The journey of STING: Guiding immune signaling through membrane trafficking

Stimulator of Interferon Genes (STING) serves as a pivotal mediator in the innate immune signaling pathway, transducing signals from various DNA receptors and playing a crucial role in natural immune processes. During cellular quiescence, STING protein resides in the endoplasmic reticulum (ER), and its activation typically occurs through the cGAS-STING signaling pathway. Upon activation, STING protein is transported to the Golgi apparatus, thereby initiating downstream signaling cascades. Vesicular transport serves as the primary mechanism for STING protein trafficking between the ER and Golgi apparatus, with COPII mediating anterograde transport from the ER to Golgi apparatus, while COPI is responsible for retrograde transport. Numerous factors influence these transport processes, thereby exerting either promoting or inhibitory effects on STING protein expression. Upon reaching the Golgi apparatus, to prevent over-activation, STING protein is transported to post-Golgi compartments for degradation. In addition to the conventional lysosomal degradation pathway, ESCRT has also been identified as one of the degradation pathways for STING protein. This review summarizes the recent findings on the membrane trafficking pathways of STING, highlighting their contributions to the regulation of cytokine production, the activation of immune cells, and the coordination of immune signaling pathways.

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.

The Role of BmTMED6 in Female Reproduction in Silkworm, Bombyx mori

Transmembrane emp24 domain (TMED) proteins have been extensively studied in mammalian embryonic development, immune regulation, and signal transduction. However, their role in insects, apart from Drosophila melanogaster, remains largely unexplored. Our previous study demonstrated the abundant expression of BmTMED6 across all stages and tissues of the silkworm. In this study, we investigate the function of BmTMED6 in reproduction. We observe significant differences in the expression of BmTMED6 between male and female silkworms, particularly in the head and fatboby, during the larval stage. Furthermore, qRT-PCR and WB analysis reveal substantial variation in BmTMED6 levels in the ovaries during pupal development, suggesting a potential association with silkworm female reproduction. We find that reducing TMED6 expression significantly decreases the number of eggs laid by female moths, leading to an accumulation of unlaid eggs in the abdomen. Moreover, downregulation of BmTMED6 leads to a decrease in the expression of BmDop2R1 and BmDop2R2, while overexpression of BmTMED6 in vitro has the opposite effect. These indicate that BmTMED6 plays a role in oviposition in female moths, potentially through the dopamine signaling pathway. This study provides a new regulatory mechanism for female reproduction in insects.

Genetic characterization of C. elegans TMED genes

BACKGROUND

p24/transmembrane Emp24 domain (TMED) proteins are a set of evolutionarily conserved, single pass transmembrane proteins that have been shown to facilitate protein secretion and selection of cargo proteins to transport vesicles in the cellular secretion pathway. However, their functions in animal development are incompletely understood.

RESULTS

The C. elegans genome encodes eight identified TMED genes, with at least one member from each defined subfamily (α, β, γ, δ). TMED gene mutants exhibit a shared set of defects in embryonic viability, animal movement, and vulval morphology. Two γ subfamily genes, tmed-1 and tmed-3, exhibit the ability to compensate for each other, as defects in movement and vulva morphology are only apparent in double mutants. TMED mutants also exhibit a delay in breakdown of basement membrane during vulva development.

CONCLUSIONS

The results establish a genetic and experimental framework for the study of TMED gene function in C. elegans, and argue that a functional protein from each subfamily is important for a shared set of developmental processes. A specific function for TMED genes is to facilitate breakdown of the basement membrane between the somatic gonad and vulval epithelial cells, suggesting a role for TMED proteins in tissue reorganization during animal development.

The many hats of transmembrane emp24 domain protein TMED9 in secretory pathway homeostasis

The secretory pathway is an intracellular highway for the vesicular transport of newly synthesized proteins that spans the endoplasmic reticulum (ER), Golgi, lysosomes and the cell surface. A variety of cargo receptors, chaperones, and quality control proteins maintain the smooth flow of cargo along this route. Among these is vesicular transport protein TMED9, which belongs to the p24/transmembrane emp24 domain (TMED) family of proteins, and is expressed across vertebrate species. The TMED family is comprised of structurally-related type I transmembrane proteins with a luminal N-terminal Golgi-dynamics domain, a luminal coiled-coil domain, a transmembrane domain and a short cytosolic C-terminal tail that binds COPI and COPII coat proteins. TMED9, like other members of the TMED family, was first identified as an abundant constituent of the COPI and COPII coated vesicles that mediate traffic between the ER and the Golgi. TMED9 is typically purified in hetero-oligomers together with TMED family members, suggesting that it may function as part of a complex. Recently, TMED family members have been discovered to play various roles in secretory pathway homeostasis including secreted protein processing, quality control and degradation of misfolded proteins, and post-Golgi trafficking. In particular, TMED9 has been implicated in autophagy, lysosomal sorting, viral replication and cancer, which we will discuss in this Mini-Review.

