Insights into COPII coat nucleation from the structure of Sec23.Sar1 complexed with the active fragment of Sec31

TFG-1 function in protein secretion and oncogenesis

Export of proteins from the endoplasmic reticulum in COPII-coated vesicles occurs at defined sites that contain the scaffolding protein Sec16. We identify TFG-1, a new conserved regulator of protein secretion that interacts directly with SEC-16 and controls the export of cargoes from the endoplasmic reticulum in Caenorhabditis elegans. Hydrodynamic studies indicate that TFG-1 forms hexamers that facilitate the co-assembly of SEC-16 with COPII subunits. Consistent with these findings, TFG-1 depletion leads to a marked decline in both SEC-16 and COPII levels at endoplasmic reticulum exit sites. The sequence encoding the amino terminus of human TFG has been previously identified in chromosome translocation events involving two protein kinases, which created a pair of oncogenes. We propose that fusion of these kinases to TFG relocalizes their activities to endoplasmic reticulum exit sites, where they prematurely phosphorylate substrates during endoplasmic reticulum export. Our findings provide a mechanism by which translocations involving TFG can result in cellular transformation and oncogenesis.

Cargo loading at the ER

Trafficking of newly synthesized cargo through the early secretory pathway defines and maintains the intracellular organization of eukaryotic cells as well as the organization of tissues and organs. The importance of this pathway is underlined by the increasing number of mutations in key components of the ER export machinery that are causative of a diversity of human diseases. Here we discuss the molecular mechanisms that dictate cargo selection during vesicle budding. While, in vitro reconstitution assays, unicellular organisms such as budding yeast, and mammalian cell culture still have much to offer in terms of gaining a full understanding of the molecular basis for secretory cargo export, such assays have to date been limited to analysis of smaller, freely diffusible cargoes. The export of large macromolecular complexes from the ER such as collagens (up to 300 nm) or lipoproteins (~500 nm) presents a clear problem in terms of maintaining both selectivity and efficiency of export. It has also become clear that in order to translate our knowledge of the molecular basis for ER export to a full understanding of the implications for normal development and disease progression, the use of metazoan models is essential. Combined, these approaches are now starting to shed light not only on the mechanisms of macromolecular cargo export from the ER but also reveal the implications of failure of this process to human development and disease.

JAK2 rearrangements, including the novel SEC31A-JAK2 fusion, are recurrent in classical Hodgkin lymphoma

The genetics of classical Hodgkin lymphoma (cHL) is poorly understood. The finding of a JAK2-involving t(4;9)(q21;p24) in 1 case of cHL prompted us to characterize this translocation on a molecular level and to determine the prevalence of JAK2 rearrangements in cHL. We showed that the t(4;9)(q21;p24) leads to a novel SEC31A-JAK2 fusion. Screening of 131 cHL cases identified 1 additional case with SEC31A-JAK2 and 2 additional cases with rearrangements involving JAK2. We demonstrated that SEC31A-JAK2 is oncogenic in vitro and acts as a constitutively activated tyrosine kinase that is sensitive to JAK inhibitors. In vivo, SEC31A-JAK2 was found to induce a T-lymphoblastic lymphoma or myeloid phenotype in a murine bone marrow transplantation model. Altogether, we identified SEC31A-JAK2 as a chromosomal aberration characteristic for cHL and provide evidence that JAK2 rearrangements occur in a minority of cHL cases. Given the proven oncogenic potential of this novel fusion, our studies provide new insights into the pathogenesis of cHL and indicate that in at least some cases, constitutive activation of the JAK/STAT pathway is caused by JAK2 rearrangements. The finding that SEC31A-JAK2 responds to JAK inhibitors indicates that patients with cHL and JAK2 rearrangements may benefit from targeted therapies.

