Glycobiology in the cytosol: the bitter side of a sweet world

NGLY1: A fascinating, multifunctional molecule

NGLY1, a cytoplasmic de-N-glycosylating enzyme is well conserved among eukaryotes. This enzyme has attracted considerable attention after mutations on the NGLY1 gene were found to cause a rare genetic disorder called NGLY1 deficiency. Recent explosive progress in NGLY1 research has revealed multi-functional aspects of this protein.

Defect in cytosolic Neu2 sialidase abrogates lipid metabolism and impairs muscle function in vivo

Sialic acid (SA) is present in glycoconjugates and important in cell–cell recognition, cell adhesion, and cell growth and as a receptor. Among the four mammalian sialidases, cytosolic NEU2 has a pivotal role in muscle and neuronal differentiation in vitro. However, its biological functions in vivo remain unclear due to its very low expression in humans. However, the presence of cytoplasmic glycoproteins, gangliosides, and lectins involved in cellular metabolism and glycan recognition has suggested the functional importance of cytosolic Neu2 sialidases. We generated a Neu2 knockout mouse model via CRISPR/Cas9-mediated genome engineering and analyzed the offspring littermates at different ages to investigate the in vivo function of cytosolic Neu2 sialidase. Surprisingly, knocking out the Neu2 gene in vivo abrogated overall lipid metabolism, impairing motor function and leading to diabetes. Consistent with these results, Neu2 knockout led to alterations in sialylated glycoproteins involved in lipid metabolism and muscle function, as shown by glycoproteomics analysis.

Intracellular leucine-rich alpha-2-glycoprotein-1 competes with Apaf-1 for binding cytochrome c in protecting MCF-7 breast cancer cells from apoptosis

Leucine-rich alpha-2-glycoprotein-1 (LRG1) has been shown to compete with apoptosis activating factor-1 (Apaf-1) for binding cytochrome c (Cyt c) and could play a role in inhibition of apoptosis. Employing MCF-7 breast cancer cells, we report that intracellular LRG1 does protect against apoptosis. Thus, cells transfected with the lrg1 gene and expressing higher levels of LRG1 were more resistant to hydrogen peroxide-induced apoptosis than parental cells, while cells in which LRG mRNA was knocked down by short hairpin (sh) RNA-induced degradation were more sensitive. The amount of Cyt c co-immunoprecipitated with Apaf-1 from the cytosol of apoptotic cells was inversely related to the level of LRG1 expression. In lrg1-transfected cells partially-glycosylated LRG1 was found in the cytosol and there was an increase in cytosolic Cyt c in live lrg1-transfected cells relative to parental cells. However, apoptosis was not spontaneously induced because Cyt c was bound to LRG1 and not to Apaf-1. Cyt c was the only detectable protein co-immunoprecipitated with LRG1. Following hydrogen peroxide treatment degradation of LRG1 allowed for induction of apoptosis. We propose that intracellular LRG1 raises the threshold of cytoplasmic Cyt c required to induce apoptosis and, thus, prevents onset of the intrinsic pathway in cells where Cyt c release from mitochondria does not result from committed apoptotic signaling. This mechanism of survival afforded by LRG1 is likely to be distinct from its extracellular survival function that has been reported by several research groups.

Soluble glycoproteins of the lacrimal sac: role in defense with special reference to prolactin-inducible protein (PIP)

Purpose: Glycoproteins play an important role in human mucosal defenses and immunity-related cell-to-cell interactions. The aim of the present study is to investigate the presence and patterns of lacrimal sac glycoproteins involved in defense mechanisms with a special reference to prolactin-inducible protein (PIP). Methods: The study was performed on healthy lacrimal sacs obtained from exenteration samples immediately after surgery and frozen at -80 degrees for subsequent analysis. Four lectins namely Concanavalin A (Con A), Dolichos lablab lectin (DLL), Wheat Germ agglutinin (WGA), and Momordica charantia lectin (MCL) were purified by affinity chromatography. Soluble proteins extract of the lacrimal sac was subjected to chromatography on lectin-affigel columns. Eluted samples from each of the lectin coupled-affigels were analyzed by 10% SDS-PAGE under reducing conditions and the protein bands were visualized using Coomassie blue stain. The protein gel bands were further subjected to mass spectrometry for glycoprotein analysis. Results: Mass spectrometry identified several glycoproteins from the lacrimal sac extracts, with known roles in defense mechanisms. The number of such glycoproteins identified were 9 each from Con A and DLL-I affinity eluted gel bands and 8 and 14 from MCL and WGA affinity eluted gel bands, respectively. Interestingly, PIP was detected in significant proportions in all the eluted gel bands with WGA showing the highest expression. Conclusions: This study is the first step towards the lacrimal sac glycoprotein profiling. PIP could be a major lead for further work on the etiopathogenesis of lacrimal drainage obstructions.