RNA aptamers specific for transmembrane p24 trafficking protein 6 and Clusterin for the targeted delivery of imaging reagents and RNA therapeutics to human β cells

The ability to detect and target β cells in vivo can substantially refine how diabetes is studied and treated. However, the lack of specific probes still hampers a precise characterization of human β cell mass and the delivery of therapeutics in clinical settings. Here, we report the identification of two RNA aptamers that specifically and selectively recognize mouse and human β cells. The putative targets of the two aptamers are transmembrane p24 trafficking protein 6 (TMED6) and clusterin (CLUS). When given systemically in immune deficient mice, these aptamers recognize the human islet graft producing a fluorescent signal proportional to the number of human islets transplanted. These aptamers cross-react with endogenous mouse β cells and allow monitoring the rejection of mouse islet allografts. Finally, once conjugated to saRNA specific for X-linked inhibitor of apoptosis (XIAP), they can efficiently transfect non-dissociated human islets, prevent early graft loss, and improve the efficacy of human islet transplantation in immunodeficient in mice.

A gold revision of the Golgi Dynamics (GOLD) domain structure and associated cell functionalities

The classical secretory pathway is the key membrane-based delivery system in eukaryotic cells. Several families of proteins involved in the secretory pathway, with functionalities going from cargo sorting receptors to the maintenance and dynamics of secretory organelles, share soluble globular domains predicted to mediate protein-protein interactions. One of them is "Golgi Dynamics" (GOLD) domain, named after its strong association with the Golgi apparatus. There are many GOLD-containing protein families, such as the Transmembrane emp24 domain-containing proteins (TMED/p24 family), animal SEC14-like proteins, Human Golgi resident protein ACBD3, a splice variant of TICAM2 called TRAM with GOLD domain and FYCO1. Here, we critically review the state-of-the-art knowledge of the structures and functions of the main representatives of GOLD-containing proteins in vertebrates. We provide the first unified description of the GOLD domain structure across different families since the first high-resolution structure was determined. With a brand-new update on the definition of the GOLD domain, we also discuss how its tertiary structure fits the β-sandwich-like fold map and give exciting new directions for forthcoming studies.

Differential use of p24 family members as cargo receptors for the transport of glycosylphosphatidylinositol-anchored proteins and Wnt1

Complexes of p24 proteins act as cargo receptors for the transport of COPII vesicles from the endoplasmic reticulum. The major cargos of p24 complexes are hydrophilic proteins tethered to the endoplasmic reticulum membrane via a covalently attached glycosylphosphatidylinositol (GPI) or fatty acid. Each p24 complex is known to contain members from all four p24 subfamilies (p24α, p24β, p24γ, and p24δ). However, it remains unclear how the cargo specificities of p24 complexes are influenced by member stoichiometry. Here, we report the subunit compositions of mammalian p24 complexes involved in the transport of GPI-anchored proteins and Wnt1. We show that at least one p24α is required for the formation of p24 complexes, and that a p24 complex consisting of p24α2, p24β1, p24γ2, and p24δ1 is required for the efficient transport of GPI-anchored proteins. On the other hand, a p24 complex containing p24α2, p24α3, p24β1, p24γ, and p24δ1 is involved in the transport of Wnt1. Further, interactions between p24α2 and p24α3 are critical for Wnt1 transport. Thus, p24α and p24γ subfamily members are important for cargo selectivity. Lastly, our data fit with an octamer, rather than a tetramer, model of p24 complexes, where each complex consists of two proteins from each p24 subfamily.

Structural and thermodynamic analyses of human TMED1 (p24γ1) Golgi dynamics

The transmembrane emp24 domain-containing (TMED) proteins, also called p24 proteins, are members of a family of sorting receptors present in all representatives of the Eukarya and abundantly present in all subcompartments of the early secretory pathway, namely the endoplasmic reticulum (ER), the Golgi, and the intermediate compartment. Although essential during the bidirectional transport between the ER and the Golgi, there is still a lack of information regarding the TMED's structure across different subfamilies. Besides, although the presence of a TMED homo-oligomerization was suggested previously based on crystallographic contacts observed for the isolated Golgi Dynamics (GOLD) domain, no further analyses of its presence in solution were done. Here, we describe the first high-resolution structure of a TMED1 GOLD representative and its biophysical characterization in solution. The crystal structure showed a dimer formation that is also present in solution in a salt-dependent manner, suggesting that the GOLD domain can form homodimers in solution even in the absence of the TMED1 coiled-coil region. A molecular dynamics description of the dimer stabilization, with a phylogenetic analysis of the residues important for the oligomerization and a model for the orientation towards the lipid membrane, are also presented.

