ER-Golgi transport defects are associated with mutations in the Sed5p-binding domain of the COPII coat subunit, Sec24p

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.

To the Surface and Back: Exo- and Endocytic Pathways in Trypanosoma brucei

Trypanosoma brucei is one of only a few unicellular pathogens that thrives extracellularly in the vertebrate host. Consequently, the cell surface plays a critical role in both immune recognition and immune evasion. The variant surface glycoprotein (VSG) coats the entire surface of the parasite and acts as a flexible shield to protect invariant proteins against immune recognition. Antigenic variation of the VSG coat is the major virulence mechanism of trypanosomes. In addition, incessant motility of the parasite contributes to its immune evasion, as the resulting fluid flow on the cell surface drags immunocomplexes toward the flagellar pocket, where they are internalized. The flagellar pocket is the sole site of endo- and exocytosis in this organism. After internalization, VSG is rapidly recycled back to the surface, whereas host antibodies are thought to be transported to the lysosome for degradation. For this essential step to work, effective machineries for both sorting and recycling of VSGs must have evolved in trypanosomes. Our understanding of the mechanisms behind VSG recycling and VSG secretion, is by far not complete. This review provides an overview of the trypanosome secretory and endosomal pathways. Longstanding questions are pinpointed that, with the advent of novel technologies, might be answered in the near future.

Assembly of γ-secretase occurs through stable dimers after exit from the endoplasmic reticulum

γ-Secretase affects many physiological processes through targeting >100 substrates; malfunctioning links γ-secretase to cancer and Alzheimer’s disease. The spatiotemporal regulation of its stoichiometric assembly remains unresolved. Fractionation, biochemical assays, and imaging support prior formation of stable dimers in the ER, which, after ER exit, assemble into full complexes. In vitro ER budding shows that none of the subunits is required for the exit of others. However, knockout of any subunit leads to the accumulation of incomplete subcomplexes in COPII vesicles. Mutating a DPE motif in presenilin 1 (PSEN1) abrogates ER exit of PSEN1 and PEN-2 but not nicastrin. We explain this by the preferential sorting of PSEN1 and nicastrin through Sec24A and Sec24C/D, respectively, arguing against full assembly before ER exit. Thus, dimeric subcomplexes aided by Sec24 paralog selectivity support a stepwise assembly of γ-secretase, controlling final levels in post-Golgi compartments.

Phylogenetic Analysis of the vesicular fusion SNARE machinery revealing its functional divergence across Eukaryotes

Proteins of the SNARE (Soluble N-ethylmaleimide-sensitive factor attachment protein receptors) family play a significant role in all vesicular fusion events involved in endocytic and exocytic pathways. These proteins act as molecular machines that assemble into tight four-helix bundle complex, bridging the opposing membranes into close proximity forming membrane fusion. Almost all SNARE proteins share a 53 amino acid coiled-coil domain, which is mostly linked to the transmembrane domain at the C-terminal end. Despite significant variations between SNARE sequences across species, the SNARE mediated membrane fusion is evolutionary conserved in all eukaryotes. It is of interest to compare the functional divergence of SNARE proteins across various eukaryotic groups during evolution. Here, we report an exhaustive phylogeny of the SNARE proteins retrieved from SNARE database including plants, animals, fungi and protists. The Initial phylogeny segregated SNARE protein sequences into five well-supported clades Qa, Qb, Qc, Qbc and R reflective of their positions in the four-helix SNARE complex. Further to improve resolution the Qa, Qb, Qc and R family specific trees were reconstructed, each of these were further segregated into organelle specific clades at first and later diverged into lineage specific subgroups. This revealed that most of the SNARE orthologs are conserved at subcellular locations or at trafficking pathways across various species during eukaryotic evolution. The paralogous expansion in SNARE repertoire was observed at metazoans (animals) and plants independently during eukaryotic evolution. However, results also show that the multi-cellular and saprophytic fungi have limited SNAREs.

The major cellulases CBH-1 and CBH-2 of Neurospora crassa rely on distinct ER cargo adaptors for efficient ER-exit

Filamentous fungi are native secretors of lignocellulolytic enzymes and are used as protein-producing factories in the industrial biotechnology sector. Despite the importance of these organisms in industry, relatively little is known about the filamentous fungal secretory pathway or how it might be manipulated for improved protein production. Here, we use Neurospora crassa as a model filamentous fungus to interrogate the requirements for trafficking of cellulase enzymes from the endoplasmic reticulum to the Golgi. We characterized the localization and interaction properties of the p24 and ERV-29 cargo adaptors, as well as their role in cellulase enzyme trafficking. We find that the two most abundantly secreted cellulases, CBH-1 and CBH-2, depend on distinct ER cargo adaptors for efficient exit from the ER. CBH-1 depends on the p24 proteins, whereas CBH-2 depends on the N. crassa homolog of yeast Erv29p. This study provides a first step in characterizing distinct trafficking pathways of lignocellulolytic enzymes in filamentous fungi.

