The organization of endoplasmic reticulum export complexes

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.

Laminin Beta 2 Is Localized at the Sites of Blood-Brain Barrier and Its Disruption Is Associated With Increased Vascular Permeability, Histochemical, and Transcriptomic Study

Heterotrimeric extracellular matrix proteins laminins are mostly deposited at basal membranes and are important in repair and neoplasia. Here, we localize laminin beta 2 (LAMB2) at the sites of blood-brain barrier (BBB). Microvasculature (MV) of normal brain is endowed with complete LAMB2 coverage. In contrast, its cognate protein laminin beta 1 (LAMB1) is absent in MV of normal brain but emerges at the sprouting tip of a growing vessels. Similarly, vascular proliferation in high-grade gliomas (HGG) is accompanied by marked overexpression of LAMB1, whereas LAMB2 shows deficient deposition. We find that many brain pathologies with presence of post-gadolinium enhancement (PGE) on magnetic resonance imaging (MRI) show disruption of LAMB2 vascular ensheathment. Inhibition of vascular endothelial growth factor signaling in HGG blocks angiogenesis, suppresses PGE in HGG, prevents expression of LAMB1, and restores LAMB2 vascular coverage. Analysis of single-cell RNA sequencing (scRNA-seq) databases shows that in quiescent brain LAMB2 is predominantly expressed by BBB-associated pericytes (PCs) and endothelial cells (ECs), whereas neither cell types produce LAMB1. In contrast, in HGG, both LAMB1 and 2 are overexpressed by endothelial precursor cells, a phenotypically unique immature group, specific to proliferating hyperplastic MV.

COPII with ALG2 and ESCRTs control lysosome-dependent microautophagy of ER exit sites

Endoplasmic reticulum exit sites (ERESs) are tubular outgrowths of endoplasmic reticulum that serve as the earliest station for protein sorting and export into the secretory pathway. How these structures respond to different cellular conditions remains unclear. Here, we report that ERESs undergo lysosome-dependent microautophagy when Ca2+ is released by lysosomes in response to nutrient stressors such as mTOR inhibition or amino acid starvation in mammalian cells. Targeting and uptake of ERESs into lysosomes were observed by super-resolution live-cell imaging and focus ion beam scanning electron microscopy (FIB-SEM). The mechanism was ESCRT dependent and required ubiquitinated SEC31, ALG2, and ALIX, with a knockout of ALG2 or function-blocking mutations of ALIX preventing engulfment of ERESs by lysosomes. In vitro, reconstitution of the pathway was possible using lysosomal lipid-mimicking giant unilamellar vesicles and purified recombinant components. Together, these findings demonstrate a pathway of lysosome-dependent ERES microautophagy mediated by COPII, ALG2, and ESCRTS induced by nutrient stress.

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.

COPII cage assembly factor Sec13 integrates information flow regulating endomembrane function in response to human variation

How information flow is coordinated for managing transit of 1/3 of the genome through endomembrane pathways by the coat complex II (COPII) system in response to human variation remains an enigma. By examining the interactome of the COPII cage-assembly component Sec13, we show that it is simultaneously associated with multiple protein complexes that facilitate different features of a continuous program of chromatin organization, transcription, translation, trafficking, and degradation steps that are differentially sensitive to Sec13 levels. For the trafficking step, and unlike other COPII components, reduction of Sec13 expression decreased the ubiquitination and degradation of wild-type (WT) and F508del variant cargo protein cystic fibrosis transmembrane conductance regulator (CFTR) leading to a striking increase in fold stability suggesting that the events differentiating export from degradation are critically dependent on COPII cage assembly at the ER Golgi intermediate compartment (ERGIC) associated recycling and degradation step linked to COPI exchange. Given Sec13's multiple roles in protein complex assemblies that change in response to its expression, we suggest that Sec13 serves as an unanticipated master regulator coordinating information flow from the genome to the proteome to facilitate spatial covariant features initiating and maintaining design and function of membrane architecture in response to human variation.

A bird's-eye view of the endoplasmic reticulum in filamentous fungi

SUMMARYThe endoplasmic reticulum (ER) is one of the most extensive organelles in eukaryotic cells. It performs crucial roles in protein and lipid synthesis and Ca2+ homeostasis. Most information on ER types, functions, organization, and domains comes from studies in uninucleate animal, plant, and yeast cells. In contrast, there is limited information on the multinucleate cells of filamentous fungi, i.e., hyphae. We provide an analytical review of existing literature to categorize different types of ER described in filamentous fungi while emphasizing the research techniques and markers used. Additionally, we identify the knowledge gaps that need to be resolved better to understand the structure-function correlation of ER in filamentous fungi. Finally, advanced technologies that can provide breakthroughs in understanding the ER in filamentous fungi are discussed.

Stay in touch with the endoplasmic reticulum

The endoplasmic reticulum (ER), which is composed of a continuous network of tubules and sheets, forms the most widely distributed membrane system in eukaryotic cells. As a result, it engages a variety of organelles by establishing membrane contact sites (MCSs). These contacts regulate organelle positioning and remodeling, including fusion and fission, facilitate precise lipid exchange, and couple vital signaling events. Here, we systematically review recent advances and converging themes on ER-involved organellar contact. The molecular basis, cellular influence, and potential physiological functions for ER/nuclear envelope contacts with mitochondria, Golgi, endosomes, lysosomes, lipid droplets, autophagosomes, and plasma membrane are summarized.

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.