TMP21 in Alzheimer's Disease: Molecular Mechanisms and a Potential Target

Alzheimer’s disease (AD) is the most common form of dementia in the elderly, which is characterized by progressive cognitive impairment. Neuritic plaques, neurofibrillary tangles and neuronal loss are the major neuropathological hallmarks in AD brains. TMP21 is a key molecule for protein trafficking in cells. Growing evidence indicates that TMP21 is dysregulated in AD, which plays a pivotal role in neuritic plaque formation. Therefore, we aim to review the dysregulation of TMP21 in AD, the role of TMP21 in neuritic plaque formation and underlying mechanisms. Moreover, the potential role of TMP21 in neurofibrillary tangle formation, synaptic impairment and neuronal loss is discussed. It will provide an outlook into the potential of regulating TMP21 as a therapeutic approach for AD treatment.

In search of chondrocyte-specific antigen

The purpose of this work was to establish, whether rat chondrocyte associated antigen, transmembrane Tmp21 protein belonging to the p24 protein family may immunize rats and thus be included into the panel of immunogens potentially involved in cartilage pathology. For immunization of rats extract from cultured chondrocytes containing surface chondrocyte proteins suspended in incomplete Freund's adjuvant was used. Control animals were injected with incomplete Freund's adjuvant without chondrocyte extract. Morphological observations indicated that both in control and experimental animals occurred subperiosteal resorption of bone, suggesting that it arised as the response to adjuvant. In trachea, however, resorption of cartilage and inflammatory changes in the respiratory epithelium and lamina propria were present only in animals exposed to antigen. Unexpectedly, sera from immunized rats strongly reacted with other antigen, which we were able to identify by Western blot and protein sequencing as cartilage oligomeric matrix protein (COMP). COMP is attached to chondrocyte membrane by integrins and its presence in chondrocyte extract is not surprising. Antibody response to COMP raises a question whether the observed changes in tracheal cartilage and epithelium represent anti-COMP reaction or were caused by some other, no specified factors. COMP is used as the marker of osteoarthritis progression, but its role in polychondritis, cartilage pathology involving i.a. tracheal cartilage resorption remains unknown. Thus, our observations may serve as the starting point for future studies in this direction.

Loss of Arabidopsis p24 function affects ERD2 trafficking and Golgi structure, and activates the unfolded protein response

The p24 family of proteins (also known as the TMED family) are key regulators of protein trafficking along the secretory pathway, but very little is known about their functions in plants. A quadruple loss-of-function mutant affecting the p24 genes from the δ-1 subclass of the p24δ subfamily (p24δ3δ4δ5δ6) showed alterations in the Golgi, suggesting that these p24 proteins play a role in the organization of the compartments of the early secretory pathway in Arabidopsis Loss of p24δ-1 proteins also induced the accumulation of the K/HDEL receptor ERD2a (ER lumen protein-retaining receptor A) at the Golgi and increased secretion of BiP family proteins, ER chaperones containing an HDEL signal, probably due to an inhibition of COPI-dependent Golgi-to-ER transport of ERD2a and thus retrieval of K/HDEL ligands. Although the p24δ3δ4δ5δ6 mutant showed enhanced sensitivity to salt stress, it did not show obvious phenotypic alterations under standard growth conditions. Interestingly, this mutant showed a constitutive activation of the unfolded protein response (UPR) and the transcriptional upregulation of the COPII subunit gene SEC31A, which may help the plant to cope with the transport defects seen in the absence of p24 proteins.

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.

p24 family proteins: key players in the regulation of trafficking along the secretory pathway

p24 family proteins have been known for a long time, but their functions have remained elusive. However, they are emerging as essential regulators of protein trafficking along the secretory pathway, influencing the composition, structure, and function of different organelles in the pathway, especially the ER and the Golgi apparatus. In addition, they appear to modulate the transport of specific cargos, including GPI-anchored proteins, G-protein-coupled receptors, or K/HDEL ligands. As a consequence, they have been shown to play specific roles in signaling, development, insulin secretion, and the pathogenesis of Alzheimer's disease. The search of new putative ligands may open the way to discover new functions for this fascinating family of proteins.

Cargo Capture and Bulk Flow in the Early Secretory Pathway

Transport of newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi complex is highly selective. As a general rule, such transport is limited to soluble and membrane-associated secretory proteins that have reached properly folded and assembled conformations. To secure the efficiency, fidelity, and control of this crucial transport step, cells use a combination of mechanisms. The mechanisms are based on selective retention of proteins in the ER to prevent uptake into transport vesicles, on selective capture of proteins in COPII carrier vesicles, on inclusion of proteins in these vesicles by default as part of fluid and membrane bulk flow, and on selective retrieval of proteins from post-ER compartments by retrograde vesicle transport.

