Involvement of the transmembrane protein p23 in biosynthetic protein transport

The schizotrophic lifestyle of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a cosmopolitan and typical necrotrophic phytopathogenic fungus that infects hundreds of plant species. Because no cultivars highly resistant to S. sclerotiorum are available, managing Sclerotinia disease caused by S. sclerotiorum is still challenging. However, recent studies have demonstrated that S. sclerotiorum has a beneficial effect and can live mutualistically as an endophyte in graminaceous plants, protecting the plants against major fungal diseases. An in-depth understanding of the schizotrophic lifestyle of S. sclerotiorum during interactions with plants under different environmental conditions will provide new strategies for controlling fungal disease. In this review, we summarize the pathogenesis mechanisms of S. sclerotiorum during its attack of host plants as a destructive pathogen and discuss its lifestyle as a beneficial endophytic fungus.

TMED3 Complex Mediates ER Stress-Associated Secretion of CFTR, Pendrin, and SARS-CoV-2 Spike

Under ER stress conditions, the ER form of transmembrane proteins can reach the plasma membrane via a Golgi-independent unconventional protein secretion (UPS) pathway. However, the targeting mechanisms of membrane proteins for UPS are unknown. Here, this study reports that TMED proteins play a critical role in the ER stress-associated UPS of transmembrane proteins. The gene silencing results reveal that TMED2, TMED3, TMED9 and TMED10 are involved in the UPS of transmembrane proteins, such as CFTR, pendrin and SARS-CoV-2 Spike. Subsequent mechanistic analyses indicate that TMED3 recognizes the ER core-glycosylated protein cargos and that the heteromeric TMED2/3/9/10 complex mediates their UPS. Co-expression of all four TMEDs improves, while each single expression reduces, the UPS and ion transport function of trafficking-deficient ΔF508-CFTR and p.H723R-pendrin, which cause cystic fibrosis and Pendred syndrome, respectively. In contrast, TMED2/3/9/10 silencing reduces SARS-CoV-2 viral release. These results provide evidence for a common role of TMED3 and related TMEDs in the ER stress-associated, Golgi-independent secretion of transmembrane proteins.

A Rare Kidney Disease To Cure Them All? Towards Mechanism-Based Therapies for Proteinopathies

Autosomal dominant tubulointerstitial kidney diseases (ADTKDs) are a group of rare genetic diseases that lead to kidney failure. Mutations in the MUC1 gene cause ADTKD-MUC1 (MUC1 kidney disease, MKD), a disorder with no available therapies. Recent studies have identified the molecular and cellular mechanisms that drive MKD disease pathogenesis. Armed with patient-derived cell lines and pluripotent stem cell (iPSC)-derived kidney organoids, it was found that MKD is a toxic proteinopathy caused by the intracellular accumulation of misfolded MUC1 protein in the early secretory pathway. We discuss the advantages of studying rare monogenic kidney diseases, describe effective patient-derived model systems, and highlight recent mechanistic insights into protein quality control that have implications for additional proteinopathies beyond rare kidney diseases.

Dual Independent Roles of the p24 Complex in Selectivity of Secretory Cargo Export from the Endoplasmic Reticulum

The cellular mechanisms that ensure the selectivity and fidelity of secretory cargo protein transport from the endoplasmic reticulum (ER) to the Golgi are still not well understood. The p24 protein complex acts as a specific cargo receptor for GPI-anchored proteins by facilitating their ER exit through a specialized export pathway in yeast. In parallel, the p24 complex can also exit the ER using the general pathway that exports the rest of secretory proteins with their respective cargo receptors. Here, we show biochemically that the p24 complex associates at the ER with other cargo receptors in a COPII-dependent manner, forming high-molecular weight multireceptor complexes. Furthermore, live cell imaging analysis reveals that the p24 complex is required to retain in the ER secretory cargos when their specific receptors are absent. This requirement does not involve neither the unfolded protein response nor the retrograde transport from the Golgi. Our results suggest that, in addition to its role as a cargo receptor in the specialized GPI-anchored protein pathway, the p24 complex also plays an independent role in secretory cargo selectivity during its exit through the general ER export pathway, preventing the non-selective bulk flow of native secretory cargos. This mechanism would ensure receptor-regulated cargo transport, providing an additional layer of regulation of secretory cargo selectivity during ER export.

RNA Interference Screening Identifies Novel Roles for RhoBTB1 and RhoBTB3 in Membrane Trafficking Events in Mammalian Cells

In the endomembrane system of mammalian cells, membrane traffic processes require a high degree of regulation in order to ensure their specificity. The range of molecules that participate in trafficking events is truly vast, and much attention to date has been given to the Rab family of small GTPases. However, in recent years, a role in membrane traffic for members of the Rho GTPase family, in particular Cdc42, has emerged. This prompted us to develop and apply an image-based high-content screen, initially focussing on the Golgi complex, using RNA interference to systematically perturb each of the 21 Rho family members and assess their importance to the overall organisation of this organelle. Analysis of our data revealed previously unreported roles for two atypical Rho family members, RhoBTB1 and RhoBTB3, in membrane traffic events. We find that depletion of RhoBTB3 affects the morphology of the Golgi complex and causes changes in the trafficking speeds of carriers operating at the interface of the Golgi and endoplasmic reticulum. In addition, RhoBTB3 was found to be present on these carriers. Depletion of RhoBTB1 was also found to cause a disturbance to the Golgi architecture, however, this phenotype seems to be linked to endocytosis and retrograde traffic pathways. RhoBTB1 was found to be associated with early endosomal intermediates, and changes in the levels of RhoBTB1 not only caused profound changes to the organisation and distribution of endosomes and lysosomes, but also resulted in defects in the delivery of two different classes of cargo molecules to downstream compartments. Together, our data reveal new roles for these atypical Rho family members in the endomembrane system.

