCFD2 (multiple coagulation factor deficiency protein 2), which encode the ER cargo receptor complex LMAN1-MCFD2. These diseases and their molecular pathogenesis are the focus of this review.

Industrialized MS-based proteomics in the search for circulating biomarkers

Proteomics is the study of the expression, structure and function of proteins under a range of cellular conditions. A rapidly evolving component of this field is clinical proteomics, which focuses on proteins involved in human disease and how they are affected by therapeutic intervention. MS is the main analytical technology for identifying and quantifying proteins whose expression is modulated across the normal to disease continuum. Applying this technology to clinical samples, however, is particularly challenging due to high biological variability in the population, a variety of disease stages, nonuniform response to therapy, multiple concomitant treatments and special requirements for handling samples from clinical trials. Given these challenges, an ‘industrialized’ approach is best suited to clinical biomarker development, with its standard operating procedures, process control and ‘chain of custody’. This review will focus, therefore, on MS-based industrialized proteomics for the discovery and verification of circulating candidate clinical protein biomarkers.

Recent developments in the understanding of the combined deficiency of FV and FVIII

Combined deficiency of factor V (FV) and factor VIII (FVIII) (F5F8D) is a genetic disorder characterized by mild-to-moderate bleeding and coordinate reduction in plasma FV and FVIII levels, as well as platelet FV level. Recent studies identified mutations in two genes (LMAN1 and MCFD2) as the cause of F5F8D. Though clinically indistinguishable, MCFD2 mutations generally exhibit lower levels of FV and FVIII than LMAN1 mutations. LMAN1 is a mannose-specific lectin that cycles between the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment. MCFD2 is an EF-hand domain protein that forms a calcium-dependent heteromeric complex with LMAN1 in cells. Missense mutations in the EF-hand domains of MCFD2 abolish the interaction with LMAN1. The LMAN1-MCFD2 complex may serve as a cargo receptor for the ER-to-Golgi transport of FV and FVIII, and perhaps a number of other glycoproteins. The B domain of FVIII may be important in mediating its interaction with the LMAN1-MCFD2 complex.

Protein quality control in the early secretory pathway

Eukaryotic cells are able to discriminate between native and non-native polypeptides, selectively transporting the former to their final destinations. Secretory proteins are scrutinized at the endoplasmic reticulum (ER)-Golgi interface. Recent findings reveal novel features of the underlying molecular mechanisms, with several chaperone networks cooperating in assisting the maturation of complex proteins and being selectively induced to match changing synthetic demands. 'Public' and 'private' chaperones, some of which enriched in specializes subregions, operate for most or selected substrates, respectively. Moreover, sequential checkpoints are distributed along the early secretory pathway, allowing efficiency and fidelity in protein secretion.

Origins of the regulated secretory pathway

Modes of transport of soluble (or luminal) secretory proteins synthesized in the endoplasmic reticulum (ER) could be divided into two groups. The socalled constitutive secretory pathway (CSP) is common to all eukaryotic cells, constantly delivering constitutive soluble secretory proteins (CSSPs) linked to the rate of protein synthesis but largely independent of external stimuli. In regulated secretion, protein is sorted from the Golgi into storage/secretory granules (SGs) whose contents are released when stimuli trigger their final fusion with the plasma membrane (Hannah et al. 1999).

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.

Receptor-mediated protein transport in the early secretory pathway

Many secretory proteins are thought to rely upon transmembrane cargo receptors for efficient endoplasmic reticulum (ER)-to-Golgi transport. These receptors recognize specific cargo-encoded sorting signals. Only a few such cargo receptors have been characterized in detail, most of them in yeast. The only well-defined cargo receptor from mammalian cells, the LMAN1-MCFD2 complex, is required for the efficient secretion of coagulation factors V and VIII. Studies of this complex, coupled with recent advances in elucidating the basic machinery that mediates ER-to-Golgi transport, have provided a more-detailed picture of the mechanisms underlying receptor-mediated transport in the early secretory pathway. In addition to yeast studies, insights have also come from investigations into several inherited disorders that have recently been attributed to defects in the secretory pathway.

