Not just a cargo receptor for large cargoes; an emerging role of TANGO1 as an organizer of ER exit sites

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.

The pathways of secretory cargo export at the endoplasmic reticulum

Palade's original model proposed that secretory cargo is transported between stable compartments via vesicles. However, recent findings challenge this view, suggesting that secretory pathway compartments are dynamic, with cargo itself dictating whether transfer occurs via vesicles or through the continuity and maturation of compartmental structures. At the heart of this process is TANGO1, a key component of a molecular machine that works in concert with COPII proteins to construct export routes tailored to the size and quantity of secretory cargo.

Small GTPase ActIvitY ANalyzing (SAIYAN) system: A method to detect GTPase activation in living cells

Small GTPases are essential in various cellular signaling pathways, and detecting their activation within living cells is crucial for understanding cellular processes. The current methods for detecting GTPase activation using fluorescent proteins rely on the interaction between the GTPase and its effector. Consequently, these methods are not applicable to factors, such as Sar1, where the effector also functions as a GTPase-activating protein. Here, we present a novel method, the Small GTPase ActIvitY ANalyzing (SAIYAN) system, for detecting the activation of endogenous small GTPases via fluorescent signals utilizing a split mNeonGreen system. We demonstrated Sar1 activation at the endoplasmic reticulum (ER) exit site and successfully detected its activation state in various cellular conditions. Utilizing the SAIYAN system in collagen-secreting cells, we discovered activated Sar1 localized both at the ER exit sites and ER-Golgi intermediate compartment (ERGIC) regions. Additionally, impaired collagen secretion confined the activated Sar1 at the ER exit sites, implying the importance of Sar1 activation through the ERGIC in collagen secretion.

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.

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

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

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.

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.

Mammalian cargo receptors for endoplasmic reticulum-to-Golgi transport: mechanisms and interactions

Proteins that are destined to enter the secretory pathway are synthesized on the rough endoplasmic reticulum (ER) and then translocated into the ER lumen, where they undergo posttranslational modifications, folding, and assembly. After passing a quality control system, the cargo proteins are packaged into coat protein complex II (COPII) vesicles to exit the ER. In metazoans, most COPII subunits have multiple paralogs, enabling COPII vesicles the flexibility to transport a diverse range of cargo. The cytoplasmic domains of transmembrane proteins can interact with SEC24 subunits of COPII to enter the ER exit sites. Some transmembrane proteins may also act as cargo receptors that bind soluble secretory proteins within the ER lumen, enabling them to enter COPII vesicles. The cytoplasmic domains of cargo receptors also contain coat protein complex I binding motifs that allow for their cycling back to the ER after unloading their cargo in the ER-Golgi intermediate compartment and cis-Golgi. Once unloaded, the soluble cargo proteins continue maturation through the Golgi before reaching their final destinations. This review provides an overview of receptor-mediated transport of secretory proteins from the ER to the Golgi, with a focus on the current understanding of two mammalian cargo receptors: the LMAN1-MCFD2 complex and SURF4, and their roles in human health and disease.

MIA3 Splice Defect in Cane Corso Dogs with Dental-Skeletal-Retinal Anomaly (DSRA)

We investigated a hereditary syndrome in Cane Corso dogs. Affected dogs developed dental-skeletal-retinal anomaly (DSRA), clinically characterized by brittle, discolored, translucent teeth, disproportionate growth and progressive retinal degeneration resulting in vision loss. Combined linkage and homozygosity mapping delineated a 5.8 Mb critical interval. The comparison of whole genome sequence data of an affected dog to 789 control genomes revealed a private homozygous splice region variant in the critical interval. It affected the MIA3 gene encoding the MIA SH3 domain ER export factor 3, which has an essential role in the export of collagen and other secreted proteins. The identified variant, XM_005640835.3:c.3822+3_3822+4del, leads to skipping of two exons from the wild type transcript, XM_005640835.3:r.3712_3822del. Genotypes at the variant were consistent with monogenic autosomal recessive mode of inheritance in a complete family and showed perfect genotype-phenotype association in 18 affected and 22 unaffected Cane Corso dogs. MIA3 variants had previously been shown to cause related phenotypes in humans and mice. Our data in dogs together with the existing functional knowledge of MIA3 variants in other mammalian species suggest the MIA3 splice defect and a near complete loss of gene function as causative molecular pathomechanism for the DSRA phenotype in the investigated dogs.

