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

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.

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.

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.

Cargo selection in the early secretory pathway of African trypanosomes

Membrane and secretory proteins are synthesized by ribosomes and then enter the endoplasmic reticulum (ER) where they undergo glycosylation and quality control for proper folding. Subsequently, proteins are transported to the Golgi apparatus and then sorted to the plasma membrane or intracellular organelles. Transport vesicles are formed at ER-exit sites (ERES) on the ER with several coat protein complexes. Cargo proteins loaded into the vesicles are selected by specific interactions with cargo receptors and/or adaptors during vesicle formation. p24 family and intracellular lectin ERGIC-53-membrane proteins are the known cargo receptors acting in the early secretory pathway (ER-Golgi). Oligomerization of the cargo receptors have been suggested to play an important role in cargo selection and sorting via posttranslational modifications in fungi and metazoans. On the other hand, the mechanisms involved in the early secretory pathway in protozoa remain unclear. In this review, we focus on Trypanosoma brucei as a representative of protozoan and discuss differences and commonalities in the molecular mechanisms of its early secretory pathway compared with other organisms.

Mammalian GPI-anchor modifications and the enzymes involved

Glycosylphosphatidylinositol (GPI) is a glycolipid added to the C-terminus of a large variety of proteins in eukaryotes, thereby anchoring these proteins to the cell surface. More than 150 different human proteins are modified with GPI, and GPI-anchored proteins (GPI-APs) play critical roles in embryogenesis, neurogenesis, immunity, and fertilization. GPI-APs are biosynthesized in the endoplasmic reticulum (ER) and transported to the plasma membrane via the Golgi apparatus. During transport, GPI-APs undergo structural remodeling that is important for the efficient folding and sorting of GPI-APs. Asparagine-linked glycan-dependent folding and deacylation by PGAP1 work together to ensure that correctly folded GPI-APs are transported from the ER to the Golgi. Remodeling of the GPI lipid moiety is critical for the association of GPI-APs with lipid rafts. On the cell surface, certain GPI-APs are cleaved by GPI cleavage enzymes and released from the membrane, a key event in processes such as spermatogenesis and neurogenesis. In this review, we discuss the enzymes involved in GPI-AP biosynthesis and the fate of GPI-APs in mammalian cells, with a focus on the assembly, folding, degradation, and cleavage of GPI-APs.

Cdc1p is a Golgi-localized glycosylphosphatidylinositol-anchored protein remodelase

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) undergo extensive posttranslational modifications and remodeling, including the addition and subsequent removal of phosphoethanolamine (EtNP) from mannose 1 (Man1) and mannose 2 (Man2) of the glycan moiety. Removal of EtNP from Man1 is catalyzed by Cdc1p, an event that has previously been considered to occur in the endoplasmic reticulum (ER). We establish that Cdc1p is in fact a cis/medial Golgi membrane protein that relies on the COPI coatomer for its retention in this organelle. We also determine that Cdc1p does not cycle between the Golgi and the ER, and consistent with this finding, when expressed at endogenous levels ER-localized Cdc1p-HDEL is unable to support the growth of cdc1Δ cells. Our cdc1 temperature-sensitive alleles are defective in the transport of a prototypical GPI-AP-Gas1p to the cell surface, a finding we posit reveals a novel Golgi-localized quality control warrant. Thus, yeast cells scrutinize GPI-APs in the ER and also in the Golgi, where removal of EtNP from Man2 (via Ted1p in the ER) and from Man1 (by Cdc1p in the Golgi) functions as a quality assurance signal.

p24 Family Proteins Are Involved in Transport to the Plasma Membrane of GPI-Anchored Proteins in Plants

p24 proteins are a family of type-I membrane proteins that cycle between the endoplasmic reticulum (ER) and the Golgi apparatus via Coat Protein I (COPI)- and COPII-coated vesicles. These proteins have been proposed to function as cargo receptors, but the identity of putative cargos in plants is still elusive. We previously generated an Arabidopsis (Arabidopsis thaliana) quadruple loss-of-function mutant affecting p24 genes from the δ-1 subclass of the p24 delta subfamily (p24δ3δ4δ5δ6 mutant). This mutant also had reduced protein levels of other p24 family proteins and was found to be sensitive to salt stress. Here, we used this mutant to test the possible involvement of p24 proteins in the transport to the plasma membrane of glycosylphosphatidylinositol (GPI)-anchored proteins. We found that GPI-anchored proteins mostly localized to the ER in p24δ3δ4δ5δ6 mutant cells, in contrast to plasma membrane proteins with other types of membrane attachment. The plasma membrane localization of GPI-anchored proteins was restored in the p24δ3δ4δ5δ6 mutant upon transient expression of a single member of the p24 δ-1 subclass, RFP-p24δ5, which was dependent on the coiled-coil domain in p24δ5. The coiled-coil domain was also important for a direct interaction between p24δ5 and the GPI-anchored protein arabinogalactan protein4 (AGP4). These results suggest that Arabidopsis p24 proteins are involved in ER export and transport to the plasma membrane of GPI-anchored proteins.

