Endoplasmic Reticulum Export of GPI-Anchored Proteins

Role of PIGM and PIGX in glycosylphosphatidylinositol biosynthesis and human health (Review)

Glycosylphosphatidylinositol-glycan (GPI) is an anchor to specific cell surface proteins known as GPI-anchored proteins (APs) that are localized in lipid rafts and may act as cell co-receptors, enzymes and adhesion molecules. The present review investigated the significance of GPI biosynthesis class phosphatidylinositol-glycan (PIG)M and PIGX in GPI synthesis and their implications in human health conditions. PIGM encodes GPI-mannosyltransferase I (MT-I) enzyme that adds the first mannose to the GPI core structure. PIGX encodes the regulatory subunit of GPI-MT-I. The present review summarizes characteristics of the coding sequences of PIGM and PIGX, and their expression in humans, as well as the relevance of GPI-MT-I and the regulatory subunit in maintaining the presence of GPI-APs on the cell surface and their secretion. In addition, the association of PIGM mutations with paroxysmal nocturnal hemoglobinuria and certain types of GPI-deficiency disease and the altered expression of PIGM and PIGX in cancer were also reviewed. In addition, their interaction with other proteins was described, suggesting a complex role in cell biology. PIGM and PIGX are critical genes for GPI synthesis. Understanding gene and protein regulation may provide valuable insights into the role of GPI-APs in cellular processes.

Recent research progress in glycosylphosphatidylinositol-anchored protein biosynthesis, chemical/chemoenzymatic synthesis, and interaction with the cell membrane

Glycosylphosphatidylinositol (GPI) attachment to the C-terminus of proteins is a prevalent posttranslational modification in eukaryotic species, and GPIs help anchor proteins to the cell surface. GPI-anchored proteins (GPI-APs) play a key role in various biological events. However, GPI-APs are difficult to access and investigate. To tackle the problem, chemical and chemoenzymatic methods have been explored for the preparation of GPI-APs, as well as GPI probes that facilitate the study of GPIs on live cells. Substantial progress has also been made regarding GPI-AP biosynthesis, which is helpful for developing new synthetic methods for GPI-APs. This article reviews the recent advancements in the study of GPI-AP biosynthesis, GPI-AP synthesis, and GPI interaction with the cell membrane utilizing synthetic probes.

Molecular basis of the inositol deacylase PGAP1 involved in quality control of GPI-AP biogenesis

The secretion and quality control of glycosylphosphatidylinositol-anchored proteins (GPI-APs) necessitates post-attachment remodeling initiated by the evolutionarily conserved PGAP1, which deacylates the inositol in nascent GPI-APs. Impairment of PGAP1 activity leads to developmental diseases in humans and fatality and infertility in animals. Here, we present three PGAP1 structures (2.66-2.84 Å), revealing its 10-transmembrane architecture and product-enzyme interaction details. PGAP1 holds GPI-AP acyl chains in an optimally organized, guitar-shaped cavity with apparent energetic penalties from hydrophobic-hydrophilic mismatches. However, abundant glycan-mediated interactions in the lumen counterbalance these repulsions, likely conferring substrate fidelity and preventing off-target hydrolysis of bulk membrane lipids. Structural and biochemical analyses uncover a serine hydrolase-type catalysis with atypical features and imply mechanisms for substrate entrance and product release involving a drawing compass movement of GPI-APs. Our findings advance the mechanistic understanding of GPI-AP remodeling.

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

Profiling Glycosylphosphatidylinositol (GPI)-Interacting Proteins in the Cell Membrane Using a Bifunctional GPI Analogue as the Probe