Two novel effectors of trafficking and maturation of the yeast plasma membrane H+ -ATPase

The endoplasmic reticulum (ER) is the entry site of proteins into the endomembrane system. Proteins exit the ER via coat protein II (COPII) vesicles in a selective manner, mediated either by direct interaction with the COPII coat or aided by cargo receptors. Despite the fundamental role of such receptors in protein sorting, only a few have been identified. To further define the machinery that packages secretory cargo and targets proteins from the ER to Golgi membranes, we used multiple systematic approaches, which revealed 2 uncharacterized proteins that mediate the trafficking and maturation of Pma1, the essential yeast plasma membrane proton ATPase. Ydl121c (Exp1) is an ER protein that binds Pma1, is packaged into COPII vesicles, and whose deletion causes ER retention of Pma1. Ykl077w (Psg1) physically interacts with Exp1 and can be found in the Golgi and coat protein I (COPI) vesicles but does not directly bind Pma1. Loss of Psg1 causes enhanced degradation of Pma1 in the vacuole. Our findings suggest that Exp1 is a Pma1 cargo receptor and that Psg1 aids Pma1 maturation in the Golgi or affects its retrieval. More generally our work shows the utility of high content screens in the identification of novel trafficking components.

Sec24 phosphorylation regulates autophagosome abundance during nutrient deprivation

Endoplasmic Reticulum (ER)-derived COPII coated vesicles constitutively transport secretory cargo to the Golgi. However, during starvation-induced stress, COPII vesicles have been implicated as a membrane source for autophagosomes, distinct organelles that engulf cellular components for degradation by macroautophagy (hereafter called autophagy). How cells regulate core trafficking machinery to fulfill dramatically different cellular roles in response to environmental cues is unknown. Here we show that phosphorylation of conserved amino acids on the membrane-distal surface of the Saccharomyces cerevisiae COPII cargo adaptor, Sec24, reprograms COPII vesicles for autophagy. We also show casein kinase 1 (Hrr25) is a key kinase that phosphorylates this regulatory surface. During autophagy, Sec24 phosphorylation regulates autophagosome number and its interaction with the C-terminus of Atg9, a component of the autophagy machinery required for autophagosome initiation. We propose that the acute need to produce autophagosomes during starvation drives the interaction of Sec24 with Atg9 to increase autophagosome abundance.

DOI:http://dx.doi.org/10.7554/eLife.21167.001

When cells experience stressful conditions, such as a shortage of nutrients, they can digest their own material via a ‘self-eating’ process called autophagy and then recycle the products for further use. When autophagy is triggered, a new membrane structure called the autophagosome forms within the cell as it engulfs the material that is to be digested. The autophagosome delivers these materials to a compartment where they are broken down into smaller parts and the resulting raw materials are reused as needed.

The membranes that make up the autophagosome are derived from other membranes within the cell. These include small membrane-bound compartments called vesicles, which carry proteins from one part of the cell to another, or to the outside of the cell. COPII vesicles, for example, carry out the first transport step in the pathway that leads out of the cell – the so-called secretory pathway. Recently it was found that, when cells are starving, COPII vesicles can be diverted to the autophagy pathway and provide a source of membrane to build the autophagosome. However, it was not understood how the membrane of a COPII vesicle is reprogrammed so that it can interact with the cellular machinery that builds autophagosomes.

Using genetic and biochemical methods, Davis et al. have now teased apart the distinct roles of COPII vesicles in autophagy and the secretory pathway in budding yeast. The results show that a protein called Sec24, a component of the coat on the vesicles, interacts with another protein called Atg9, which is needed for the first steps of autophagosome formation. Davis et al. observed that Sec24 could be modified by the attachment of phosphate groups at a distinct site on the surface of Sec24. This modification promotes Sec24 to interact with Atg9 and increases the number of autophagosomes that form when cells are starving. Davis et al. also found that the enzyme casein kinase 1 is one of the enzymes responsible for attaching phosphate groups to Sec24.

Following on from this work, it will be important to test whether modification of vesicle coat proteins is a widespread mechanism for reprogramming membranes for different uses in other situations as well.