Biosynthesis of GPI-anchored proteins: special emphasis on GPI lipid remodeling

Glycosylphosphatidylinositols (GPIs) act as membrane anchors of many eukaryotic cell surface proteins. GPIs in various organisms have a common backbone consisting of ethanolamine phosphate (EtNP), three mannoses (Mans), one non-N-acetylated glucosamine, and inositol phospholipid, whose structure is EtNP-6Manα-2Manα-6Manα-4GlNα-6myoinositol-P-lipid. The lipid part is either phosphatidylinositol of diacyl or 1-alkyl-2-acyl form, or inositol phosphoceramide. GPIs are attached to proteins via an amide bond between the C-terminal carboxyl group and an amino group of EtNP. Fatty chains of inositol phospholipids are inserted into the outer leaflet of the plasma membrane. More than 150 different human proteins are GPI anchored, whose functions include enzymes, adhesion molecules, receptors, protease inhibitors, transcytotic transporters, and complement regulators. GPI modification imparts proteins with unique characteristics, such as association with membrane microdomains or rafts, transient homodimerization, release from the membrane by cleavage in the GPI moiety, and apical sorting in polarized cells. GPI anchoring is essential for mammalian embryogenesis, development, neurogenesis, fertilization, and immune system. Mutations in genes involved in remodeling of the GPI lipid moiety cause human diseases characterized by neurological abnormalities. Yeast Saccharomyces cerevisiae has >60 GPI-anchored proteins (GPI-APs). GPI is essential for growth of yeast. In this review, we discuss biosynthesis of GPI-APs in mammalian cells and yeast with emphasis on the lipid moiety.

Drosophila p24 and Sec22 regulate Wingless trafficking in the early secretory pathway

The Wnt signaling pathway is crucial for development and disease. The regulation of Wnt protein trafficking is one of the pivotal issues in the Wnt research field. Here we performed a genetic screen in Drosophila melanogaster for genes involved in Wingless/Wnt secretion, and identified the p24 protein family members Baiser, CHOp24, Eclair and a v-SNARE protein Sec22, which are involved in the early secretory pathway of Wingless/Wnt. We provided genetic evidence demonstrating that loss of p24 proteins or Sec22 impedes Wingless (Wg) secretion in Drosophila wing imaginal discs. We found that Baiser cannot replace other p24 proteins (CHOp24 or Eclair) in escorting Wg, and only Baiser and CHOp24 interact with Wg. Moreover, we showed that the v-SNARE protein Sec22 and Wg are packaged together with p24 proteins. Taken together, our data provide important insights into the early secretory pathway of Wg/Wnt.

Trafficking and Membrane Organization of GPI-Anchored Proteins in Health and Diseases

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are a class of lipid-anchored proteins attached to the membranes by a glycolipid anchor that is added, as posttranslation modification, in the endoplasmic reticulum. GPI-APs are expressed at the cell surface of eukaryotes where they play diverse vital functions. Like all plasma membrane proteins, GPI-APs must be correctly sorted along the different steps of the secretory pathway to their final destination. The presence of both a glycolipid anchor and a protein portion confers special trafficking features to GPI-APs. Here, we discuss the recent advances in the field of GPI-AP trafficking, focusing on the mechanisms regulating their biosynthetic pathway and plasma membrane organization. We also discuss how alterations of these mechanisms can result in different diseases. Finally, we will examine the strict relationship between the trafficking and function of GPI-APs in epithelial cells.

Pre-synaptic localization of the γ-secretase-inhibiting protein p24α2 in the mammalian brain

Dysregulated metabolism and consequent extracellular accumulation of amyloid-β (Aβ) peptides in the brain underlie the pathogenesis of Alzheimer's disease. Extracellular Aβ in the brain parenchyma is mainly secreted from the pre-synaptic terminals of neuronal cells in a synaptic activity-dependent manner. The p24 family member p24α2 reportedly attenuates Aβ generation by inhibiting γ-secretase processing of amyloid precursor protein; however, the pattern of expression and localization of p24α2 in the brain remains unknown. We performed immunohistochemical staining and subcellular fractionation for p24α2 in the mouse brain. Immunostaining showed that p24α2 is broadly distributed in the gray matter of the central nervous system and is predominantly localized to synapses. Subcellular fractionation revealed prominent localization of p24α2 in the pre-synaptic terminals. Immunoisolation of synaptic vesicles (SV) indicated that p24α2 is condensed at active zone-docked SV. During development, p24α2 expression is highest in the post-natal period and gradually decreases with age. We also confirmed that amyloid precursor protein and γ-secretase components are localized at active zone-docked SV. Our results suggest a novel functional role for p24α2 in the regulation of synaptic transmission and synaptogenesis, and provide evidence for the participation of p24α2 in the regulation of Aβ generation and secretion in the brain. The p24 family member p24α2 attenuates amyloid-β (Aβ) generation by inhibiting the γ-secretase processing. We report that p24α2 is condensed at active zone-docked synaptic vesicles in the brain. p24α2 expression is highest in the post-natal period and gradually decreases with age. Our results suggest a novel function for p24α2 at the synapse, including the regulation of brain Aβ generation.