Models of Intracellular Transport: Pros and Cons

Intracellular transport is one of the most confusing issues in the field of cell biology. Many different models and their combinations have been proposed to explain the experimental data on intracellular transport. Here, we analyse the data related to the mechanisms of endoplasmic reticulum-to-Golgi and intra-Golgi transport from the point of view of the main models of intracellular transport; namely: the vesicular model, the diffusion model, the compartment maturation–progression model, and the kiss-and-run model. This review initially describes our current understanding of Golgi function, while highlighting the recent progress that has been made. It then continues to discuss the outstanding questions and potential avenues for future research with regard to the models of these transport steps. To compare the power of these models, we have applied the method proposed by K. Popper; namely, the formulation of prohibitive observations according to, and the consecutive evaluation of, previous data, on the basis on the new models. The levels to which the different models can explain the experimental observations are different, and to date, the most powerful has been the kiss-and-run model, whereas the least powerful has been the diffusion model.

Regulation of global gene expression in brain by TMP21

TMP21, a type I transmembrane protein of thep24 protein family, mediates protein trafficking and maturation. Dysregulation of TMP21 is implicated in the pathogenesis of Alzheimer’s disease (AD). However, underlying mechanisms remain elusive. To reveal the function of TMP21 in the brain and the pathogenic role of TMP21 in the brain of AD, the global gene expression was profiled in the brain of TMP21 knockdown mice. We found that 8196 and 8195 genes are significantly altered in the hippocampus and cortex, respectively. The genes are involved in a number of brain function-related pathways, including glutamatergic synapse pathway, serotonergic synapse pathway, synaptic vesicle pathway, and long-term depression pathway. Moreover, the network analysis suggests that the TMP21 may contribute to the pathogenesis of AD by regulatingPI3K/Akt/GSK3β signalling pathway. Our study provides an insight into the physiological function of TMP21 in the brain and pathological role of TMP21 in AD.

A Novel Alzheimer-Associated SNP in Tmp21 Increases Amyloidogenesis

Recent studies suggest that TMP21 is a selective modulator of γ-secretase and its dysregulation affects APP processing, leading to increased Aβ generation. However, the genetic association between Tmp21 and Alzheimer's disease (AD) remains elusive. In this study, we identified that a novel single-nucleotide polymorphism (SNP) rs12435391 (IVS4-28T>C) in intron 4 of Tmp21 was genetically associated with AD. We found that allele C of the SNP rs12435391 did not affect splicing site recognition, but it significantly increased TMP21 gene expression. The stability of Tmp21 pre-mRNA and the transcription of Tmp21 were not affected by allele C of the SNP rs12435391. However, allele C of the SNP rs12435391 significantly increased the splicing efficiency of Tmp21 pre-mRNA, leading to the elevation of mature mRNA. Furthermore, allele C of the SNP rs12435391 significantly reduced C83 level and increased Aβ generation. Taken together, our study suggests that TMP21 is genetically associated with Alzheimer's disease, with the novel Tmp21 SNP as a risk factor for Alzheimer's pathogenesis.

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.

Trafficking of glycosylphosphatidylinositol anchored proteins from the endoplasmic reticulum to the cell surface

In eukaryotes, many cell surface proteins are attached to the plasma membrane via a glycolipid glycosylphosphatidylinositol (GPI) anchor. GPI-anchored proteins (GPI-APs) receive the GPI anchor as a conserved posttranslational modification in the lumen of the endoplasmic reticulum (ER). After anchor attachment, the GPI anchor is structurally remodeled to function as a transport signal that actively triggers the delivery of GPI-APs from the ER to the plasma membrane, via the Golgi apparatus. The structure and composition of the GPI anchor confer a special mode of interaction with membranes of GPI-APs within the lumen of secretory organelles that lead them to be differentially trafficked from other secretory membrane proteins. In this review, we examine the mechanisms by which GPI-APs are selectively transported through the secretory pathway, with special focus on the recent progress made in their actively regulated export from the ER and the trans-Golgi network.

COPII coat composition is actively regulated by luminal cargo maturation

BACKGROUND

Export from the ER is an essential process driven by the COPII coat, which forms vesicles at ER exit sites (ERESs) to transport mature secretory proteins to the Golgi. Although the basic mechanism of COPII assembly is known, how COPII machinery is regulated to meet varying cellular secretory demands is unclear.

RESULTS

Here, we report a specialized COPII system that is actively recruited by luminal cargo maturation. Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are luminal secretory proteins anchored to the membrane by the glycolipid GPI. After protein attachment in the ER lumen, lipid and glycan parts of the GPI anchor are remodeled. In yeast, GPI-lipid remodeling concentrates GPI-APs into specific ERESs. We found that GPI-glycan remodeling induces subsequent recruitment of the specialized ER export machinery that enables vesicle formation from these specific ERESs. First, the transmembrane cargo receptor p24 complex binds GPI-APs as a lectin by recognizing the remodeled GPI-glycan. Binding of remodeled cargo induces the p24 complex to recruit the COPII subtype Lst1p, specifically required for GPI-AP ER export.

CONCLUSIONS

Our results show that COPII coat recruitment by cargo receptors is not constitutive but instead is actively regulated by binding of mature ligands. Therefore, we reveal a novel functional link between luminal cargo maturation and COPII vesicle budding, providing a mechanism to adjust specialized COPII vesicle production to the amount and quality of their luminal cargos that are ready for ER exit. This helps to understand how the ER export machinery adapts to different needs for luminal cargo secretion.