Glycosylphosphatidylinositol (GPI) anchorage of cell surface proteins to the membrane is biologically important and ubiquitous in eukaryotes. However, GPIs do not contain long enough lipids to span the entire membrane bilayer. To transduce binding signals, GPIs must interact with other membrane components, but such interactions are difficult to define. Here, a new method was developed to explore GPI-interacting membrane proteins in live cell with a bifunctional analogue of the glucosaminylphosphatidylinositol motif conserved in all GPIs as a probe. This probe contained a diazirine functionality in the lipid and an alkynyl group on the glucosamine residue to respectively facilitate the cross-linkage of GPI-binding membrane proteins with the probe upon photoactivation and then the installation of biotin to the cross-linked proteins via a click reaction for affinity-based protein isolation and analysis. Profiling the proteins pulled down from the Hela cells revealed 94 unique and 18 overrepresented proteins compared to the control, and most of them are membrane proteins and many are GPI-related. The results have proved not only the concept of using the new bifunctional GPI probe to investigate GPI-binding membrane proteins but also the important role of inositol in the biological functions of GPI anchors and GPI-anchored proteins.

Transfer of Proteins from Cultured Human Adipose to Blood Cells and Induction of Anabolic Phenotype Are Controlled by Serum, Insulin and Sulfonylurea Drugs

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are anchored at the outer leaflet of eukaryotic plasma membranes (PMs) only by carboxy-terminal covalently coupled GPI. GPI-APs are known to be released from the surface of donor cells in response to insulin and antidiabetic sulfonylureas (SUs) by lipolytic cleavage of the GPI or upon metabolic derangement as full-length GPI-APs with the complete GPI attached. Full-length GPI-APs become removed from extracellular compartments by binding to serum proteins, such as GPI-specific phospholipase D (GPLD1), or insertion into the PMs of acceptor cells. Here, the interplay between the lipolytic release and intercellular transfer of GPI-APs and its potential functional impact was studied using transwell co-culture with human adipocytes as insulin-/SU-responsive donor cells and GPI-deficient erythroleukemia as acceptor cells (ELCs). Measurement of the transfer as the expression of full-length GPI-APs at the ELC PMs by their microfluidic chip-based sensing with GPI-binding α-toxin and GPI-APs antibodies and of the ELC anabolic state as glycogen synthesis upon incubation with insulin, SUs and serum yielded the following results: (i) Loss of GPI-APs from the PM upon termination of their transfer and decline of glycogen synthesis in ELCs, as well as prolongation of the PM expression of transferred GPI-APs upon inhibition of their endocytosis and upregulated glycogen synthesis follow similar time courses. (ii) Insulin and SUs inhibit both GPI-AP transfer and glycogen synthesis upregulation in a concentration-dependent fashion, with the efficacies of the SUs increasing with their blood glucose-lowering activity. (iii) Serum from rats eliminates insulin- and SU-inhibition of both GPI-APs' transfer and glycogen synthesis in a volume-dependent fashion, with the potency increasing with their metabolic derangement. (iv) In rat serum, full-length GPI-APs bind to proteins, among them (inhibited) GPLD1, with the efficacy increasing with the metabolic derangement. (v) GPI-APs are displaced from serum proteins by synthetic phosphoinositolglycans and then transferred to ELCs with accompanying stimulation of glycogen synthesis, each with efficacies increasing with their structural similarity to the GPI glycan core. Thus, both insulin and SUs either block or foster transfer when serum proteins are depleted of or loaded with full-length GPI-APs, respectively, i.e., in the normal or metabolically deranged state. The transfer of the anabolic state from somatic to blood cells over long distance and its "indirect" complex control by insulin, SUs and serum proteins support the (patho)physiological relevance of the intercellular transfer of GPI-APs.

ERV14 receptor impacts mycelial growth via its interactions with cell wall synthase and transporters in Aspergillus niger

Efficient protein secretion is closely correlated with vesicle sorting and packaging, especially with cargo receptor-mediated selective transport for ER exit. Even though Aspergillus niger is considered an industrially natural host for protein production due to its exceptional secretion capacity, the trafficking mechanism in the early secretory pathway remains a black box for us to explore. Here, we identified and characterized all putative ER cargo receptors of the three families in A. niger. We successfully constructed overexpression and deletion strains of each receptor and compared the colony morphology and protein secretion status of each strain. Among them, the deletion of Erv14 severely inhibited mycelial growth and secretion of extracellular proteins such as glucoamylase. To gain a comprehensive understanding of the proteins associated with Erv14, we developed a high-throughput method by combining yeast two-hybrid (Y2H) with next-generation sequencing (NGS) technology. We found Erv14 specifically interacted with transporters. Following further validation of the quantitative membrane proteome, we determined that Erv14 was associated with the transport of proteins involved in processes such as cell wall synthesis, lipid metabolism, and organic substrate metabolism.