Mapping Gene Circuits Essential for Germ Layer Differentiation via Loss-of-Function Screens in Haploid Human Embryonic Stem Cells

Pluripotent stem cells can differentiate into all embryonic germ layers, yet the genes essential for these cell fate transitions in human remain elusive. Here, we mapped the essential genes for the differentiation of human pluripotent stem cells (hPSCs) into the three germ layers by using a genome-wide loss-of-function library established in haploid hPSCs. Strikingly, we observed a high fraction of essential genes associated with plasma membrane, highlighting signaling pathways needed for each lineage differentiation. Interestingly, analysis of all hereditary neurological disorders uncovered high essentiality among microcephaly-causing genes. Furthermore, we demonstrated lineage-specific hierarchies among essential transcription factors and a set of Golgi- and endoplasmic reticulum-related genes needed for the differentiation into all germ layers. Our work sheds light on the gene networks regulating early gastrulation events in human by defining essential drivers of specific embryonic germ layer fates and essential genes for the exit from pluripotency.

Biosynthesis and biology of mammalian GPI-anchored proteins

At least 150 human proteins are glycosylphosphatidylinositol-anchored proteins (GPI-APs). The protein moiety of GPI-APs lacking transmembrane domains is anchored to the plasma membrane with GPI covalently attached to the C-terminus. The GPI consists of the conserved core glycan, phosphatidylinositol and glycan side chains. The entire GPI-AP is anchored to the outer leaflet of the lipid bilayer by insertion of fatty chains of phosphatidylinositol. Because of GPI-dependent membrane anchoring, GPI-APs have some unique characteristics. The most prominent feature of GPI-APs is their association with membrane microdomains or membrane rafts. In the polarized cells such as epithelial cells, many GPI-APs are exclusively expressed in the apical surfaces, whereas some GPI-APs are preferentially expressed in the basolateral surfaces. Several GPI-APs act as transcytotic transporters carrying their ligands from one compartment to another. Some GPI-APs are shed from the membrane after cleavage within the GPI by a GPI-specific phospholipase or a glycosidase. In this review, I will summarize the current understanding of GPI-AP biosynthesis in mammalian cells and discuss examples of GPI-dependent functions of mammalian GPI-APs.

Transmembrane emp24 domain proteins in development and disease

Regulated transport through the secretory pathway is essential for embryonic development and homeostasis. Disruptions in this process impact cell fate, differentiation and survival, often resulting in abnormalities in morphogenesis and in disease. Several congenital malformations are caused by mutations in genes coding for proteins that regulate cargo protein transport in the secretory pathway. The severity of mutant phenotypes and the unclear aetiology of transport protein-associated pathologies have motivated research on the regulation and mechanisms through which these proteins contribute to morphogenesis. This review focuses on the role of the p24/transmembrane emp24 domain (TMED) family of cargo receptors in development and disease.

RAB21 interacts with TMED10 and modulates its localization and abundance

Membrane trafficking controls vesicular transport of cargo between cellular compartments. Vesicular trafficking is essential for cellular homeostasis and dysfunctional trafficking is linked to several pathologies such as neurodegenerative diseases. Following endocytosis, early endosomes act as sorting stations of internalized materials, routing cargo toward various fates. One important class of membrane trafficking regulators are RAB GTPases. RAB21 has been associated with multiple functions and regulates integrin internalization, endosomal sorting of specific clathrin-independent cargo and autophagy. Although RAB21 is mostly associated with early endosomes, it has been shown to mediate a specific sorting event at the Golgi. From mass spectrometry data, we identified a GTP-favored interaction between RAB21 and TMED10 and 9, essential regulators of COPI and COPII vesicles. Using RAB21 knockout cells, we describe the role of RAB21 in modulating TMED10 Golgi localization. Taken together, our study suggests a new potential function of RAB21 in modulating TMED10 trafficking, with relevance to neurodegenerative disorders.