Functional implications of the Golgi and microtubular network in gonadotropes

In contrast to the widely accepted images of the Golgi apparatus as a cup-like shape, the Golgi in pituitary gonadotropes is organized as a spherical shape in which the outer and inner faces are cis- and trans-Golgi elements, respectively. At the center of the spherical Golgi, a pair of centrioles is situated as a microtubule-organizing center from which radiating microtubules isotropically extend toward the cell periphery. This review focuses on the significance of the characteristic organization of the Golgi and microtubule network in gonadotropes, considering the roles of microtubule-dependent membrane transport in the formation and maintenance of the Golgi structure. Because the highly symmetrical organization of the Golgi is possibly perturbed in response to experimental treatments of gonadotropes, monitoring of the Golgi structure in gonadotropes under various experimental conditions will be a novel in vivo approach to elucidate the biogenesis of the Golgi apparatus.

ARF1 and SAR1 GTPases in endomembrane trafficking in plants

Small GTPases largely control membrane traffic, which is essential for the survival of all eukaryotes. Among the small GTP-binding proteins, ARF1 (ADP-ribosylation factor 1) and SAR1 (Secretion-Associated RAS super family 1) are commonly conserved among all eukaryotes with respect to both their functional and sequential characteristics. The ARF1 and SAR1 GTP-binding proteins are involved in the formation and budding of vesicles throughout plant endomembrane systems. ARF1 has been shown to play a critical role in COPI (Coat Protein Complex I)-mediated retrograde trafficking in eukaryotic systems, whereas SAR1 GTPases are involved in intracellular COPII-mediated protein trafficking from the ER to the Golgi apparatus. This review offers a summary of vesicular trafficking with an emphasis on the ARF1 and SAR1 expression patterns at early growth stages and in the de-etiolation process.

SNX12 role in endosome membrane transport

In this paper, we investigated the role of sorting nexin 12 (SNX12) in the endocytic pathway. SNX12 is a member of the PX domain-containing sorting nexin family and shares high homology with SNX3, which plays a central role in the formation of intralumenal vesicles within multivesicular endosomes. We found that SNX12 is expressed at very low levels compared to SNX3. SNX12 is primarily associated with early endosomes and this endosomal localization depends on the binding to 3-phosphoinositides. We find that overexpression of SNX12 prevents the detachment (or maturation) of multivesicular endosomes from early endosomes. This in turn inhibits the degradative pathway from early to late endosomes/lysosomes, much like SNX3 overexpression, without affecting endocytosis, recycling and retrograde transport. In addition, while previous studies showed that Hrs knockdown prevents EGF receptor sorting into multivesicular endosomes, we find that overexpression of SNX12 restores the sorting process in an Hrs knockdown background. Altogether, our data show that despite lower expression level, SNX12 shares redundant functions with SNX3 in the biogenesis of multivesicular endosomes.

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.

Drosophila melanogaster p24 trafficking proteins have vital roles in development and reproduction

p24 proteins comprise a family of type-I transmembrane proteins of ~24kD that are present in yeast and plants as well as metazoans ranging from Drosophila to humans. These proteins are most commonly localized to the endoplasmic reticulum (ER)-Golgi interface and are incorporated in anterograde and retrograde transport vesicles. Little is known about how disruption of p24 signaling affects individual tissue function or whole animals. Drosophila melanogaster express nine p24 genes, grouped into four subfamilies. Based upon our mRNA and protein expression data, Drosophila p24 family members are expressed in a variety of tissues. To identify functions for particular Drosophila p24 proteins, we used RNA interference (RNAi) to reduce p24 expression. Ubiquitous reduction of most p24 genes resulted in complete or partial lethality during development. We found that reducing p24 levels in adults caused defects in female fecundity (egg laying) and also reduced male fertility. We attributed reduced female fecundity to decreased neural p24 expression. These results provide the first genetic analysis of all p24 family members in a multicellular animal and indicate vital roles for Drosophila p24s in development and reproduction, implicating neural expression of p24s in the regulation of female behavior.

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.

Transgenic neuronal overexpression reveals that stringently regulated p23 expression is critical for coordinated movement in mice

Background

p23 belongs to the highly conserved p24 family of type I transmembrane proteins, which participate in the bidirectional protein transport between the endoplasmic reticulum and Golgi apparatus. Mammalian p23 has been shown to interact with γ-secretase complex, and modulate secretory trafficking as well as intramembranous processing of amyloid precursor protein in cultured cells. Negative modulation of β-amyloid production by p23 in cultured cell lines suggested that elevation of p23 expression in neurons might mitigate cerebral amyloid burden.

Results

We generated several lines of transgenic mice expressing human p23 in neurons under the control of Thy-1.2 promoter. We found that even a 50% increase in p23 levels in the central nervous system of mice causes post-natal growth retardation, severe neurological problems characterized by tremors, seizure, ataxia, and uncoordinated movements, and premature death. The severity of the phenotype closely correlated with the level of p23 overexpression in multiple transgenic lines. While the number and general morphology of neurons in Hup23 mice appeared to be normal throughout the brain, abnormal non-Golgi p23 localization was observed in a subset of neurons with high transgene expression in brainstem. Moreover, detailed immunofluorescence analysis revealed marked proliferation of astrocytes, activation of microglia, and thinning of myelinated bundles in brainstem of Hup23 mice.

Conclusions

These results demonstrate that proper level of p23 expression is critical for neuronal function, and perturbing p23 function by overexpression initiates a cascade of cellular reactions in brainstem that leads to severe motor deficits and other neurological problems, which culminate in premature death. The neurological phenotype observed in Hup23 mice highlights significant adverse effects associated with manipulating neuronal expression of p23, a previously described negative modulator of γ-secretase activity and β-amyloid production. Moreover, our report has broader relevance to molecular mechanisms in several neurodegenerative diseases as it highlights the inherent vulnerability of the early secretory pathway mechanisms that ensure proteostasis in neurons.