DOI:http://dx.doi.org/10.7554/eLife.21167.002

Sec24C/D-isoform-specific sorting of the preassembled ER-Golgi Q-SNARE complex

SNAREs are incorporated into COPII vesicles by direct interaction with Sec24. In mammals, Sec24 isoforms recruit either Sec22b or the Q-SNARE complex comprising Syntaxin5, GS27, and Bet1. Analysis of immunoisolated COPII vesicles and intracellular localization of Sec24 isoforms indicates that all ER-Golgi SNAREs are present on the same vesicles.

Secretory proteins are exported from the endoplasmic reticulum in COPII vesicles. SNARE proteins—core machinery for membrane fusion—are incorporated into COPII vesicles by direct interaction with Sec24. Here we report a novel mechanism for sorting of the ER–Golgi Q-SNAREs into COPII vesicles. Different mammalian Sec24 isoforms recruit either the R-SNARE Sec22b or the Q-SNAREs Syntaxin5, GS27, and Bet1. Syntaxin5 is the only Q-SNARE that directly interacts with Sec24C, requiring its “open” conformation. Mutation within the IxM cargo-binding site of Sec24C led to a drastic reduction in sorting of all three Q-SNAREs into COPII vesicles, implying their ER export as a preassembled complex. Analysis of immunoisolated COPII vesicles and intracellular localization of Sec24 isoforms indicate that all ER–Golgi SNAREs are present on the same vesicle. Combined with existing data, our findings yield a general concept of how Sec24 isoforms can recruit fusogenic SNARE subunits to keep them functionally apart and thus prime mammalian COPII vesicles for homotypic fusion.

Multiple ER-Golgi SNARE transmembrane domains are dispensable for trafficking but required for SNARE recycling

In biochemical assays, the transmembrane domains of SNAREs have been found to be crucial in mediating membrane fusion. Yeast cells in which a particular essential ER–Golgi SNARE’s transmembrane domain has been removed remain viable and devoid of forward-trafficking defects.

The formation of soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) complexes between opposing membranes is an essential prerequisite for fusion between vesicles and their target compartments. The composition and length of a SNARE’s transmembrane domain (TMD) is also an indicator for their steady-state distribution in cells. The evolutionary conservation of the SNARE TMD, together with the strict requirement of this feature for membrane fusion in biochemical studies, implies that the TMD represents an essential protein module. Paradoxically, we find that for several essential ER- and Golgi-localized SNAREs, a TMD is unnecessary. Moreover, in the absence of a covalent membrane tether, such SNAREs can still support ER–Golgi vesicle transport and recapitulate established genetic interactions. Transport anomalies appear to be restricted to retrograde trafficking, but these defects are overcome by the attachment of a C-terminal lipid anchor to the SNARE. We conclude that the TMD functions principally to support the recycling of Qb-, Qc-, and R-SNAREs and, in so doing, retrograde transport.

Bax Inhibitor-1 regulates hepatic lipid accumulation via ApoB secretion

In this study, we explored the effects of Bax Inhibitor-1 (BI-1) on ApoB aggregation in high-fat diet (HFD)-induced hepatic lipid accumulation. After 1 week on a HFD, triglycerides and cholesterol accumulated more in the liver and were not effectively secreted into the plasma, whereas after 8 weeks, lipids were highly accumulated in both the liver and plasma, with a greater effect in BI-1 KO mice compared with BI-1 WT mice. ApoB, a lipid transfer protein, was accumulated to a greater extent in the livers of HFD-BI-1 KO mice compared with HFD-BI-1 WT mice. Excessive post-translational oxidation of protein disulfide isomerase (PDI), intra-ER ROS accumulation and folding capacitance alteration were also observed in HFD-BI-1 KO mice. Higher levels of endoplasmic reticulum (ER) stress were consistently observed in KO mice compared with the WT mice. Adenovirus-mediated hepatic expression of BI-1 in the BI-1 KO mice rescued the above phenotypes. Our results suggest that BI-1-mediated enhancement of ApoB secretion regulates hepatic lipid accumulation, likely through regulation of ER stress and ROS accumulation.

Sec24 is a coincidence detector that simultaneously binds two signals to drive ER export

BACKGROUND

Incorporation of secretory proteins into ER-derived vesicles involves recognition of cytosolic signals by the COPII coat protein, Sec24. Additional cargo diversity is achieved through cargo receptors, which include the Erv14/Cornichon family that mediates export of transmembrane proteins despite the potential for such clients to directly interact with Sec24. The molecular function of Erv14 thus remains unclear, with possible roles in COPII binding, membrane domain chaperoning, and lipid organization.