Integrins: versatile receptors controlling melanocyte adhesion, migration and proliferation

From the onset of melanocyte specification from the neural crest, throughout their migration during embryogenesis and until they reside in their niche in the basal keratinocyte layer, melanocytes interact in dynamic ways with the extracellular environment of the growing embryo. To recognize and to adhere to their environment, melanocytes depend on heterodimeric cell surface receptors of the family of integrins. In addition to the control of adhesive interactions between melanocytes and the extracellular matrix scaffold secreted by fibroblasts and keratinocytes, the integrin receptors allow cells also to sense the mechanical condition of the extracellular environment, responding by intracellular signaling, triggering cell survival, proliferation or migration events. In this review, we summarize the recently emerged concepts that explain integrin-dependent adhesion and how this adhesion system interfaces with integrin-dependent signaling events. The gained information will help to understand melanocyte behavior in pathological situations such as melanoma growth and metastasis formation.

Cranio-lenticulo-sutural dysplasia associated with defects in collagen secretion

Cranio-lenticulo-sutural dysplasia (CLSD) is a rare autosomal recessive syndrome manifesting with large and late-closing fontanels and calvarial hypomineralization, Y-shaped cataracts, skeletal defects, and hypertelorism and other facial dysmorphisms. The CLSD locus was mapped to chromosome 14q13-q21 and a homozygous SEC23A F382L missense mutation was identified in the original family. Skin fibroblasts from these patients exhibit features of a secretion defect with marked distension of the endoplasmic reticulum (ER), consistent with SEC23A function in protein export from the ER. We report an unrelated family where a male proband presented with clinical features of CLSD. A heterozygous missense M702V mutation in a highly conserved residue of SEC23A was inherited from the clinically unaffected father, but no maternal SEC23A mutation was identified. Cultured skin fibroblasts from this new patient showed a severe secretion defect of collagen and enlarged ER, confirming aberrant protein export from the ER. Milder collagen secretion defects and ER distention were present in paternal fibroblasts, indicating that an additional mutation(s) is present in the proband. Our data suggest that defective ER export is the cause of CLSD and genetic element(s) besides SEC23A may influence its presentation.

The ALG-2 binding site in Sec31A influences the retention kinetics of Sec31A at the endoplasmic reticulum exit sites as revealed by live-cell time-lapse imaging

ALG-2, a member of the penta-EF-hand protein family, interacts Ca²+-dependently with a COPII component, Sec31A. In this study, we first established HeLa cells stably expressing green fluorescent protein-fused ALG-2 (GFP-ALG-2) and red fluorescent protein-fused Sec31A (Sec31A-RFP). After inducing Ca²+-mobilization, the cytoplasmic distribution of GFP-ALG-2 changed from a diffuse to a punctate pattern, which extensively overlapped with the Sec31A-RFP-positive structures, indicating that ALG-2 is recruited to the endoplasmic reticulum exit sites (ERES) in living cells. Next, overlay experiments with biotin-labeled ALG-2 were done to dissect the ALG-2 binding site (ABS). They revealed that a sequence comprising amino acid residues 839-851 in the Pro-rich region was necessary and sufficient for direct binding to ALG-2. Finally, fluorescence recovery after photobleaching analysis indicated that the ABS deletion reduced the high-affinity population of Sec31A to the ERES, suggesting that the ABS is one of the key determinants of the retention kinetics of Sec31A at ERES.

p125A exists as part of the mammalian Sec13/Sec31 COPII subcomplex to facilitate ER-Golgi transport

p125A is an accessory protein for COPII-mediated vesicle budding that links the Sec13/Sec31 and Sec23/24 subcomplexes.

Coat protein II (COPII)–mediated export from the endoplasmic reticulum (ER) involves sequential recruitment of COPII complex components, including the Sar1 GTPase, the Sec23/Sec24 subcomplex, and the Sec13/Sec31 subcomplex. p125A was originally identified as a Sec23A-interacting protein. Here we demonstrate that p125A also interacts with the C-terminal region of Sec31A. The Sec31A-interacting domain of p125A is between residues 260–600, and is therefore a distinct domain from that required for interaction with Sec23A. Gel filtration and immunodepletion studies suggest that the majority of cytosolic p125A exists as a ternary complex with the Sec13/Sec31A subcomplex, suggesting that Sec 13, Sec31A, and p125A exist in the cytosol primarily as preassembled Sec13/Sec31A/p125A heterohexamers. Golgi morphology and protein export from the ER were affected in p125A-silenced cells. Our results suggest that p125A is part of the Sec13/Sec31A subcomplex and facilitates ER export in mammalian cells.