Role of SNX16 in the dynamics of tubulo-cisternal membrane domains of late endosomes

In this paper, we report that the PX domain-containing protein SNX16, a member of the sorting nexin family, is associated with late endosome membranes. We find that SNX16 is selectively enriched on tubulo-cisternal elements of this membrane system, whose highly dynamic properties and formation depend on intact microtubules. By contrast, SNX16 was not found on vacuolar elements that typically contain LBPA, and thus presumably correspond to multivesicular endosomes. We conclude that SNX16, together with its partner phosphoinositide, define a highly dynamic subset of late endosomal membranes, supporting the notion that late endosomes are organized in distinct morphological and functional regions. Our data also indicate that SNX16 is involved in tubule formation and cholesterol transport as well as trafficking of the tetraspanin CD81, suggesting that the protein plays a role in the regulation of late endosome membrane dynamics.

The yeast p24 complex regulates GPI-anchored protein transport and quality control by monitoring anchor remodeling

Two functions of the p24 complex are described: one connects GPI-anchored proteins to COPII proteins at ER exit sites to facilitate their incorporation into ER-derived vesicles, and the other serves in quality control of GPI-anchored proteins to retrieve unremodeled GPI-anchored proteins from the Golgi back to the ER.

Glycosylphosphatidylinositol (GPI)-anchored proteins are secretory proteins that are attached to the cell surface of eukaryotic cells by a glycolipid moiety. Once GPI anchoring has occurred in the lumen of the endoplasmic reticulum (ER), the structure of the lipid part on the GPI anchor undergoes a remodeling process prior to ER exit. In this study, we provide evidence suggesting that the yeast p24 complex, through binding specifically to GPI-anchored proteins in an anchor-dependent manner, plays a dual role in their selective trafficking. First, the p24 complex promotes efficient ER exit of remodeled GPI-anchored proteins after concentration by connecting them with the COPII coat and thus facilitates their incorporation into vesicles. Second, it retrieves escaped, unremodeled GPI-anchored proteins from the Golgi to the ER in COPI vesicles. Therefore the p24 complex, by sensing the status of the GPI anchor, regulates GPI-anchored protein intracellular transport and coordinates this with correct anchor remodeling.

Role of AP1 and Gadkin in the traffic of secretory endo-lysosomes

Whereas lysosome-related organelles (LRO) of specialized cells display both exocytic and endocytic features, lysosomes in nonspecialized cells can also acquire the property to fuse with the plasma membrane upon an acute rise in cytosolic calcium. Here, we characterize this unconventional secretory pathway in fibroblast-like cells, by monitoring the appearance of Lamp1 on the plasma membrane and the release of lysosomal enzymes into the medium. After sequential ablation of endocytic compartments in living cells, we find that donor membranes primarily derive from a late compartment, but that an early compartment is also involved. Strikingly, this endo-secretory process is not affected by treatments that inhibit endosome dynamics (microtubule depolymerization, cholesterol accumulation, overexpression of Rab7 or its effector Rab-interacting lysosomal protein [RILP], overexpression of Rab5 mutants), but depends on Rab27a, a GTPase involved in LRO secretion, and is controlled by F-actin. Moreover, we find that this unconventional endo-secretory pathway requires the adaptor protein complexes AP1, Gadkin (which recruits AP1 by binding to the γ1 subunit), and AP2, but not AP3. We conclude that a specific fraction of the AP2-derived endocytic pathway is dedicated to secretory purposes under the control of AP1 and Gadkin.

p23/Tmp21 associates with protein kinase Cdelta (PKCdelta) and modulates its apoptotic function

There is emerging evidence that C1 domains, motifs originally identified in PKC isozymes and responsible for binding of phorbol esters and diacylglycerol, interact with the Golgi/endoplasmic reticulum protein p23 (Tmp21). In this study, we investigated whether PKCδ, a kinase widely implicated in apoptosis and inhibition of cell cycle progression, associates with p23 and determined the potential functional implications of this interaction. Using a yeast two-hybrid approach, we found that the PKCδ C1b domain associates with p23 and identified two key residues (Asp(245) and Met(266)) implicated in this interaction. Interestingly, silencing p23 from LNCaP prostate cancer cells using RNAi markedly enhanced PKCδ-dependent apoptosis and activation of PKCδ downstream effectors ROCK and JNK by phorbol 12-myristate 13-acetate. Moreover, translocation of PKCδ to the plasma membrane by phorbol 12-myristate 13-acetate was enhanced in p23-depleted LNCaP cells. Notably, a PKCδ mutant that failed to interact with p23 triggered a strong apoptotic response when expressed in LNCaP cells. In summary, our data compellingly support the concept that C1 domains have dual roles both in lipid and protein associations and provide strong evidence that p23 acts as an anchoring protein that retains PKCδ at the perinuclear region, thus limiting the availability of this kinase for activation in response to stimuli.

Role of EBAG9 protein in coat protein complex I-dependent glycoprotein maturation and secretion processes in tumor cells

Many proteins mature within the secretory pathway by the acquisition of glycans. Failure to maintain the proper distribution of the glycosylation machinery might lead to disease. High expression levels of the ubiquitous Golgi protein estrogen receptor-binding fragment-associated gene 9 (EBAG9) in human tumors correlate with poor clinical prognosis, and EBAG9 overexpression in epithelial cell lines induces truncated glycans, typical of many carcinomas. Here, we addressed the pathogenetic link between EBAG9 expression and the alteration of the cellular glycome. We applied confocal microscopy, live imaging, pulse-chase labeling in conjunction with immunoprecipitation, and enzymatic activity assays in a variety of EBAG9-overexpressing or depleted epithelial tumor cell lines. EBAG9 shuttles between the ER-Golgi intermediate compartment and the cis-Golgi, and we demonstrate association of EBAG9 with coat protein complex I (COPI)-coated transport vesicles. EBAG9 overexpression imposes delay of endoplasmic reticulum-to-Golgi transport and mislocalizes components of the ER quality control and glycosylation machinery. Conversely, EBAG9 down-regulation accelerates glycoprotein transport through the Golgi and enhances mannosidase activity. Thus, EBAG9 acts as a negative regulator of a COPI-dependent ER-to-Golgi transport pathway in epithelial cells and represents a novel pathogenetic principle in which interference with intracellular membrane trafficking results in the emergence of a tumor-associated glycome.