Anterograde trafficking of Toll-like receptors requires the cargo sorting adaptors TMED-2 and 7

Toll-Like Receptors (TLRs) play a pivotal role in immunity by recognising conserved structural features of pathogens and initiating the innate immune response. TLR signalling is subject to complex regulation that remains poorly understood. Here we show that two small type I transmembrane receptors, TMED2 and 7, that function as cargo sorting adaptors in the early secretory pathway are required for transport of TLRs from the ER to Golgi. Protein interaction studies reveal that TMED7 interacts with TLR2, TLR4 and TLR5 but not with TLR3 and TLR9. On the other hand, TMED2 interacts with TLR2, TLR4 and TLR3. Dominant negative forms of TMED7 suppress the export of cell surface TLRs from the ER to the Golgi. By contrast TMED2 is required for the ER-export of both plasma membrane and endosomal TLRs. Together, these findings suggest that association of TMED2 and TMED7 with TLRs facilitates anterograde transport from the ER to the Golgi.

Intrinsically disordered region-mediated condensation of IFN-inducible SCOTIN/SHISA-5 inhibits ER-to-Golgi vesicle transport

Newly synthesized proteins in the endoplasmic reticulum (ER) are sorted by coat protein complex II (COPII) at the ER exit site en route to the Golgi. Under cellular stresses, COPII proteins become targets of regulation to control the transport. Here, we show that the COPII outer coat proteins Sec31 and Sec13 are selectively sequestered into the biomolecular condensate of SCOTIN/SHISA-5, which interferes with COPII vesicle formation and inhibits ER-to-Golgi transport. SCOTIN is an ER transmembrane protein with a cytosolic intrinsically disordered region (IDR), which is required and essential for the formation of condensates. Upon IFN-γ stimulation, which is a cellular condition that induces SCOTIN expression and condensation, ER-to-Golgi transport was inhibited in a SCOTIN-dependent manner. Furthermore, cancer-associated mutations of SCOTIN perturb its ability to form condensates and control transport. Together, we propose that SCOTIN impedes the ER-to-Golgi transport through its ability to form biomolecular condensates at the ER membrane.

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.

Unique asymmetric distribution of phosphatidylserine and phosphatidylethanolamine in Toxoplasma gondii revealed by nanoscale analysis

Toxoplasma gondii is a highly prevalent obligate apicomplexan parasite that is important in clinical and veterinary medicine. It is known that glycerophospholipids phosphatidylserine (PtdSer) and phosphatidylethanolamine (PtdEtn), especially their expression levels and flip-flops between cytoplasmic and exoplasmic leaflets, in the membrane of T. gondii play important roles in efficient growth in host mammalian cells, but their distributions have still not been determined because of technical difficulties in studying intracellular lipid distribution at the nanometer level. In this study, we developed an electron microscopy method that enabled us to determine the distributions of PtdSer and PtdEtn in individual leaflets of cellular membranes by using quick-freeze freeze-fracture replica labeling. Our findings show that PtdSer and PtdEtn are asymmetrically distributed, with substantial amounts localized at the luminal leaflet of the inner membrane complex (IMC), which comprises flattened vesicles located just underneath the plasma membrane (see Figs. 2B and 7). We also found that PtdSer was absent in the cytoplasmic leaflet of the inner IMC membrane, but was present in considerable amounts in the cytoplasmic leaflet of the middle IMC membrane, suggesting a barrier-like mechanism preventing the diffusion of PtdSer in the cytoplasmic leaflets of the two membranes. In addition, the expression levels of both PtdSer and PtdEtn in the luminal leaflet of the IMC membrane in the highly virulent RH strain were higher than those in the less virulent PLK strain. We also found that the amount of glycolipid GM3, a lipid raft component, was higher in the RH strain than in the PLK strain. These results suggest a correlation between lipid raft maintenance, virulence, and the expression levels of PtdSer and PtdEtn in T. gondii.

Acute manipulation and real-time visualization of membrane trafficking and exocytosis in Drosophila

Intracellular trafficking of secretory proteins plays key roles in animal development and physiology, but so far, tools for investigating the dynamics of membrane trafficking have been limited to cultured cells. Here, we present a system that enables acute manipulation and real-time visualization of membrane trafficking through the reversible retention of proteins in the endoplasmic reticulum (ER) in living multicellular organisms. By adapting the "retention using selective hooks" (RUSH) approach to Drosophila, we show that trafficking of GPI-linked, secreted, and transmembrane proteins can be controlled with high temporal precision in intact animals and cultured organs. We demonstrate the potential of this approach by analyzing the kinetics of ER exit and apical secretion and the spatiotemporal dynamics of tricellular junction assembly in epithelia of living embryos. Furthermore, we show that controllable ER retention enables tissue-specific depletion of secretory protein function. The system is broadly applicable to visualizing and manipulating membrane trafficking in diverse cell types in vivo.

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.

Insights into Cell-Specific Functions of Microtubules in Skeletal Muscle Development and Homeostasis

The contractile cells of skeletal muscles, called myofibers, are elongated multinucleated syncytia formed and maintained by the fusion of proliferative myoblasts. Human myofibers can be hundreds of microns in diameter and millimeters in length. Myofibers are non-mitotic, obviating the need for microtubules in cell division. However, microtubules have been adapted to the unique needs of these cells and are critical for myofiber development and function. Microtubules in mature myofibers are highly dynamic, and studies in several experimental systems have demonstrated the requirements for microtubules in the unique features of muscle biology including myoblast fusion, peripheral localization of nuclei, assembly of the sarcomere, transport and signaling. Microtubule-binding proteins have also been adapted to the needs of the skeletal muscle including the expression of skeletal muscle-specific protein isoforms generated by alternative splicing. Here, we will outline the different roles microtubules play in skeletal muscle cells, describe how microtubule abnormalities can lead to muscle disease and discuss the broader implications for microtubule function.