Arabidopsis p24δ5 and p24δ9 facilitate Coat Protein I-dependent transport of the K/HDEL receptor ERD2 from the Golgi to the endoplasmic reticulum

The p24 proteins belong to a family of type I membrane proteins which cycle between the endoplasmic reticulum (ER) and Golgi via coat protein I (COPI) and COPII vesicles. Current nomenclature classifies them into four subfamilies, although plant p24 proteins belong to either the p24β or the p24δ subfamilies. Here, we show that Arabidopsis p24δ5/δ9 and HDEL ligands shift the steady-state distribution of the K/HDEL receptor ERD2 from the Golgi to the ER. We also show that p24δ5/δ9 interact directly with ERD2. This interaction requires the Golgi dynamics (GOLD) domain in p24δ5 and is much higher at acidic than at neutral pH, consistent with both proteins interacting at the cis-Golgi. In addition, p24δ5 also inhibits the secretion of HDEL ligands, but not constitutive secretion, showing a role for p24δ5 in retrograde Golgi-to-ER transport. Both p24δ5 and ERD2 interact with ADP-ribosylation factor 1 (ARF1) and COPI subunits, mostly at acidic pH, consistent with COPI vesicles being involved in retrograde transport of both proteins. In contrast, both proteins interact with the COPII subunit Sec23, mostly at neutral pH, consistent with this interaction taking place at the ER for anterograde transport to the Golgi apparatus.

Transcriptional regulation of secretory capacity by bZip transcription factors

Cells of specialized secretory organs expand their secretory pathways to accommodate the increased protein load necessary for their function. The endoplasmic reticulum (ER), the Golgi apparatus and the secretory vesicles, expand not only the membrane components but also the protein machinery required for increased protein production and transport. Increased protein load causes an ER stress response akin to the Unfolded Protein Response (UPR). Recent work has implicated several bZip transcription factors in the regulation of protein components of the early secretory pathway necessary to alleviate this stress. Here, we highlight eight bZip transcription factors in regulating secretory pathway component genes. These include components of the three canonical branches of the UPR-ATF4, XBP1, and ATF6, as well as the five members of the Creb3 family of transcription factors.We review findings from both invertebrate and vertebrate model systems suggesting that all of these proteins increase secretory capacity in response to increased protein load. Finally, we propose that the Creb3 family of factors may have a dual role in secretory cell differentiation by also regulating the pathways necessary for cell cycle exit during terminal differentiation.

Molecular and cellular regulation of toll-like receptor-4 activity induced by lipopolysaccharide ligands

As well as being the primary signaling receptor for bacterial endotoxin or lipopolysaccharide Toll-like receptor-4 function is modulated by numerous factors not only in the context of microbial pathogenesis but also autoimmune and allergic diseases. TLR4 is subject to multiple levels of endogenous control and regulation from biosynthesis and trafficking to signal transduction and degradation. On the other hand regulation of TLR4 activity breaks down during Gram −ve sepsis leading to systemic damage, multi organ failure, and death. In this article, we review how TLR4 traffics from the early secretory pathway, the cis/trans Golgi to the cell surface and endolysosomal compartments. We will present evidence about how these processes influence signaling and can potentially lead to increased sensitivity to ligand-dependent activation as well as ligand-independent constitutive activation that may contribute to pathogenesis in sepsis. We will also discuss how sustained signaling may be coupled to endocytosis and consider the potential molecular mechanisms of immuno-modulators that modify TLR4 signaling function including the cat allergen FelD1 and endogenous protein ligands such as the extracellular matrix protein tenascin C and calprotectin (MRP8/14).

The COP II adaptor protein TMED7 is required to initiate and mediate the delivery of TLR4 to the plasma membrane

Toll-like receptor 4 (TLR4), the receptor for the bacterial product endotoxin, is subject to multiple points of regulation at the levels of signaling, biogenesis, and trafficking. Dysregulation of TLR4 signaling can cause serious inflammatory diseases, such as sepsis. We found that the p24 family protein TMED7 (transmembrane emp24 protein transport domain containing 7) is required for the trafficking of TLR4 from the endoplasmic reticulum to the cell surface through the Golgi. TMED7 formed a stable complex with the ectodomain of TLR4, an interaction that required the coiled-coil and Golgi dynamics (GOLD) domains, but not the cytosolic, coat protein complex II (COP II) sorting motif, of TMED7. Depletion of TMED7 reduced TLR4 signaling mediated by the adaptor protein MyD88 (myeloid differentiation marker 88), but not that mediated by the adaptor proteins TRIF [Toll-interleukin-1 receptor (TIR) domain-containing adaptor protein inducing interferon-β] and TRAM (TRIF-related adaptor molecule). Truncated forms of TMED7 lacking the COP II sorting motif or the transmembrane domain were mislocalized and resulted in ligand-independent signaling that probably arises from receptors accumulated intracellularly. Together, these results support the hypothesis that p24 proteins perform a quality control step by recognizing correctly folded anterograde cargo, such as TLR4, in early secretory compartments and facilitating the translocation of this cargo to the cell surface.