p28, a novel ERGIC/cis Golgi protein, required for Golgi ribbon formation

The mammalian Golgi apparatus consists of individual cisternae that are stacked in a polarized manner to form the compact zones of the Golgi. Several stacks are linked to form a ribbon via dynamic lateral bridges. The determinants required for maintaining the characteristic Golgi structure are incompletely understood. Here, we have characterized p28, a new gamma-subfamily member of p24 membrane proteins. p28 localized to endoplasmic reticulum-Golgi intermediate compartment (ERGIC) and cis Golgi and accumulated in the ERGIC upon Brefeldin A treatment, typical for a protein cycling in the early secretory pathway. p28 interacted with a subset of p24 proteins. Its depletion by small interfering RNA (siRNA) led to fragmentation of the Golgi without affecting the overall organization of microtubules but considerably reducing the amount of acetylated tubulin. The distribution of COPI and tethers, including GM130, was not affected. At the ultrastructural level, the Golgi fragments appeared as mini-stacks with apparently unchanged cis-trans topology. Golgi fragmentation did not impair anterograde or retrograde traffic. Fluorescence recovery after photobleaching (FRAP) experiments revealed that silencing p28 prevents protein exchange between Golgi stacks during reassembly after Brefeldin A-induced Golgi breakdown. These results show that the formation of a Golgi ribbon requires the structural membrane protein p28 in addition to previously identified SNAREs, coat proteins and tethers.

Using transgenic animal models in neuroendocrine research: lessons from Xenopus laevis

Transgenic animals are commonly employed to explore the function of individual proteins. Transgenic animal models include the mouse, the zebrafish, and the South African clawed toad Xenopus laevis. In contrast to mice and zebrafish, with Xenopus transgenesis DNA integration is mostly achieved in the one-cell stage. Moreover, Xenopus (as well as zebrafish) eggs are relatively large, the embryos are transparent, a large offspring is generated, and maintenance of the offspring is easy. In our transgenic studies in Xenopus, we focus on the well-characterized neuroendocrine melanotrope cells of the pituitary pars intermedia that are regulated during the process of adaptation of Xenopus to a changing environment. When the animal is placed on a black background, the melanotrope cells produce and process large amounts of the prohormone proopiomelanocortin (POMC). We apply stable melanotrope-specific transgenesis that is achieved by mixing a Xenopus POMC-promoter/transgene construct with sperm nuclei and injecting this mixture into unfertilized eggs. Since in the melanotrope cells the POMC promoter is much more active in black-adapted animals, the level of transgene expression can be manipulated by placing the animal on either a black or a white background. In this paper we review the possibilities of the Xenopus melanotrope-specific transgenic approach. Following a brief overview of the functioning of Xenopus melanotrope cells, stable melanotrope-specific transgenesis is discussed and our transgenic studies on brain-derived neurotrophic factor and secretory pathway components are described as examples of the transgenic approach in a physiological context and close to the in vivo situation.

Functional diversity among p24 subfamily members

BACKGROUND INFORMATION

The p24 protein family plays an important but unclear role at the ER (endoplasmic reticulum)-Golgi interface. A p24 member from each subfamily (p24alpha(3), beta(1), gamma(3) and delta(2)) is upregulated with the prohormone POMC (pro-opiomelanocortin) when Xenopus laevis intermediate pituitary melanotrope cells are physiologically activated. Here we explored the role of p24 by generating and analysing Xenopus with melanotrope cell-specific transgene expression of p24beta(1) or p24gamma(3), two of the p24 proteins coexpressed with POMC, and compared the results with those previously reported for the two other coexpressed p24s (p24alpha(3) and p24delta(2)).

RESULTS

The transgene expression of p24beta(1) or p24gamma(3) did not affect the endogenous p24 proteins or affected only endogenous p24gamma(3) respectively, whereas in transgenics expressing p24alpha(3) and p24delta(2), the levels of all endogenous p24 proteins were strongly decreased. Nevertheless, as for p24alpha(3) but albeit to a lesser extent, in the p24beta(1)-transgenic melanotrope cells the rate of cargo cleavage was reduced, probably reflecting reduced cargo transport from the ER, and POMC glycosylation and sulfation in the Golgi were not affected. The p24gamma(3)-transgenic cells displayed features of both the p24alpha(3)-transgenics (reduced cargo cleavage, normal POMC sulfation) and the p24delta(2)-transgenics (affected POMC glycosylation).

CONCLUSIONS

Our results show that the four upregulated proteins p24alpha(3), beta(1), gamma(3) and delta(2) have non-redundant roles in the early secretory pathway, and suggest that each p24 subfamily member provides a proper ER/Golgi subcompartmental microenvironment, together allowing correct secretory protein transport and processing.

Concentration of GPI-anchored proteins upon ER exit in yeast

Previous biochemical work has revealed two parallel routes of exit from the endoplasmic reticulum (ER) in the yeast Saccharomyces cerevisiae, one seemingly specific for glycosyl-phosphatidylinositol (GPI)-anchored proteins. Using the coat protein II (COPII) mutant sec31-1, we visualized ER exit sites (ERES) and identified three distinct ERES populations in vivo. One contains glycosylated pro-alpha-factor, the second contains the GPI-anchored proteins Cwp2p, Ccw14p and Tos6p and the third is enriched with the hexose transporter, Hxt1p. Concentration of GPI-anchored proteins prior to budding requires anchor remodeling, and Hxt1p incorporation into ERES requires the COPII components Sec12p and Sec16p. Additionally, we have found that GPI-anchored protein ER exit is controlled by the p24 family member Emp24p, whereas ER export of most transmembrane proteins requires the Cornichon homologue Erv14p.