Selenoprotein K contributes to CD36 subcellular trafficking in hepatocytes by accelerating nascent COPII vesicle formation and aggravates hepatic steatosis

SelenoproteinK (SelK), an endoplasmic reticulum (ER) - resident protein, possesses the property of mediate oxidation resistance and ER - associated protein degradation (ERAD) in several tissues. Here, we found that increased SelK markedly promotes fatty acid translocase (CD36) subcellular trafficking and aggravates lipid accumulation in hepatocytes. We demonstrated that SelK is required for the assembly of COPII vesicles and accelerates transport of palmitoylated-CD36 from the ER to Golgi, thus facilitating CD36 plasma membrane distribution both in vivo and in vitro. The mechanism is that SelK increases the stability of Sar1B and triggers CD36-containing nascent COPII vesicle formation, consequently, promotes CD36 subcellular trafficking. Furthermore, we verified that the intervention of SelK SH3 binding domain can inhibit the vesicle formation and CD36 subcellular trafficking, significantly ameliorates NAFLD in mice. Collectively, our findings disclose an unexpected role of SelK in regulating NAFLD development, suggesting that targeting the SelK of hepatocytes may be a new therapeutic strategy for the treatment of NAFLD.

Convergence of secretory, endosomal, and autophagic routes in trans-Golgi-associated lysosomes

At the trans-Golgi, complex traffic connections exist to the endolysosomal system additional to the main Golgi-to-plasma membrane secretory route. Here, we investigated three hits in a Drosophila screen displaying secretory cargo accumulation in autophagic vesicles: ESCRT-III component Vps20, SNARE-binding Rop, and lysosomal pump subunit VhaPPA1-1. We found that Vps20, Rop, and lysosomal markers localize near the trans-Golgi. Furthermore, we document that the vicinity of the trans-Golgi is the main cellular location for lysosomes and that early, late, and recycling endosomes associate as well with a trans-Golgi-associated degradative compartment where basal microautophagy of secretory cargo and other materials occurs. Disruption of this compartment causes cargo accumulation in our hits, including Munc18 homolog Rop, required with Syx1 and Syx4 for Rab11-mediated endosomal recycling. Finally, besides basal microautophagy, we show that the trans-Golgi-associated degradative compartment contributes to the growth of autophagic vesicles in developmental and starvation-induced macroautophagy. Our results argue that the fly trans-Golgi is the gravitational center of the whole endomembrane system.

On the nature of the plant ER exit sites

In plants, the endoplasmic reticulum (ER) and Golgi bodies are not only in close proximity, but are also physically linked. This unique organization raises questions about the nature of the transport vectors carrying cargo between the two organelles. Same as in metazoan and yeast cells, it was suggested that cargo is transported from the ER to Golgi cisternae via COPII-coated vesicles produced at ribosome-free ER exit sites (ERES). Recent developments in mammalian cell research suggest, though, that COPII helps to select secretory cargo, but does not coat the carriers leaving the ER. Furthermore, it was shown that mammalian ERES expand into a tubular network containing secretory cargo, but no COPII components. Because of the close association of the ER and Golgi bodies in plant cells, it was previously proposed that ERES and the Golgi comprise a secretory unit that travels over or with a motile ER membrane. In this study, we aimed to explore the nature of ERES in plant cells and took advantage of high-resolution confocal microscopy and imaged ERES labelled with canonical markers (Sar1a, Sec16, Sec24). We found that ERES are dynamically connected to Golgi bodies and most likely represent pre-cis-Golgi cisternae. Furthermore, we showed fine tubular connections from the ER to Golgi compartments (ERGo tubules) as well as fine protrusions from ERES/Golgi cisternae connecting with the ER. We suggest that these tubules observed between the ER and Golgi as well as between the ER and ERES are involved in stabilizing the physical connection between ER and ERES/Golgi cisternae, but may also be involved in cargo transport from the ER to Golgi bodies.

Identification of two pathways mediating protein targeting from ER to lipid droplets

Pathways localizing proteins to their sites of action are essential for eukaryotic cell organization and function. Although mechanisms of protein targeting to many organelles have been defined, how proteins, such as metabolic enzymes, target from the endoplasmic reticulum (ER) to cellular lipid droplets (LDs) is poorly understood. Here we identify two distinct pathways for ER-to-LD protein targeting: early targeting at LD formation sites during formation, and late targeting to mature LDs after their formation. Using systematic, unbiased approaches in Drosophila cells, we identified specific membrane-fusion machinery, including regulators, a tether and SNARE proteins, that are required for the late targeting pathway. Components of this fusion machinery localize to LD–ER interfaces and organize at ER exit sites. We identified multiple cargoes for early and late ER-to-LD targeting pathways. Our findings provide a model for how proteins target to LDs from the ER either during LD formation or by protein-catalysed formation of membrane bridges.

Song et al. identify two protein-targeting pathways from the endoplasmic reticulum to (1) early lipid droplets (LDs) and (2) mature lipid droplets. They define key factors mediating the second, late pathway and its many cargoes.