Sar1 assembly regulates membrane constriction and ER export

While dynamin pinches vesicles from the plasma membrane, the Sar1 GTPase specializes in cinching ER membrane tubules.

The guanosine triphosphatase Sar1 controls the assembly and fission of COPII vesicles. Sar1 utilizes an amphipathic N-terminal helix as a wedge that inserts into outer membrane leaflets to induce vesicle neck constriction and control fission. We hypothesize that Sar1 organizes on membranes to control constriction as observed with fission proteins like dynamin. Sar1 activation led to membrane-dependent oligomerization that transformed giant unilamellar vesicles into small vesicles connected through highly constricted necks. In contrast, membrane tension provided through membrane attachment led to organization of Sar1 in ordered scaffolds that formed rigid, uniformly nonconstricted lipid tubules to suggest that Sar1 organization regulates membrane constriction. Sar1 organization required conserved residues located on a unique C-terminal loop. Mutations in this loop did not affect Sar1 activation or COPII recruitment and enhanced membrane constriction, yet inhibited Sar1 organization and procollagen transport from the endoplasmic reticulum (ER). Sar1 activity was directed to liquid-disordered lipid phases. Thus, lipid-directed and tether-assisted Sar1 organization controls membrane constriction to regulate ER export.

Structure of coatomer cage proteins and the relationship among COPI, COPII, and clathrin vesicle coats

COPI-coated vesicles form at the Golgi apparatus from two cytosolic components, ARF G protein and coatomer, a heptameric complex that can polymerize into a cage to deform the membrane into a bud. Although coatomer shares a common evolutionary origin with COPII and clathrin vesicle coat proteins, the architectural relationship among the three cages is unclear. Strikingly, the alphabeta'-COP core of coatomer crystallizes as a triskelion in which three copies of a beta'-COP beta-propeller domain converge through their axial ends. We infer that the trimer constitutes the vertex of the COPI cage. Our model proposes that the COPI cage is intermediate in design between COPII and clathrin: COPI shares with clathrin an arrangement of three curved alpha-solenoid legs radiating from a common center, and COPI shares with COPII highly similar vertex interactions involving the axial ends of beta-propeller domains.

Crystal structure of alpha-COP in complex with epsilon-COP provides insight into the architecture of the COPI vesicular coat

The heptameric coatomer complex forms the protein shell of membrane-bound vesicles that are involved in transport from the Golgi to the endoplasmatic reticulum and in intraGolgi trafficking. The heptamer can be dissected into a heterotetrameric F-subcomplex, which displays similarities to the adapter complex of the "inner" coat in clathrin-coated vesicles, and a heterotrimeric B-subcomplex, which is believed to form an "outer" coat with a morphology distinct from that of clathrin-coated vesicles. We have determined the crystal structure of the complex between the C-terminal domain (CTD) of alpha-COP and full-length epsilon-COP, two components of the B-subcomplex, at a 2.9 A resolution. The alpha-COP(CTD) x epsilon-COP heterodimer forms a rod-shaped structure, in which epsilon-COP adopts a tetratricopeptide repeat (TPR) fold that deviates substantially from the canonical superhelical conformation. The alpha-COP CTD adopts a U-shaped architecture that complements the TPR fold of epsilon-COP. The epsilon-COP TPRs form a circular bracelet that wraps around a protruding beta-hairpin of the alpha-COP CTD, thus interlocking the two proteins. The alpha-COP(CTD) x epsilon-COP complex forms heterodimers in solution, and we demonstrate biochemically that the heterodimer directly interacts with the Dsl1 tethering complex. These data suggest that the heterodimer is exposed on COPI vesicles, while the remaining part of the B-subcomplex oligomerizes underneath into a cage.