Localization and regional distribution of p23/TMP21 in the brain

Sequential processing of amyloid precursor protein by beta- and gamma-secretases generates Alzheimer's disease (AD)-associated beta-amyloid peptides. Recently it was reported that the transmembrane protein p23/TMP21 associates with gamma-secretase, and negatively regulates beta-amyloid production. Despite the link between p23 function and AD pathogenesis, the expression of p23 has not been examined in the brain. Here, we describe the detailed immunohistochemical characterization of p23 expression in rodent and human brain. We report that p23 is co-expressed with gamma-secretase subunits in select neuronal cell populations in rodent brain. Interestingly, the steady-state level of p23 in the brain is high during embryonic development and then declines after birth. Furthermore, the steady-state p23 levels are reduced in the brains of individuals with AD. We conclude that p23 is expressed in neurons throughout the brain and the decline in p23 expression during postnatal development may significantly contribute to enhanced beta-amyloid production in the adult brain.

In vitro budding of intralumenal vesicles into late endosomes is regulated by Alix and Tsg101

Endosomes along the degradation pathway leading to lysosomes accumulate membranes in their lumen and thus exhibit a characteristic multivesicular appearance. These lumenal membranes typically incorporate down-regulated EGF receptor destined for degradation, but the mechanisms that control their formation remain poorly characterized. Here, we describe a novel quantitative biochemical assay that reconstitutes the formation of lumenal vesicles within late endosomes in vitro. Vesicle budding into the endosome lumen was time-, temperature-, pH-, and energy-dependent and required cytosolic factors and endosome membrane components. Our light and electron microscopy analysis showed that the compartment supporting the budding process was accessible to endocytosed bulk tracers and EGF receptor. We also found that the EGF receptor became protected against trypsin in our assay, indicating that it was sorted into the intraendosomal vesicles that were formed in vitro. Our data show that the formation of intralumenal vesicles is ESCRT-dependent, because the process was inhibited by the K173Q dominant negative mutant of hVps4. Moreover, we find that the ESCRT-I subunit Tsg101 and its partner Alix control intralumenal vesicle formation, by acting as positive and negative regulators, respectively. We conclude that budding of the limiting membrane toward the late endosome lumen, which leads to the formation of intraendosomal vesicles, is controlled by the positive and negative functions of Tsg101 and Alix, respectively.

Novel oxidative stress-responsive gene ERS25 functions as a regulator of the heat-shock and cell death response

Members of the yeast p24 family, including Emp24p and Erv25p, exist as heteromeric complexes that have been proposed to cycle between the endoplasmic reticulum (ER) and Golgi compartments. The specific functions and sites of action of p24 proteins are still unknown. Here we identified a human homolog of the yeast p24 family of proteins, named ERS25 (endoplasmic reticulum stress-response protein 25), and investigated its role in stress response. ERS25 is predicted to have an ER localization signal peptide, a GOLD (Golgi dynamics) domain, which is found in several eukaryotic Golgi and lipid-trafficking proteins, a coiled-coil region, and a transmembrane domain. We demonstrate that ERS25 is localized to the ER and is induced by ER-specific stress, heat shock, and oxidative stress. The selective induction of ERS25 by brefeldin A, but not tunicamycin, implicates the involvement of ERS25 in protein trafficking between the ER and the Golgi. Small interfering RNA-mediated inhibition of ERS25 results in a significant decrease in apoptosis as well as a reduction of reactive oxygen species induced by oxidative stress. Moreover, ERS25 depletion results in a significant increase in the levels of the ER chaperone HSP70 in response to heat-shock stress through increased levels of HSF-1. We also found that inhibition of ERS25 induction in response to heat shock enhanced the binding of HSP70 to Apaf-1, which is likely to interfere in stress-mediated apoptosis. Together, the data presented here demonstrate that ERS25 may play a critical role in regulation of heat-shock response and apoptosis.

The yeast p24 complex is required for the formation of COPI retrograde transport vesicles from the Golgi apparatus

The p24 family members are transmembrane proteins assembled into heteromeric complexes that continuously cycle between the ER and the Golgi apparatus. These cargo proteins were assumed to play a structural role in COPI budding because of their major presence in mammalian COPI vesicles. However, this putative function has not been proved conclusively so far. Furthermore, deletion of all eight yeast p24 family members does not produce severe transport phenotypes, suggesting that the p24 complex is not essential for COPI function. In this paper we provide direct evidence that the yeast p24 complex plays an active role in retrograde transport from Golgi to ER by facilitating the formation of COPI-coated vesicles. Therefore, our results demonstrate that p24 proteins are important for vesicle formation instead of simply being a passive traveler, supporting the model in which cargo together with a small GTPase of the ARF superfamily and coat subunits act as primer for vesicle formation.

Disparate effects of p24alpha and p24delta on secretory protein transport and processing

Background

The p24 family is thought to be somehow involved in endoplasmic reticulum (ER)-to-Golgi protein transport. A subset of the p24 proteins (p24α₃, -β₁, -γ₃ and -δ₂) is upregulated when Xenopus laevis intermediate pituitary melanotrope cells are physiologically activated to produce vast amounts of their major secretory cargo, the prohormone proopiomelanocortin (POMC).