Cargo receptor Surf4 regulates endoplasmic reticulum export of proinsulin in pancreatic β-cells

Insulin is an essential peptide hormone that maintains blood glucose levels. Although the mechanisms underlying insulin exocytosis have been investigated, the mechanism of proinsulin export from the endoplasmic reticulum (ER) remains unclear. Here, we demonstrated that Surf4, a cargo receptor homolog, regulates the ER export of proinsulin via its recruitment to ER exit sites (ERES). Under high-glucose conditions, Surf4 expression was upregulated, and Surf4 proteins mainly localized to the ER at a steady state and accumulated in the ERES, along with proinsulin in rat insulinoma INS-1 cells. Surf4-knockdown resulted in proinsulin retention in the ER and decreased the levels of mature insulin in secretory granules, thereby significantly reducing insulin secretion. Surf4 forms an oligomer and can physically interact with proinsulin and Sec12, essential for COPII vesicle formation. Our findings suggest that Surf4 interacts with proinsulin and delivers it into COPII vesicles for ER export in co-operation with Sec12 and COPII.

The Golgi Apparatus and its Next-Door Neighbors

The Golgi apparatus represents a central compartment of membrane traffic. Its apparent architecture, however, differs considerably among species, from unstacked and scattered cisternae in the budding yeast Saccharomyces cerevisiae to beautiful ministacks in plants and further to gigantic ribbon structures typically seen in mammals. Considering the well-conserved functions of the Golgi, its fundamental structure must have been optimized despite seemingly different architectures. In addition to the core layers of cisternae, the Golgi is usually accompanied by next-door compartments on its cis and trans sides. The trans-Golgi network (TGN) can be now considered as a compartment independent from the Golgi stack. On the cis side, the intermediate compartment between the ER and the Golgi (ERGIC) has been known in mammalian cells, and its functional equivalent is now suggested for yeast and plant cells. High-resolution live imaging is extremely powerful for elucidating the dynamics of these compartments and has revealed amazing similarities in their behaviors, indicating common mechanisms conserved along the long course of evolution. From these new findings, I would like to propose reconsideration of compartments and suggest a new concept to describe their roles comprehensively around the Golgi and in the post-Golgi trafficking.

SURF4-induced tubular ERGIC selectively expedites ER-to-Golgi transport

The endoplasmic reticulum (ER)-to-Golgi transport is critical to protein secretion and intracellular sorting. Here, we report a highly elongated tubular ER-Golgi intermediate compartment (t-ERGIC) that selectively expedites the ER-to-Golgi transport for soluble cargoes of the receptor SURF4. Lacking the canonical ERGIC marker ERGIC-53 yet positive for the small GTPases Rab1A/B, the t-ERGIC is further marked by its extraordinarily elongated and thinned shape. With its large surface-to-volume ratio, high intracellular traveling speeds, and ER-Golgi recycling capabilities, the t-ERGIC accelerates the trafficking of SURF4-bound cargoes. The biogenesis and cargo selectivity of t-ERGIC both depend on SURF4, which recognizes the N terminus of soluble cargoes and co-clusters with the selected cargoes to expand the ER-exit site. In the steady state, the t-ERGIC-mediated fast ER-to-Golgi transport is antagonized by the KDEL-mediated ER retrieval. Together, our results argue that specific cargo-receptor interactions give rise to distinct transport carriers that regulate the trafficking kinetics.

The Dendritic Ergic: Microtubule And Actin Cytoskeletons Participate In Stop-And-Go Movement Of Mobile Carriers Between Stable Structures

The ER-to-Golgi intermediate compartment (ERGIC) is a membranous organelle that mediates protein transport between the endoplasmic reticulum (ER) and the Golgi apparatus. In neurons, clusters of these vesiculotubular structures are situated throughout the cell in proximity to the ER, passing cargo to the cis-Golgi cisternae, located mainly in the perinuclear region. Although ERGIC markers have been identified in neurons, the distribution and dynamics of neuronal ERGIC structures have not been characterized yet. Here, we show that long-distance ERGIC transport occurs via an intermittent mechanism in dendrites, with mobile elements moving between stationary structures. Slow and fast live-cell imaging have captured stable ERGIC structures remaining in place over long periods of time, as well as mobile ERGIC structures advancing very short distances along dendrites. These short distances have been consistent with the lengths between the stationary ERGIC structures. Kymography revealed ERGIC elements that moved intermittently, emerging from and fusing with stationary ERGIC structures. Interestingly, this movement apparently depends not only on the integrity of the microtubule cytoskeleton, as previously reported, but on the actin cytoskeleton as well. Our results indicate that the dendritic ERGIC has a dual nature, with both stationary and mobile structures. The neural ERGIC network transports proteins via a stop-and-go movement in which both the microtubule and the actin cytoskeletons participate. This article is protected by copyright. All rights reserved.

Branched Actin Maintains Acetylated Microtubule Network in the Early Secretory Pathway

In the early secretory pathway, the delivery of anterograde cargoes from the endoplasmic reticulum (ER) exit sites (ERES) to the Golgi apparatus is a multi-step transport process occurring via the ER-Golgi intermediate compartment (IC, also called ERGIC). While the role microtubules in ER-to-Golgi transport has been well established, how the actin cytoskeleton contributes to this process remains poorly understood. Here, we report that Arp2/3 inhibition affects the network of acetylated microtubules around the Golgi and induces the accumulation of unusually long RAB1/GM130-positive carriers around the centrosome. These long carriers are less prone to reach the Golgi apparatus, and arrival of anterograde cargoes to the Golgi is decreased upon Arp2/3 inhibition. Our data suggest that Arp2/3-dependent actin polymerization maintains a stable network of acetylated microtubules, which ensures efficient cargo trafficking at the late stage of ER to Golgi transport.