Generation of clade- and symbiont-specific antibodies to characterize marker molecules during Cnidaria-Symbiodinium endosymbiosis

The endosymbiosis between cnidarians and dinoflagellates is responsible for the formation of coral reefs. Changes in molecules have been identified during the process of cnidaria-Symbiodinium endosymbiosis. However, the complexity of the molecular interaction has prevented the establishment of a mechanistic explanation of cellular regulation in this mutualistic symbiosis. To date, no marker molecules have been identified to specifically represent the symbiotic status. Because the endosymbiotic association occurs in the symbiotic gastrodermal cells (SGCs), whole cells of isolated SGCs were used as an antigen to generate monoclonal antibodies (mAb) to screen possible molecular candidates of symbiotic markers. The results showed that one of the generated monoclonal antibodies, 2–6F, specifically recognized clade C symbiotic Symbiodinium but not its free-living counterpart or other Symbiodinium clades. The expression levels of 2–6F mAb-recognized proteins are highly correlated with the symbiotic status, and these proteins were characterized as N-linked glycoproteins via treatment with peptide N-glycosidase F. Furthermore, their glycan moieties were markedly different from those of free-living Symbiodinium, potentially suggesting host regulation of post-translational modification. Consequently, the 2–6F mAb can be used to detect the symbiotic state of corals and investigate the complex molecular interactions in cnidaria-Symbiodinium endosymbiosis.

Chlorophyll Catabolites in Fall Leaves of the Wych Elm Tree Present a Novel Glycosylation Motif

Fall leaves of the common wych elm tree (Ulmus glabra) were studied with respect to chlorophyll catabolites. Over a dozen colorless, non‐fluorescent chlorophyll catabolites (NCCs) and several yellow chlorophyll catabolites (YCCs) were identified tentatively. Three NCC fractions were isolated and their structures were characterized by spectroscopic means. Two of these, Ug‐NCC‐27 and Ug‐NCC‐43, carried a glucopyranosyl appendage. Ug‐NCC‐53, the least polar of these NCCs, was identified as the formal product of an intramolecular esterification of the propionate and primary glucopyranosyl hydroxyl groups of Ug‐NCC‐43. Thus, the glucopyranose moiety and three of the pyrrole units of Ug‐NCC‐53 span a 20‐membered ring, installing a bicyclo[17.3.1]glycoside moiety. This structural motif is unprecedented in heterocyclic natural products, according to a thorough literature search. The remarkable, three‐dimensional bicyclo[17.3.1]glycoside architecture reduces the flexibility of the linear tetrapyrrole. This feature of Ug‐NCC‐53 is intriguing, considering the diverse biological effects of known bicyclo[n.3.1]glycosidic natural products.

GalNAc-T4 putatively modulates the estrogen regulatory network through FOXA1 glycosylation in human breast cancer cells

GALNT4 belongs to a family of N-acetylgalactosaminyltransferases, which catalyze the transfer of GalNAc to Serine or Threonine residues in the initial step of mucin-type O-linked protein glycosylation. This glycosylation type is the most complex post-translational modification of proteins, playing important roles during cellular differentiation and in pathological disorders. Most of the breast cancer subtypes are estrogen receptor positive, and hence, the estrogen pathway represents a key regulatory network. We investigated the expression of GalNAc-T4 in a panel of mammary epithelial cell lines and found its expression is associated with the estrogen status of the cells. FOXA1, a key transcription factor, functions to promote estrogen responsive gene expression by acting as a cofactor to estrogen receptor alpha (ERα), but all the aspects of this regulatory mechanism are not fully explored. This study found that knockdown of GALNT4 expression in human breast cancer cells attenuated the protein expression of ERα, FOXA1, and Cyclin D1. Further, our immunoprecipitation assays depicted the possibility of FOXA1 to undergo O-GalNAc modifications with a decrease of GalNAc residues in the GALNT4 knockdown cells and also impairment in the FOXA1-ERα association. Rescuing GALNT4 expression could restore the interaction as well as the glycosylation of FOXA1. Together, these findings suggest a key role for GalNAc-T4 in the estrogen pathway through FOXA1 glycosylation.