Trafficking through COPII stabilises cell polarity and drives secretion during Drosophila epidermal differentiation

Background

The differentiation of an extracellular matrix (ECM) at the apical side of epithelial cells implies massive polarised secretion and membrane trafficking. An epithelial cell is hence engaged in coordinating secretion and cell polarity for a correct and efficient ECM formation.

Principal Findings

We are studying the molecular mechanisms that Drosophila tracheal and epidermal cells deploy to form their specific apical ECM during differentiation. In this work we demonstrate that the two genetically identified factors haunted and ghost are essential for polarity maintenance, membrane topology as well as for secretion of the tracheal luminal matrix and the cuticle. We show that they code for the Drosophila COPII vesicle-coating components Sec23 and Sec24, respectively, that organise vesicle transport from the ER to the Golgi apparatus.

Conclusion

Taken together, epithelial differentiation during Drosophila embryogenesis is a concerted action of ECM formation, plasma membrane remodelling and maintenance of cell polarity that all three rely mainly, if not absolutely, on the canonical secretory pathway from the ER over the Golgi apparatus to the plasma membrane. Our results indicate that COPII vesicles constitute a central hub for these processes.

Regulation of coat assembly--sorting things out at the ER

The small GTPase Sar1 resides at the core of a regulatory cycle that controls protein export from the ER in COPII vesicles. Recent advances in minimally reconstituted systems indicate continual flux of Sar1 through GTPase cycles facilitates cargo concentration into forming vesicles that ultimately bud from membranes. During export from ER membranes, this GTPase cycle is harnessed through the combinatorial power of multiple coat subunits and cargo adaptors to sort an expanding array of proteins into ER-derived vesicles. The COPII budding machinery is further organized into higher-order structures at transitional zones on the ER surface where the large multi-domain Sec16 protein appears to perform a central function.

Modifications to the C-terminus of Arf1 alter cell functions and protein interactions

Arf family proteins are approximately 21-kDa GTP-binding proteins that are critical regulators of membrane traffic and the actin cytoskeleton. Studies examining the complex signaling pathways underlying Arf action have relied on recombinant proteins comprised of Arf fused to epitope tags or proteins, such as glutathione S-transferase or green fluorescent protein, for both cell-based mammalian cell studies and bacterially expressed recombinant proteins for biochemical assays. However, the effects of such protein fusions on the biochemical properties relevant to the cellular function have been only incompletely studied at best. Here, we have characterized the effect of C-terminal tagging of Arf1 on (i) function in Saccharomyces cerevisiae, (ii) in vitro nucleotide exchange and (iii) interaction with guanine nucleotide exchange factors and GTPase-activating proteins. We found that the tagged Arfs were substantially impaired or altered in each assay, compared with the wild-type protein, and these changes are certain to alter actions in cells. We discuss the results related to the interpretation of experiments using these reagents and we propose that authors and editors consistently adopt a few simple rules for describing and discussing results obtained with Arf family members that can be readily applied to other proteins.

Vesicular calcium regulates coat retention, fusogenicity, and size of pre-Golgi intermediates

This study establishes a role for luminal Ca²⁺ in ER/Golgi transport organelles and elucidates an effector mechanism involving the EF-hand protein ALG-2 and regulation of COPII coat retention.

The significance and extent of Ca²⁺ regulation of the biosynthetic secretory pathway have been difficult to establish, and our knowledge of regulatory relationships integrating Ca²⁺ with vesicle coats and function is rudimentary. Here, we investigated potential roles and mechanisms of luminal Ca²⁺ in the early secretory pathway. Specific depletion of luminal Ca²⁺ in living normal rat kidney cells using cyclopiazonic acid (CPA) resulted in the extreme expansion of vesicular tubular cluster (VTC) elements. Consistent with this, a suppressive role for vesicle-associated Ca²⁺ in COPII vesicle homotypic fusion was demonstrated in vitro using Ca²⁺ chelators. The EF-hand–containing protein apoptosis-linked gene 2 (ALG-2), previously implicated in the stabilization of sec31 at endoplasmic reticulum exit sites, inhibited COPII vesicle fusion in a Ca²⁺-requiring manner, suggesting that ALG-2 may be a sensor for the effects of vesicular Ca²⁺ on homotypic fusion. Immunoisolation established that Ca²⁺ chelation inhibits and ALG-2 specifically favors residual retention of the COPII outer shell protein sec31 on pre-Golgi fusion intermediates. We conclude that vesicle-associated Ca²⁺, acting through ALG-2, favors the retention of residual coat molecules that seem to suppress membrane fusion. We propose that in cells, these Ca²⁺-dependent mechanisms temporally regulate COPII vesicle interactions, VTC biogenesis, cargo sorting, and VTC maturation.