Single-cell analysis reveals androgen receptor regulates the ER-to-Golgi trafficking pathway with CREB3L2 to drive prostate cancer progression

Androgen receptor (AR) plays a central role in driving prostate cancer (PCa) progression. How AR promotes this process is still not completely clear. Herein, we used single-cell transcriptome analysis to reconstruct the transcriptional network of AR in PCa. Our work shows AR directly regulates a set of signature genes in the ER-to-Golgi protein vesicle-mediated transport pathway. The expression of these genes is required for maximum androgen-dependent ER-to-Golgi trafficking, cell growth, and survival. Our analyses also reveal the signature genes are associated with PCa progression and prognosis. Moreover, we find inhibition of the ER-to-Golgi transport process with a small molecule enhanced antiandrogen-mediated tumor suppression of hormone-sensitive and insensitive PCa. Finally, we demonstrate AR collaborates with CREB3L2 in mediating ER-to-Golgi trafficking in PCa. In summary, our findings uncover a critical role for dysregulation of ER-to-Golgi trafficking expression and function in PCa progression, provide detailed mechanistic insights for how AR tightly controls this process, and highlight the prospect of targeting the ER-to-Golgi pathway as a therapeutic strategy for advanced PCa.

A systematic review of Sec24 cargo interactome

Endoplasmic reticulum (ER)-to-Golgi trafficking is an essential and highly conserved cellular process. The coat protein complex-II (COPII) arm of the trafficking machinery incorporates a wide array of cargo proteins into vesicles through direct or indirect interactions with Sec24, the principal subunit of the COPII coat. Approximately one-third of all mammalian proteins rely on the COPII-mediated secretory pathway for membrane insertion or secretion. There are four mammalian Sec24 paralogs and three yeast Sec24 paralogs with emerging evidence of paralog-specific cargo interaction motifs. Furthermore, individual paralogs also differ in their affinity for a subset of sorting motifs present on cargo proteins. As with many aspects of protein trafficking, we lack a systematic and thorough understanding of the interaction of Sec24 with cargoes. This systematic review focuses on the current knowledge of cargo binding to both yeast and mammalian Sec24 paralogs and their ER export motifs. The analyses show that Sec24 paralog specificity of cargo (and cargo receptors) range from exclusive paralog dependence or preference to partial redundancy. We also discuss how the Sec24 secretion system is hijacked by viral (eg, VSV-G, Hepatitis B envelope protein) and bacterial (eg, the enteropathogenic Escherichia coli type III secretion system effector NleA/EspI) pathogens.

ER exit sites in Drosophila display abundant ER-Golgi vesicles and pearled tubes but no megacarriers

Secretory cargos are collected at endoplasmic reticulum (ER) exit sites (ERES) before transport to the Golgi apparatus. Decades of research have provided many details of the molecular events underlying ER-Golgi exchanges. Essential questions, however, remain about the organization of the ER-Golgi interface in cells and the type of membrane structures mediating traffic from ERES. To investigate these, we use transgenic tagging in Drosophila flies, 3D-structured illumination microscopy (SIM), and focused ion beam scanning electron microscopy (FIB-SEM) to characterize ERES-Golgi units in collagen-producing fat body, imaginal discs, and imaginal discs overexpressing ERES determinant Tango1. Facing ERES, we find a pre-cis-Golgi region, equivalent to the vertebrate ER-Golgi intermediate compartment (ERGIC), involved in both anterograde and retrograde transport. This pre-cis-Golgi is continuous with the rest of the Golgi, not a separate compartment or collection of large carriers, for which we find no evidence. We observe, however, many vesicles, as well as pearled tubules connecting ERES and Golgi.

Intertwined and Finely Balanced: Endoplasmic Reticulum Morphology, Dynamics, Function, and Diseases

The endoplasmic reticulum (ER) is an organelle that is responsible for many essential subcellular processes. Interconnected narrow tubules at the periphery and thicker sheet-like regions in the perinuclear region are linked to the nuclear envelope. It is becoming apparent that the complex morphology and dynamics of the ER are linked to its function. Mutations in the proteins involved in regulating ER structure and movement are implicated in many diseases including neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis (ALS). The ER is also hijacked by pathogens to promote their replication. Bacteria such as Legionella pneumophila and Chlamydia trachomatis, as well as the Zika virus, bind to ER morphology and dynamics-regulating proteins to exploit the functions of the ER to their advantage. This review covers our understanding of ER morphology, including the functional subdomains and membrane contact sites that the organelle forms. We also focus on ER dynamics and the current efforts to quantify ER motion and discuss the diseases related to ER morphology and dynamics.

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.

The molecular mechanisms underlying acrosome biogenesis elucidated by gene-manipulated mice

Sexual reproduction requires the fusion of two gametes in a multistep and multifactorial process termed fertilization. One of the main steps that ensures successful fertilization is acrosome reaction. The acrosome, a special kind of organelle with a cap-like structure that covers the anterior portion of sperm head, plays a key role in the process. Acrosome biogenesis begins with the initial stage of spermatid development, and it is typically divided into four successive phases: the Golgi phase, cap phase, acrosome phase, and maturation phase. The run smoothly of above processes needs an active and specific coordination between the all kinds of organelles (endoplasmic reticulum, trans-golgi network and nucleus) and cytoplasmic structures (acroplaxome and manchette). During the past two decades, an increasingly genes have been discovered to be involved in modulating acrosome formation. Most of these proteins interact with each other and show a complicated molecular regulatory mechanism to facilitate the occurrence of this event. This Review focuses on the progresses of studying acrosome biogenesis using gene-manipulated mice and highlights an emerging molecular basis of mammalian acrosome formation.