Generation and degradation of free asparagine-linked glycans

Asparagine (N)-linked protein glycosylation, which takes place in the eukaryotic endoplasmic reticulum (ER), is important for protein folding, quality control and the intracellular trafficking of secretory and membrane proteins. It is known that, during N-glycosylation, considerable amounts of lipid-linked oligosaccharides (LLOs), the glycan donor substrates for N-glycosylation, are hydrolyzed to form free N-glycans (FNGs) by unidentified mechanisms. FNGs are also generated in the cytosol by the enzymatic deglycosylation of misfolded glycoproteins during ER-associated degradation. FNGs derived from LLOs and misfolded glycoproteins are eventually merged into one pool in the cytosol and the various glycan structures are processed to a near homogenous glycoform. This article summarizes the current state of our knowledge concerning the formation and catabolism of FNGs.

Non-lysosomal degradation pathway for N-linked glycans and dolichol-linked oligosaccharides

There is growing evidence that asparagine (N)-linked glycans play pivotal roles in protein folding and intra- or intercellular trafficking of N-glycosylated proteins. During the N-glycosylation of proteins, significant amounts of free oligosaccharides (fOSs) and phosphorylated oligosaccharides (POSs) are generated at the endoplasmic reticulum (ER) membrane by unclarified mechanisms. fOSs are also formed in the cytosol by the enzymatic deglycosylation of misfolded glycoproteins destined for proteasomal degradation. This article summarizes the current knowledge of the molecular and regulatory mechanisms underlying the formation of fOSs and POSs in mammalian cells and Saccharomyces cerevisiae.

Aberrant sialylation and fucosylation of intracellular proteins in cervical tissue are critical markers of cervical carcinogenesis

Numerous studies have suggested that increased sialylation and fucosylation levels are signs of cancer progression. The majority of studies have focused on cell surface and bloodstream glycosylation changes associated with cancer progression, while little attention has been paid to changes in the glycosylation of cytosolic proteins. We compared the mannosylation, sialylation and fucosylation levels of cytosolic proteins obtained from human cervical tissues without neoplastic lesions vs. with cancer, using lectin blot and enzyme-linked lectin assay (ELLA) systems. There were no quantitative differences in mannosylation levels between the cytosolic proteins of normal and cancer tissues. However, we found markedly reduced sialylation (P<0.001) and fucosylation (P<0.01) in the proteins of cancer tissues. The ELLA system for detecting sialylation had extremely high sensitivity (91-100%) and specificity (82-100%) in distinguishing normal and cancer tissues. Thus, the changes in the glycosylation of cytosolic proteins during carcinogenesis of the cervix are quite different from previous observations concerning glycoconjugates in the bloodstream or on the cell surface. We suggest that changes in the glycosylation of intracellular proteins may be useful markers of the development of cervical cancer.

CA 125 concentration in portal blood as a predictor of resectability in pancreatic tumor

AIM OF THE STUDY

Pancreatic cancer is one of the most frequent cancers in the world. Only 20% of patients seem to have disease confined to the pancreas, but in only every second case the tumor turns out to be resectable during surgery. Tumor markers may be a useful tool in differentiating benign from malignant pancreatic tumors and in clinical staging. The purpose of the study is to assess CA 125 utility as a predictor of resectability in pancreatic tumor.

MATERIAL AND METHODS

66 patients were operated on for pancreatic tumor between October 2010 and July 2012. CA 125 concentration was measured in peripheral and portal blood. 57 patients were diagnosed with malignant and 9 with inflammatory tumor. Seven patients had metastases to the liver. Radical surgery was performed in 34 patients.

RESULTS

Significantly higher CA 125 concentration in portal blood was found in the pancreatic cancer than in the inflammatory tumor group (36.5 ±99.6 vs. 16.4 ±26.5; p < 0.05). CA 125 concentration in peripheral blood and in portal blood as well of patients with malignant pancreatic tumors and with metastases to the liver was significantly higher than in the group without metastases (146.15 ±256.1 vs. 18.5 ±17.5; p < 0.01 and 147.5 ±261.2 vs. 19.7 ±24.3; p < 0.05, respectively). CA 125 values in the group without metastases to the liver and in the case of radical surgery were significantly higher in portal than in peripheral blood (19.7 ±24.3 vs. 18.5 ±17.5; p < 0.001 and 13.2 ±15.0 vs. 13.0 ±15.2; p < 0.001, respectively).