Molecular analysis of 42 patients with congenital dyserythropoietic anemia type II: new mutations in the SEC23B gene and a search for a genotype-phenotype relationship

BACKGROUND

The most frequent form of congenital dyserythropoietic anemia is the type II form. Recently it was shown that the vast majority of patients with congenital dyserythropoietic anemia type II carry mutations in the SEC23B gene. Here we established the molecular basis of 42 cases of congenital dyserythropoietic anemia type II and attempted to define a genotype-phenotype relationship.

DESIGN AND METHODS

SEC23B gene sequencing analysis was performed to assess the diversity and incidence of each mutation in 42 patients with congenital dyserythropoietic anemia type II (25 described exclusively in this work), from the Italian and the French Registries, and the relationship of these mutations with the clinical presentation. To this purpose, we divided the patients into two groups: (i) patients with two missense mutations and (ii) patients with one nonsense and one missense mutation.

RESULTS

We found 22 mutations of uneven frequency, including seven novel mutations. Compound heterozygosity for a missense and a nonsense mutation tended to produce a more severe clinical presentation, a lower reticulocyte count, a higher serum ferritin level, and, in some cases, more pronounced transfusion needs, than homozygosity or compound heterozygosity for two missense mutations. Homozygosity or compound heterozygosity for two nonsense mutations was never found.

CONCLUSIONS

This study allowed us to determine the most frequent mutations in patients with congenital dyserythropoietic anemia type II. Correlations between the mutations and various biological parameters suggested that the association of one missense mutation and one nonsense mutation was significantly more deleterious that the association of two missense mutations. However, there was an overlap between the two categories.

ER exit sites--localization and control of COPII vesicle formation

The first membrane trafficking step in the biosynthetic secretory pathway, the export of proteins and lipids from the endoplasmic reticulum (ER), is mediated by COPII-coated vesicles. In mammalian cells, COPII vesicle budding occurs at specialized sites on the ER, the so-called transitional ER (tER). Here, we discuss aspects of the formation and maintenance of these sites, the mechanisms by which cargo becomes segregated within them, and the propagation of ER exit sites (ERES) during cell division. All of these features are inherently linked to the formation, maintenance and function of the Golgi apparatus underlining the importance of ERES to Golgi function and more widely in terms of intracellular organization and cellular function.

Vesicle coating and uncoating: controlling the formation of large COPII-coated carriers

The basic mechanisms underlying the formation of coated vesicles are now defined in considerable detail. This article highlights recent developments in our understanding of the problem of exporting large macromolecular cargo such as procollagen from the endoplasmic reticulum and discusses the implications that this has for cell and tissue organisation and human disease.

TANGO1 facilitates cargo loading at endoplasmic reticulum exit sites

A genome-wide screen revealed previously unidentified components required for transport and Golgi organization (TANGO). We now provide evidence that one of these proteins, TANGO1, is an integral membrane protein localized to endoplasmic reticulum (ER) exit sites, with a luminal SH3 domain and a cytoplasmic proline-rich domain (PRD). Knockdown of TANGO1 inhibits export of bulky collagen VII from the ER. The SH3 domain of TANGO1 binds to collagen VII; the PRD binds to the COPII coat subunits, Sec23/24. In this scenario, PRD binding to Sec23/24 subunits could stall COPII carrier biogenesis to permit the luminal domain of TANGO1 to guide SH3-bound cargo into a growing carrier. All cells except those of hematopoietic origin express TANGO1. We propose that TANGO1 exports other cargoes in cells that do not secrete collagen VII. However, TANGO1 does not enter the budding carrier, which represents a unique mechanism to load cargo into COPII carriers.

