A new class of endoplasmic reticulum export signal PhiXPhiXPhi for transmembrane proteins and its selective interaction with Sec24C

Partitioning to ordered membrane domains regulates the kinetics of secretory traffic

The organelles of eukaryotic cells maintain distinct protein and lipid compositions required for their specific functions. The mechanisms by which many of these components are sorted to their specific locations remain unknown. While some motifs mediating subcellular protein localization have been identified, many membrane proteins and most membrane lipids lack known sorting determinants. A putative mechanism for sorting of membrane components is based on membrane domains known as lipid rafts, which are laterally segregated nanoscopic assemblies of specific lipids and proteins. To assess the role of such domains in the secretory pathway, we applied a robust tool for synchronized secretory protein traffic (RUSH, Retention Using Selective Hooks) to protein constructs with defined affinity for raft phases. These constructs consist solely of single-pass transmembrane domains (TMDs) and, lacking other sorting determinants, constitute probes for membrane domain-mediated trafficking. We find that while raft affinity can be sufficient for steady-state PM localization, it is not sufficient for rapid exit from the endoplasmic reticulum (ER), which is instead mediated by a short cytosolic peptide motif. In contrast, we find that Golgi exit kinetics are highly dependent on raft affinity, with raft preferring probes exiting the Golgi ~2.5-fold faster than probes with minimal raft affinity. We rationalize these observations with a kinetic model of secretory trafficking, wherein Golgi export can be facilitated by protein association with raft domains. These observations support a role for raft-like membrane domains in the secretory pathway and establish an experimental paradigm for dissecting its underlying machinery.

Two compound heterozygous variants in the CLN8 gene are responsible for neuronal cereidolipofuscinoses disorder in a child: a case report

Background

Neuronal Ceroid Lipofuscinosis (NCL) disorders, recognized as the primary cause of childhood dementia globally, constitute a spectrum of genetic abnormalities. CLN8, a subtype within NCL, is characterized by cognitive decline, motor impairment, and visual deterioration. This study focuses on an atypical case with congenital onset and a remarkably slow disease progression.

Methods

Whole-genome sequencing at 30× coverage was employed as part of a national genomics program to investigate the genetic underpinnings of rare diseases. This genomic approach aimed to challenge established classifications (vLINCL and EPMR) and explore the presence of a continuous phenotypic spectrum associated with CLN8.

Results

The whole-genome sequencing revealed two novel likely pathogenic mutations in the CLN8 gene on chromosome 8p23.3. These mutations were not previously associated with CLN8-related NCL. Contrary to established classifications (vLINCL and EPMR), our findings suggest a continuous phenotypic spectrum associated with CLN8. Pathological subcellular markers further validated the genomic insights.

Discussion

The identification of two previously undescribed likely pathogenic CLN8 gene mutations challenges traditional classifications and highlights a more nuanced phenotypic spectrum associated with CLN8. Our findings underscore the significance of genetic modifiers and interactions with unrelated genes in shaping variable phenotypic outcomes. The inclusion of pathological subcellular markers further strengthens the validity of our genomic insights. This research enhances our understanding of CLN8 disorders, emphasizing the need for comprehensive genomic analyses to elucidate the complexity of phenotypic presentations and guide tailored therapeutic strategies. The identification of new likely pathogenic mutations underscores the dynamic nature of CLN8-related NCL and the importance of individualized approaches to patient management.

Partitioning to ordered membrane domains regulates the kinetics of secretory traffic