The α-helical region in p24γ2 subunit of p24 protein cargo receptor is pivotal for the recognition and transport of glycosylphosphatidylinositol-anchored proteins

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are group of proteins that depend on p24 cargo receptors for their transport from the endoplasmic reticulum to the Golgi apparatus. The GPI anchor is expected to act as a sorting and transport signal, but so far little is known about the recognition mechanism. In the present study we investigate the GPI-AP transport in cell knockdown of p24γ, the most diverse p24 subfamily. Knockdown of p24γ2 but not of other p24γ family members impaired the transport of a reporter GPI-AP. Restoration of the knockdown-induced phenotype using chimeric constructs between p24γ2 and the related p24γ1 further implied a role of the α-helical region of p24γ2 but not its GOLD domain in the specific binding of GPI-APs. We conclude that motifs in the membrane-adjacent α-helical region of p24γ2 are involved in recognition of GPI-APs and are consequently responsible for the incorporation of these proteins into coat protein complex II-coated transport vesicles.

Rat chondrocyte-associated antigen identified as sialylated transmembrane protein Tmp21 belonging to the p24 protein family

Rabbit serum produced after transplantation of isolated rat chondrocytes [sensitized rabbit serum (SRS)] demonstrated M r ~ 74- and ~23-kDa (western blot analysis) antigens in rat chondrocyte extracts. Only the latter remained after reduction in 2-mercaptoethanol. Protein sequence analysis of 23-kDa chondrocyte-associated antigen (CAA) revealed that it corresponds to transmembrane Tmp21 protein belonging to the p24 protein family. These proteins mainly participate in the traffic between the endoplasmic reticulum and Golgi complex and in some cells appear also in the membrane of secretory granules and plasmalemma. Tmp21 extracted from chondrocytes was sialylated and ceased to bind SRS after deglycosylation. A previous study from our laboratory indicated that expression of CAA, now identified as sialylated Tmp21, decreased in cultured chondrocytes concomitantly with the decline of collagen type II and aggrecan and the rise of collagen type I and versican expression. Since the sialylated form of Tmp21 (also known as emp24) was not described in other tissues and seems to be specific for chondrocytes, we assume that CAA may be considered a chondrocyte differentiation antigen.

Isoform-selective oligomer formation of Saccharomyces cerevisiae p24 family proteins

p24 family proteins are evolutionarily conserved transmembrane proteins involved in the early secretory pathway. Saccharomyces cerevisiae has 8 known p24 proteins that are classified into four subfamilies (p24α, -β, -γ, and -δ). Emp24 and Erv25 are the sole members of p24β and -δ, respectively, and deletion of either destabilizes the remaining p24 proteins, resulting in p24 null phenotype (p24Δ). We studied genetic and physical interactions of p24α (Erp1, -5, and -6) and γ (Erp2, -3, and -4). Deletion of the major p24α (Erp1) partially inhibited p24 activity as reported previously. A second mutation in either Erp5 or Erp6 aggravated the erp1Δ phenotype, and the triple mutation gave a full p24Δ phenotype. Similar genetic interactions were observed among the major p24γ (Erp2) and the other two γ members. All the p24α/γ isoforms interacted with both p24β and -δ. Interaction between p24β and -δ was isoform-selective, and five major α/γ pairs were detected. These results suggest that the yeast p24 proteins form functionally redundant αβγδ complexes. We also identified Rrt6 as a novel p24δ isoform. Rrt6 shows only limited sequence identity (∼15%) to known p24 proteins but was found to have structural properties characteristic of p24. Rrt6 was induced when cells were grown on glycerol and form an additional αβγδ complex with Erp3, Erp5, and Emp24. This complex was mainly localized to the Golgi, whereas the p24 complex containing Erv25, instead of Rrt6 but otherwise with the same isoform composition, was found mostly in the ER.

Functional diversity of the p24γ homologue Erp reveals physiological differences between two filamentous fungi

The protein hyper-secreting filamentous fungi impact their surrounding environments by secreting cellulases and digesting plant cell wall via microbe-plant interspecies interaction. This process is of paramount importance in biofuel production from the renewable lignocellulosic biomass, because cellulase production is the key factor in cost determination. Despite the importance of protein secretion, p24 protein, a key factor in eukaryotic protein maturation and secretion, was never investigated in filamentous fungi. The erp genes encoding p24γ homologues were identified in Trichoderma reesei and Penicillium decumbens. The roles of Erp and their participated cellular pathways were investigated via disruption of erp, revealing significant differences: sporulation was hampered in T. reesei Δerp but not in P. decumbens Δerp; in both species Erp maintains membrane integrity; Erp is likely involved in hyphae polarity maintenance in T. reesei. Protein- and transcription-level investigations of Erp participation in cellulase production revealed distinct regulatory mechanisms. In T. reesei, cellulase encoding genes were repressed under secretion stress. In contrast, activation of the same genes under the same stress was identified in P. decumbens. These observations revealed a novel cellulase gene regulation mechanism, clearly suggested the different physiological roles of Erp, and further demonstrated the different physiology of T. reesei and P. decumbens, despite above 75% sequence identity between the proteins and the close evolutionary relationship between the two species.