The Golgi-resident protein ACBD3 concentrates STING at ER-Golgi contact sites to drive export from the ER

STING, an endoplasmic reticulum (ER)-resident receptor for cyclic di-nucleotides (CDNs), is essential for innate immune responses. Upon CDN binding, STING moves from the ER to the Golgi, where it activates downstream type-I interferon (IFN) signaling. General cargo proteins exit from the ER via concentration at ER exit sites. However, the mechanism of STING concentration is poorly understood. Here, we visualize the ER exit sites of STING by blocking its transport at low temperature or by live-cell imaging with the cell-permeable ligand bis-^pivSATE-2'F-c-di-dAMP, which we have developed. After ligand binding, STING forms punctate foci at non-canonical ER exit sites. Unbiased proteomic screens and super-resolution microscopy show that the Golgi-resident protein ACBD3/GCP60 recognizes and concentrates ligand-bound STING at specialized ER-Golgi contact sites. Depletion of ACBD3 impairs STING ER-to-Golgi trafficking and type-I IFN responses. Our results identify the ACBD3-mediated non-canonical cargo concentration system that drives the ER exit of STING.

Overexpression of LAS21 in Cellulase-Displaying Saccharomyces cerevisiae for High-Yield Ethanol Production from Pretreated Sugarcane Bagasse

The valorization of lignocellulosic feedstocks into biofuels and biochemicals has received much attention due to its environmental friendliness and sustainability. However, engineering an ideal microorganism that can both produce sufficient cellulases and ferment ethanol is highly challenging. In this study, we have tested seven different genes that are involved in glycosylphosphatidylinositol (GPI) biosynthesis and remodeling for the improvement of cellulase activity tethered on the S. cerevisiae cell surface. It was found that the overexpression of LAS21 can improve β-glucosidase activity by 48.8% compared to the original strain. Then, the three cellulase genes (cellobiohydrolase, endoglucanase, and β-glucosidase) and the LAS21 gene were co-introduced into a diploid thermotolerant S. cerevisiae strain by a multiple-round transformation approach, resulting in the cellulolytic ECBLCCE5 strain. Further optimization of the bioprocess parameters was found to enhance the ethanol yield of the ECBLCCE5 strain. Scaling up the valorization of pretreated sugarcane bagasses in a 1 L bioreactor resulted in a maximum ethanol concentration of 28.0 g/L (86.5% of theoretical yield). Our study provides a promising way to improve the economic viability of second-generation ethanol production. Moreover, the engineering of genes involved in GPI biosynthesis and remodeling can be applied to other yeast cell surface display applications.

Stage-Specific COPII-Mediated Cargo Selectivity in African Trypanosomes

A hallmark of eukaryotic cells is the ability to form a secretory pathway connecting many intracellular compartments. In the early secretory pathway, coated protein complex II (COPII)-coated vesicles mediate the anterograde transport of newly synthesized secretory cargo from the endoplasmic reticulum to the Golgi apparatus. The COPII coat complex is comprised of an inner layer of Sec23/Sec24 heterodimers and an outer layer of Sec13/Sec31 heterotetramers. In African trypanosomes, there are two paralogues each of Sec23 and Sec24, that form obligate heterodimers (TbSec23.2/TbSec24.1, TbSec23.1/TbSec24.2). It is not known if these form distinct homotypic classes of vesicles or one heterotypic class, but it is known that TbSec23.2/TbSec24.1 specifically mediate forward trafficking of GPI-anchored proteins (GPI-APs) in bloodstream-form trypanosomes (BSF). Here, we showed that this selectivity was lost in insect procyclic stage parasites (PCF). All isoforms of TbSec23 and TbSec24 are essential in PCF parasites as judged by RNAi knockdowns. RNAi silencing of each subunit had equivalent effects on the trafficking of GPI-APs and p67, a transmembrane lysosomal protein. However, silencing of the TbSec23.2/TbSec24.1 had heterodimer had a significant impact on COPII mediated trafficking of soluble TbCatL from the ER to the lysosome. This finding suggests a model in which selectivity of COPII transport was altered between the BSF and PCF trypanosomes, possibly as an adaptation to a digenetic life cycle.