The COPII cage: unifying principles of vesicle coat assembly

Communication between compartments of the exocytic and endocytic pathways in eukaryotic cells involves transport carriers - vesicles and tubules - that mediate the vectorial movement of cargo. Recent studies of transport-carrier formation in the early secretory pathway have provided new insights into the mechanisms of cargo selection by coat protein complex-II (COPII) adaptor proteins, the construction of cage-protein scaffolds and fission. These studies are beginning to produce a unifying molecular and structural model of coat function in the formation and fission of vesicles and tubules in endomembrane traffic.

Not all secretory granules are created equal: Partitioning of soluble content proteins

Secretory granules carrying fluorescent cargo proteins are widely used to study granule biogenesis, maturation, and regulated exocytosis. We fused the soluble secretory protein peptidylglycine alpha-hydroxylating monooxygenase (PHM) to green fluorescent protein (GFP) to study granule formation. When expressed in AtT-20 or GH3 cells, the PHM-GFP fusion protein partitioned from endogenous hormone (adrenocorticotropic hormone, growth hormone) into separate secretory granule pools. Both exogenous and endogenous granule proteins were stored and released in response to secretagogue. Importantly, we found that segregation of content proteins is not an artifact of overexpression nor peculiar to GFP-tagged proteins. Neither luminal acidification nor cholesterol-rich membrane microdomains play essential roles in soluble content protein segregation. Our data suggest that intrinsic biophysical properties of cargo proteins govern their differential sorting, with segregation occurring during the process of granule maturation. Proteins that can self-aggregate are likely to partition into separate granules, which can accommodate only a few thousand copies of any content protein; proteins that lack tertiary structure are more likely to distribute homogeneously into secretory granules. Therefore, a simple "self-aggregation default" theory may explain the little acknowledged, but commonly observed, tendency for both naturally occurring and exogenous content proteins to segregate from each other into distinct secretory granules.

Sorting ourselves out: seeking consensus on trafficking in the beta-cell

Biogenesis of the regulated secretory pathway in the pancreatic beta-cell involves packaging of products, notably proinsulin, into immature secretory granules derived from the trans-Golgi network. Proinsulin is converted to insulin and C-peptide as granules mature. Secretory proteins not entering granules are conveyed by transport intermediates directly to the plasma membrane for constitutive secretion. One of the co-authors, Peter Arvan, has proposed that in addition, small vesicles bud from granules to traffic to the endosomal system. From there, some proteins are secreted by a (post-granular) constitutive-like pathway. He argues that retention in granules is facilitated by condensation, rendering soluble products (notably C-peptide and proinsulin) more available for constitutive-like secretion. Thus he argues that prohormone conversion is potentially important in secretory granule biogenesis. The other co-author, Philippe Halban, argues that the post-granular secretory pathway is not of physiological relevance in primary beta-cells, and contests the importance of proinsulin conversion for retention in granules. Both, however, agree that trafficking from granules to endosomes is important, purging granules of unwanted newly synthesized proteins and allowing their traffic to other destinations. In this Traffic Interchange, the two co-authors attempt to reconcile their differences, leading to a common vision of proinsulin trafficking in primary and transformed cells.

Binding of the Golgi sorting receptor muclin to pancreatic zymogens through sulfated O-linked oligosaccharides

Sorting and packaging of regulated secretory proteins involves protein aggregation in the trans-Golgi network and secretory granules. In this work, we characterized the pH-dependent interactions of pancreatic acinar cell-regulated secretory proteins (zymogens) with Muclin, a putative Golgi cargo receptor. In solution, purified Muclin co-aggregated with isolated zymogens at mildly acidic pH. In an overlay assay, [35S]sulfate biosynthetically labeled Muclin bound directly at mildly acidic pH to the zymogen granule content proteins amylase, prolipase, pro-carboxypeptidase A1, pro-elastase II, chymotrypsinogen B, and Reg1. Denaturation of Muclin with reducing agents to break the numerous intrachain disulfide bonds in Muclin's scavenger receptor cysteine-rich and CUB domains did not interfere with binding. Non-sulfated [35S]Met/Cys-labeled Muclin showed decreased binding in the overlay assay. Extensive Pronase E digestion of unlabeled Muclin was used to produce glycopeptides, which competed for binding of [35S]sulfate-labeled Muclin to zymogens. The results demonstrate that the sulfated, O-glycosylated groups are responsible for the pH-dependent interactions of Muclin with the zymogens. The behavior of Muclin fulfils the requirement of a Golgi cargo receptor to bind to regulated secretory proteins under the mildly acidic pH conditions that exist in the trans-Golgi network.

