TANGO1/cTAGE5 receptor as a polyvalent template for assembly of large COPII coats

[Research progress on collagen secretion mechanisms in scarring]

Scar formation is characterized by dynamic alterations in collagen secretion, which critically determine scar morphology and pathological progression. In fibroblasts, collagen secretion is initiated through the activation of cytokine- and integrin-mediated signaling pathways, which promote collagen gene transcription. The procollagen polypeptide α chains undergo extensive post-translational modifications, including hydroxylation and glycosylation, within the endoplasmic reticulum (ER), followed by folding and assembly into triple-helical procollagen. Subsequent intracellular trafficking involves the sequential transport of procollagen through the ER, Golgi apparatus, and plasma membrane, accompanied by further structural refinements prior to extracellular secretion. Once secreted, procollagen is enzymatically processed to form mature collagen fibrils, which drive scar tissue remodeling. Recent advances in elucidating regulation of collagen secretion have identified pivotal molecular targets, such as transforming growth factor-beta 1 (TGF-β1), prolyl 4-hydroxylase (P4H), heat shock protein 47 (HSP47), and transport and Golgi organization protein 1 (TANGO1), providing novel therapeutic strategies to mitigate pathological scar hyperplasia and improve regenerative outcomes. This review provides a comprehensive analysis of the molecular mechanisms governing collagen secretion during scar formation, with emphasis on signaling cascades, procollagen biosynthesis, intracellular transport dynamics, and post-translational modifications, thereby offering a framework for developing targeted anti-scar therapies.

Mechanisms of COPII coat assembly and cargo recognition in the secretory pathway

One third of all proteins in eukaryotes transit between the endoplasmic reticulum (ER) and the Golgi to reach their functional destination inside or outside of the cell. During export, secretory proteins concentrate at transitional zones of the ER known as ER exit sites, where they are packaged into transport carriers formed by the highly conserved coat protein complex II (COPII). Despite long-standing knowledge of many of the fundamental pathways that govern traffic in the early secretory pathway, we still lack a complete mechanistic model to explain how the various steps of COPII-mediated ER exit are regulated to efficiently transport diverse cargoes. In this Review, we discuss the current understanding of the mechanisms underlying COPII-mediated vesicular transport, highlighting outstanding knowledge gaps. We focus on how coat assembly and disassembly dictate carrier morphogenesis, how COPII selectively recruits a vast number of cargo and cargo adaptors, and finally discuss how COPII mechanisms in mammals might have adapted to enable transport of large proteins.

Organizing principles underlying COPII-mediated transport

The early secretory pathway governs the transport of thousands of secreted and transmembrane proteins and lipids from the endoplasmic reticulum (ER) to juxtaposed ER-Golgi Intermediate Compartments (ERGIC). This process is largely directed by Coat Protein complex II (COPII), which accumulates on distinct, ribosome-free ER subdomains (transitional ER) to generate highly curved transport intermediates of various sizes and shapes. The rate of secretory flux from the ER can vary significantly, depending on cell type, environmental cues, and other factors, but the mechanisms that regulate COPII-mediated trafficking have been slow to emerge. Here, we focus on recent progress that has contributed to our understanding of how the early secretory pathway is structured to facilitate the export of cargoes from the ER into a chasm approximately 300-500-nm in size, prior to fusion with ERGIC membranes without the aid of cytoskeletal elements to guide their journey.

Cryo-electron tomography reveals how COPII assembles on cargo-containing membranes

Proteins traverse the eukaryotic secretory pathway through membrane trafficking between organelles. The coat protein complex II (COPII) mediates the anterograde transport of newly synthesized proteins from the endoplasmic reticulum, engaging cargoes with a wide range of size and biophysical properties. The native architecture of the COPII coat and how cargo might influence COPII carrier morphology remain poorly understood. Here we reconstituted COPII-coated membrane carriers using purified Saccharomyces cerevisiae proteins and cell-derived microsomes as a native membrane source. Using cryo-electron tomography with subtomogram averaging, we demonstrate that the COPII coat binds cargo and forms largely spherical vesicles from native membranes. We reveal the architecture of the inner and outer coat layers and shed light on how spherical carriers are formed. Our results provide insights into the architecture and regulation of the COPII coat and advance our current understanding of how membrane curvature is generated.

A distant TANGO1 family member promotes vitellogenin export from the ER in C. elegans

Vitellogenin is thought to share a common ancestor with human apolipoprotein B (ApoB) for systemic lipid transport. In Caenorhabditis elegans, although a general route for inter-tissue vitellogenin transport has been described, the full mechanism that underlies its intracellular trafficking within the intestine remains obscure. In humans, the TANGO1 family of proteins generates membrane carriers to accommodate bulky ApoB-containing lipoprotein particles for their endoplasmic reticulum (ER) export. TANGO1 orthologs have hitherto been discovered in most metazoans, except nematodes. Here, we report the C. elegans TNGL-1 as a mediator of vitellogenin export from the ER. Depletion of TNGL-1 causes the retention of vitellogenin in the ER lumen. Furthermore, the TNGL-1 C-terminal unstructured domain and its luminal globular domain are required for its proper localization and cargo engagement, respectively. Our findings support TNGL-1 as a distant TANGO1 family member and point to the universal requirement of TANGO1-based mechanisms for the secretion of specific metazoan proteins.

Endoplasmic reticulum exit sites are segregated for secretion based on cargo size

TANGO1, TANGO1-Short, and cTAGE5 form stable complexes at the endoplasmic reticulum exit sites (ERES) to preferably export bulky cargoes. Their C-terminal proline-rich domain (PRD) binds Sec23A and affects COPII assembly. The PRD in TANGO1-Short was replaced with light-responsive domains to control its binding to Sec23A in U2OS cells (human osteosarcoma). TANGO1-ShortΔPRD was dispersed in the ER membrane but relocated rapidly, reversibly, to pre-existing ERES by binding to Sec23A upon light activation. Prolonged binding between the two, concentrated ERES in the juxtanuclear region, blocked cargo export and relocated ERGIC53 into the ER, minimally impacting the Golgi complex organization. Bulky collagen VII and endogenous collagen I were collected at less than 47% of the stalled ERES, whereas small cargo molecules were retained uniformly at almost all the ERES. We suggest that ERES are segregated to handle cargoes based on their size, permitting cells to traffic them simultaneously for optimal secretion.