IMPORTANCE African trypanosomes synthesize dense surface coats composed of stage-specific glycosylphosphatidylinositol lipid anchored proteins. We previously defined specific machinery in bloodstream stage parasites that mediate the exit of these proteins from the endoplasmic reticulum. Here, we performed similar analyses in the procyclic insect stage and found significant differences in this process. These findings contribute to our understanding of secretory processes in this unusual eukaryotic model system.

Self-incompatibility requires GPI anchor remodeling by the poppy PGAP1 ortholog HLD1

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are tethered to the outer leaflet of the plasma membrane where they function as key regulators of a plethora of biological processes in eukaryotes. Self-incompatibility (SI) plays a pivotal role regulating fertilization in higher plants through recognition and rejection of "self" pollen. Here, we used Arabidopsis thaliana lines that were engineered to be self-incompatible by expression of Papaver rhoeas SI determinants for an SI suppressor screen. We identify HLD1/AtPGAP1, an ortholog of the human GPI-inositol deacylase PGAP1, as a critical component required for the SI response. Besides a delay in flowering time, no developmental defects were observed in HLD1/AtPGAP1 knockout plants, but SI was completely abolished. We demonstrate that HLD1/AtPGAP1 functions as a GPI-inositol deacylase and that this GPI-remodeling activity is essential for SI. Using GFP-SKU5 as a representative GPI-AP, we show that the HLD1/AtPGAP1 mutation does not affect GPI-AP production and targeting but affects their cleavage and release from membranes in vivo. Our data not only implicate GPI-APs in SI, providing new directions to investigate SI mechanisms, but also identify a key functional role for GPI-AP remodeling by inositol deacylation in planta.

Quality-controlled ceramide-based GPI-anchored protein sorting into selective ER exit sites

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) exit the endoplasmic reticulum (ER) through a specialized export pathway in the yeast Saccharomyces cerevisiae. We have recently shown that a very-long acyl chain (C26) ceramide present in the ER membrane drives clustering and sorting of GPI-APs into selective ER exit sites (ERES). Now, we show that this lipid-based ER sorting also involves the C26 ceramide as a lipid moiety of GPI-APs, which is incorporated into the GPI anchor through a lipid-remodeling process after protein attachment in the ER. Moreover, we also show that a GPI-AP with a C26 ceramide moiety is monitored by the GPI-glycan remodelase Ted1, which, in turn, is required for receptor-mediated export of GPI-APs. Therefore, our study reveals a quality-control system that ensures lipid-based sorting of GPI-APs into selective ERESs for differential ER export, highlighting the physiological need for this specific export pathway.

Molecular mechanisms of endomembrane trafficking in plants

Endomembrane trafficking is essential for all eukaryotic cells. The best-characterized membrane trafficking organelles include the endoplasmic reticulum (ER), Golgi apparatus, early and recycling endosomes, multivesicular body, or late endosome, lysosome/vacuole, and plasma membrane. Although historically plants have given rise to cell biology, our understanding of membrane trafficking has mainly been shaped by the much more studied mammalian and yeast models. Whereas organelles and major protein families that regulate endomembrane trafficking are largely conserved across all eukaryotes, exciting variations are emerging from advances in plant cell biology research. In this review, we summarize the current state of knowledge on plant endomembrane trafficking, with a focus on four distinct trafficking pathways: ER-to-Golgi transport, endocytosis, trans-Golgi network-to-vacuole transport, and autophagy. We acknowledge the conservation and commonalities in the trafficking machinery across species, with emphasis on diversity and plant-specific features. Understanding the function of organelles and the trafficking machinery currently nonexistent in well-known model organisms will provide great opportunities to acquire new insights into the fundamental cellular process of membrane trafficking.