Methodology/Principal Findings

Here we find that transgene expression of p24α₃ or p24δ₂ specifically in the Xenopus melanotrope cells in both cases causes an effective displacement of the endogenous p24 proteins, resulting in severely distorted p24 systems and disparate melanotrope cell phenotypes. Transgene expression of p24α₃ greatly reduces POMC transport and leads to accumulation of the prohormone in large, ER-localized electron-dense structures, whereas p24δ₂-transgenesis does not influence the overall ultrastructure of the cells nor POMC transport and cleavage, but affects the Golgi-based processes of POMC glycomaturation and sulfation.

Conclusions/Significance

Transgenic expression of two distinct p24 family members has disparate effects on secretory pathway functioning, illustrating the specificity and non-redundancy of our transgenic approach. We conclude that members of the p24 family furnish subcompartments of the secretory pathway with specific sets of machinery cargo to provide the proper microenvironments for efficient and correct secretory protein transport and processing.

Drosophila melanogaster p24 genes have developmental, tissue-specific, and sex-specific expression patterns and functions

Genes encoding members of the p24 family of intracellular trafficking proteins are present throughout animal and plant lineages. However, very little is known about p24 developmental, spatial, or sex-specific expression patterns or how localized expression affects function. We investigated these problems in Drosophila melanogaster, which contains nine genes encoding p24 proteins. One of these genes, logjam (loj), is expressed in the adult female nervous system and ovaries and is essential for oviposition. Nervous system-specific expression of loj, but not ovary-specific expression, rescues the behavioral defect of mutants. The Loj protein localizes to punctate structures in the cellular cytoplasm. These structures colocalize with a marker specific to the intermediate compartment and cis-Golgi, consistent with experimental evidence from other systems suggesting that p24 proteins function in intracellular transport between the endoplasmic reticulum and Golgi. Our findings reveal that Drosophila p24 transcripts are developmentally and tissue-specifically expressed. CG31787 is male-specifically expressed gene that is present during the larval, pupal, and adult stages. Female CG9053 mRNA is limited to the head, whereas males express this gene widely. Together, our studies provide experimental evidence indicating that some p24 genes have sex-specific expression patterns and tissue- and sex-limited functions.

Differential localization of coatomer complex isoforms within the Golgi apparatus

Coatomer, the coat protein of coat protein complex (COP)I-vesicles, is a soluble protein complex made up of seven subunits, alpha-, beta-, beta'-, gamma-, delta-, epsilon-, and zeta-COP. Higher eukaryotes have two paralogous versions of the gamma- and zeta- subunits, termed gamma1- and gamma2-COP and zeta1- and zeta2-COP. Different combinations of these subunits are known to exist within coatomer complexes, and gamma1/zeta1-, gamma1/zeta2-, and gamma2/zeta1-COP represent the major coatomer populations in mammals. The role of COPI vesicles in the early secretory pathway is the subject of considerable debate. To help to resolve this discussion, we used quantitative immunoelectron microscopy and found that significant localization differences for COPI-isoforms do exist, with a preference for gamma1zeta1- and gamma1zeta2-coatomer in the early Golgi apparatus and gamma2zeta1-coatomer in the late Golgi apparatus. These differences suggest distinct functions for coatomer isoforms in a manner similar to clathrin/adaptor vesicles, where different adaptor proteins serve particular transport routes.

Dual roles of the transmembrane protein p23/TMP21 in the modulation of amyloid precursor protein metabolism

BACKGROUND

Alzheimer's disease (AD) is characterized by cerebral deposition of beta-amyloid (Abeta) peptides. Abeta is released from ectodomain cleaved amyloid precursor protein (APP) via intramembranous proteolysis by gamma-secretase, a complex consisting of presenilin and a few other proteins. p23/TMP21, a member of the p24 family type I transmembrane proteins, was recently identified as a presenilin complex component capable of modulating gamma-secretase cleavage. The p24 family proteins form oligomeric complexes and regulate vesicular trafficking in the early secretory pathway, but their role in APP trafficking has not been investigated.

RESULTS

Here, we report that siRNA-mediated depletion of p23 in N2a neuroblastoma and HeLa cells produces concomitant knockdown of additional p24 family proteins and increases secretion of sAPP. Furthermore, intact cell and cell-free Abeta production increases following p23 knockdown, similar to data reported earlier using HEK293 cells. However, we find that p23 is not present in mature gamma-secretase complexes isolated using an active-site gamma-secretase inhibitor. Depletion of p23 and expression of a familial AD-linked PS1 mutant have additive effects on Abeta42 production. Knockdown of p23 expression confers biosynthetic stability to nascent APP, allowing its efficient maturation and surface accumulation. Moreover, immunoisolation analyses show decrease in co-residence of APP and the APP adaptor Mint3. Thus, multiple lines of evidence indicate that p23 function influences APP trafficking and sAPP release independent of its reported role in gamma-secretase modulation.

CONCLUSION

These data assign significance to p24 family proteins in regulating APP trafficking in the continuum of bidirectional transport between the ER and Golgi, and ascribe new relevance to the regulation of early trafficking in AD pathogenesis.

Mutations in the SPG3A gene encoding the GTPase atlastin interfere with vesicle trafficking in the ER/Golgi interface and Golgi morphogenesis

Mutations in SPG3A causing autosomal dominant pure spastic paraplegia led to identification of atlastin, a new dynamin-like large GTPase. Atlastin is localized in the endoplasmic reticulum, the Golgi, neurites and growth cones and has been implicated in neurite outgrowth. To investigate whether it exerts its activity in the early secretory system, we expressed normal and mutant atlastin in cell culture. Pathogenic mutations in the GTPase domain interfered with the maturation of Golgi complexes by preventing the budding of vesicles from the endoplasmic reticulum, whereas mutations in other regions of the protein disrupted fission of endoplasmic reticulum-derived vesicles or their migration to their Golgi target. Atlastin, therefore, plays a role in vesicle trafficking in the ER/Golgi interface. Furthermore, atlastin partially co-localized with proteins of the p24/emp/gp25L family that regulate vesicle budding and trafficking in the early secretory pathway, and co-immunoprecipitated p24, suggesting a functional relationship that should be further explored.