CONCLUSIONS

Determination of CA 125 concentration in peripheral blood and in portal blood as well might be a useful tool in differentiating between malignant and inflammatory pancreatic tumors and when decisions on surgery extensiveness are being made.

Genome-wide association study of serum selenium concentrations

Selenium is an essential trace element and circulating selenium concentrations have been associated with a wide range of diseases. Candidate gene studies suggest that circulating selenium concentrations may be impacted by genetic variation; however, no study has comprehensively investigated this hypothesis. Therefore, we conducted a two-stage genome-wide association study to identify genetic variants associated with serum selenium concentrations in 1203 European descents from two cohorts: the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening and the Women’s Health Initiative (WHI). We tested association between 2,474,333 single nucleotide polymorphisms (SNPs) and serum selenium concentrations using linear regression models. In the first stage (PLCO) 41 SNPs clustered in 15 regions had p < 1 × 10⁻⁵. None of these 41 SNPs reached the significant threshold (p = 0.05/15 regions = 0.003) in the second stage (WHI). Three SNPs had p < 0.05 in the second stage (rs1395479 and rs1506807 in 4q34.3/AGA-NEIL3; and rs891684 in 17q24.3/SLC39A11) and had p between 2.62 × 10⁻⁷ and 4.04 × 10⁻⁷ in the combined analysis (PLCO + WHI). Additional studies are needed to replicate these findings. Identification of genetic variation that impacts selenium concentrations may contribute to a better understanding of which genes regulate circulating selenium concentrations.

Dual functions for cytosolic α-mannosidase (Man2C1): its down-regulation causes mitochondria-dependent apoptosis independently of its α-mannosidase activity

Cytosolic α-mannosidase (Man2C1) trims free oligosaccharides in mammalian cells, and its down-regulation reportedly delays cancer growth by inducing mitotic arrest or apoptosis. However, the mechanism by which Man2C1 down-regulation induces apoptosis is unknown. Here, we demonstrated that silencing of Man2C1 via small hairpin RNAs induced mitochondria-dependent apoptosis in HeLa cells. Expression of CHOP (C/EBP homologous protein), a transcription factor critical to endoplasmic reticulum stress-induced apoptosis, was significantly up-regulated in Man2C1 knockdown cells. However, this enhanced CHOP expression was not caused by endoplasmic reticulum stress. Interestingly, Man2C1 catalytic activity was not required for this regulation of apoptosis; introduction of mutant, enzymatically inactive Man2C1 rescued apoptotic phenotypes of Man2C1 knockdown cells. These results show that Man2C1 has dual functions: one in glycan catabolism and another in apoptotic signaling.

Identification and origin of N-linked β-D-N-acetylglucosamine monosaccharide modifications on Arabidopsis proteins

Many plant proteins are modified with N-linked oligosaccharides at asparagine-X-serine/threonine sites during transit through the endoplasmic reticulum and the Golgi. We have identified a number of Arabidopsis (Arabidopsis thaliana) proteins with modifications consisting of an N-linked N-acetyl-D-glucosamine monosaccharide (N-GlcNAc). Electron transfer dissociation mass spectrometry analysis of peptides bearing this modification mapped the modification to asparagine-X-serine/threonine sites on proteins that are predicted to transit through the endoplasmic reticulum and Golgi. A mass labeling method was developed and used to study N-GlcNAc modification of two thioglucoside glucohydrolases (myrosinases), TGG1 and TGG2 (for thioglucoside glucohydrolase). These myrosinases are also modified with high-mannose (Man)-type glycans. We found that N-GlcNAc and high-Man-type glycans can occur at the same site. It has been hypothesized that N-GlcNAc modifications are generated when endo-β-N-acetylglucosaminidase (ENGase) cleaves N-linked glycans. We examined the effects of mutations affecting the two known Arabidopsis ENGases on N-GlcNAc modification of myrosinase and found that modification of TGG2 was greatly reduced in one of the single mutants and absent in the double mutant. Surprisingly, N-GlcNAc modification of TGG1 was not affected in any of the mutants. These data support the hypothesis that ENGases hydrolyze high-Man glycans to produce some of the N-GlcNAc modifications but also suggest that some N-GlcNAc modifications are generated by another mechanism. Since N-GlcNAc modification was detected at only one site on each myrosinase, the production of the N-GlcNAc modification may be regulated.