RESULTS

Using a targeted mutagenesis approach to define the mechanism of Erv14 function, we identify conserved residues in the second transmembrane domain of Erv14 that mediate interaction with a subset of Erv14 clients. We further show that interaction of Erv14 with a novel cargo-binding surface on Sec24 is necessary for efficient trafficking of all of its clients. However, we also determine that some Erv14 clients also directly engage an adjacent cargo-binding domain of Sec24, suggesting a novel mode of dual interaction between cargo and coat.

CONCLUSIONS

We conclude that Erv14 functions as a canonical cargo receptor that couples membrane proteins to the COPII coat, but that maximal export requires a bivalent signal that derives from motifs on both the cargo protein and Erv14. Sec24 can thus be considered a coincidence detector that binds simultaneously to multiple signals to drive packaging of polytopic membrane proteins. This mode of dual signal binding to a single coat protein might serve as a general mechanism to trigger efficient capture, or may be specifically employed in ER export to control deployment of nascent proteins.

The back and forth of cargo exit from the endoplasmic reticulum

Endoplasmic reticulum (ER) to Golgi trafficking is an essential step in sorting mature, correctly folded, processed and assembled proteins (cargo) from immature proteins and ER-resident proteins. However, the mechanisms governing trafficking selectivity, specificity and regulation are not yet fully understood. To date, three complementary mechanisms have been described that enable regulation of this trafficking step: ER retention of immature proteins in the ER; selective uptake of fully mature proteins into Golgi-bound vesicles; and retrieval from the Golgi of immature cargo that has erroneously exited the ER. Together, these three mechanisms allow incredible specificity and enable the cell to carry out protein quality control and regulate protein processing, oligomerization and expression. This review will focus on the current knowledge of selectivity mechanisms acting during the ER-to-Golgi sorting step and their significance in health and disease. The review will also highlight several key questions that have remained unanswered and discuss the future frontiers.

The B7-1 cytoplasmic tail enhances intracellular transport and mammalian cell surface display of chimeric proteins in the absence of a linear ER export motif

Membrane-tethered proteins (mammalian surface display) are increasingly being used for novel therapeutic and biotechnology applications. Maximizing surface expression of chimeric proteins on mammalian cells is important for these applications. We show that the cytoplasmic domain from the B7-1 antigen, a commonly used element for mammalian surface display, can enhance the intracellular transport and surface display of chimeric proteins in a Sar1 and Rab1 dependent fashion. However, mutational, alanine scanning and deletion analysis demonstrate the absence of linear ER export motifs in the B7 cytoplasmic domain. Rather, efficient intracellular transport correlated with the presence of predicted secondary structure in the cytoplasmic tail. Examination of the cytoplasmic domains of 984 human and 782 mouse type I transmembrane proteins revealed that many previously identified ER export motifs are rarely found in the cytoplasmic tail of type I transmembrane proteins. Our results suggest that efficient intracellular transport of B7 chimeric proteins is associated with the structure rather than to the presence of a linear ER export motif in the cytoplasmic tail, and indicate that short (less than ~ 10-20 amino acids) and unstructured cytoplasmic tails should be avoided to express high levels of chimeric proteins on mammalian cells.

Redundant function of two Arabidopsis COPII components, AtSec24B and AtSec24C, is essential for male and female gametogenesis

Anterograde vesicle transport from the endoplasmic reticulum to the Golgi apparatus is the start of protein transport through the secretory pathway, in which the transport is mediated by coat protein complex II (COPII)-coated vesicles. Therefore, most proteins synthesized on the endoplasmic reticulum are loaded as cargo into COPII vesicles. The COPII is composed of the small GTPase Sar1 and two types of protein complexes (Sec23/24 and Sec13/31). Of these five COPII components, Sec24 is thought to recognize cargo that is incorporated into COPII vesicles by directly interacting with the cargo. The Arabidopsis genome encodes three types of Sec24 homologs (AtSec24A, AtSec24B, and AtSec24C). The subcellular dynamics and function of AtSec24A have been characterized. The intracellular distributions and functions of other AtSec24 proteins are not known, and the functional differences among the three AtSec24s remain unclear. Here, we found that all three AtSec24s were expressed in similar parts of the plant body and showed the same subcellular localization pattern. AtSec24B knockout plant, but not AtSec24C knockdown plant, showed mild male sterility with reduction of pollen germination. Significant decrease of AtSec24B and AtSec24C expression affected male and female gametogenesis in Arabidopsis thaliana. Our results suggested that the redundant function of AtSec24B and AtSec24C is crucial for the development of plant reproductive cells. We propose that the COPII transport is involved in male and female gametogenesis in planta.