KDEL and KKXX retrieval signals appended to the same reporter protein determine different trafficking between endoplasmic reticulum, intermediate compartment, and Golgi complex

Many endoplasmic reticulum (ER) proteins maintain their residence by dynamic retrieval from downstream compartments of the secretory pathway. In previous work we compared the retrieval process mediated by the two signals, KKMP and KDEL, by appending them to the same neutral reporter protein, CD8, and found that the two signals determine a different steady-state localization of the reporter. CD8-K (the KDEL-bearing form) was restricted mainly to the ER, whereas CD8-E19 (the KKMP-bearing form) was distributed also to the intermediate compartment and Golgi complex. To investigate whether this different steady-state distribution reflects a difference in exit rates from the ER and/or in retrieval, we have now followed the first steps of export of the two constructs from the ER and their trafficking between ER and Golgi complex. Contrary to expectation, we find that CD8-K is efficiently recruited into transport vesicles, whereas CD8-E19 is not. Thus, the more restricted ER localization of CD8-K must be explained by a more efficient retrieval to the ER. Moreover, because most of ER resident CD8-K is not O-glycosylated but almost all CD8-E19 is, the results suggest that CD8-K is retrieved from the intermediate compartment, before reaching the Golgi, where O-glycosylation begins. These results illustrate how different retrieval signals determine different trafficking patterns and pose novel questions on the underlying molecular mechanisms.

Colocalization of Ca2+-ATPase and GRP94 with p58 and the effects of thapsigargin on protein recycling suggest the participation of the pre-Golgi intermediate compartment in intracellular Ca2+ storage

We have studied the localization of functional components of cellular Ca2+ transport and storage and the effects of thapsigargin (TG), a specific inhibitor of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA), with respect to the p58-containing pre-Golgi intermediate compartment (IC). The depletion of Ca2+ stores in normal rat kidney (NRK) cells by TG abolished the retention of the KDEL-containing, Ca2+-binding, luminal ER chaperones GRP94/endoplasmin and GRP78/BiP, and resulted in the appearance of the proteins in the culture medium before inducing their synthesis. Immunolocalization of GRP94 in TG-treated cells showed that the protein was transported to the Golgi complex and, in parallel, the KDEL receptor was redistributed from the Golgi to p58-positive IC structures, but was not transported further to the ER. Similarly, p58 that normally cycles between the ER, IC, and cis-Golgi, was largely depleted from the cell periphery and arrested in large-sized IC elements and numerous vesicles or buds in the Golgi region, showing that TG selectively blocks its recycling from the IC back to the ER. Importantly, cell fractionation analyses and confocal fluorescence microscopy provided evidence that the IC elements in unperturbed cells contain SERCA and a considerable pool of GRP94. Thus, the observed effects of TG on protein retention and recycling can be explained by a change in the luminal Ca2+ concentration of the IC. Moreover, the compositional properties of the IC elements suggest that they participate in intracellular Ca2+ storage.

Lumenal protein multimerization in the distal secretory pathway/secretory granules

Differences in protein solubility appear to play an important role in lumenal protein trafficking through Golgi/post-Golgi compartments. Recent advances indicate that multimeric protein assembly is one of the factors regulating the efficiency of protein storage within secretory granules, by mechanisms that, with slight modification, might be considered to represent the culmination of a process of Golgi cisternal maturation.

Peri-Golgi vesicles contain retrograde but not anterograde proteins consistent with the cisternal progression model of intra-Golgi transport

A cisternal progression mode of intra-Golgi transport requires that Golgi resident proteins recycle by peri-Golgi vesicles, whereas the alternative model of vesicular transport predicts anterograde cargo proteins to be present in such vesicles. We have used quantitative immuno-EM on NRK cells to distinguish peri-Golgi vesicles from other vesicles in the Golgi region. We found significant levels of the Golgi resident enzyme mannosidase II and the transport machinery proteins giantin, KDEL-receptor, and rBet1 in coatomer protein I-coated cisternal rims and peri-Golgi vesicles. By contrast, when cells expressed vesicular stomatitis virus protein G this anterograde marker was largely absent from the peri-Golgi vesicles. These data suggest a role of peri-Golgi vesicles in recycling of Golgi residents, rather than an important role in anterograde transport.