Sialidase significance for cancer progression

Aberrant glycosylation is a characteristic feature of cancer cells. In particular, altered sialylation is closely associated with malignant properties, including invasiveness and metastatic potential. To elucidate the molecular mechanisms underlying the aberrancy, our studies have focused on mammalian sialidase, which catalyzes the removal of sialic acid residues from glycoproteins and glycolipids. The four types of mammalian sialidase identified to date show altered expression and behave in different manners during carcinogenesis. The present review briefly summarizes results on altered expression of sialidases and their possible roles in cancer progression. These enzymes are indeed factors defining cancer malignancy and thus potential targets for cancer diagnosis and therapy.

Mammalian sialidases: physiological and pathological roles in cellular functions

Sialic acids are terminal acidic monosaccharides, which influence the chemical and biological features of glycoconjugates. Their removal catalyzed by a sialidase modulates various biological processes through change in conformation and creation or loss of binding sites of functional molecules. Sialidases exist widely in vertebrates and also in a variety of microorganisms. Recent research on mammalian sialidases has provided evidence for great importance of these enzymes in various cellular functions, including lysosomal catabolism, whereas microbial sialidases appear to play roles limited to nutrition and pathogenesis. Four types of mammalian sialidases have been identified and characterized to date, designated as NEU1, NEU2, NEU3 and NEU4. They are encoded by different genes and differ in major subcellular localization and enzymatic properties including substrate specificity, and each has been found to play a unique role depending on its particular properties. This review is an attempt to concisely summarize current knowledge concerning mammalian sialidases, with a special focus on their properties and physiological and pathological roles in cellular functions.

Skp1 prolyl 4-hydroxylase of dictyostelium mediates glycosylation-independent and -dependent responses to O2 without affecting Skp1 stability

Cytoplasmic prolyl 4-hydroxylases (PHDs) have a primary role in O(2) sensing in animals via modification of the transcriptional factor subunit HIFα, resulting in its polyubiquitination by the E3(VHL)ubiquitin (Ub) ligase and degradation in the 26 S proteasome. Previously thought to be restricted to animals, a homolog (P4H1) of HIFα-type PHDs is expressed in the social amoeba Dictyostelium where it also exhibits characteristics of an O(2) sensor for development. Dictyostelium lacks HIFα, and P4H1 modifies a different protein, Skp1, an adaptor of the SCF class of E3-Ub ligases related to the E3(VHL)Ub ligase that targets animal HIFα. Normally, the HO-Skp1 product of the P4H1 reaction is capped by a GlcNAc sugar that can be subsequently extended to a pentasaccharide by novel glycosyltransferases. To analyze the role of glycosylation, the Skp1 GlcNAc-transferase locus gnt1 was modified with a missense mutation to block catalysis or a stop codon to truncate the protein. Despite the accumulation of the hydroxylated form of Skp1, Skp1 was not destabilized based on metabolic labeling. However, hydroxylation alone allowed for partial correction of the high O(2) requirement of P4H1-null cells, therefore revealing both glycosylation-independent and glycosylation-dependent roles for hydroxylation. Genetic complementation of the latter function required an enzymatically active form of Gnt1. Because the effect of the gnt1 deficiency depended on P4H1, and Skp1 was the only protein labeled when the GlcNAc-transferase was restored to mutant extracts, Skp1 apparently mediates the cellular functions of both P4H1 and Gnt1. Although Skp1 stability itself is not affected by hydroxylation, its modification may affect the stability of targets of Skp1-dependent Ub ligases.

Nutriproteomics: technologies and applications for identification and quantification of biomarkers and ingredients

Nutrition refers to the process by which a living organism ingests and digests food and uses the nutrients therein for growth, tissue maintenance and all other functions essential to life. Food components interact with our body at molecular, cellular, organ and system level. Nutrients come in complex mixtures, in which the presence and concentration of single compounds as well as their interactions with other compounds and the food matrix influence their bioavailability and bioefficacy. Traditionally, nutrition research mainly concentrated on supplying nutrients of quality to nourish populations and on preventing specific nutrient deficiencies. More recently, it investigates health-related aspects of individual ingredients or of complete diets, in view of health promotion, performance optimisation, disease prevention and risk assessment. This review focuses on proteins and peptides, their role as nutrients and biomarkers and on the technologies developed for their analysis. In the first part of this review, we provide insights into the way proteins are currently characterised and analysed using classical and emerging proteomic approaches. The scope of the second part is to review major applications of proteomics to nutrition, from characterisation of food proteins and peptides, via investigation of health-related food benefits to understanding disease-related mechanisms.