Accurate prediction of peptide binding sites on protein surfaces

Many important protein-protein interactions are mediated by the binding of a short peptide stretch in one protein to a large globular segment in another. Recent efforts have provided hundreds of examples of new peptides binding to proteins for which a three-dimensional structure is available (either known experimentally or readily modeled) but where no structure of the protein-peptide complex is known. To address this gap, we present an approach that can accurately predict peptide binding sites on protein surfaces. For peptides known to bind a particular protein, the method predicts binding sites with great accuracy, and the specificity of the approach means that it can also be used to predict whether or not a putative or predicted peptide partner will bind. We used known protein-peptide complexes to derive preferences, in the form of spatial position specific scoring matrices, which describe the binding-site environment in globular proteins for each type of amino acid in bound peptides. We then scan the surface of a putative binding protein for sites for each of the amino acids present in a peptide partner and search for combinations of high-scoring amino acid sites that satisfy constraints deduced from the peptide sequence. The method performed well in a benchmark and largely agreed with experimental data mapping binding sites for several recently discovered interactions mediated by peptides, including RG-rich proteins with SMN domains, Epstein-Barr virus LMP1 with TRADD domains, DBC1 with Sir2, and the Ago hook with Argonaute PIWI domain. The method, and associated statistics, is an excellent tool for predicting and studying binding sites for newly discovered peptides mediating critical events in biology.

Dynamic organization of COPII coat proteins at endoplasmic reticulum export sites in plant cells

Protein export from the endoplasmic reticulum (ER) is mediated by the accumulation of COPII proteins such as Sar1, Sec23/24 and Sec13/31 at specialized ER export sites (ERES). Although the distribution of COPII components in mammalian and yeast systems is established, a unified model of ERES dynamics has yet to be presented in plants. To investigate this, we have followed the dynamics of fluorescent fusions to inner and outer components of the coat, AtSec24 and AtSec13, in three different plant model systems: tobacco and Arabidopsis leaf epidermis, as well as tobacco BY-2 suspension cells. In leaves, AtSec24 accumulated at Golgi-associated ERES, whereas AtSec13 showed higher levels of cytosolic staining compared with AtSec24. However, in BY-2 cells, both AtSec13 and AtSec24 labelled Golgi-associated ERES, along with AtSec24. To correlate the distribution of the COPII coat with the dynamics of organelle movement, quantitative live-cell imaging analyses demonstrated that AtSec24 and AtSec13 maintained a constant association with Golgi-associated ERES, irrespective of their velocity. However, recruitment of AtSec24 and AtSec13 to ERES, as well as the number of ERES marked by these proteins, was influenced by export of membrane cargo proteins from the ER to the Golgi. Additionally, the increased availability of AtSec24 affected the distribution of AtSec13, inducing recruitment of this outer COPII coat component to ERES. These results provide a model that, in plants, protein export from the ER occurs via sequential recruitment of inner and outer COPII components to form transport intermediates at mobile, Golgi-associated ERES.

Discrete determinants in ArfGAP2/3 conferring Golgi localization and regulation by the COPI coat

From yeast to mammals, two types of GTPase-activating proteins, ArfGAP1 and ArfGAP2/3, control guanosine triphosphate (GTP) hydrolysis on the small G protein ADP-ribosylation factor (Arf) 1 at the Golgi apparatus. Although functionally interchangeable, they display little similarity outside the catalytic GTPase-activating protein (GAP) domain, suggesting differential regulation. ArfGAP1 is controlled by membrane curvature through its amphipathic lipid packing sensor motifs, whereas Golgi targeting of ArfGAP2 depends on coatomer, the building block of the COPI coat. Using a reporter fusion approach and in vitro assays, we identified several functional elements in ArfGAP2/3. We show that the Golgi localization of ArfGAP3 depends on both a central basic stretch and a carboxy-amphipathic motif. The basic stretch interacts directly with coatomer, which we found essential for the catalytic activity of ArfGAP3 on Arf1-GTP, whereas the carboxy-amphipathic motif interacts directly with lipid membranes but has minor role in the regulation of ArfGAP3 activity. Our findings indicate that the two types of ArfGAP proteins that reside at the Golgi use a different combination of protein-protein and protein-lipid interactions to promote GTP hydrolysis in Arf1-GTP.