Secretory protein biogenesis and traffic in the early secretory pathway

The secretory pathway is responsible for the synthesis, folding, and delivery of a diverse array of cellular proteins. Secretory protein synthesis begins in the endoplasmic reticulum (ER), which is charged with the tasks of correctly integrating nascent proteins and ensuring correct post-translational modification and folding. Once ready for forward traffic, proteins are captured into ER-derived transport vesicles that form through the action of the COPII coat. COPII-coated vesicles are delivered to the early Golgi via distinct tethering and fusion machineries. Escaped ER residents and other cycling transport machinery components are returned to the ER via COPI-coated vesicles, which undergo similar tethering and fusion reactions. Ultimately, organelle structure, function, and cell homeostasis are maintained by modulating protein and lipid flux through the early secretory pathway. In the last decade, structural and mechanistic studies have added greatly to the strong foundation of yeast genetics on which this field was built. Here we discuss the key players that mediate secretory protein biogenesis and trafficking, highlighting recent advances that have deepened our understanding of the complexity of this conserved and essential process.

Disruption of the Sec24d gene results in early embryonic lethality in the mouse

Transport of newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi is mediated by the coat protein complex COPII. The inner coat of COPII is assembled from heterodimers of SEC23 and SEC24. Though mice with mutations in one of the four Sec24 paralogs, Sec24b, exhibit a neural tube closure defect, deficiency in humans or mice has not yet been described for any of the other Sec24 paralogs. We now report characterization of mice with targeted disruption of Sec24d. Early embryonic lethality is observed in mice completely deficient in SEC24D, while a hypomorphic Sec24d allele permits survival to mid-embryogenesis. Mice haploinsufficient for Sec24d exhibit no phenotypic abnormality. A BAC transgene containing Sec24d rescues the embryonic lethality observed in Sec24d-null mice. These results demonstrate an absolute requirement for SEC24D expression in early mammalian development that is not compensated by the other three Sec24 paralogs. The early embryonic lethality resulting from loss of SEC24D in mice contrasts with the previously reported mild skeletal phenotype of SEC24D deficiency in zebrafish and restricted neural tube phenotype of SEC24B deficiency in mice. Taken together, these observations suggest that the multiple Sec24 paralogs have developed distinct functions over the course of vertebrate evolution.

Sec24p and Sec16p cooperate to regulate the GTP cycle of the COPII coat

Vesicle budding from the endoplasmic reticulum (ER) employs a cycle of GTP binding and hydrolysis to regulate assembly of the COPII coat. We have identified a novel mutation (sec24-m11) in the cargo-binding subunit, Sec24p, that specifically impacts the GTP-dependent generation of vesicles in vitro. Using a high-throughput approach, we defined genetic interactions between sec24-m11 and a variety of trafficking components of the early secretory pathway, including the candidate COPII regulators, Sed4p and Sec16p. We defined a fragment of Sec16p that markedly inhibits the Sec23p- and Sec31p-stimulated GTPase activity of Sar1p, and demonstrated that the Sec24p-m11 mutation diminished this inhibitory activity, likely by perturbing the interaction of Sec24p with Sec16p. The consequence of the heightened GTPase activity when Sec24p-m11 is present is the generation of smaller vesicles, leading to accumulation of ER membranes and more stable ER exit sites. We propose that association of Sec24p with Sec16p creates a novel regulatory complex that retards the GTPase activity of the COPII coat to prevent premature vesicle scission, pointing to a fundamental role for GTP hydrolysis in vesicle release rather than in coat assembly/disassembly.

Functional anatomy of the Arabidopsis cytokinesis-specific syntaxin KNOLLE

In plant cytokinesis, Golgi/trans-Golgi network-derived vesicles are targeted to the plane of cell division where they fuse with one another to form the partitioning membrane (cell plate). This membrane fusion requires a specialised syntaxin (Qa-SNARE), named KNOLLE in Arabidopsis. KNOLLE is only made during the M-phase of the cell cycle, targeted to the plane of cell division and degraded in the vacuole at the end of cytokinesis. To identify the parts of KNOLLE required for proper targeting and function in membrane fusion, we generated chimeric syntaxins comprising complementary fragments from KNOLLE and MVB-localized PEP12 (SYP21). Surprisingly, targeting of the chimeric protein was not specified by the C-terminal membrane anchor. Rather the N-terminal region including helix Ha and the adjacent linker to helix Hb appeared to played a critical role. However, deletion of this N-terminal fragment from KNOLLE (KN(Δ1-82) ) had the same effect as its presence in the chimeric protein (KN(1-82) -PEP12(64-279) ), suggesting that targeting to the plane of cell division occurs by default, i.e. when no sorting signal would target the syntaxin to a specific endomembrane compartment. Once the full-length syntaxin accumulated at the plane of division, phenotypic rescue of the knolle mutant only required the SNARE domain plus the adjacent linker connecting helix Hc to the SNARE domain from KNOLLE. Our results suggest that targeting of syntaxin to the plane of cell division occurs without active sorting, whereas syntaxin-mediated membrane fusion requires sequence-specific features.