How Golgi glycosylation meets and needs trafficking: the case of the COG complex

Protein glycosylation is one of the major biosynthetic functions occurring in the endoplasmic reticulum and Golgi compartments. It requires an amazing number of enzymes, chaperones, lectins and transporters whose actions delicately secure the fidelity of glycan structures. Over the past 30 years, glycobiologists hammered that glycan structures are not mere decorative elements but serve crucial cellular functions. This becomes dramatically illustrated by a group of mostly severe, inherited human disorders named congenital disorders of glycosylation (CDG). To date, many types of CDG have been defined genetically and most of the time the defects impair the biosynthesis, transfer and remodeling of N-glycans. Recently, the identification of the several types of CDG caused by deficiencies in the conserved oligomeric Golgi (COG) complex, a complex involved in vesicular Golgi trafficking, expanded the field of CDG but also brought novel insights in glycosylation. The molecular mechanisms underlying the complex pathway of N-glycosylation in the Golgi are far from understood. The availability of COG-deficient CDG patients and patients' cells offered a new way to study how COG, and its different subunits, could influence the Golgi N-glycosylation machinery and localization. This review summarizes the recent findings on the implication of COG in Golgi glycosylation. It highlights the need for a dynamic, finely tuned balance between anterograde and retrograde trafficking for the correct localization of Golgi enzymes to assure the stepwise maturation of N-glycan chains.

Promiscuous activity of ER glucosidase II discovered through donor specificity analysis of UGGT

In glycoprotein quality control system in the endoplasmic reticulum (ER), UGGT (UDP-glucose:glycoprotein glucosyltransferase) and glucosidase II (G-II) play key roles. UGGT serves as a glycoprotein folding sensor by virtue of its unique specificity to glucosylate glycoproteins at incompletely folded stage. By using various UDP-Glc analogues, we first analyzed donor specificity of UGGT, which was proven to be rather narrow. However, marginal activity was observed with UDP-galactose and UDP-glucuronic acid as well as with 3-, 4- and 6-deoxy glucose analogues to give corresponding transfer products. Intriguingly, G-II smoothly converted all of them back to Man(9)GlcNAc(2), providing an indication that G-II has a promiscuous activity as a broad specificity hexosidase.

Serum tumor markers in pancreatic cancer-recent discoveries

The low prevalence of pancreatic cancer remains an obstacle to the development of effective screening tools in an asymptomatic population. However, development of effective serologic markers still offers the potential for improvement of diagnostic capabilities, especially for subpopulations of patients with high risk for pancreatic cancer. The accurate identification of patients with pancreatic cancer and the exclusion of disease in those with benign disorders remain important goals. While clinical experience largely dismissed many candidate markers as useful markers of pancreatic cancer, CA19-9 continues to show promise. The present review highlights the development and the properties of different tumor markers in pancreatic cancer and their impact on the diagnostic and treatment of this aggressive disease.

A cytoplasmic prolyl hydroxylation and glycosylation pathway modifies Skp1 and regulates O2-dependent development in Dictyostelium

The soil amoeba Dictyostelium is an obligate aerobe that monitors O(2) for informational purposes in addition to utilizing it for oxidative metabolism. Whereas low O(2) suffices for proliferation, a higher level is required for slugs to culminate into fruiting bodies, and O(2) influences slug polarity, slug migration, and cell-type proportioning. Dictyostelium expresses a cytoplasmic prolyl 4-hydroxylase (P4H1) known to mediate O(2)-sensing in animals, but lacks HIFalpha, a major hydroxylation target whose accumulation directly induces animal hypoxia-dependent transcriptional changes. The O(2)-requirement for culmination is increased by P4H1-gene disruption and reduced by P4H1 overexpression. A target of Dictyostelium P4H1 is Skp1, a subunit of the SCF-class of E3-ubiquitin ligases related to the VBC-class that mediates hydroxylation-dependent degradation of animal HIFalpha. Skp1 is a target of a novel cytoplasmic O-glycosylation pathway that modifies HyPro143 with a pentasaccharide, and glycosyltransferase mutants reveal that glycosylation intermediates have antagonistic effects toward P4H1 in O(2)-signaling. Current evidence indicates that Skp1 is the only glycosylation target in cells, based on metabolic labeling, biochemical complementation, and enzyme specificity studies. Bioinformatics studies suggest that the HyPro-modification pathway existed in the ancestral eukaryotic lineage and was retained in selected modern day unicellular organisms whose life cycles experience varying degrees of hypoxia. It is proposed that, in Dictyostelium and other protists including the agent for human toxoplasmosis Toxoplasma gondii, prolyl hydroxylation and glycosylation mediate O(2)-signaling in hierarchical fashion via Skp1 to control the proteome, directly via degradation rather than indirectly via transcription as found in animals.