The organelles of eukaryotic cells maintain distinct protein and lipid compositions required for their specific functions. The mechanisms by which many of these components are sorted to their specific locations remain unknown. While some motifs mediating subcellular protein localization have been identified, many membrane proteins and most membrane lipids lack known sorting determinants. A putative mechanism for sorting of membrane components is based on membrane domains known as lipid rafts, which are laterally segregated nanoscopic assemblies of specific lipids and proteins. To assess the role of such domains in the secretory pathway, we applied a robust tool for synchronized secretory protein traffic (RUSH, Retention Using Selective Hooks) to protein constructs with defined affinity for raft phases. These constructs consist solely of single-pass transmembrane domains (TMDs) and, lacking other sorting determinants, constitute probes for membrane domain-mediated trafficking. We find that while raft affinity can be sufficient for steady-state PM localization, it is not sufficient for rapid exit from the endoplasmic reticulum (ER), which is instead mediated by a short cytosolic peptide motif. In contrast, we find that Golgi exit kinetics are highly dependent on raft affinity, with raft preferring probes exiting Golgi ~2.5-fold faster than probes with minimal raft affinity. We rationalize these observations with a kinetic model of secretory trafficking, wherein Golgi export can be facilitated by protein association with raft domains. These observations support a role for raft-like membrane domains in the secretory pathway and establish an experimental paradigm for dissecting its underlying machinery.

Membrane skeleton hyperstability due to a novel alternatively spliced 4.1R can account for ellipsoidal camelid red cells with decreased deformability

The red blood cells (RBCs) of vertebrates have evolved into two basic shapes, with nucleated nonmammalian RBCs having a biconvex ellipsoidal shape and anuclear mammalian RBCs having a biconcave disk shape. In contrast, camelid RBCs are flat ellipsoids with reduced membrane deformability, suggesting altered membrane skeletal organization. However, the mechanisms responsible for their elliptocytic shape and reduced deformability have not been determined. We here showed that in alpaca RBCs, protein 4.1R, a major component of the membrane skeleton, contains an alternatively spliced exon 14-derived cassette (e14) not observed in the highly conserved 80 kDa 4.1R of other highly deformable biconcave mammalian RBCs. The inclusion of this exon, along with the preceding unordered proline- and glutamic acid-rich peptide (PE), results in a larger and unique 90 kDa camelid 4.1R. Human 4.1R containing e14 and PE, but not PE alone, showed markedly increased ability to form a spectrin-actin-4.1R ternary complex in viscosity assays. A similar facilitated ternary complex was formed by human 4.1R possessing a duplication of the spectrin-actin-binding domain, one of the mutations known to cause human hereditary elliptocytosis. The e14- and PE-containing mutant also exhibited an increased binding affinity to β-spectrin compared with WT 4.1R. Taken together, these findings indicate that 4.1R protein with the e14 cassette results in the formation and maintenance of a hyperstable membrane skeleton, resulting in rigid red ellipsoidal cells in camelid species, and suggest that membrane structure is evolutionarily regulated by alternative splicing of exons in the 4.1R gene.

Mechanisms regulating the sorting of soluble lysosomal proteins

Lysosomes are key regulators of many fundamental cellular processes such as metabolism, autophagy, immune response, cell signalling and plasma membrane repair. These highly dynamic organelles are composed of various membrane and soluble proteins, which are essential for their proper functioning. The soluble proteins include numerous proteases, glycosidases and other hydrolases, along with activators, required for catabolism. The correct sorting of soluble lysosomal proteins is crucial to ensure the proper functioning of lysosomes and is achieved through the coordinated effort of many sorting receptors, resident ER and Golgi proteins, and several cytosolic components. Mutations in a number of proteins involved in sorting soluble proteins to lysosomes result in human disease. These can range from rare diseases such as lysosome storage disorders, to more prevalent ones, such as Alzheimer’s disease, Parkinson’s disease and others, including rare neurodegenerative diseases that affect children. In this review, we discuss the mechanisms that regulate the sorting of soluble proteins to lysosomes and highlight the effects of mutations in this pathway that cause human disease. More precisely, we will review the route taken by soluble lysosomal proteins from their translation into the ER, their maturation along the Golgi apparatus, and sorting at the trans-Golgi network. We will also highlight the effects of mutations in this pathway that cause human disease.

