F-Box Proteins That Contain Sugar-Binding Domains

Harnessing the power of mollusc lectins as immuno-protective biomolecules

The rapid advancement of molecular research on macromolecules has contributed to the discovery of 'Lectin', a carbohydrate-binding protein which specifically interacts with receptors on the surface of glycans and regulates various cellular activities thereby stimulating immunological functions. Considering the wide variety of sources and immunological significance, research has led to the discovery of lectins in invertebrate molluscs. Such lectins in molluscs mediate active immune response as they lack adaptive immunity. Phylum Mollusca is identified with different types of lectins such as C-lectin, Galectin, P-lectin, I-lectin, and H-lectin, along with other immunologically significant lectin molecules such as F- lectin, R-lectin, ficolins, chitinase like lectin etc., all of these with specific ligand binding and structural diversity. Molluscan C-type lectins are the most functional ones that increase the activity of phagocytic cells through specific carbohydrate binding of antigenic ligands and haemocyte adhesion thereby enhancing the immune response. Helix pomatia agglutinin and Helix aspersa agglutinin are the two H-lectins that were identified within molluscs that could even target cancer-progressing cells through specific binding. Also, these lectins identified in molluscs are proven to be efficient in antibacterial and immunomodulatory functions. These insights attract researchers to identify novel lectins in molluscs and their characterization that play a key role in protection against diseases. This review discusses the structural features of mollusc lectins, their specific binding, molecular interactions and their immunological applications.

CircGPRC5A enhances colorectal cancer progress by stabilizing PPP1CA and inducing YAP dephosphorylation

BACKGROUND

With the advancements in bioinformatic technology, an increasing number of circular RNAs (circRNAs) have been discovered and their crucial roles in the development and progression of various malignancies have been confirmed through multiple pathways. However, the specific mechanisms involving protein-binding circRNAs in colorectal cancer (CRC) remain largely unexplored.

METHODS

Differential circRNA expression was assessed using a human circRNA microarray in five CRC tissue and paired normal samples. CircGPRC5A expression was then confirmed in the CRC tissues and paired normal samples using qRT-PCR. The biological function of circGPRC5A in CRC were studied in vitro and in vivo. Western blotting, fluorescence in situ hybridization, immunofluorescence, RNA pulldown, mass spectrometry, immunoprecipitation, quantitative phosphoproteomics, and RNA-binding protein immunoprecipitation assays were used to study circGPRC5A.

RESULTS

Our analysis revealed that circGPRC5A expression was higher in CRC tissues compared to normal tissues and was associated with tumor size, tumor stage and lymph node status. CircGPRC5A promoted CRC cell proliferation, migration, and metastasis in vitro and in vivo. CircGPRC5A could stabilize PPP1CA protein by inhibiting the binding between UBA1 and PPP1CA, and increasing YAP dephosphorylation.

CONCLUSIONS

Our study revealed that circGPRC5A plays an essential function in CRC progression by stabilizing PPP1CA protein and enhancing YAP dephosphorylation. CircGPRC5A could act as a novel and potential target for CRC.

Human Lectins, Their Carbohydrate Affinities and Where to Find Them

Lectins are a class of proteins responsible for several biological roles such as cell-cell interactions, signaling pathways, and several innate immune responses against pathogens. Since lectins are able to bind to carbohydrates, they can be a viable target for targeted drug delivery systems. In fact, several lectins were approved by Food and Drug Administration for that purpose. Information about specific carbohydrate recognition by lectin receptors was gathered herein, plus the specific organs where those lectins can be found within the human body.

Frontal affinity chromatography: An excellent method of analyzing weak biomolecular interactions based on a unique principle

BACKGROUND

Not only strong biomolecular interactions but also weak interactions play important roles in ensuring appropriate operations of many biological systems. Although a thorough investigation of the latter is essential in understanding life science, few suitable research tools are available because of inherent difficulties.

SCOPE OF REVIEW

Frontal affinity chromatography (FAC) is a versatile method that overcomes the inherent difficulties to provide accurate information on weak interactions. Since its concept and merit are not widely recognized, a comprehensive interpretation of FAC is provided in this review to encourage its application among researchers.

MAJOR CONCLUSION

FAC is based on a unique principle of measuring the binding strength by the delayed migration of an analyte through an affinity column. Its utility was elucidated via the lectin-glycan interactions.