Putative p24 complexes in Arabidopsis contain members of the delta and beta subfamilies and cycle in the early secretory pathway

p24 proteins are a family of type I membrane proteins localized to compartments of the early secretory pathway and to coat protein I (COPI)- and COPII-coated vesicles. They can be classified, by sequence homology, into four subfamilies, named p24α, p24β, p24γ, and p24δ. In contrast to animals and fungi, plants contain only members of the p24β and p24δ subfamilies, the latter probably including two different subclasses. It has previously been shown that transiently expressed red fluorescent protein (RFP)-p24δ5 (p24δ1 subclass) localizes to the endoplasmic reticulum (ER) at steady state as a consequence of highly efficient COPI-based recycling from the Golgi apparatus. It is now shown that transiently expressed RFP-p24δ9 (p24δ2 subclass) also localizes to the ER. In contrast, transiently expressed green fluorescent protein (GFP)-p24β3 mainly localizes to the Golgi apparatus (as p24β2) and exits the ER in a COPII-dependent manner. Immunogold electron microscopy in Arabidopsis root tip cells using specific antibodies shows that endogenous p24δ9 localizes mainly to the ER but also partially to the cis-Golgi. In contrast, endogenous p24β3 mainly localizes to the Golgi apparatus. By a combination of experiments using transient expression, knock-out mutants, and co-immunoprecipitation, it is proposed that Arabidopsis p24 proteins form different heteromeric complexes (including members of the β and δ subfamilies) which are important for their stability and their coupled trafficking at the ER-Golgi interface. Evidence is also provided for a role for p24δ5 in retrograde Golgi-ER transport of the KDEL-receptor ERD2.

Transport of glycosylphosphatidylinositol-anchored proteins from the endoplasmic reticulum

In this review on the transport of glycosylphosphatidylinositol-anchored proteins (GPI-APs), we focus on events that occur in the endoplasmic reticulum after the transfer of GPI to proteins. These events include structural remodeling of both the lipid and glycan moieties of GPI, recruitment of GPI-APs into ER exit sites, association with the cargo receptor, p24 protein complex, and packaging into COPII coated transport vesicles. Similarities with the transport of Wnt proteins are also discussed. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Coupled transport of Arabidopsis p24 proteins at the ER-Golgi interface

p24 proteins are a family of type I membrane proteins localized to compartments of the early secretory pathway and to coat protein I (COPI)- and COPII-coated vesicles. They can be classified, by sequence homology, into four subfamilies, named p24α, p24β, p24γ, and p24δ. In contrast to animals and fungi, plants contain only members of the p24β and p24δ subfamilies. It has previously been shown that transiently expressed red fluorescent protein (RFP)-p24δ5 localizes to the endoplasmic reticulum (ER) as a consequence of highly efficient COPI-based recycling from the Golgi apparatus. Using specific antibodies, endogenous p24δ5 has now been localized to the ER and p24β2 to the Golgi apparatus in Arabidopsis root tip cells by immunogold electron microscopy. The relative contributions of the cytosolic tail and the luminal domains to p24δ5 trafficking have also been characterized. It is demonstrated that whereas the dilysine motif in the cytoplasmic tail determines the location of p24δ5 in the early secretory pathway, the luminal domain may contribute to its distribution downstream of the Golgi apparatus. By using knock-out mutants and co-immunoprecipitation experiments, it is shown that p24δ5 and p24β2 interact with each other. Finally, it is shown that p24δ5 and p24β2 exhibit coupled trafficking at the ER-Golgi interface. It is proposed that p24δ5 and p24β2 interact with each other at ER export sites for ER exit and coupled transport to the Golgi apparatus. Once in the Golgi, p24δ5 interacts very efficiently with the COPI machinery for retrograde transport back to the ER.

COPII and COPI traffic at the ER-Golgi interface

Protein traffic is necessary to maintain homeostasis in all eukaryotic organisms. All newly synthesized secretory proteins destined to the secretory and endolysosmal systems are transported from the endoplasmic reticulum to the Golgi before delivery to their final destinations. Here, we describe the COPII and COPI coating machineries that generate carrier vesicles and the tethers and SNAREs that mediate COPII and COPI vesicle fusion at the ER-Golgi interface.

Subclass-specific localization and trafficking of Arabidopsis p24 proteins in the ER-Golgi interface

We describe a comprehensive analysis of the subcellular localization and in vivo trafficking of Arabidopsis p24 proteins. In Arabidopsis, there are 11 p24 proteins, which fall into only δ and β subfamilies. Interestingly, the δ subfamily of p24 proteins in Arabidopsis is elaborated spectacularly in evolution, which can be grouped into two subclasses: p24δ1 and p24δ2. We found that, although all p24δ proteins possess classic COPII/COPI binding motifs in their cytosolic C-termini, p24δ1 proteins are localized to the endoplasmic reticulum (ER), p24δ2 proteins are localized to both ER and Golgi. Two p24β proteins reside largely in Golgi. Similar to Atp24 (termed p24δ1c in this study), p24δ2d also cycles between the ER and Golgi. Interestingly, coexpression with p24β1 could retain p24δ2d, but not p24δ1d in Golgi. We revealed that the lumenal coiled-coil domain of p24δ2d is required for its steady-state localization in Golgi, probably through its interaction with p24β1. In p24β1, there is no classic COPII or COPI binding motif in its C-terminus. However, the protein also cycles between the ER and Golgi. We found that a conserved RV motif located at the extreme end of the C-terminus of p24β1 plays an important role in its Golgi target.