A subset of p23 localized on secretory granules in pancreatic beta-cells

Proteins on the membrane of secretory granules (SGs) involved in their biogenesis and exocytosis are poorly characterized compared with those of synaptic vesicle in neurons. Thus the secretory granule membrane was prepared from a mouse pancreatic beta-cell line MIN6 by subcellular fractionation, and protein constituents were analyzed by microscale two-dimensional liquid chromatography coupled with electrospray ionization tandem mass spectrometry. Using this proteomics approach, one of the p24 family proteins, p23, was unexpectedly found in the granule fraction, although p24 proteins are generally regarded as functioning in the early secretory pathways between the endoplasmic reticulum and the Golgi apparatus. We further showed that p23 is expressed at high levels in endocrine cells. Furthermore, immunocytochemical analyses of pancreatic beta-cells at the light and electron microscopic levels demonstrated that a significant amount of p23 is localized on the insulin granule membrane, although it is most intensely concentrated at the cis-Golgi compartment as previously shown in non-endocrine cells. These findings suggest that a fraction of p23 enters post-Golgi compartments and may function in the biogenesis and/or quality control of SGs.

Evidence for a role of transmembrane protein p25 in localization of protein tyrosine phosphatase TC48 to the ER

T-cell protein tyrosine phosphatase gives rise to two splice isoforms: TC48, which is localized to the endoplasmic reticulum (ER) and TC45, a nuclear protein. The present study was undertaken to identify proteins that are involved in targeting TC48 to the ER. We identified two TC48-interacting proteins, p25 and p23, from a yeast two-hybrid screen. p23 and p25 are members of a family of putative cargo receptors that are important for vesicular trafficking between Golgi complex and ER. Both p23 and p25 associate with overexpressed TC48 in Cos-1 cells as determined by coimmunoprecipitation. A significant amount of TC48 colocalized initially with ERGIC and Golgi complex markers (in addition to ER and nuclear membrane localization) and was then retrieved to the ER. Coexpression with p25 enhanced ER localization of TC48, whereas coexpression with p23 resulted in its trapping in membranous structures. Coexpression of a p25 mutant lacking the ER-localization signal KKxx resulted in enhanced Golgi localization of TC48. Forty C-terminal amino acid residues of TC48 (position 376-415) were sufficient for interaction with p23 (but not with p25) and targeted green fluorescence protein (GFP) to the Golgi complex. Targeting of GFP to the ER required 66 C-terminal amino acid residues of TC48 (position 350-415), which showed interaction with p25 and p23. We suggest that TC48 translocates to the Golgi complex along the secretory pathway, whereas its ER localization is maintained by selective retrieval enabled by interactions with p25 and p23.

Endosome-to-cytosol transport of viral nucleocapsids

During viral infection, fusion of the viral envelope with endosomal membranes and nucleocapsid release were thought to be concomitant events. We show here that for the vesicular stomatitis virus they occur sequentially, at two successive steps of the endocytic pathway. Fusion already occurs in transport intermediates between early and late endosomes, presumably releasing the nucleocapsid within the lumen of intra-endosomal vesicles, where it remains hidden. Transport to late endosomes is then required for the nucleocapsid to be delivered to the cytoplasm. This last step, which initiates infection, depends on the late endosomal lipid lysobisphosphatidic acid (LBPA) and its putative effector Alix/AIP1, and is regulated by phosphatidylinositol-3-phosphate (PtdIns3P) signalling via the PtdIns3P-binding protein Snx16. We conclude that the nucleocapsid is exported into the cytoplasm after the back-fusion of internal vesicles with the limiting membrane of late endosomes, and that this process is controlled by the phospholipids LBPA and PtdIns3P and their effectors.

Golgin tethers define subpopulations of COPI vesicles

Coiled-coil proteins of the golgin family have been implicated in intra-Golgi transport through tethering coat protein complex I (COPI) vesicles. The p115-golgin tether is the best studied, and here we characterize the golgin-84-CASP tether. The vesicles bound by this tether were strikingly different from those bound by the p115-golgin tether in that they lacked members of the p24 family of putative cargo receptors and contained enzymes instead of anterograde cargo. Microinjected golgin-84 or CASP also inhibited Golgi-enzyme transport to the endoplasmic reticulum, further implicating this tether in retrograde transport. These and other golgins may modulate the flow patterns within the Golgi stack.

Tethering assays for COPI vesicles mediated by golgins

A method is described that allows the attachment of COPI vesicles and Golgi membranes to glass slides that can then be analyzed using electron microscopy (EM) and immuno-EM methods. Subpopulations of COPI vesicles can be bound selectively using recombinant golgins. Alternatively, COPI vesicles can be attached to prebound Golgi membranes. Marking these vesicles selectively with biotin allows their site of attachment to be identified.

Sorting signals in the cytosolic tail of plant p24 proteins involved in the interaction with the COPII coat

The ability of the cytosolic tail of a plant p24 protein to bind COPI and COPII subunits from plant and animal sources in vitro has been examined. We have found that a dihydrophobic motif in the -7,-8 position (relative to the cytosolic carboxy-terminus), which strongly cooperates with a dilysine motif in the -3,-4 position for COPI binding, is required for COPII binding. In addition, we show that COPI and COPII coat proteins from plant cytosol compete for binding to the sorting motifs in these tails. Only in the absence of the dilysine motif in the -3,-4 position or after COPI depletion could we observe COPII binding to the p24 tail. This competition is not observed when using rat liver cytosol.