Evidence for the involvement of the Arabidopsis SEC24A in male transmission

Eukaryotic cells use COPII-coated carriers for endoplasmic reticulum (ER)-to-Golgi protein transport. Selective cargo capture into ER-derived carriers is largely driven by the SEC24 component of the COPII coat. The Arabidopsis genome encodes three AtSEC24 genes with overlapping expression profiles but it is yet to be established whether the AtSEC24 proteins have overlapping roles in plant growth and development. Taking advantage of Arabidopsis thaliana as a model plant system for studying gene function in vivo, through reciprocal crosses, pollen characterization, and complementation tests, evidence is provided for a role for AtSEC24A in the male gametophyte. It is established that an AtSEC24A loss-of-function mutation is tolerated in the female gametophyte but that it causes defects in pollen leading to failure of male transmission of the AtSEC24A mutation. These data provide a characterization of plant SEC24 family in planta showing incompletely overlapping functions of the AtSEC24 isoforms. The results also attribute a novel role to SEC24 proteins in a multicellular model system, specifically in male fertility.

Differential selection of Golgi proteins by COPII Sec24 isoforms in procyclic Trypanosoma brucei

The Sec24 subunit of the coat protein complex II (COPII) has been implicated in selecting newly synthesized cargo from the endoplasmic reticulum (ER) for delivery to the Golgi. The protozoan parasite, Trypanosoma brucei, contains two paralogs, TbSec24.1 and TbSec24.2, which were depleted using RNA interference in the insect form of the parasite. Depletion of either TbSec24.1 or TbSec24.2 resulted in growth arrest and modest inhibition of anterograde transport of the putative Golgi enzyme, TbGntB, and the secretory marker, BiPNAVRG-HA9. In contrast, depletion of TbSec24.1, but not TbSec24.2, led to reversible mislocalization of the Golgi stack proteins, TbGRASP and TbGolgin63. The latter accumulated in the ER. The localization of the COPI coatomer subunit, TbεCOP, and the trans Golgi network (TGN) protein, TbGRIP70, was largely unaffected, although the latter was preferentially lost from those Golgi that were not associated with the bilobe, a structure previously implicated in Golgi biogenesis. Together, these data suggest that TbSec24 paralogs can differentiate among proteins destined for the Golgi.

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.

Genetic analysis of yeast Sec24p mutants suggests cargo binding is not co-operative during ER export

Many eukaryotic secretory proteins are selected for export from the endoplasmic reticulum (ER) through their interaction with the Sec24p subunit of the coat protein II (COPII) coat. Three distinct cargo-binding sites on yeast Sec24p have been described by biochemical, genetic and structural studies. Each site recognizes a limited set of peptide motifs or a folded structural domain, however, the breadth of cargo recognized by a given site and the dynamics of cargo engagement remain poorly understood. We aimed to gain further insight into the broader molecular function of one of these cargo-binding sites using a non-biased genetic approach. We exploited the in vivo lethality associated with mutation of the Sec24p B-site to identify genes that suppress this phenotype when overexpressed. We identified SMY2 as a general suppressor that rescued multiple defects in Sec24p, and SEC22 as a specific suppressor of two adjacent cargo-binding sites, raising the possibility of allosteric regulation of these domains. We generated a novel set of mutations in Sec24p that distinguish these two sites and examined the ability of Sec22p to rescue these mutations. Our findings suggest that co-operativity does not influence cargo capture at these sites, and that Sec22p rescue occurs via its function as a retrograde SNARE.

Blocking ER export of the Golgi SNARE SYP31 affects plant growth

We recently identified a novel and transplantable di-acidic motif (EXXD) that facilitates ER export of the Golgi syntaxin SYP31 (type IV protein) and which may function also for type I and type II proteins in plants. By mutagenesis of Arabidopsis thaliana SYP31 and live cell imaging experiments in tobacco leaf epidermal cells, we determined that replacing the MELAD sequence of SYP31 with gagag retained SYP31 in the ER, which demonstrates that the di-acidic motif ELAD is critical for SYP31 ER export. To investigate whether blockage of a Golgi SNARE in the ER have consequences for plant growth, we produced tobacco plants stably overexpressing either the wild type MELAD or the mutant gagag form of SYP31. Whereas tobacco plants overexpressing the wild-type SYP31 developed to set seed, tobacco plants overexpressing the mutant form gagag rapidly became chlorotic, ceased their growth and invariably died after several weeks. This indicated that retention of overexpressed SYP31 in the ER is likely toxic for the secretory pathway and, therefore, plant development. Putative explanations for this observation are discussed taking into account SNARE properties and possible interactions.