Critical Determinants in ER-Golgi Trafficking of Enzymes Involved in Glycosylation

All living cells generate structurally complex and compositionally diverse spectra of glycans and glycoconjugates, critical for organismal evolution, development, functioning, defense, and survival. Glycosyltransferases (GTs) catalyze the glycosylation reaction between activated sugar and acceptor substrate to synthesize a wide variety of glycans. GTs are distributed among more than 130 gene families and are involved in metabolic processes, signal pathways, cell wall polysaccharide biosynthesis, cell development, and growth. Glycosylation mainly takes place in the endoplasmic reticulum (ER) and Golgi, where GTs and glycosidases involved in this process are distributed to different locations of these compartments and sequentially add or cleave various sugars to synthesize the final products of glycosylation. Therefore, delivery of these enzymes to the proper locations, the glycosylation sites, in the cell is essential and involves numerous secretory pathway components. This review presents the current state of knowledge about the mechanisms of protein trafficking between ER and Golgi. It describes what is known about the primary components of protein sorting machinery and trafficking, which are recognition sites on the proteins that are important for their interaction with the critical components of this machinery.

A systematic review of Sec24 cargo interactome

Endoplasmic reticulum (ER)-to-Golgi trafficking is an essential and highly conserved cellular process. The coat protein complex-II (COPII) arm of the trafficking machinery incorporates a wide array of cargo proteins into vesicles through direct or indirect interactions with Sec24, the principal subunit of the COPII coat. Approximately one-third of all mammalian proteins rely on the COPII-mediated secretory pathway for membrane insertion or secretion. There are four mammalian Sec24 paralogs and three yeast Sec24 paralogs with emerging evidence of paralog-specific cargo interaction motifs. Furthermore, individual paralogs also differ in their affinity for a subset of sorting motifs present on cargo proteins. As with many aspects of protein trafficking, we lack a systematic and thorough understanding of the interaction of Sec24 with cargoes. This systematic review focuses on the current knowledge of cargo binding to both yeast and mammalian Sec24 paralogs and their ER export motifs. The analyses show that Sec24 paralog specificity of cargo (and cargo receptors) range from exclusive paralog dependence or preference to partial redundancy. We also discuss how the Sec24 secretion system is hijacked by viral (eg, VSV-G, Hepatitis B envelope protein) and bacterial (eg, the enteropathogenic Escherichia coli type III secretion system effector NleA/EspI) pathogens.

Cholesterol-binding protein TSPO2 coordinates maturation and proliferation of terminally differentiating erythroblasts

TSPO2 (translocator protein 2) is a transmembrane protein specifically expressed in late erythroblasts and has been postulated to mediate intracellular redistribution of cholesterol. We identified TSPO2 as the causative gene for the HK (high-K⁺) trait with immature red cell phenotypes in dogs and investigated the effects of the TSPO2 defects on erythropoiesis in HK dogs with the TSPO2 mutation and Tspo2 knockout (Tspo2^−/−) mouse models. Bone marrow–derived erythroblasts from HK dogs showed increased binucleated and apoptotic cells at various stages of maturation and shed large nuclei with incomplete condensation when cultured in the presence of erythropoietin, indicating impaired maturation and cytokinesis. The canine TSPO2 induces cholesterol accumulation in the endoplasmic reticulum and could thereby regulate cholesterol availability by changing intracellular cholesterol distribution in erythroblasts. Tspo2^−/− mice consistently showed impaired cytokinesis with increased binucleated erythroblasts, resulting in compensated anemia, and their red cell membranes had increased Na,K-ATPase, resembling the HK phenotype in dogs. Tspo2-deficient mouse embryonic stem cell–derived erythroid progenitor (MEDEP) cells exhibited similar morphological defects associated with a cell-cycle arrest at the G₂/M phase, resulting in decreased cell proliferation and had a depletion in intracellular unesterified and esterified cholesterol. When the terminal maturation was induced, Tspo2^−/− MEDEP cells showed delays in hemoglobinization; maturation-associated phenotypic changes in CD44, CD71, and TER119 expression; and cell-cycle progression. Taken together, these findings imply that TSPO2 is essential for coordination of maturation and proliferation of erythroblasts during normal erythropoiesis.