A cargo sorting receptor mediates chloroplast protein trafficking through the secretory pathway

Nucleus-encoded chloroplast proteins can be transported via the secretory pathway. The molecular mechanisms underlying the trafficking of chloroplast proteins between the intracellular compartments are largely unclear, and a cargo sorting receptor has not previously been identified in the secretory pathway. Here, we report a cargo sorting receptor that is specifically present in Viridiplantae and mediates the transport of cargo proteins to the chloroplast. Using a forward genetic analysis, we identified a gene encoding a transmembrane protein (MtTP930) in barrel medic (Medicago truncatula). Mutation of MtTP930 resulted in impaired chloroplast function and a dwarf phenotype. MtTP930 is highly expressed in the aerial parts of the plant and is localized to the endoplasmic reticulum (ER) exit sites and Golgi. MtTP930 contains typical cargo sorting receptor motifs, interacts with Sar1, Sec12, and Sec24, and participates in coat protein complex II vesicular transport. Importantly, MtTP930 can recognize the cargo proteins plastidial N-glycosylated nucleotide pyrophosphatase/phosphodiesterase (MtNPP) and α-carbonic anhydrase (MtCAH) in the ER and then transport them to the chloroplast via the secretory pathway. Mutation of a homolog of MtTP930 in Arabidopsis (Arabidopsis thaliana) resulted in a similar dwarf phenotype. Furthermore, MtNPP-GFP failed to localize to chloroplasts when transgenically expressed in Attp930 protoplasts, implying that these cargo sorting receptors are conserved in plants. These findings fill a gap in our understanding of the mechanism by which chloroplast proteins are sorted and transported via the secretory pathway.

TFG regulates inner COPII coat recruitment to facilitate anterograde secretory protein transport

Coat protein complex II (COPII) governs the initial steps of biosynthetic secretory protein transport from the endoplasmic reticulum (ER), facilitating the movement of a wide variety of cargoes. Here, we demonstrate that Trk-fused gene (TFG) regulates the rate at which inner COPII coat proteins are concentrated at ER subdomains. Specifically, in cells lacking TFG, the GTPase-activating protein (GAP) Sec23 accumulates more rapidly at budding sites on the ER as compared with control cells, potentially altering the normal timing of GTP hydrolysis on Sar1. Under these conditions, anterograde trafficking of several secretory cargoes is delayed, irrespective of their predicted size. We propose that TFG controls the local, freely available pool of Sec23 during COPII coat formation and limits its capacity to prematurely destabilize COPII complexes on the ER. This function of TFG enables it to act akin to a rheostat, promoting the ordered recruitment of Sec23, which is critical for efficient secretory cargo export.

Unraveling Chylomicron Retention Disease Enhances Insight into SAR1B GTPase Functions and Mechanisms of Actions, While Shedding Light of Intracellular Chylomicron Trafficking

Over the past three decades, significant efforts have been focused on unraveling congenital intestinal disorders that disrupt the absorption of dietary lipids and fat-soluble vitamins. The primary goal has been to gain deeper insights into intra-enterocyte sites, molecular steps, and crucial proteins/regulatory pathways involved, while simultaneously identifying novel therapeutic targets and diagnostic tools. This research not only delves into specific and rare malabsorptive conditions, such as chylomicron retention disease (CRD), but also contributes to our understanding of normal physiology through the utilization of cutting-edge cellular and animal models alongside advanced research methodologies. This review elucidates how modern techniques have facilitated the decoding of CRD gene defects, the identification of dysfunctional cellular processes, disease regulatory mechanisms, and the essential role of coat protein complex II-coated vesicles and cargo receptors in chylomicron trafficking and endoplasmic reticulum (ER) exit sites. Moreover, experimental approaches have shed light on the multifaceted functions of SAR1B GTPase, wherein loss-of-function mutations not only predispose individuals to CRD but also exacerbate oxidative stress, inflammation, and ER stress, potentially contributing to clinical complications associated with CRD. In addition to dissecting the primary disease pathology, genetically modified animal models have emerged as invaluable assets in exploring various ancillary aspects, including responses to environmental challenges such as dietary alterations, gender-specific disparities in disease onset and progression, and embryonic lethality or developmental abnormalities. In summary, this comprehensive review provides an in-depth and contemporary analysis of CRD, offering a meticulous examination of the CRD current landscape by synthesizing the latest research findings and advancements in the field.

Protein networking: nicotinic acetylcholine receptors and their protein-protein-associations

Nicotinic acetylcholine receptors (nAChR) are complex transmembrane proteins involved in neurotransmission in the nervous system and at the neuromuscular junction. nAChR disorders may lead to severe, potentially fatal pathophysiological states. To date, the receptor has been the focus of basic and applied research to provide novel therapeutic interventions. Since most studies have investigated only the nAChR itself, it is necessary to consider the receptor as part of its protein network to understand or elucidate-specific pathways. On its way through the secretory pathway, the receptor interacts with several chaperones and proteins. This review takes a closer look at these molecular interactions and focuses especially on endoplasmic reticulum biogenesis, secretory pathway sorting, Golgi maturation, plasma membrane presentation, retrograde internalization, and recycling. Additional knowledge regarding the nAChR protein network may lead to a more detailed comprehension of the fundamental pathomechanisms of diseases or may lead to the discovery of novel therapeutic drug targets.

The Versatility of Collagen in Pharmacology: Targeting Collagen, Targeting with Collagen

Collagen, a versatile family of proteins with 28 members and 44 genes, is pivotal in maintaining tissue integrity and function. It plays a crucial role in physiological processes like wound healing, hemostasis, and pathological conditions such as fibrosis and cancer. Collagen is a target in these processes. Direct methods for collagen modulation include enzymatic breakdown and molecular binding approaches. For instance, Clostridium histolyticum collagenase is effective in treating localized fibrosis. Polypeptides like collagen-binding domains offer promising avenues for tumor-specific immunotherapy and drug delivery. Indirect targeting of collagen involves regulating cellular processes essential for its synthesis and maturation, such as translation regulation and microRNA activity. Enzymes involved in collagen modification, such as prolyl-hydroxylases or lysyl-oxidases, are also indirect therapeutic targets. From another perspective, collagen is also a natural source of drugs. Enzymatic degradation of collagen generates bioactive fragments known as matrikines and matricryptins, which exhibit diverse pharmacological activities. Overall, collagen-derived peptides present significant therapeutic potential beyond tissue repair, offering various strategies for treating fibrosis, cancer, and genetic disorders. Continued research into specific collagen targeting and the application of collagen and its derivatives may lead to the development of novel treatments for a range of pathological conditions.