Heterogeneity in the transcriptional response of the human pathogen Aspergillus fumigatus to the antifungal agent caspofungin

Aspergillus fumigatus is the main causative agent of invasive pulmonary aspergillosis (IPA), a severe disease that affects immunosuppressed patients worldwide. The fungistatic drug caspofungin (CSP) is the second line of therapy against IPA but has increasingly been used against clinical strains that are resistant to azoles, the first line antifungal therapy. In high concentrations, CSP induces a tolerance phenotype with partial reestablishment of fungal growth called CSP paradoxical effect (CPE), resulting from a change in the composition of the cell wall. An increasing number of studies has shown that different isolates of A. fumigatus exhibit phenotypic heterogeneity, including heterogeneity in their CPE response. To gain insights into the underlying molecular mechanisms of CPE response heterogeneity, we analyzed the transcriptomes of two A. fumigatus reference strains, Af293 and CEA17, exposed to low and high CSP concentrations. We found that there is a core transcriptional response that involves genes related to cell wall remodeling processes, mitochondrial function, transmembrane transport, and amino acid and ergosterol metabolism, and a variable response related to secondary metabolite (SM) biosynthesis and iron homeostasis. Specifically, we show here that the overexpression of a SM pathway that works as an iron chelator extinguishes the CPE in both backgrounds, whereas iron depletion is detrimental for the CPE in Af293 but not in CEA17. We next investigated the function of the transcription factor CrzA, whose deletion was previously shown to result in heterogeneity in the CPE response of the Af293 and CEA17 strains. We found that CrzA constitutively binds to and modulates the expression of several genes related to processes involved in CSP tolerance and that crzA deletion differentially impacts the SM production and growth of Af293 and CEA17. As opposed to the ΔcrzACEA17 mutant, the ΔcrzAAf293 mutant fails to activate cell wall remodeling genes upon CSP exposure, which most likely severely affects its macrostructure and extinguishes its CPE. This study describes how heterogeneity in the response to an antifungal agent between A. fumigatus strains stems from heterogeneity in the function of a transcription factor and its downstream target genes.

Remodeling-defective GPI-anchored proteins on the plasma membrane activate the spindle assembly checkpoint

Newly synthesized glycosylphosphatidylinositol-anchored proteins (GPI-APs) undergo extensive remodeling prior to transport to the plasma membrane. GPI-AP remodeling events serve as quality assurance signatures, and complete remodeling of the anchor functions as a transport warrant. Using a genetic approach in yeast cells, we establish that one remodeling event, the removal of ethanolamine-phosphate from mannose 2 via Ted1p (yPGAP5), is essential for cell viability in the absence of the Golgi-localized putative phosphodiesterase Dcr2p. While GPI-APs in which mannose 2 has not been remodeled in dcr2 ted1-deficient cells can still be delivered to the plasma membrane, their presence elicits a unique stress response. Stress is sensed by Mid2p, a constituent of the cell wall integrity pathway, whereupon signal promulgation culminates in activation of the spindle assembly checkpoint. Our results are consistent with a model in which cellular stress response and chromosome segregation checkpoint pathways are functionally interconnected.

Leptin Induces Apoptotic and Pyroptotic Cell Death via NLRP3 Inflammasome Activation in Rat Hepatocytes

Leptin, a hormone that is predominantly produced by adipose tissue, is closely associated with various liver diseases. However, there is a lack of understanding as to whether leptin directly induces cytotoxic effects in hepatocytes as well as the mechanisms that are involved. Inflammasomes, which are critical components in the innate immune system, have been recently shown to modulate cell death. In this study, we examined the effect of leptin on the viability of rat hepatocytes and the underlying mechanisms, with a particular focus on the role of inflammasomes activation. Leptin treatment induced cytotoxicity in rat hepatocytes, as determined by decreased cell viability, increased caspase-3 activity, and the enhanced release of lactate dehydrogenase. NLRP3 inflammasomes were activated by leptin both in vitro and in vivo, as determined by the maturation of interleukin-1β and caspase-1, and the increased expression of inflammasome components, including NLRP3 and ASC. Mechanistically, leptin-induced inflammasome activation is mediated via the axis of ROS production, ER stress, and autophagy. Notably, the inhibition of inflammasomes by treatment with the NLRP3 inhibitor or the IL-1 receptor antagonist protected the hepatocytes from leptin-induced cell death. Together, these results indicate that leptin exerts cytotoxic effects in hepatocytes, at least in part, via the activation of NLRP3 inflammasomes.