Tales of tails in transporters

Cell nutrition, detoxification, signalling, homeostasis and response to drugs, processes related to cell growth, differentiation and survival are all mediated by plasma membrane (PM) proteins called transporters. Despite their distinct fine structures, mechanism of function, energetic requirements, kinetics and substrate specificities, all transporters are characterized by a main hydrophobic body embedded in the PM as a series of tightly packed, often intertwined, α-helices that traverse the lipid bilayer in a zigzag mode, connected with intracellular or extracellular loops and hydrophilic N- and C-termini. Whereas longstanding genetic, biochemical and biophysical evidence suggests that specific transmembrane segments, and also their connecting loops, are responsible for substrate recognition and transport dynamics, emerging evidence also reveals the functional importance of transporter N- and C-termini, in respect to transport catalysis, substrate specificity, subcellular expression, stability and signalling. This review highlights selected prototypic examples of transporters in which their termini play important roles in their functioning.

CLN8 is an endoplasmic reticulum cargo receptor that regulates lysosome biogenesis

Organelle biogenesis requires proper transport of proteins from their site of synthesis to their target subcellular compartment1-3. Lysosomal enzymes are synthesized in the endoplasmic reticulum (ER) and traffic through the Golgi complex before being transferred to the endolysosomal system4-6, but how they are transferred from the ER to the Golgi is unknown. Here, we show that ER-to-Golgi transfer of lysosomal enzymes requires CLN8, an ER-associated membrane protein whose loss of function leads to the lysosomal storage disorder, neuronal ceroid lipofuscinosis 8 (a type of Batten disease)7. ER-to-Golgi trafficking of CLN8 requires interaction with the COPII and COPI machineries via specific export and retrieval signals localized in the cytosolic carboxy terminus of CLN8. CLN8 deficiency leads to depletion of soluble enzymes in the lysosome, thus impairing lysosome biogenesis. Binding to lysosomal enzymes requires the second luminal loop of CLN8 and is abolished by some disease-causing mutations within this region. Our data establish an unanticipated example of an ER receptor serving the biogenesis of an organelle and indicate that impaired transport of lysosomal enzymes underlies Batten disease caused by mutations in CLN8.

Trafficking of the amino acid transporter B0,+ (SLC6A14) to the plasma membrane involves an exclusive interaction with SEC24C for its exit from the endoplasmic reticulum

A plasma membrane amino acid transporter B^0,+ (ATB^0,+), encoded by the SLC6A14 gene, is specific for neutral and basic amino acids. It is up-regulated in several types of malignant cancers. Neurotransmitter transporters of the SLC6 family interact with specific SEC24 proteins of the COPII complex along their pathway from the endoplasmic reticulum (ER) to Golgi. This study focused on the possible role of SEC24 proteins in ATB^0,+ trafficking. Rat ATB^0,+ was expressed in HEK293 cells, its localization and trafficking were examined by Western blot, deglycosylation, immunofluorescence (co-localization with ER and trans-Golgi markers) and biotinylation. The expression of ATB^0,+ at the plasma membrane was decreased by dominant negative mutants of SAR1, a GTPase, whose activity triggers the formation of the COPII complex. ATB^0,+ co-precipitated with SEC24C (but not with the remaining isoforms A, B and D). This interaction was confirmed by immunocytochemistry and the proximity ligation assay. Co-localization of SEC24C with endogenous ATB^0,+ was also observed in MCF-7 breast cancer cells. Contrary to the endogenous transporter, part of the overexpressed ATB^0,+ is directed to proteolysis, a process significantly reversed by a proteasome inhibitor bortezomib. Co-transfection with a SEC24C dominant negative mutant attenuated ATB^0,+ expression at the plasma membrane, due to proteolytic degradation. These results support a hypothesis that lysine at position +2 downstream of the ER export "RI" motif on the cargo protein is crucial for SEC24C binding and for further trafficking to the Golgi. Moreover, there is an equilibrium between ER export and degradation mechanisms in case of overexpressed transporter.