GENERAL SIGNIFICANCE

FAC has a great potential as a research tool to solve many difficult problems in general bioscience that are relevant to almost all researchers.

Crosstalk between Ubiquitination and Other Post-translational Protein Modifications in Plant Immunity

Post-translational modifications (PTMs) are central to the modulation of protein activity, stability, subcellular localization, and interaction with partners. They greatly expand the diversity and functionality of the proteome and have taken the center stage as key players in regulating numerous cellular and physiological processes. Increasing evidence indicates that in addition to a single regulatory PTM, many proteins are modified by multiple different types of PTMs in an orchestrated manner to collectively modulate the biological outcome. Such PTM crosstalk creates a combinatorial explosion in the number of proteoforms in a cell and greatly improves the ability of plants to rapidly mount and fine-tune responses to different external and internal cues. While PTM crosstalk has been investigated in depth in humans, animals, and yeast, the study of interplay between different PTMs in plants is still at its infant stage. In the past decade, investigations showed that PTMs are widely involved and play critical roles in the regulation of interactions between plants and pathogens. In particular, ubiquitination has emerged as a key regulator of plant immunity. This review discusses recent studies of the crosstalk between ubiquitination and six other PTMs, i.e., phosphorylation, SUMOylation, poly(ADP-ribosyl)ation, acetylation, redox modification, and glycosylation, in the regulation of plant immunity. The two basic ways by which PTMs communicate as well as the underlying mechanisms and diverse outcomes of the PTM crosstalk in plant immunity are highlighted.

FBXO2 modulates STAT3 signaling to regulate proliferation and tumorigenicity of osteosarcoma cells

Background

Osteosarcoma (OS) is the most common primary bone malignancy in children and adolescents, and hyperproliferation of cells is a major problem of OS. FBXO2 belongs to the family of F-box proteins, and is a substrate recognition component of the Skp1-Cul1-F-box protein (SCF) E3 ubiquitin ligase complex with specificity for high-mannose glycoproteins. The aim of the present study was to investigate the critical role of FBXO2 in OS cells.

Methods

The protein and mRNA expression levels of FBXO2 in clinic OS patients were measured by quantitative real time-polymerase chain reaction (qRT-PCR), Western blot and Immunohistochemical (IHC) staining assays, respectively. The FBXO2 overexpression model was constructed by retro-virus transfection in OS cells. FBXO2 knockout (KO) cells were generated by Clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated protein 9 (Cas9) assay. Cell counting and colony formation assays were used to analyze the effect of FBXO2 on the biological function of OS cells. FBXO2 KO cells were injected into nude mice to observe tumor growth in vivo. The interaction between FBXO2 and IL-6 was detected by immunoprecipitation. Luciferase assay was used to determine the transcriptional activity of STAT3.

Results

Here, we show that FBXO2 is significantly up-regulated in clinical OS samples compared to adjacent normal tissues. Ectopic expression of FBXO2 leads to increased OS cell proliferation and colony-forming ability, while FBXO2 knockout by CRISPR-Cas9-based gene editing has the opposite effect. In addition, the glycoprotein recognition activity of FBXO2 is required for its biological function in OS. In vivo experiments showed that FBXO2 knockout greatly impaired the tumorigenicity of OS cells in nude mice. At the molecular level, we found that knocking out FBXO2 can significantly inhibit STAT3 phosphorylation and downstream target gene expression through IL-6R stabilization.

Conclusion

Together, these results indicate that FBXO2 promotes OS development by activating the STAT3 signaling pathway, suggesting that FBXO2 may be a new target for OS treatment.

Effects of nicotinamide and sodium butyrate on meat quality and muscle ubiquitination degradation genes in broilers reared at a high stocking density