Multiple roles for actin in secretory and endocytic pathways

Actin filaments play multiple roles in the secretory pathway and in endosome dynamics in mammals, including maintenance of Golgi structure, release of membrane cargo from the trans-Golgi network (TGN), endocytosis, and endosomal sorting dynamics. In addition, TGN carrier transport and endocytosis both occur by multiple mechanisms in mammals. Actin likely plays a role in at least four mammalian endocytic pathways, five pathways for membrane release from the TGN, and three processes involving endosomes. Also, the mammalian Golgi structure is highly dynamic, and actin is likely important for these dynamics. One challenge for many of these processes is the need to deal with other membrane-associated structures, such as the cortical actin network at the plasma membrane or the matrix that surrounds the Golgi. Arp2/3 complex is a major actin assembly factor in most of the processes mentioned, but roles for formins and tandem WH2-motif-containing assembly factors are being elucidated and are anticipated to grow with further study. The specific role for actin has not been defined for most of these processes, but is likely to involve the generation of force for membrane dynamics, either by actin polymerization itself or by myosin motor activity. Defining these processes mechanistically is necessary for understanding membrane dynamics in general, as well as pathways that utilize these processes, such as autophagy.

Cellular and sub-cellular mechanisms of lipid transport from gut to lymph

Although the general structure of the barrier between the gut and the blood is well known, many details are still missing. Here, we analyse the literature and our own data related to lipid transcytosis through adult mammalian enterocytes, and their absorption into lymph at the tissue level of the intestine. After starvation, the Golgi complex (GC) of enterocytes is in a resting state. The addition of lipids in the form of chyme leads to the initial appearance of pre-chylomicrons (ChMs) in the tubules of the smooth endoplasmic reticulum, which are attached at the basolateral plasma membrane, immediately below the 'belt' of the adhesive junctions. Then pre-ChMs move into the cisternae of the rough endoplasmic reticulum and then into the expansion of the perforated Golgi cisternae. Next, they pass through the GC, and are concentrated in the distensions of the perforated cisternae on the trans-side of the GC. The arrival of pre-ChMs at the GC leads to the transition of the GC to a state of active transport, with formation of intercisternal connections, attachment of cis-most and trans-most perforated cisternae to the medial Golgi cisternae, and disappearance of COPI vesicles. Post-Golgi carriers then deliver ChMs to the basolateral plasma membrane, fuse with it, and secret ChMs into the intercellular space between enterocytes at the level of their interdigitating contacts. Finally, ChMs are squeezed out into the interstitium through pores in the basal membrane, most likely due to the function of the actin-myosin 'cuff' around the interdigitating contacts. These pores appear to be formed by protrusions of the dendritic cells and the enterocytes per se. ChMs are absorbed from the interstitium into the lymphatic capillaries through the special oblique contacts between endothelial cells, which function as valves through the contraction-relaxation of bundles of smooth muscle cells in the interstitium. Lipid overloading of enterocytes results in accumulation of cytoplasmic lipid droplets, an increase in diameter of ChMs, inhibition of intra-Golgi transport, and fusion of ChMs in the interstitium. Here, we summarise and analyse recent findings, and discuss their functional implications.

ER-to-Golgi protein delivery through an interwoven, tubular network extending from ER

Cellular versatility depends on accurate trafficking of diverse proteins to their organellar destinations. For the secretory pathway (followed by approximately 30% of all proteins), the physical nature of the vessel conducting the first portage (endoplasmic reticulum [ER] to Golgi apparatus) is unclear. We provide a dynamic 3D view of early secretory compartments in mammalian cells with isotropic resolution and precise protein localization using whole-cell, focused ion beam scanning electron microscopy with cryo-structured illumination microscopy and live-cell synchronized cargo release approaches. Rather than vesicles alone, the ER spawns an elaborate, interwoven tubular network of contiguous lipid bilayers (ER exit site) for protein export. This receptacle is capable of extending microns along microtubules while still connected to the ER by a thin neck. COPII localizes to this neck region and dynamically regulates cargo entry from the ER, while COPI acts more distally, escorting the detached, accelerating tubular entity on its way to joining the Golgi apparatus through microtubule-directed movement.

The ER-mitochondria Ca2+ signaling in cancer progression: Fueling the monster

Cancer is a leading cause of death worldwide. All major tumor suppressors and oncogenes are now recognized to have fundamental connections with metabolic pathways. A hallmark feature of cancer cells is a reprogramming of their metabolism even when nutrients are available. Increasing evidence indicates that most cancer cells rely on mitochondrial metabolism to sustain their energetic and biosynthetic demands. Mitochondria are functionally and physically coupled to the endoplasmic reticulum (ER), the major calcium (Ca²⁺) storage organelle in mammalian cells, through special domains known as mitochondria-ER contact sites (MERCS). In this domain, the release of Ca²⁺ from the ER is mainly regulated by inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs), a family of Ca²⁺ release channels activated by the ligand IP3. IP3R mediated Ca²⁺ release is transferred to mitochondria through the mitochondrial Ca²⁺ uniporter (MCU). Once in the mitochondrial matrix, Ca²⁺ activates several proteins that stimulate mitochondrial performance. The role of IP3R and MCU in cancer, as well as the other proteins that enable the Ca²⁺ communication between these two organelles is just beginning to be understood. Here, we describe the function of the main players of the ER mitochondrial Ca²⁺ communication and discuss how this particular signal may contribute to the rise and development of cancer traits.