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

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

The murine cytomegalovirus immunoevasin gp40/m152 inhibits NKG2D receptor RAE-1γ by intracellular retention and cell surface masking

NKG2D (also known as KLRK1) is a crucial natural killer (NK) cell-activating receptor, and the murine cytomegalovirus (MCMV) employs multiple immunoevasins to avoid NKG2D-mediated activation. One of the MCMV immunoevasins, gp40 (m152), downregulates the cell surface NKG2D ligand RAE-1γ (also known as Raet1c) thus limiting NK cell activation. This study establishes the molecular mechanism by which gp40 retains RAE-1γ in the secretory pathway. Using flow cytometry and pulse-chase analysis, we demonstrate that gp40 retains RAE-1γ in the early secretory pathway, and that this effect depends on the binding of gp40 to a host protein, TMED10, a member of the p24 protein family. We also show that the TMED10-based retention mechanism can be saturated, and that gp40 has a backup mechanism as it masks RAE-1γ on the cell surface, blocking the interaction with the NKG2D receptor and thus NK cell activation.

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.

Lamin is essential for nuclear localization of the GPI synthesis enzyme PIG-B and GPI-anchored protein production in Drosophila

Membrane lipid biosynthesis is a complex process that occurs in various intracellular compartments. In Drosophila, phosphatidylinositol glycan-B (PIG-B), which catalyzes addition of the third mannose in glycosylphosphatidylinositol (GPI), localizes to the nuclear envelope (NE). Although this NE localization is essential for Drosophila development, the underlying molecular mechanism remains unknown. To elucidate this mechanism, we identified PIG-B-interacting proteins by performing immunoprecipitation followed by proteomic analysis. We then examined which of these proteins are required for the NE localization of PIG-B. Knockdown of Lamin Dm0, a B-type lamin, led to mislocalization of PIG-B from the NE to the endoplasmic reticulum. Lamin Dm0 associated with PIG-B at the inner nuclear membrane, a process that required the tail domain of Lamin Dm0. Furthermore, GPI moieties were distributed abnormally in the Lamin Dm0 mutant. These data indicate that Lamin Dm0 is involved in the NE localization of PIG-B and is required for proper GPI-anchor modification of proteins.

Highlighted Article: Lamin plays a role in post-translational modification of plasma membrane proteins by tethering the GPI modification enzyme PIG-B to the inner nuclear membrane.

Modification of the Mammalian Endomembrane System in Healthy and Diseased Cells

One remarkable characteristic of eukaryotic cells is the complexity of their membrane systems [...].

On the Potential Function of Type II Arabinogalactan O-Glycosylation in Regulating the Fate of Plant Secretory Proteins

In a plant-specific mode of protein glycosylation, various sugars and glycans are attached to hydroxyproline giving rise to a variety of diverse O-glycoproteins. The sub-family of arabinogalactan proteins is implicated in a multitude of biological functions, however, the mechanistic role of O-glycosylation on AGPs by type II arabinogalactans is largely elusive. Some models suggest roles of the O-glycans such as in ligand-receptor interactions and as localized calcium ion store. Structurally different but possibly analogous types of protein O-glycosylation exist in animal and yeast models and roles for O-glycans were suggested in determining the fate of O-glycoproteins by affecting intracellular sorting or proteolytic activation and degradation. At present, only few examples exist that describe how the fate of artificial and endogenous arabinogalactan proteins is affected by O-glycosylation with type II arabinogalactans. In addition to other roles, these glycans might act as a molecular determinant for cellular localization and protein lifetime of many endogenous proteins.