ER exit in physiology and disease

The biosynthetic secretory pathway is comprised of multiple steps, modifications and interactions that form a highly precise pathway of protein trafficking and secretion, that is essential for eukaryotic life. The general outline of this pathway is understood, however the specific mechanisms are still unclear. In the last 15 years there have been vast advancements in technology that enable us to advance our understanding of this complex and subtle pathway. Therefore, based on the strong foundation of work performed over the last 40 years, we can now build another level of understanding, using the new technologies available. The biosynthetic secretory pathway is a high precision process, that involves a number of tightly regulated steps: Protein folding and quality control, cargo selection for Endoplasmic Reticulum (ER) exit, Golgi trafficking, sorting and secretion. When deregulated it causes severe diseases that here we categorise into three main groups of aberrant secretion: decreased, excess and altered secretion. Each of these categories disrupts organ homeostasis differently, effecting extracellular matrix composition, changing signalling events, or damaging the secretory cells due to aberrant intracellular accumulation of secretory proteins. Diseases of aberrant secretion are very common, but despite this, there are few effective therapies. Here we describe ER exit sites (ERES) as key hubs for regulation of the secretory pathway, protein quality control and an integratory hub for signalling within the cell. This review also describes the challenges that will be faced in developing effective therapies, due to the specificity required of potential drug candidates and the crucial need to respect the fine equilibrium of the pathway. The development of novel tools is moving forward, and we can also use these tools to build our understanding of the acute regulation of ERES and protein trafficking. Here we review ERES regulation in context as a therapeutic strategy.

Nutrient deprivation alters the rate of COPII subunit recruitment at ER subdomains to tune secretory protein transport

Co-assembly of the multilayered coat protein complex II (COPII) with the Sar1 GTPase at subdomains of the endoplasmic reticulum (ER) enables secretory cargoes to be concentrated efficiently within nascent transport intermediates, which subsequently deliver their contents to ER-Golgi intermediate compartments. Here, we define the spatiotemporal accumulation of native COPII subunits and secretory cargoes at ER subdomains under differing nutrient availability conditions using a combination of CRISPR/Cas9-mediated genome editing and live cell imaging. Our findings demonstrate that the rate of inner COPII coat recruitment serves as a determinant for the pace of cargo export, irrespective of COPII subunit expression levels. Moreover, increasing inner COPII coat recruitment kinetics is sufficient to rescue cargo trafficking deficits caused by acute nutrient limitation. Our findings are consistent with a model in which the rate of inner COPII coat addition acts as an important control point to regulate cargo export from the ER.

TANGO1 Dances to Export of Procollagen from the Endoplasmic Reticulum

The endoplasmic reticulum (ER) to Golgi secretory pathway is an elegantly complex process whereby protein cargoes are manufactured, folded, and distributed from the ER to the cisternal layers of the Golgi stack before they are delivered to their final destinations. The export of large bulky cargoes such as procollagen and its trafficking to the Golgi is a sophisticated mechanism requiring TANGO1 (Transport ANd Golgi Organization protein 1. It is also called MIA3 (Melanoma Inhibitory Activity protein 3). TANGO1 has two prominent isoforms, TANGO1-Long and TANGO1-Short, and each isoform has specific functions. On the luminal side, TANGO1-Long has an HSP47 recruitment domain and uses this protein to collect collagen. It can also tether its paralog isoforms cTAGE5 and TALI and along with these proteins enlarges the vesicle to accommodate procollagen. Recent studies show that TANGO1-Long combines retrograde membrane flow with anterograde cargo transport. This complex mechanism is highly activated in fibrosis and promotes the excessive deposition of collagen in the tissues. The therapeutic targeting of TANGO1 may prove successful in the control of fibrotic disorders. This review focuses on TANGO1 and its complex interaction with other procollagen export factors that modulate increased vesicle size to accommodate the export of procollagen.

DYRK3 enables secretory trafficking by maintaining the liquid-like state of ER exit sites

The dual-specificity kinase DYRK3 controls the formation and dissolution of multiple biomolecular condensates, regulating processes including stress recovery and mitotic progression. Here, we report that DYRK3 functionally interacts with proteins associated with endoplasmic reticulum (ER) exit sites (ERESs) and that inhibition of DYRK3 perturbs the organization of the ERES-Golgi interface and secretory trafficking. DYRK3-mediated regulation of ERES depends on the N-terminal intrinsically disordered region (IDR) of the peripheral membrane protein SEC16A, which co-phase separates with ERES components to form liquid-like condensates on the surface of the ER. By modulating the liquid-like properties of ERES, we show that their physical state is essential for functional cargo trafficking through the early secretory pathway. Our findings support a mechanism whereby phosphorylation by DYRK3 and its reversal by serine-threonine phosphatases regulate the material properties of ERES to create a favorable physicochemical environment for directional membrane traffic in eukaryotic cells.

Caspase 1 Enhances Transport and Golgi Organization Protein 1 Expression to Promote Procollagen Export From the Endoplasmic Reticulum in Systemic Sclerosis Contributing to Fibrosis

OBJECTIVE

Transport and Golgi Organization protein 1 (TANGO1) is a protein that regulates the export of procollagen from the endoplasmic reticulum and has a role in the organization of exit sites for general protein export. What regulates the expression of TANGO1 and the role of TANGO1 in fibrosis is poorly understood and has never been studied in the setting of systemic sclerosis (SSc). We undertook this study to determine the role of TANGO1 in SSc fibrosis.

METHODS

SSc (n = 15) and healthy (n = 12) primary fibroblast lung cell lines were investigated for the expression of TANGO1. Histologic analyses for TANGO1 were performed on lung biopsy samples (n = 12 SSc patient samples and n = 8 healthy control samples).

RESULTS

SSc fibroblasts showed increased expression of TANGO1 protein in cultured fibroblasts. TANGO1 colocalizes with α-smooth muscle actin (α-SMA)-positive cells in SSc lung tissue and is highly up-regulated in the neointima of SSc vessels. TANGO1 expression was dependent on the inflammasome activation of caspase 1. It was also dependent on signaling from the interleukin-1 (IL-1) and transforming growth factor β (TGFβ) receptors. The decrease in TANGO1 down-regulated export of larger cargos including collagen and laminin. Reduced TANGO1 protein had no effect on smaller molecular weight cargoes; however, the secretion of elastin was significantly reduced.