Assembly and Cellular Exit of Coronaviruses: Hijacking an Unconventional Secretory Pathway from the Pre-Golgi Intermediate Compartment via the Golgi Ribbon to the Extracellular Space

Coronaviruses (CoVs) assemble by budding into the lumen of the intermediate compartment (IC) at the endoplasmic reticulum (ER)-Golgi interface. However, why CoVs have chosen the IC as their intracellular site of assembly and how progeny viruses are delivered from this compartment to the extracellular space has remained unclear. Here we address these enigmatic late events of the CoV life cycle in light of recently described properties of the IC. Of particular interest are the emerging spatial and functional connections between IC elements and recycling endosomes (REs), defined by the GTPases Rab1 and Rab11, respectively. The establishment of IC-RE links at the cell periphery, around the centrosome and evidently also at the noncompact zones of the Golgi ribbon indicates that—besides traditional ER-Golgi communication—the IC also promotes a secretory process that bypasses the Golgi stacks, but involves its direct connection with the endocytic recycling system. The initial confinement of CoVs to the lumen of IC-derived large transport carriers and their preferential absence from Golgi stacks is consistent with the idea that they exit cells following such an unconventional route. In fact, CoVs may share this pathway with other intracellularly budding viruses, lipoproteins, procollagen, and/or protein aggregates experimentally introduced into the IC lumen.

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.

Mitotic ER exit site dynamics: insights into blockade of secretion from the ER during mitosis

How ER exit sites disassemble during mitosis is not well understood. Transport ANd Golgi Organization 1 (TANGO1, also known as MIA3), a cargo receptor originally identified for collagens, acts as a hub for ER exit site disassembly under the control of Casein Kinase 1 (CK1)-mediated phosphorylation and Protein Phosphatase 1 (PP1)-mediated dephosphorylation. Impaired dephosphorylation during mitosis induces ER exit site disassembly.

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.

Folded or Degraded in Endoplasmic Reticulum

In consistent with other membrane-bound and secretory proteins, immune checkpoint proteins go through a set of modifications in the endoplasmic reticulum (ER) to acquire their native functional structures before they function at their destinations. There are various ER-resident chaperones and enzymes synergistically regulate and catalyze the glycosylation, folding and transporting of proteins. The whole processing is under the surveillance of ER quality control system which allows the correctly folded proteins to exit from the ER with the help of coat proteinII(COPII) coated vesicles, while retains the rest of terminally misfolded ones in the ER and then eliminates them via ER-associated degradation (ERAD) or ER-to-lysosomes-associated degradation (ERLAD). The dysfunction of the ER causes ER stress which triggers unfolded protein response (UPR) to restore ER proteostasis. Unsolvable prolonged ER stress ultimately results in cell death. This chapter reviews the process that proteins undergo in the ER, and the glycosylation, folding and degradation of immune checkpoint proteins as well as the associated potential immunotherapies to date.

Molecular mechanisms and clinical manifestations of rare genetic disorders associated with type I collagen

Type I collagen is an important structural protein of bone, skin, tendon, ligament and other connective tissues. It is initially synthesized as a precursor form, procollagen, consisting of two identical pro-α1(I) and one proα2(I) chains, encoded by COL1A1 and COL1A2, respectively. The N- and C- terminal propeptides of procollagen are cleavage by N-proteinase and C-proteinase correspondingly, to form the central triple helix structure with Gly-X-Y repeat units. Mutations of COL1A1 and COL1A2 genes are associated with osteogenesis imperfecta, some types of Ehlers-Danlos syndrome, Caffey diseases, and osteogenesis imperfect/Ehlers- Danlos syndrome overlapping diseases. Clinical symptoms caused by different variations can be variable or similar, mild to lethal, and vice versa. We reviewed the relationship between clinical manifestations and type I collagen - related rare genetic disorders and their possible molecular mechanisms for different mutations and disorders.