Interactions of Lipid Droplets with the Intracellular Transport Machinery

Historically, studies of intracellular membrane trafficking have focused on the secretory and endocytic pathways and their major organelles. However, these pathways are also directly implicated in the biogenesis and function of other important intracellular organelles, the best studied of which are peroxisomes and lipid droplets. There is a large recent body of work on these organelles, which have resulted in the introduction of new paradigms regarding the roles of membrane trafficking organelles. In this review, we discuss the roles of membrane trafficking in the life cycle of lipid droplets. This includes the complementary roles of lipid phase separation and proteins in the biogenesis of lipid droplets from endoplasmic reticulum (ER) membranes, and the attachment of mature lipid droplets to membranes by lipidic bridges and by more conventional protein tethers. We also discuss the catabolism of neutral lipids, which in part results from the interaction of lipid droplets with cytosolic molecules, but with important roles for both macroautophagy and microautophagy. Finally, we address their eventual demise, which involves interactions with the autophagocytotic machinery. We pay particular attention to the roles of small GTPases, particularly Rab18, in these processes.

Interorganelle communication and membrane shaping in the early secretory pathway

Biomolecules in the secretory pathway use membrane trafficking for reaching their final intracellular destination or for secretion outside the cell. This highly dynamic and multipartite process involves different organelles that communicate to one another while maintaining their identity, shape, and function. Recent studies unraveled new mechanisms of interorganelle communication that help organize the early secretory pathway. We highlight how the spatial proximity between endoplasmic reticulum (ER) exit sites and early Golgi elements provides novel means of ER-Golgi communication for ER export. We also review recent findings on how membrane contact sites between the ER and the trans-Golgi membranes can sustain anterograde traffic out of the Golgi complex.

Carbon tetrachloride suppresses ER-Golgi transport by inhibiting COPII vesicle formation on the ER membrane in the RLC-16 hepatocyte cell line

Carbon tetrachloride (CCl4 ) causes hepatotoxicity in mammals, with its hepatocytic metabolism producing radicals that attack the intracellular membrane system and destabilize intracellular vesicle transport. Inhibition of intracellular transport causes lipid droplet retention and abnormal protein distribution. The intracellular transport of synthesized lipids and proteins from the endoplasmic reticulum (ER) to the Golgi apparatus is performed by coat complex II (COPII) vesicle transport, but how CCl4 inhibits COPII vesicle transport has not been elucidated. COPII vesicle formation on the ER membrane is initiated by the recruitment of Sar1 protein from the cytoplasm to the ER membrane, followed by that of the COPII coat constituent proteins (Sec23, Sec24, Sec13, and Sec31). In this study, we evaluated the effect of CCl4 on COPII vesicle formation using the RLC-16 rat hepatocyte cell line. Our results showed that CCl4 suppressed ER-Golgi transport in RLC-16 cells. Using a reconstituted system of rat liver tissue-derived cytoplasm and RLC-16 cell-derived ER membranes, CCl4 treatment inhibited the recruitment of Sar1 and Sec13 from the cytosolic fraction to ER membranes. CCl4 -induced changes in the ER membrane accordingly inhibited the accumulation of COPII vesicle-coated constituent proteins on the ER membrane, as well as the formation of COPII vesicles, which suppressed lipid and protein transport between the ER and Golgi apparatus. Our data suggest that CCl4 inhibits ER-Golgi intracellular transport by inhibiting COPII vesicle formation on the ER membrane in hepatocytes.

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.

Export Control: Post-transcriptional Regulation of the COPII Trafficking Pathway

The coat protein complex II (COPII) mediates forward trafficking of protein and lipid cargoes from the endoplasmic reticulum. COPII is an ancient and essential pathway in all eukaryotes and COPII dysfunction underlies a range of human diseases. Despite this broad significance, major aspects of COPII trafficking remain incompletely understood. For example, while the biochemical features of COPII vesicle formation are relatively well characterized, much less is known about how the COPII system dynamically adjusts its activity to changing physiologic cues or stresses. Recently, post-transcriptional mechanisms have emerged as a major mode of COPII regulation. Here, we review the current literature on how post-transcriptional events, and especially post-translational modifications, govern the COPII pathway.

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.

ER-to-Golgi Transport in HeLa Cells Displays High Resilience to Ca2+ and Energy Stresses

One third of all human proteins are either transmembrane or soluble secretory proteins that first target the endoplasmic reticulum (ER). These proteins subsequently leave the ER and enter the Golgi apparatus via ER-Golgi intermediate vesicular structures. Live-cell imaging of cargos fused to fluorescent proteins (FPs) enables the high-resolution visualization and characterization of secretory transport processes. Here, we performed fluorescence time-lapse imaging to assess the Ca2+ and energy dependency of ER-to-Golgi transport in living HeLa cells, a cancer cell model which has been well investigated. Our data revealed that ER-to-Golgi transport remained highly efficient in the absence of ATP-generating substrates, despite clear reductions in cytosolic and mitochondrial ATP levels under these energy stress conditions. However, cell treatment with 2-deoxy-D-glucose (2-DG), which severely diminished subcellular ATP levels, abolished ER-to-Golgi transport. Interestingly, while 2-DG elevated cytosolic Ca2+ levels and reduced long-distance movements of glycosylphosphatidylinositol (GPI)-positive vesicles, robust short-term ER Ca2+ mobilizations, which strongly affected the motility of these vesicles, did not considerably impair ER-to-Golgi transport. In summary, we highlight that ER-to-Golgi transport in HeLa cells remains functional despite high energy and Ca2+ stress levels.