Streamlined architecture and glycosylphosphatidylinositol-dependent trafficking in the early secretory pathway of African trypanosomes

The variant surface glycoprotein (VSG) of bloodstream form Trypanosoma brucei (Tb) is a critical virulence factor. The VSG glycosylphosphatidylinositol (GPI)-anchor strongly influences passage through the early secretory pathway. Using a dominant-negative mutation of TbSar1, we show that endoplasmic reticulum (ER) exit of secretory cargo in trypanosomes is dependent on the coat protein complex II (COPII) machinery. Trypanosomes have two orthologues each of the Sec23 and Sec24 COPII subunits, which form specific heterodimeric pairs: TbSec23.1/TbSec24.2 and TbSec23.2/TbSec24.1. RNA interference silencing of each subunit is lethal but has minimal effects on trafficking of soluble and transmembrane proteins. However, silencing of the TbSec23.2/TbSec24.1 pair selectively impairs ER exit of GPI-anchored cargo. All four subunits colocalize to one or two ER exit sites (ERES), in close alignment with the postnuclear flagellar adherence zone (FAZ), and closely juxtaposed to corresponding Golgi clusters. These ERES are nucleated on the FAZ-associated ER. The Golgi matrix protein Tb Golgi reassembly stacking protein defines a region between the ERES and Golgi, suggesting a possible structural role in the ERES:Golgi junction. Our results confirm a selective mechanism for GPI-anchored cargo loading into COPII vesicles and a remarkable degree of streamlining in the early secretory pathway. This unusual architecture probably maximizes efficiency of VSG transport and fidelity in organellar segregation during cytokinesis.

A novel di-acidic motif facilitates ER export of the syntaxin SYP31

It is generally accepted that ER protein export is largely influenced by the transmembrane domain (TMD). The situation is unclear for membrane-anchored proteins such as SNAREs, which are anchored to the membrane by their TMD at the C-terminus. For example, in plants, Sec22 and SYP31 (a yeast Sed5 homologue) have a 17 aa TMD but different locations (ER/Golgi and Golgi), indicating that TMD length alone is not sufficient to explain their targeting. To establish the identity of factors that influence SNARE targeting, mutagenesis and live cell imaging experiments were performed on SYP31. It was found that deletion of the entire N-terminus domain of SYP31 blocked the protein in the ER. Several deletion mutants of different parts of this N-terminus domain indicated that a region between the SNARE helices Hb and Hc is required for Golgi targeting. In this region, replacement of the aa sequence MELAD by GAGAG or MALAG retained the protein in the ER, suggesting that MELAD may function as a di-acidic ER export motif EXXD. This suggestion was further verified by replacing the established di-acidic ER export motif DLE of a type II Golgi protein AtCASP and a membrane-anchored type I chimaera, TMcCCASP, by MELAD or GAGAG. The MELAD motif allowed the proteins to reach the Golgi, whereas the motif GAGAG was found to be insufficient to facilitate ER protein export. Our analyses indicate that we have identified a novel and transplantable di-acidic motif that facilitates ER export of SYP31 and may function for type I and type II proteins in plants.

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.

Smy2p participates in COPII vesicle formation through the interaction with Sec23p/Sec24p subcomplex

The coat protein complex II (COPII) is essential for vesicle formation from the endoplasmic reticulum (ER) and is composed of two heterodimeric subcomplexes, Sec23p/Sec24p and Sec13p/Sec31p, and the small guanosine triphosphatase Sar1p. In an effort to identify novel factors that may participate in COPII vesicle formation, we isolated SMY2, a yeast gene encoding a protein of unknown function, as a multicopy suppressor of the temperature-sensitive sec24-20 mutant. We found that even a low-copy expression of SMY2 was sufficient for the suppression of the sec24-20 phenotypes, and the chromosomal deletion of SMY2 led to a severe growth defect in the sec24-20 background. In addition, SMY2 exhibited genetic interactions with several other genes involved in the ER-to-Golgi transport. Subcellular fractionation analysis showed that Smy2p was a peripheral membrane protein fractionating together with COPII components. However, Smy2p was not loaded onto COPII vesicles generated in vitro. Interestingly, coimmunoprecipitation between Smy2p and the Sec23p/Sec24p subcomplex was specifically observed in sec23-1 and sec24-20 backgrounds, suggesting that this interaction was a prerequisite for the suppression of the sec24-20 phenotypes by overexpression of SMY2. We propose that Smy2p is located on the surface of the ER and facilitates COPII vesicle formation through the interaction with Sec23p/Sec24p subcomplex.