CONCLUSION

TANGO1 is markedly increased in SSc fibroblasts and was found to be elevated in lung tissue in association with α-SMA-positive cells. TANGO1 expression is driven by inflammasome-dependent caspase 1 activation and is mediated by IL-1 and TGFβ downstream signaling. These observations suggest that during fibrosis, caspase 1 promotes the up-regulation of TANGO1 and the organization of endoplasmic reticulum exits sites, ultimately contributing to procollagen export and fibrosis.

The COPII subunit CsSEC23 mediates fruit glossiness in cucumber

To improve our understanding of the mechanism underlying cucumber glossiness regulation, a novel cucumber mutant with a glossy peel (Csgp) was identified. MutMap, genotyping, and gene editing results demonstrated that CsSEC23, which is the core component of COPII vesicles, mediates the glossiness of cucumber fruit peel. CsSEC23 is functionally conserved and located in the Golgi and endoplasmic reticulum. CsSEC23 could interact with CsSEC31, but this interaction was absent in the Csgp mutant, which decreased the efficiency of COPII vesicle transportation. Genes related to wax and cutin transport were upregulated in the Csgp mutant, and the cuticle structure of the Csgp-mutant peel became thinner. Moreover, the wax and cutin contents were also changed due to CsSEC23 mutation. Taken together, the results obtained from this study revealed that CsSEC23 mediates cucumber glossiness, and this mediating might be affected by COPII vesicle transportation.

TANGO1 interacts with NRTN to promote hepatocellular carcinoma progression by regulating the PI3K/AKT/mTOR signaling pathway

Transport and Golgi organization 1 (TANGO1) also known as MIA3, belongs to the melanoma inhibitory activity gene (MIA) family together with MIA, MIA2 and OTOR; these members play different roles in different tumors, but the mechanism underlying TANGO1s effect on hepatocellular carcinoma (HCC) is unclear. Our study confirmed that TANGO1 is a promoter of HCC, In HCC cells, TANGO1 can promote proliferation, inhibit apoptosis, promote EMT. These changes were reversed after TANGO1 inhibition. We explored the molecular mechanism of TANGO1 and HCC and found that the promoting effect of TANGO1 on HCC related to neurturin (NRTN) and the PI3K/AKT/mTOR signaling pathway based on RNA-seq results. NRTN is not only related to neuronal growth, differentiation and maintenance but is also involved in a variety of tumorigenic processes, and PI3K/AKT/mTOR signaling pathway has been shown to be involved in HCC progression. We verified that TANGO1 interacts with NRTN in HCC cells using endogenous Co-IP and confocal localization, and both promote HCC progression by activating the PI3K/AKT/mTOR signaling pathway. Our results reveal the mechanism by which TANGO1 promotes HCC progression, suggesting that the TANGO1/NRTN axis may be a potential therapeutic target for HCC worthy of further investigation.

The Sar1 GTPase is dispensable for COPII-dependent cargo export from the ER

Coat protein complex II (COPII) plays an integral role in the packaging of secretory cargoes within membrane-enclosed transport carriers that leave the endoplasmic reticulum (ER) from discrete subdomains. Lipid bilayer remodeling necessary for this process is driven initially by membrane penetration mediated by the Sar1 GTPase and further stabilized by assembly of a multilayered complex of several COPII proteins. However, the relative contributions of these distinct factors to transport carrier formation and protein trafficking remain unclear. Here, we demonstrate that anterograde cargo transport from the ER continues in the absence of Sar1, although the efficiency of this process is dramatically reduced. Specifically, secretory cargoes are retained nearly five times longer at ER subdomains when Sar1 is depleted, but they ultimately remain capable of being translocated to the perinuclear region of cells. Taken together, our findings highlight alternative mechanisms by which COPII promotes transport carrier biogenesis.

O-GlcNAc Dynamics: The Sweet Side of Protein Trafficking Regulation in Mammalian Cells

The transport of proteins between the different cellular compartments and the cell surface is governed by the secretory pathway. Alternatively, unconventional secretion pathways have been described in mammalian cells, especially through multivesicular bodies and exosomes. These highly sophisticated biological processes rely on a wide variety of signaling and regulatory proteins that act sequentially and in a well-orchestrated manner to ensure the proper delivery of cargoes to their final destination. By modifying numerous proteins involved in the regulation of vesicular trafficking, post-translational modifications (PTMs) participate in the tight regulation of cargo transport in response to extracellular stimuli such as nutrient availability and stress. Among the PTMs, O-GlcNAcylation is the reversible addition of a single N-acetylglucosamine monosaccharide (GlcNAc) on serine or threonine residues of cytosolic, nuclear, and mitochondrial proteins. O-GlcNAc cycling is mediated by a single couple of enzymes: the O-GlcNAc transferase (OGT) which catalyzes the addition of O-GlcNAc onto proteins, and the O-GlcNAcase (OGA) which hydrolyses it. Here, we review the current knowledge on the emerging role of O-GlcNAc modification in the regulation of protein trafficking in mammalian cells, in classical and unconventional secretory pathways.

Sorting and Export of Proteins at the Endoplasmic Reticulum

Secretory proteins are transported from the endoplasmic reticulum (ER) to the Golgi complex in carriers that are formed by the concerted activities of cytoplasmic proteins in the coat protein complex II (COPII). COPII was first described in Saccharomyces cerevisiae and its basic functions are largely conserved throughout eukaryotes. The discovery of the TANGO1 (transport and Golgi organization 1) family of proteins is revealing insights into how cells can adapt COPII proteins to reorganize the ER exit site for the export of the most abundant and bulky molecules, collagens.

Nutrient deprivation alters the rate of COPII coat assembly to tune secretory protein transport

Co-assembly of the multilayered coat protein complex II (COPII) with the Sari GTPase at subdomains of the endoplasmic reticulum (ER) enables secretory cargoes to be concentrated efficiently within nascent transport intermediates, which subsequently deliver their contents to ER-Golgi intermediate compartments. Here, we define the spatiotemporal accumulation of native COPII subunits and secretory cargoes at ER subdomains under differing nutrient availability conditions using a combination of CRISPR/Cas9-mediated genome editing and live cell imaging. Our findings demonstrate that the rate of inner COPII coat assembly serves as a determinant for the pace of cargo export, irrespective of COPII subunit expression levels. Moreover, increasing inner COPII coat assembly kinetics is sufficient to rescue cargo trafficking deficits caused by acute nutrient limitation in a manner dependent on Sar1 GTPase activity. Our findings are consistent with a model in which the rate of inner COPII coat formation acts as an important control point to regulate cargo export from the ER.