A novel 800 kb microduplication of chromosome 16q22.1 resulting in learning disability and epilepsy may explain phenotypic variability in a family with 15q13 microdeletion

The phenotype of 15q13.3 microdeletion is variable and can be non-penetrant. Recently, "second-hit hypothesis" has been proposed as a possible explanation for some variability in recurrent microdeletion syndromes. We present a family with a 1.9 Mb 15q13.3 deletion and a novel 800 kb 16q22.1 duplication. We show that the 16q22.1 duplication may be a phenotypic modifier in this family and likely results in epilepsy and learning difficulties. We state the possible genes in this region that may be important in neurological development and function.

Dilysine retrieval signal-containing p24 proteins collaborate in inhibiting γ-cleavage of amyloid precursor protein

γ-Secretase mediates intramembranous γ-cleavage and ε-cleavage of β-amyloid precursor protein (APP) to liberate β-amyloid peptide (Aβ) and APP intracellular domain respectively from the membrane. Although the regulatory mechanism of γ-secretase cleavage remains unresolved, a member of the p24 cargo protein family, named p24δ(1) or TMP21, has been identified as an activity-modulating component. The p24 family proteins are divided into four subfamilies (p24α, β, δ and γ). In contrast to p24δ(1), p24β(1) has reportedly no effect on γ-cleavage. In this study, we determined whether p24α(2), p24γ(3) or p24γ(4) modulates APP processing. Knockdown of cellular p24α(2) induced a significant increase in Aβ generation but not in APP intracellular domain production in cell-based and cell-free assays, whereas p24α(2) over-expression suppressed Aβ secretion. By contrast, Aβ secretion was not altered by p24γ(3) or p24γ(4) knockdown. Endogenous p24α(2) co-immunoprecipitated with core components of the γ-secretase complex, and the anti-p24α(2) immunoprecipitate exhibited γ-secretase activity. Mutational disruption of the conserved dilysine ER-retrieval motifs of p24α(2) and p24δ(1) perturbed inhibition of γ-cleavage. Simultaneous knockdown, or co-over-expression, of these proteins had no additive or synergistic effect on Aβ generation. Our findings suggest that dilysine ER-retrieval signal-containing p24 proteins, p24α(2) and p24δ(1), bind with γ-secretase complexes and collaborate in attenuating γ-cleavage of APP.

The trafficking protein Tmed2/p24beta(1) is required for morphogenesis of the mouse embryo and placenta

During vesicular transport between the endoplasmic reticulum and the Golgi, members of the TMED/p24 protein family form hetero-oligomeric complexes that facilitate protein-cargo recognition as well as vesicle budding. In addition, they regulate each other's level of expression. Despite analyses of TMED/p24 protein distribution in mammalian cells, yeast, and C. elegans, little is known about the role of this family in vertebrate embryogenesis. We report the presence of a single point mutation in Tmed2/p24beta(1) in a mutant mouse line, 99J, identified in an ENU mutagenesis screen for recessive developmental abnormalities. This mutation does not affect Tmed2/p24beta(1) mRNA levels but results in loss of TMED2/p24beta(1) protein. Prior to death at mid-gestation, 99J homozygous mutant embryos exhibit developmental delay, abnormal rostral-caudal elongation, randomized heart looping, and absence of the labyrinth layer of the placenta. We find that Tmed2/p24beta(1) is normally expressed in tissues showing morphological defects in 99J mutant embryos and that these affected tissues lack the TMED2/p24beta(1) oligomerization partners, TMED7/p24gamma(3) and TMED10/p24delta(1). Our data reveal a requirement for TMED2/p24beta(1) protein in the morphogenesis of the mouse embryo and placenta.

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.

p24 proteins play a role in peroxisome proliferation in yeast

Emp24 is a member of the p24 protein family, which was initially localized to the endoplasmic reticulum, Golgi and COP vesicles, but has recently shown to be associated with Saccharomyces cerevisiae peroxisomes as well. Using cell fractionation and electron- and fluorescence microscopy, we show that in the yeast Hansenula polymorpha, Emp24 also associates with peroxisomes. In addition, we show that peroxisome numbers are strongly decreased in H. polymorpha cells lacking two proteins of the p24 complex, Emp24 and Erp3. Detailed fluorescence microscopy analyses suggest that emp24.erp3 cells are disturbed in peroxisome fission and inheritance.