Summary: A small early endosomal RAB GTPase is found to interact with p24 family members, with potential impacts on p24 functions.

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.

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.

Crystallographic analysis of murine p24γ2 Golgi dynamics domain

The p24 family proteins form homo- and hetero-oligomeric complexes for efficient transport of cargo proteins from the endoplasmic reticulum to the Golgi apparatus. It consists of four subfamilies (p24α, p24β, p24γ, and p24δ). p24γ2 plays crucial roles in the selective transport of glycosylphosphatidylinositol-anchored proteins. Here, we determined the crystal structure of mouse p24γ2 Golgi dynamics (GOLD) domain at 2.8 Å resolution by the single anomalous diffraction method using intrinsic sulfur atoms. In spite of low sequence identity among p24 family proteins, p24γ2 GOLD domain assumes a β-sandwich fold, similar to that of p24β1 or p24δ1. An additional short α-helix is observed at the C-terminus of the p24γ2 GOLD domain. Intriguingly, p24γ2 GOLD domains crystallize as dimers, and dimer formation seems assisted by the short α-helix. Dimerization modes of GOLD domains are compared among p24 family proteins. Proteins 2017; 85:764-770. © 2016 Wiley Periodicals, Inc.

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.

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.

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.

Aspergillus fumigatus Cap59-like protein A is involved in α1,3-mannosylation of GPI-anchors

Glycosylphosphatidylinositol (GPI) attaches a variety of eukaryotic proteins to the outer leaflet of the plasma membrane. In fungi, these proteins may also be transferred to the cell wall, to which they are covalently linked via a remnant of the GPI-anchor. They play crucial physiological roles in cell-cell interactions, adhesion or cell wall biogenesis. The biosynthesis of GPI-anchors in the endoplasmic reticulum, their transfer to proteins, early remodelling and transport to the Golgi apparatus has been fairly well described. In contrast, almost nothing is known about the genes and enzymes involved in adding glycan side chains to GPI after protein attachment. In this study, we characterized an α1,3-mannosyltransferase involved in maturation of GPI-anchors from the pathogenic fungus Aspergillus fumigatus. This enzyme shows homology to Cryptococcus neoformans Cap59p, a putative glycosyltransferase involved in capsule formation and virulence, and was thus named Cap59-like protein A (ClpA). Targeted deletion of the clpA gene in A. fumigatus led to absence of α1,3-mannose from mature GPI-anchors. The enzyme was further located to the Golgi-like apparatus of A. fumigatus and was shown to be active in the yeast Saccharomyces cerevisiae.

Post-Golgi anterograde transport requires GARP-dependent endosome-to-TGN retrograde transport

GARP (tethering factor)- and VAMP4 (v-SNARE)-dependent endosome-to-TGN retrograde transport is required for the efficient post-Golgi anterograde transport of cell-surface integral membrane proteins. Golgi-resident membrane proteins TMEM87A and TMEM87B are involved in endosome-to-TGN retrograde transport.

The importance of endosome-to–trans-Golgi network (TGN) retrograde transport in the anterograde transport of proteins is unclear. In this study, genome-wide screening of the factors necessary for efficient anterograde protein transport in human haploid cells identified subunits of the Golgi-associated retrograde protein (GARP) complex, a tethering factor involved in endosome-to-TGN transport. Knockout (KO) of each of the four GARP subunits, VPS51–VPS54, in HEK293 cells caused severely defective anterograde transport of both glycosylphosphatidylinositol (GPI)-anchored and transmembrane proteins from the TGN. Overexpression of VAMP4, v-SNARE, in VPS54-KO cells partially restored not only endosome-to-TGN retrograde transport, but also anterograde transport of both GPI-anchored and transmembrane proteins. Further screening for genes whose overexpression normalized the VPS54-KO phenotype identified TMEM87A, encoding an uncharacterized Golgi-resident membrane protein. Overexpression of TMEM87A or its close homologue TMEM87B in VPS54-KO cells partially restored endosome-to-TGN retrograde transport and anterograde transport. Therefore GARP- and VAMP4-dependent endosome-to-TGN retrograde transport is required for recycling of molecules critical for efficient post-Golgi anterograde transport of cell-surface integral membrane proteins. In addition, TMEM87A and TMEM87B are involved in endosome-to-TGN retrograde transport.

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.

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.