Molecular mechanism of GTP binding- and dimerization-induced enhancement of Sar1-mediated membrane remodeling

The Sar1 GTPase initiates coat protein II (COPII)-mediated protein transport by generating membrane curvature at subdomains on the endoplasmic reticulum, where it is activated by the guanine nucleotide exchange factor (GEF) Sec12. Crystal structures of GDP- and GTP-bound forms of Sar1 suggest that it undergoes a conformational switch in which GTP binding enhances the exposure of an amino-terminal amphipathic helix necessary for efficient membrane penetration. However, key residues in the amino terminus were not resolved in crystal structures, and experimental studies have suggested that the amino terminus of Sar1 is solvent-exposed in the absence of a membrane, even in the GDP-bound state. Therefore, the molecular mechanism by which GTP binding activates the membrane-remodeling activity of Sar1 remains unclear. Using atomistic molecular dynamics simulations, we compare the membrane-binding and curvature generation activities of Sar1 in its GDP- and GTP-bound states. We show that in the GTP-bound state, Sar1 inserts into the membrane with its complete (residues 1 to 23) amphipathic amino-terminal helix, while Sar1-GDP binds to the membrane only through its first 12 residues. Such differential membrane-binding modes translate into significant differences in the protein volume inserted into the membrane. As a result, Sar1-GTP generates positive membrane curvature 10 to 20 times higher than Sar1-GDP. Dimerization of the GTP-bound form of Sar1 further amplifies curvature generation. Taken together, our results present a detailed molecular mechanism for how the nucleotide-bound state of Sar1 regulates its membrane-binding and remodeling activities in a concentration-dependent manner, paving the way toward a better understanding COPII-mediated membrane transport.

The small GTPase Sar1, control centre of COPII trafficking

Sar1 is a small GTPase of the ARF family. Upon exchange of GDP for GTP, Sar1 associates with the endoplasmic reticulum (ER) membrane and recruits COPII components, orchestrating cargo concentration and membrane deformation. Many aspects of the role of Sar1 and regulation of its GTP cycle remain unclear, especially as complexity increases in higher organisms that secrete a wider range of cargoes. This review focusses on the regulation of GTP hydrolysis and its role in coat assembly, as well as the mechanism of Sar1-induced membrane deformation and scission. Finally, we highlight the additional specialisation in higher eukaryotes and the outstanding questions on how Sar1 functions are orchestrated.

Ultrastructure of COPII vesicle formation in yeast characterized by correlative light and electron microscopy

Traffic of proteins out of the endoplasmic reticulum (ER) is driven by the COPII coat, a layered protein scaffold that mediates the capture of cargo proteins and the remodeling of the ER membrane into spherical vesicular carriers. Although the components of this machinery have been genetically defined, and the mechanisms of coat assembly extensively explored in vitro, understanding the physical mechanisms of membrane remodeling in cells remains a challenge. Here we use correlative light and electron microscopy (CLEM) to visualize the nanoscale ultrastructure of membrane remodeling at ER exit sites (ERES) in yeast cells. Using various COPII mutants, we have determined the broad contribution that each layer of the coat makes to membrane remodeling. Our data suggest that inner coat components define the radius of curvature, whereas outer coat components facilitate membrane fission. The organization of the coat in conjunction with membrane biophysical properties determines the ultrastructure of vesicles and thus the efficiency of protein transport.

Secretory defects in pediatric osteosarcoma result from downregulation of selective COPII coatomer proteins

Pediatric osteosarcomas (OS) exhibit extensive genomic instability that has complicated the identification of new targeted therapies. We found the vast majority of 108 patient tumor samples and patient-derived xenografts (PDXs), which display an unusually dilated endoplasmic reticulum (ER), have reduced expression of four COPII vesicle components that trigger aberrant accumulation of procollagen-I protein within the ER. CRISPR activation technology was used to increase the expression of two of these, SAR1A and SEC24D, to physiological levels. This was sufficient to resolve the dilated ER morphology, restore collagen-I secretion, and enhance secretion of some extracellular matrix (ECM) proteins. However, orthotopic xenograft growth was not adversely affected by restoration of only SAR1A and SEC24D. Our studies reveal the mechanism responsible for the dilated ER that is a hallmark characteristic of OS and identify a highly conserved molecular signature for this genetically unstable tumor. Possible relationships of this phenotype to tumorigenesis are discussed.

ER-to-Golgi trafficking of procollagen III via conventional vesicular and tubular carriers

Collagen is the major protein component of the extracellular matrix. Synthesis of procollagens starts in the endoplasmic reticulum (ER), and three ⍺ chains form a rigid triple helix 300-400 nm in length. It remains unclear how such a large cargo is transported from the ER to the Golgi apparatus. In this study, to elucidate the intracellular transport of fibril-forming collagens, we fused cysteine-free GFP to the N-telopeptide region of procollagen III (GFP-COL3A1) and analyzed transport by live-cell imaging. We found that the maturation dynamics of procollagen III were largely different from those of network-forming procollagen IV (Matsui et al. 2020). Proline hydroxylation of procollagen III uniquely triggered the formation of intralumenal droplet-like structures similar to events caused by liquid-liquid phase separation, and ER exit sites surrounded large droplets containing chaperones. Procollagen III was transported to the Golgi apparatus via vesicular and tubular carriers containing ERGIC53 and RAB1B; this process required TANGO1 and CUL3, which we previously reported were dispensable for procollagen IV. GFP-COL3A1 and mCherry-⍺1AT were co-transported in the same vesicle. Based on these findings, we propose that shortly after ER exit, enlarged carriers containing procollagen III fuse to ERGIC for transport to the Golgi apparatus by conventional cargo carriers. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Collagen has a unique SEC24 preference for efficient export from the endoplasmic reticulum

SEC24 is mainly involved in cargo sorting during COPII vesicle assembly. There are four SEC24 paralogs (A-D) in vertebrates, which are classified into two subgroups (SEC24A/B and SEC24C/D). Pathological mutations in SEC24D cause osteogenesis imperfecta with craniofacial dysplasia in humans. sec24d mutant fish also recapitulate the phenotypes. Consistent with the skeletal phenotypes, the secretion of collagen was severely defective in mutant fish, emphasizing the importance of SEC24D in collagen secretion. However, SEC24D patient-derived fibroblasts show only a mild secretion phenotype, suggesting tissue-specificity in the secretion process. Using Sec24d KO mice and cultured cells, we show that SEC24A and SEC24B also contribute to endoplasmic reticulum (ER) export of procollagen. In contrast, fibronectin 1 requires either SEC24C or SEC24D for ER export. On the basis of our results, we propose that procollagen interacts with multiple SEC24 paralogs for efficient export from the ER, and that this is the basis for tissue-specific phenotypes resulting from SEC24 paralog deficiency.