Structural basis of cargo membrane protein discrimination by the human COPII coat machinery

Genomic analysis shows that the increased complexity of trafficking pathways in mammalian cells involves an expansion of the number of SNARE, Rab and COP proteins. Thus, the human genome encodes four forms of Sec24, the cargo selection subunit of the COPII vesicular coat, and this is proposed to increase the range of cargo accommodated by human COPII-coated vesicles. In this study, we combined X-ray crystallographic and biochemical analysis with functional assays of cargo packaging into COPII vesicles to establish molecular mechanisms for cargo discrimination by human Sec24 subunits. A conserved IxM packaging signal binds in a surface groove of Sec24c and Sec24d, but the groove is occluded in the Sec24a and Sec24b subunits. Conversely, LxxLE class transport signals and the DxE signal of VSV glycoprotein are selectively bound by Sec24a and Sec24b subunits. A comparative analysis of crystal structures of the four human Sec24 isoforms establishes the structural determinants for discrimination among these transport signals, and provides a framework to understand how an expansion of coat subunits extends the range of cargo proteins packaged into COPII-coated vesicles.

Structural basis for cargo regulation of COPII coat assembly

Using cryo-electron microscopy, we have solved the structure of an icosidodecahedral COPII coat involved in cargo export from the endoplasmic reticulum (ER) coassembled from purified cargo adaptor Sec23-24 and Sec13-31 lattice-forming complexes. The coat structure shows a tetrameric assembly of the Sec23-24 adaptor layer that is well positioned beneath the vertices and edges of the Sec13-31 lattice. Fitting the known crystal structures of the COPII proteins into the density map reveals a flexible hinge region stemming from interactions between WD40 beta-propeller domains present in Sec13 and Sec31 at the vertices. The structure shows that the hinge region can direct geometric cage expansion to accommodate a wide range of bulky cargo, including procollagen and chylomicrons, that is sensitive to adaptor function in inherited disease. The COPII coat structure leads us to propose a mechanism by which cargo drives cage assembly and membrane curvature for budding from the ER.

Efficient coupling of Sec23-Sec24 to Sec13-Sec31 drives COPII-dependent collagen secretion and is essential for normal craniofacial development

The COPII coat assembles on endoplasmic reticulum membranes to coordinate the collection of secretory cargo with the formation of transport vesicles. During COPII assembly, Sar1 deforms the membrane and recruits the Sec23-Sec24 complex (Sec23/24), which is the primary cargo-binding adaptor for the system, and Sec13-Sec31 (Sec13/31), which provides a structural outer layer for vesicle formation. Here we show that Sec13 depletion results in concomitant loss of Sec31 and juxtanuclear clustering of pre-budding complexes containing Sec23/24 and cargo. Electron microscopy reveals the presence of curved coated profiles on distended endoplasmic reticulum, indicating that Sec13/31 is not required for the generation or maintenance of the curvature. Surprisingly, export of tsO45-G-YFP, a marker of secretory cargo, is unaffected by Sec13/31 depletion; by contrast, secretion of collagen from primary fibroblasts is strongly inhibited. Suppression of Sec13 expression in zebrafish causes defects in proteoglycan deposition and skeletal abnormalities that are grossly similar to the craniofacial abnormalities of crusher mutant zebrafish and patients with cranio-lenticulo-sutural dysplasia. We conclude that efficient coupling of the inner (Sec23/24) and outer (Sec13/31) layers of the COPII coat is required to drive the export of collagen from the endoplasmic reticulum, and that highly efficient COPII assembly is essential for normal craniofacial development during embryogenesis.