Targeting DNA to the endoplasmic reticulum efficiently enhances gene delivery and therapy

Gene therapy mediated by non-viral carriers is gaining an increasing popularity due to its high biosafety and the convenience of production on a large scale, yet inefficient gene delivery is a limiting obstacle. Few gene vectors can avoid the endosome-lysosome route, and as a result, their DNA payloads are easily decomposed during transfection. Herein, a peptide (pardaxin, PAR)-modified cationic liposome (PAR-Lipo) targeting the endoplasmic reticulum (ER) was developed for improving the gene delivery efficiency. Interestingly, compared to non-PAR-modified cationic liposomes (Non-Lipos) and Lipofectamine 2000 (Lipo 2000, a commercial genetic vector), PAR-Lipos showed remarkably higher gene delivery efficiency in vitro and better antitumor efficacy in vivo. It was demonstrated that PAR-Lipos could be accumulated into the ER via a non-lysosome intracellular route after cellular internalization, which induced the retention of the DNA payload in the ER close to the nucleus, while Non-Lipos, like most conventional cationic carriers, mainly presented lysosomal retention after their endocytosis. The unique intracellular transport behavior of PAR-Lipos can enhance the protection of the DNA payload, prolong their residence time in the cell, and promote their entry into the nucleus relying on the intimate relationship between the ER and nuclear membrane, which is the explanation for the enhanced gene-therapy effect mediated by PAR-Lipos. Our research may provide alternative means of efficiently delivering genes in cells.

Vesicular and uncoated Rab1-dependent cargo carriers facilitate ER to Golgi transport

Secretory cargo is recognized, concentrated and trafficked from endoplasmic reticulum (ER) exit sites (ERES) to the Golgi. Cargo export from the ER begins when a series of highly conserved COPII coat proteins accumulate at the ER and regulate the formation of cargo-loaded COPII vesicles. In animal cells, capturing live de novo cargo trafficking past this point is challenging; it has been difficult to discriminate whether cargo is trafficked to the Golgi in a COPII-coated vesicle. Here, we describe a recently developed live-cell cargo export system that can be synchronously released from ERES to illustrate de novo trafficking in animal cells. We found that components of the COPII coat remain associated with the ERES while cargo is extruded into COPII-uncoated, non-ER associated, Rab1 (herein referring to Rab1a or Rab1b)-dependent carriers. Our data suggest that, in animal cells, COPII coat components remain stably associated with the ER at exit sites to generate a specialized compartment, but once cargo is sorted and organized, Rab1 labels these export carriers and facilitates efficient forward trafficking.This article has an associated First Person interview with the first author of the paper.

Visualization of Procollagen IV Reveals ER-to-Golgi Transport by ERGIC-independent Carriers

Collagen is the most abundant protein in animal tissues and is critical for their proper organization. Nascent procollagens in the endoplasmic reticulum (ER) are considered too large to be loaded into coat protein complex II (COPII) vesicles, which have a diameter of 60-80 nm, for exit from the ER and transport to the Golgi complex. To study the transport mechanism of procollagen IV, which generates basement membranes, we introduced a cysteine-free GFP tag at the N-terminus of the triple helical region of the α1(IV) chain (cfSGFP2-col4a1), and examined the dynamics of this protein in HT-1080 cells, which produce endogenous collagen IV. cfSGFP2-col4a1 was transported from the ER to the Golgi by vesicles, which were a similar size as small cargo carriers. However, mCherry-ERGIC53 was recruited to α1-antitrypsin-containing vesicles, but not to cfSGFP2-col4a1-containing vesicles. Knockdown analysis revealed that Sar1 and SLY1/SCFD1 were required for transport of cfSGFP2-col4a1. TANGO1, CUL3, and KLHL12 were not necessary for the ER-to-Golgi trafficking of procollagen IV. Our data suggest that procollagen IV is exported from the ER via an enlarged COPII coat carrier and is transported to the Golgi by unique transport vesicles without recruitment of ER-Golgi intermediate compartment membranes.Key words: collagen, procollagen IV, endoplasmic reticulum, ER-to-Golgi transport, ERGIC.

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.

CDP-diacylglycerol, a critical intermediate in lipid metabolism

The roles of lipids expand beyond the basic building blocks of biological membranes. In addition to forming complex and dynamic barriers, the thousands of different lipid species in the cell contribute to essentially all the processes of life. Specific lipids are increasingly identified in cellular processes, including signal transduction, membrane trafficking, metabolic control and protein regulation. Tight control of their synthesis and degradation is essential for homeostasis. Most of the lipid molecules in the cell originate from a small number of critical intermediates. Thus, regulating the synthesis of intermediates is essential for lipid homeostasis and optimal biological functions. Cytidine diphosphate diacylglycerol (CDP-DAG) is an intermediate which occupies a branch point in lipid metabolism. CDP-DAG is incorporated into different synthetic pathways to form distinct phospholipid end-products depending on its location of synthesis. Identification and characterization of CDP-DAG synthases which catalyze the synthesis of CDP-DAG has been hampered by difficulties extracting these membrane-bound enzymes for purification. Recent developments have clarified the cellular localization of the CDP-DAG synthases and identified a new unrelated CDP-DAG synthase enzyme. These findings have contributed to a deeper understanding of the extensive synthetic and signaling networks stemming from this key lipid intermediate.