Computed structures of core eukaryotic protein complexes

Protein-protein interactions play critical roles in biology, but the structures of many eukaryotic protein complexes are unknown, and there are likely many interactions not yet identified. We take advantage of advances in proteome-wide amino acid coevolution analysis and deep-learning–based structure modeling to systematically identify and build accurate models of core eukaryotic protein complexes within the Saccharomyces cerevisiae proteome. We use a combination of RoseTTAFold and AlphaFold to screen through paired multiple sequence alignments for 8.3 million pairs of yeast proteins, identify 1505 likely to interact, and build structure models for 106 previously unidentified assemblies and 806 that have not been structurally characterized. These complexes, which have as many as five subunits, play roles in almost all key processes in eukaryotic cells and provide broad insights into biological function.

Fighting the Fiber: Targeting Collagen in Lung Fibrosis

Organ fibrosis is characterized by epithelial injury and aberrant tissue repair, where activated effector cells, mostly fibroblasts and myofibroblasts, excessively deposit collagen into the extracellular matrix. Fibrosis frequently results in organ failure and has been estimated to contribute to at least one third of all global deaths. Also lung fibrosis, in particular idiopathic pulmonary fibrosis (IPF), is a fatal disease with rising incidence worldwide. As current treatment options targeting fibrogenesis are insufficient, there is an urgent need for novel therapeutic strategies. During the last decade, several studies have proposed to target intra- and extracellular components of the collagen biosynthesis, maturation, and degradation machinery. This includes intra- and extracellular targets directly acting on collagen gene products, but also such that anabolize essential building blocks of collagen, in particular glycine and proline biosynthetic enzymes. Collagen, however, is a ubiquitous molecule in the body and fulfils essential functions as a macromolecular scaffold, growth factor reservoir, and receptor binding site in virtually every tissue. This review summarizes recent advances and future directions in this field. Evidence for the proposed therapeutic targets and where they currently stand in terms of clinical drug development for treatment of fibrotic disease is provided. The drug targets are furthermore discussed in light of (1) specificity for collagen biosynthesis, maturation and degradation, and (2) specificity for disease-associated collagen. As therapeutic success and safety of these drugs may largely depend on targeted delivery, different strategies for specific delivery to the main effector cells and to the extracellular matrix are discussed.

A general role for TANGO1, encoded by MIA3, in secretory pathway organization and function

Complex machinery is required to drive secretory cargo export from the endoplasmic reticulum (ER), which is an essential process in eukaryotic cells. In vertebrates, the MIA3 gene encodes two major forms of transport and Golgi organization protein 1 (TANGO1S and TANGO1L), which have previously been implicated in selective trafficking of procollagen. Using genome engineering of human cells, light microscopy, secretion assays, genomics and proteomics, we show that disruption of the longer form, TANGO1L, results in relatively minor defects in secretory pathway organization and function, including having limited impacts on procollagen secretion. In contrast, loss of both long and short forms results in major defects in cell organization and secretion. These include a failure to maintain the localization of ERGIC53 (also known as LMAN1) and SURF4 to the ER-Golgi intermediate compartment and dramatic changes to the ultrastructure of the ER-Golgi interface. Disruption of TANGO1 causes significant changes in early secretory pathway gene and protein expression, and impairs secretion not only of large proteins, but of all types of secretory cargo, including small soluble proteins. Our data support a general role for MIA3/TANGO1 in maintaining secretory pathway structure and function in vertebrate cells.

TANGO1 marshals the early secretory pathway for cargo export

TANGO1 protein facilitates the endoplasmic reticulum (ER) export of large cargoes that cannot be accommodated in 60 nm transport vesicles. It assembles into a ring in the plane of the ER membrane to create a distinct domain. Its lumenal portion collects and sorts folded cargoes while its cytoplasmic domains collar COPII coats, recruit retrograde COPI-coated membranes that fuse within the TANGO1 ring, thus opening a tunnel for cargo transfer from the ER into a growing export conduit. This mode of cargo transfer bypasses the need for vesicular intermediates and is used to export the most abundant and bulky cargoes. The evolution of TANGO1 and its activities defines the difference between yeast and animal early secretory pathways.

Collagen transport and related pathways in Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI) comprises a heterogeneous group of patients who share bone fragility and deformities as the main characteristics, albeit with different degrees of severity. Phenotypic variation also exists in other connective tissue aspects of the disease, complicating disease classification and disease course prediction. Although collagen type I defects are long established as the primary cause of the bone pathology, we are still far from comprehending the complete mechanism. In the last years, the advent of next generation sequencing has triggered the discovery of many new genetic causes for OI, helping to draw its molecular landscape. It has become clear that, in addition to collagen type I genes, OI can be caused by multiple proteins connected to different parts of collagen biosynthesis. The production of collagen entails a complex process, starting from the production of the collagen Iα1 and collagen Iα2 chains in the endoplasmic reticulum, during and after which procollagen is subjected to a plethora of posttranslational modifications by chaperones. After reaching the Golgi organelle, procollagen is destined to the extracellular matrix where it forms collagen fibrils. Recently discovered mutations in components of the retrograde transport of chaperones highlight its emerging role as critical contributor of OI development. This review offers an overview of collagen regulation in the context of recent gene discoveries, emphasizing the significance of transport disruptions in the OI mechanism. We aim to motivate exploration of skeletal fragility in OI from the perspective of these pathways to identify regulatory points which can hint to therapeutic targets.