Both layers of the COPII coat come into view

Anterograde transport in the early secretory pathway is mediated by COPII-coated vesicles. Stagg et al. (2008) have now visualized the double-layered COPII coat using electron cryomicroscopy, providing insight into how coats are assembled to accommodate cargo of different sizes.

Peptide-mediated interactions in biological systems: new discoveries and applications

Peptide-mediated interactions play very important roles in cellular processes. Recent years have seen much activity in the discovery of new bioactive peptides, and interactions mediated by protein-peptide binding events. At the same time, computational approaches continue to be developed that allow protein-peptide interactions to be discovered with great accuracy. There are also a growing number of chemicals that can target these interactions with various applications in disease. Both new discoveries and predictions suggest that these protein-peptide interactions play greater roles in cellular processes than previously thought. We propose that projects to uncover the protein-peptide repertoire used in Nature in a systematic way will have numerous applications in molecular biology and medicine.

Drosophila Sec16 mediates the biogenesis of tER sites upstream of Sar1 through an arginine-rich motif

tER sites are specialized cup-shaped ER subdomains characterized by the focused budding of COPII vesicles. Sec16 has been proposed to be involved in the biogenesis of tER sites by binding to COPII coat components and clustering nascent-coated vesicles. Here, we show that Drosophila Sec16 (dSec16) acts instead as a tER scaffold upstream of the COPII machinery, including Sar1. We show that dSec16 is required for Sar1-GTP concentration to the tER sites where it recruits in turn the components of the COPII machinery to initiate coat assembly. Last, we show that the dSec16 domain required for its localization maps to an arginine-rich motif located in a nonconserved region. We propose a model in which dSec16 binds ER cups via its arginine-rich domain, interacts with Sar1-GTP that is generated on ER membrane by Sec12 and concentrates it in the ER cups where it initiates the formation of COPII vesicles, thus acting as a tER scaffold.

The genetic basis of a craniofacial disease provides insight into COPII coat assembly

Proteins trafficking through the secretory pathway must first exit the endoplasmic reticulum (ER) through membrane vesicles created and regulated by the COPII coat protein complex. Cranio-lenticulo-sutural dysplasia (CLSD) was recently shown to be caused by a missense mutation in SEC23A, a gene encoding one of two paralogous COPII coat proteins. We now elucidate the molecular mechanism underlying this disease. In vitro assays reveal that the mutant form of SEC23A poorly recruits the Sec13-Sec31 complex, inhibiting vesicle formation. Surprisingly, this effect is modulated by the Sar1 GTPase paralog used in the reaction, indicating distinct affinities of the two human Sar1 paralogs for the Sec13-Sec31 complex. Patient cells accumulate numerous tubular cargo-containing ER exit sites devoid of observable membrane coat, likely representing an intermediate step in COPII vesicle formation. Our results indicate that the Sar1-Sec23-Sec24 prebudding complex is sufficient to form cargo-containing tubules in vivo, whereas the Sec13-Sec31 complex is required for membrane fission.

Assembly, organization, and function of the COPII coat

A full mechanistic understanding of how secretory cargo proteins are exported from the endoplasmic reticulum for passage through the early secretory pathway is essential for us to comprehend how cells are organized, maintain compartment identity, as well as how they selectively secrete proteins and other macromolecules to the extracellular space. This process depends on the function of a multi-subunit complex, the COPII coat. Here we describe progress towards a full mechanistic understanding of COPII coat function, including the latest findings in this area. Much of our understanding of how COPII functions and is regulated comes from studies of yeast genetics, biochemical reconstitution and single cell microscopy. New developments arising from clinical cases and model organism biology and genetics enable us to gain far greater insight in to the role of membrane traffic in the context of a whole organism as well as during embryogenesis and development. A significant outcome of such a full understanding is to reveal how the machinery and processes of membrane trafficking through the early secretory pathway fail in disease states.