Elizabeth Miller: Sleuthing the details of the secretory pathway. Interviewed by Caitlin Sedwick

Liz Miller uses biochemistry and chemical and genetic screens in yeast to probe the mechanisms of intracellular protein transport.

The Transport Signal on Sec22 for Packaging into COPII-Coated Vesicles Is a Conformational Epitope

The mechanism of cargo concentration into ER-derived vesicles involves interactions between the COPII vesicular coat complex and cargo transport signals--peptide sequences of 10-15 residues. The SNARE protein Sec22 contains a signal that binds the COPII subcomplex Sec23/24 and specifies its endoplasmic reticulum (ER) exit as an unassembled SNARE. The 200 kDa crystal structure of Sec22 bound to Sec23/24 reveals that the transport signal is a folded epitope rather than a conventional short peptide sequence. The NIE segment of the SNARE motif folds against the N-terminal longin domain, and this closed form of Sec22 binds at the Sec23/24 interface. Thus, COPII recognizes unassembled Sec22 via a folded epitope, whereas Sec22 assembly into SNARE complexes would mask the NIE segment. The concept of a conformational epitope as a transport signal suggests packaging mechanisms in which a coat is sensitive to the folded state of a cargo protein or the assembled state of a multiprotein complex.

Liver fatty acid-binding protein initiates budding of pre-chylomicron transport vesicles from intestinal endoplasmic reticulum

The rate-limiting step in the transit of absorbed dietary fat across the enterocyte is the generation of the pre-chylomicron transport vesicle (PCTV) from the endoplasmic reticulum (ER). This vesicle does not require coatomer-II (COPII) proteins for budding from the ER membrane and contains vesicle-associated membrane protein 7, found in intestinal ER, which is a unique intracellular location for this SNARE protein. We wished to identify the protein(s) responsible for budding this vesicle from ER membranes in the absence of the requirement for COPII proteins. We chromatographed rat intestinal cytosol on Sephacryl S-100 and found that PCTV budding activity appeared in the low molecular weight fractions. Additional chromatographic steps produced a single major and several minor bands on SDS-PAGE. By tandem mass spectroscopy, the bands contained both liver and intestinal fatty acid-binding proteins (L- and I-FABP) as well as four other proteins. Recombinant proteins for each of the six proteins identified were tested for PCTV budding activity; only L-FABP and I-FABP (23% the activity of L-FABP) were active. The vesicles generated by L-FABP were sealed, contained apolipoproteins B48 and AIV, were of the same size as PCTV on Sepharose CL-6B, and by electron microscopy, excluded calnexin and calreticulin but did not fuse with cis-Golgi nor did L-FABP generate COPII-dependent vesicles. Gene-disrupted L-FABP mouse cytosol had 60% the activity of wild type mouse cytosol. We conclude that L-FABP can select cargo for and bud PCTV from intestinal ER membranes.

Genetic and molecular interactions of the Erv41p-Erv46p complex involved in transport between the endoplasmic reticulum and Golgi complex

Erv41p and Erv46p are integral membrane proteins conserved across species. They were originally identified as abundant constituents of COPII-coated vesicles, and form a complex which cycles between the endoplasmic reticulum and Golgi complex. Yeast strains lacking these proteins are viable but display subtle secretory phenotypes. In order to obtain information about possible biological roles of this protein complex in endoplasmic reticulum to Golgi transport, we employed the Synthetic Genetic Array approach to screen for synthetic genetic interactions with the erv46 null mutation. We identified synthetic interactions with vma12, vma21, vma22 and vps1 deletion mutations. The vma21Δ mutation exacerbates transport defects caused by the erv46Δ mutation. Unexpectedly, yeast strains lacking Vma21p fail to sort the endoplasmic reticulum to Golgi v-SNARE, Bos1p, efficiently into COPII vesicles, yet these vesicles are fully fusion competent. In addition, we set out to identify, by a biochemical approach, proteins interacting with the Erv41p-Erv46p complex. We report a strong interaction between the Erv41p-Erv46p complex and endoplasmic reticulum glucosidase II. Strains lacking a cycling Erv41p-Erv46p complex display a mild glycoprotein processing defect.