Collagen Biosynthesis, Processing, and Maturation in Lung Ageing

In addition to providing a macromolecular scaffold, the extracellular matrix (ECM) is a critical regulator of cell function by virtue of specific physical, biochemical, and mechanical properties. Collagen is the main ECM component and hence plays an essential role in the pathogenesis and progression of chronic lung disease. It is well-established that many chronic lung diseases, e.g., chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) primarily manifest in the elderly, suggesting increased susceptibility of the aged lung or accumulated alterations in lung structure over time that favour disease. Here, we review the main steps of collagen biosynthesis, processing, and turnover and summarise what is currently known about alterations upon lung ageing, including changes in collagen composition, modification, and crosslinking. Recent proteomic data on mouse lung ageing indicates that, while the ER-resident machinery of collagen biosynthesis, modification and triple helix formation appears largely unchanged, there are specific changes in levels of type IV and type VI as well as the two fibril-associated collagens with interrupted triple helices (FACIT), namely type XIV and type XVI collagens. In addition, levels of the extracellular collagen crosslinking enzyme lysyl oxidase are decreased, indicating less enzymatically mediated collagen crosslinking upon ageing. The latter contrasts with the ageing-associated increase in collagen crosslinking by advanced glycation endproducts (AGEs), a result of spontaneous reactions of protein amino groups with reactive carbonyls, e.g., from monosaccharides or reactive dicarbonyls like methylglyoxal. Given the slow turnover of extracellular collagen such modifications accumulate even more in ageing tissues. In summary, the collective evidence points mainly toward age-induced alterations in collagen composition and drastic changes in the molecular nature of collagen crosslinks. Future work addressing the consequences of these changes may provide important clues for prevention of lung disease and for lung bioengineering and ultimately pave the way to novel targeted approaches in lung regenerative medicine.

Structure of the complete, membrane-assembled COPII coat reveals a complex interaction network

COPII mediates Endoplasmic Reticulum to Golgi trafficking of thousands of cargoes. Five essential proteins assemble into a two-layer architecture, with the inner layer thought to regulate coat assembly and cargo recruitment, and the outer coat forming cages assumed to scaffold membrane curvature. Here we visualise the complete, membrane-assembled COPII coat by cryo-electron tomography and subtomogram averaging, revealing the full network of interactions within and between coat layers. We demonstrate the physiological importance of these interactions using genetic and biochemical approaches. Mutagenesis reveals that the inner coat alone can provide membrane remodelling function, with organisational input from the outer coat. These functional roles for the inner and outer coats significantly move away from the current paradigm, which posits membrane curvature derives primarily from the outer coat. We suggest these interactions collectively contribute to coat organisation and membrane curvature, providing a structural framework to understand regulatory mechanisms of COPII trafficking and secretion.

Defects in early secretory pathway transport machinery components and neurodevelopmental disorders

The early secretory pathway, provisionally comprising of vesicular traffic between the endoplasmic reticulum (ER) and the Golgi apparatus, occurs constitutively in mammalian cells. Critical for a constant supply of secretory and plasma membrane (PM) materials, the pathway is presumably essential for general cellular function and survival. Neurons exhibit a high intensity in membrane dynamics and protein/lipid trafficking, with differential and polarized trafficking towards the somatodendritic and axonal PM domains. Mutations in genes encoding early secretory pathway membrane trafficking machinery components are known to result in neurodevelopmental or neurological disorders with disease manifestation in early life. Here, such rare disorders associated with autosomal recessive mutations in coat proteins, membrane tethering complexes and membrane fusion machineries responsible for trafficking in the early secretory pathway are summarily discussed. These mutations affected genes encoding subunits of coat protein complex I and II, subunits of transport protein particle (TRAPP) complexes, members of the YIP1 domain family (YIPF) and a SNAP receptor (SNARE) family member. Why the ubiquitously present and constitutively acting early secretory pathway machinery components could specifically affect neurodevelopment is addressed, with the plausible underlying disease etiologies and neuropathological mechanisms resulting from these mutations explored.

Architecture and mechanism of metazoan retromer:SNX3 tubular coat assembly

Retromer is a master regulator of cargo retrieval from endosomes, which is critical for many cellular processes including signaling, immunity, neuroprotection, and virus infection. The retromer core (VPS26/VPS29/VPS35) is present on cargo-transporting, tubular carriers along with a range of sorting nexins. Here, we elucidate the structural basis of membrane tubulation and coupled cargo recognition by metazoan and fungal retromer coats assembled with the non-Bin1/Amphiphysin/Rvs (BAR) sorting nexin SNX3 using cryo-electron tomography. The retromer core retains its arched, scaffolding structure but changes its mode of membrane recruitment when assembled with different SNX adaptors, allowing cargo recognition at subunit interfaces. Thus, membrane bending and cargo incorporation can be modulated to allow retromer to traffic cargoes along different cellular transport routes.

Linton Mark Traub (1962-2020)

Meir Aridor and David Owen discuss the life and achievements of Linton Traub, who passed away on October 19, 2020.

Tunnels for Protein Export from the Endoplasmic Reticulum

The functions of coat protein complex II (COPII) coats in cargo packaging and the creation of vesicles at the endoplasmic reticulum are conserved in eukaryotic protein secretion. Standard COPII vesicles, however, cannot handle the secretion of metazoan-specific cargoes such as procollagens, apolipoproteins, and mucins. Metazoans have thus evolved modules centered on proteins like TANGO1 (transport and Golgi organization 1) to engage COPII coats and early secretory pathway membranes to engineer a novel mode of cargo export at the endoplasmic reticulum.

Tales of the ER-Golgi Frontier: Drosophila-Centric Considerations on Tango1 Function

In the secretory pathway, the transfer of cargo from the ER to the Golgi involves dozens of proteins that localize at specific regions of the ER called ER exit sites (ERES), where cargos are concentrated preceding vesicular transport to the Golgi. Despite many years of research, we are missing crucial details of how this highly dynamic ER-Golgi interface is defined, maintained and functions. Mechanisms allowing secretion of large cargos such as the very abundant collagens are also poorly understood. In this context, Tango1, discovered in the fruit fly Drosophila and widely conserved in animal evolution, has received a lot of attention in recent years. Tango1, an ERES-localized transmembrane protein, is the single fly member of the MIA/cTAGE family, consisting in humans of TANGO1 and at least 14 different related proteins. After its discovery in flies, a specific role of human TANGO1 in mediating secretion of collagens was reported. However, multiple studies in Drosophila have demonstrated that Tango1 is required for secretion of all cargos. At all ERES, through self-interaction and interactions with other proteins, Tango1 aids ERES maintenance and tethering of post-ER membranes. In this review, we discuss discoveries on Drosophila Tango1 and put them in relation with research on human MIA/cTAGE proteins. In doing so, we aim to offer an integrated view of Tango1 function and the nature of ER-Golgi transport from an evolutionary perspective.