Relax, Cool Down and Scaffold: How to Restore Surface Expression of Folding-Deficient Mutant GPCRs and SLC6 Transporters

Many diseases arise from mutations, which impair protein folding. The study of folding-deficient variants of G protein-coupled receptors and solute carrier 6 (SLC6) transporters has shed light on the folding trajectory, how it is monitored and how misfolding can be remedied. Reducing the temperature lowers the energy barrier between folding intermediates and thereby eliminates stalling along the folding trajectory. For obvious reasons, cooling down is not a therapeutic option. One approach to rescue misfolded variants is to use membrane-permeable orthosteric ligands. Antagonists of GPCRs are—in many instances—effective pharmacochaperones: they restore cell surface expression provided that they enter cells and bind to folding intermediates. Pharmacochaperoning of SLC6 transporters is less readily achieved because the ionic conditions in the endoplasmic reticulum (ER) are not conducive to binding of typical inhibitors. The second approach is to target the heat-shock protein (HSP) relay, which monitors the folding trajectory on the cytosolic side. Importantly, orthosteric ligands and HSP-inhibitors are not mutually exclusive. In fact, pharmacochaperones and HSP-inhibitors can act in an additive or synergistic manner. This was exemplified by rescuing disease-causing, folding-deficient variants of the human dopamine transporters with the HSP70 inhibitor pifithrin-μ and the pharmacochaperone noribogaine in Drosophila melanogaster.

Acid-Sensing Ion Channel 2a (ASIC2a) Promotes Surface Trafficking of ASIC2b via Heteromeric Assembly

Acid-sensing ion channels (ASICs) are proton-activated cation channels that play important roles as typical proton sensors during pathophysiological conditions and normal synaptic activities. Among the ASIC subunits, ASIC2a and ASIC2b are alternative splicing products from the same gene, ACCN1. It has been shown that ASIC2 isoforms have differential subcellular distribution: ASIC2a targets the cell surface by itself, while ASIC2b resides in the ER. However, the underlying mechanism for this differential subcellular localization remained to be further elucidated. By constructing ASIC2 chimeras, we found that the first transmembrane (TM1) domain and the proximal post-TM1 domain (17 amino acids) of ASIC2a are critical for membrane targeting of the proteins. We also observed that replacement of corresponding residues in ASIC2b by those of ASIC2a conferred proton-sensitivity as well as surface expression to ASIC2b. We finally confirmed that ASIC2b is delivered to the cell surface from the ER by forming heteromers with ASIC2a, and that the N-terminal region of ASIC2a is additionally required for the ASIC2a-dependent membrane targeting of ASIC2b. Together, our study supports an important role of ASIC2a in membrane targeting of ASIC2b.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

A positive signal prevents secretory membrane cargo from recycling between the Golgi and the ER

The Golgi complex and ER are dynamically connected by anterograde and retrograde trafficking pathways. To what extent and by what mechanism outward-bound cargo proteins escape retrograde trafficking has been poorly investigated. Here, we analysed the behaviour of several membrane proteins at the ER/Golgi interface in live cells. When Golgi-to-plasma membrane transport was blocked, vesicular stomatitis virus glycoprotein (VSVG), which bears an ER export signal, accumulated in the Golgi, whereas an export signal-deleted version of VSVG attained a steady state determined by the balance of retrograde and anterograde traffic. A similar behaviour was displayed by EGF receptor and by a model tail-anchored protein, whose retrograde traffic was slowed by addition of VSVG's export signal. Retrograde trafficking was energy- and Rab6-dependent, and Rab6 inhibition accelerated signal-deleted VSVG's transport to the cell surface. Our results extend the dynamic bi-directional relationship between the Golgi and ER to include surface-directed proteins, uncover an unanticipated role for export signals at the Golgi complex, and identify recycling as a novel factor that regulates cargo transport out of the early secretory pathway.

Exiting the ER: what we know and what we don't

The vast majority of proteins that are transported to different cellular compartments and secreted from the cell require coat protein complex II (COPII) for export from the endoplasmic reticulum (ER). Many of the molecular mechanisms underlying COPII assembly are understood in great detail, but it is becoming increasingly evident that this basic machinery is insufficient to account for diverse aspects of protein export from the ER that are observed in vivo. Here we review recent data that have furthered our mechanistic understanding of COPII assembly and, in particular, how genetic diseases associated with the early secretory pathway have added fundamental insights into the regulation of ER-derived carrier formation. We also highlight some unresolved issues that future work should address to better understand the physiology of COPII-mediated transport.