Role of a cytoplasmic dual-function glycosyltransferase in O2 regulation of development in Dictyostelium

In the social amoeba Dictyostelium, a terminal step in development is regulated by environmental O(2). Prolyl 4-hydroxylase-1 (P4H1) was previously implicated in mediating the O(2) signal, and P4H1-null cells require elevated O(2) to culminate. The E3-ubiquitin ligase adaptor Skp1 is a P4H1 substrate, and here we investigate the function of PgtA, a dual function beta3-galactosyltransferase/alpha2-fucosyltransferase that contributes the 2nd and 3rd sugars of the pentasaccharide cap formed on Skp1 hydroxyproline. Although pgtA-null cells, whose Skp1 contains only a single sugar (N-acetylglucosamine or GlcNAc), show wild-type O(2) dependence of culmination, cells lacking AgtA, an alpha3-galactosyltransferase required to extend the trisaccharide, require elevated O(2) as for P4H1-null cells. Skp1 is the only detectable protein modified by purified PgtA added to pgtA-null extracts. The basis for specificity of PgtA was investigated using native Skp1 acceptor glycoforms and a novel synthetic peptide containing GlcNAcalpha1,4-hydroxy(trans)proline. Cysteine-alkylation of Skp1 strongly inhibited modification by the PgtA galactosyltransferase but not the fucosyltransferase. Furthermore, native and synthetic Skp1 glycopeptides were poorly galactosylated, not processively fucosylated, and negligibly inhibitory, whereas the fucosyltransferase was active toward small substrates. In addition, the galactosyltransferase exhibited an atypical concentration dependence on UDP-galactose. The results provide the first evidence that Skp1 is the functional target of P4H1 in O(2) regulation, indicate a gatekeeper function for the beta3-galactosyltransferase in the PgtA dual reaction, and identify an unexpected P4H1-dependent yet antagonistic function for PgtA that is reversed by AgtA.

Lectin-like ERAD players in ER and cytosol

Protein quality control in the endoplasmic reticulum (ER) is an elaborate process conserved from yeast to mammals, ensuring that only newly synthesized proteins with correct conformations in the ER are sorted further into the secretory pathway. It is well known that high-mannose type N-glycans are involved in protein-folding events. In the quality control process, proteins that fail to achieve proper folding or proper assembly are degraded in a process known as ER-associated degradation (ERAD). The ERAD pathway comprises multiple steps including substrate recognition and targeting to the retro-translocation machinery, retrotranslocation from the ER into the cytosol, and proteasomal degradation through ubiquitination. Recent studies have documented the important roles of sugar-recognition (lectin-type) molecules for trimmed high-mannose type N-glycans and glycosidases in the ERAD pathways in both ER and cytosol. In this review, we discuss a fundamental system that monitors glycoprotein folding in the ER and the unique roles of the sugar-recognizing ubiquitin ligase and peptide:N-glycanase (PNGase) in the cytosolic ERAD pathway.

Nucleocytoplasmic plant lectins

During the last decade it was unambiguously shown that plants synthesize minute amounts of carbohydrate-binding proteins upon exposure to stress situations like drought, high salt, hormone treatment, pathogen attack or insect herbivory. In contrast to the 'classical' plant lectins, which are typically found in storage vacuoles or in the extracellular compartment this new class of lectins is located in the cytoplasm and the nucleus. Based on these observations the concept was developed that lectin-mediated protein-carbohydrate interactions in the cytoplasm and the nucleus play an important role in the stress physiology of the plant cell. Hitherto, six families of nucleocytoplasmic lectins have been identified. This review gives an overview of our current knowledge on the occurrence of nucleocytoplasmic plant lectins. The carbohydrate-binding properties of these lectins and potential ligands in the nucleocytoplasmic compartment are discussed in view of the physiological role of the lectins in the plant cell.