A physical mechanism of TANGO1-mediated bulky cargo export

The endoplasmic reticulum (ER)-resident protein TANGO1 assembles into a ring around ER exit sites (ERES), and links procollagens in the ER lumen to COPII machinery, tethers, and ER-Golgi intermediate compartment (ERGIC) in the cytoplasm (Raote et al., 2018). Here, we present a theoretical approach to investigate the physical mechanisms of TANGO1 ring assembly and how COPII polymerization, membrane tension, and force facilitate the formation of a transport intermediate for procollagen export. Our results indicate that a TANGO1 ring, by acting as a linactant, stabilizes the open neck of a nascent COPII bud. Elongation of such a bud into a transport intermediate commensurate with bulky procollagens is then facilitated by two complementary mechanisms: (i) by relieving membrane tension, possibly by TANGO1-mediated fusion of retrograde ERGIC membranes and (ii) by force application. Altogether, our theoretical approach identifies key biophysical events in TANGO1-driven procollagen export.

Combinatorial multivalent interactions drive cooperative assembly of the COPII coat

Protein secretion is initiated at the endoplasmic reticulum by the COPII coat, which self-assembles to form vesicles. Here, we examine the mechanisms by which a cargo-bound inner coat layer recruits and is organized by an outer scaffolding layer to drive local assembly of a stable structure rigid enough to enforce membrane curvature. An intrinsically disordered region in the outer coat protein, Sec31, drives binding with an inner coat layer via multiple distinct interfaces, including a newly defined charge-based interaction. These interfaces combinatorially reinforce each other, suggesting coat oligomerization is driven by the cumulative effects of multivalent interactions. The Sec31 disordered region could be replaced by evolutionarily distant sequences, suggesting plasticity in the binding interfaces. Such a multimodal assembly platform provides an explanation for how cells build a powerful yet transient scaffold to direct vesicle traffic.

Biophysical forces in membrane bending and traffic

Intracellular trafficking requires extensive changes in membrane morphology. Cells use several distinct molecular factors and physical cues to remodel membranes. Here, we highlight recent advances in identifying the biophysical mechanisms of membrane curvature generation. In particular, we focus on the cooperation of molecular and physical drivers of membrane bending during three stages of vesiculation: budding, cargo selection, and scission. Taken together, the studies reviewed here emphasize that, rather than a single dominant mechanism, several mechanisms typically work in parallel during each step of membrane remodeling. Important challenges for the future of this field are to understand how multiple mechanisms work together synergistically and how a series of stochastic events can be combined to achieve a deterministic result-assembly of the trafficking vesicle.

TANGO1 membrane helices create a lipid diffusion barrier at curved membranes

We have previously shown TANGO1 organises membranes at the interface of the endoplasmic reticulum (ER) and ERGIC/Golgi (Raote et al., 2018). TANGO1 corrals retrograde membranes at ER exit sites to create an export conduit. Here the retrograde membrane is, in itself, an anterograde carrier. This mode of forward transport necessitates a mechanism to prevent membrane mixing between ER and the retrograde membrane. TANGO1 has an unusual membrane helix organisation, composed of one membrane-spanning helix (TM) and another that penetrates the inner leaflet (IM). We have reconstituted these membrane helices in model membranes and shown that TM and IM together reduce the flow of lipids at a region of defined shape. We have also shown that the helices align TANGO1 around an ER exit site. We suggest this is a mechanism to prevent membrane mixing during TANGO1-mediated transfer of bulky secretory cargos from the ER to the ERGIC/Golgi via a tunnel.

Small sequence variations between two mammalian paralogs of the small GTPase SAR1 underlie functional differences in coat protein complex II assembly

Vesicles that are coated by coat protein complex II (COPII) are the primary mediators of vesicular traffic from the endoplasmic reticulum to the Golgi apparatus. Secretion-associated Ras-related GTPase 1 (SAR1) is a small GTPase that is part of COPII and, upon GTP binding, recruits the other COPII proteins to the endoplasmic reticulum membrane. Mammals have two SAR1 paralogs that genetic data suggest may have distinct physiological roles, e.g. in lipoprotein secretion in the case of SAR1B. Here we identified two amino acid clusters that have conserved SAR1 paralog–specific sequences. We observed that one cluster is adjacent to the SAR1 GTP-binding pocket and alters the kinetics of GTP exchange. The other cluster is adjacent to the binding site for two COPII components, SEC31 homolog A COPII coat complex component (SEC31) and SEC23. We found that the latter cluster confers to SAR1B a binding preference for SEC23A that is stronger than that of SAR1A for SEC23A. Unlike SAR1B, SAR1A was prone to oligomerize on a membrane surface. SAR1B knockdown caused loss of lipoprotein secretion, overexpression of SAR1B but not of SAR1A could restore secretion, and a divergent cluster adjacent to the SEC31/SEC23-binding site was critical for this SAR1B function. These results highlight that small primary sequence differences between the two mammalian SAR1 paralogs lead to pronounced biochemical differences that significantly affect COPII assembly and identify a specific function for SAR1B in lipoprotein secretion, providing insights into the mechanisms of large cargo secretion that may be relevant for COPII-related diseases.

Tango1 coordinates the formation of endoplasmic reticulum/Golgi docking sites to mediate secretory granule formation

Regulated secretion is a conserved process occurring across diverse cells and tissues. Current models suggest that the conserved cargo receptor Tango1 mediates the packaging of collagen into large coat protein complex II (COPII) vesicles that move from the endoplasmic reticulum (ER) to the Golgi apparatus. However, how Tango1 regulates the formation of COPII carriers and influences the secretion of other cargo remains unknown. Here, through high-resolution imaging of Tango1, COPII, Golgi, and secretory cargo (mucins) in Drosophila larval salivary glands, we found that Tango1 forms ring-like structures that mediate the formation of COPII rings rather than vesicles. These COPII rings act as docking sites for the cis-Golgi. Moreover, we observed nascent secretory mucins emerging from the Golgi side of these Tango1-COPII-Golgi complexes, suggesting that these structures represent functional docking sites/fusion points between the ER exit sites and the Golgi. Loss of Tango1 disrupted the formation of COPII rings, the association of COPII with the cis-Golgi, mucin O-glycosylation, and secretory granule biosynthesis. Additionally, we identified a Tango1 self-association domain that is essential for formation of this structure. Our results provide evidence that Tango1 organizes an interaction site where secretory cargo is efficiently transferred from the ER to Golgi and then to secretory vesicles. These findings may explain how the loss of Tango1 can influence Golgi/ER morphology and affect the secretion of diverse proteins across many tissues.