This study was conducted to investigate the effects of supplementation with nicotinamide (NAM) and sodium butyrate (BA) on meat quality and expression of muscle development genes in broilers reared at a high stocking density. A total of 567, 21-day-old AA broilers were randomly assigned to 5 treatment groups and 2 control groups, with 7 replicates of each group. The control groups included a low stocking density (LD; 12.9 birds/m²) and were fed a basal diet. The treatment groups were kept at a high stocking density (HD; 18.6 birds/m²) and received either a low dose of NAM (50 mg/kg; treatment LN), a high dose of NAM (100 mg/kg; treatment HN), a low dose of BA (500 mg/kg; treatment LB), a high dose of BA (1,000 mg/kg; treatment HB), or a compound supplement (50 mg/kg NAM+500 mg/kg BA; treatment COMB); broilers were reared till 42 D of age. The control groups were kept at HD or at LD (12.9 birds/m²) and were fed a basal diet. The heterophil-to-lymphocyte ratio was significantly higher in the HD control group than that in the LD group; this ratio was significantly lower in treatments LN, HN, HB, and COMB than that in the HD control group. The lightness of breast muscles at 45 min and 24 h after slaughter was significantly higher in the HD group than that in the LD group, and the HD group showed the highest drip loss at 24 h and 48 h. Lightness and drip loss were lower in the HN, LB, and COMB treatments than those in the HD group. HD rearing significantly reduced gene expression of myogenic regulatory factor 5 (MYF5) while significantly increased expression of the protein ubiquitin degradation genes FBXO9, FBXO22, and FBXO32. All treatments significantly reduced FBXO9 and FBXO32 expression. Our results suggest dietary supplementation with NAM and BA can improve meat quality of broilers under high stocking density by upregulating the expression of myogenic genes, and inhibiting protein ubiquitination.

The sugar code: letters and vocabulary, writers, editors and readers and biosignificance of functional glycan-lectin pairing

Ubiquitous occurrence in Nature, abundant presence at strategically important places such as the cell surface and dynamic shifts in their profile by diverse molecular switches qualifies the glycans to serve as versatile biochemical signals. However, their exceptional structural complexity often prevents one noting how simple the rules of objective-driven assembly of glycan-encoded messages are. This review is intended to provide a tutorial for a broad readership. The principles of why carbohydrates meet all demands to be the coding section of an information transfer system, and this at unsurpassed high density, are explained. Despite appearing to be a random assortment of sugars and their substitutions, seemingly subtle structural variations in glycan chains by a sophisticated enzymatic machinery have emerged to account for their specific biological meaning. Acting as 'readers' of glycan-encoded information, carbohydrate-specific receptors (lectins) are a means to turn the glycans' potential to serve as signals into a multitude of (patho)physiologically relevant responses. Once the far-reaching significance of this type of functional pairing has become clear, the various modes of spatial presentation of glycans and of carbohydrate recognition domains in lectins can be explored and rationalized. These discoveries are continuously revealing the intricacies of mutually adaptable routes to achieve essential selectivity and specificity. Equipped with these insights, readers will gain a fundamental understanding why carbohydrates form the third alphabet of life, joining the ranks of nucleotides and amino acids, and will also become aware of the importance of cellular communication via glycan-lectin recognition.

Compartmentalization and Selective Tagging for Disposal of Misfolded Glycoproteins

The ability of mammalian cells to correctly identify and degrade misfolded secretory proteins, most of them bearing N-glycans, is crucial for their correct function and survival. An inefficient disposal mechanism results in the accumulation of misfolded proteins and consequent endoplasmic reticulum (ER) stress. N-glycan processing creates a code that reveals the folding status of each molecule, enabling continued folding attempts or targeting of the doomed glycoprotein for disposal. We review here the main steps involved in the accurate processing of unfolded glycoproteins. We highlight recent data suggesting that the processing is not stochastic, but that there is selective accelerated glycan trimming on misfolded glycoprotein molecules.

Epstein-Barr virus activates F-box protein FBXO2 to limit viral infectivity by targeting glycoprotein B for degradation

Epstein-Barr virus (EBV) is a human cancer-related virus closely associated with lymphoid and epithelial malignancies, and EBV glycoprotein B (gB) plays an essential role in viral entry into both B cells and epithelial cells by promoting cell-cell fusion. EBV gB is exclusively modified with high-mannose-linked N-glycans and primarily localizes to the endoplasmic reticulum (ER) with low levels on the plasma membrane (PM). However, the mechanism through which gB is regulated within host cells is largely unknown. Here, we report the identification of F-box only protein 2 (FBXO2), an SCF ubiquitin ligase substrate adaptor that preferentially binds high-mannose glycans and attenuates EBV infectivity by targeting N-glycosylated gB for degradation. gB possesses seven N-glycosylation sites, and FBXO2 directly binds to these high-mannose moieties through its sugar-binding domain. The interaction promotes the degradation of glycosylated gB via the ubiquitin-proteasome pathway. Depletion of FBXO2 not only stabilizes gB but also promotes its transport from the ER to the PM, resulting in enhanced membrane fusion and viral entry. FBXO2 is expressed in epithelial cells but not B cells, and EBV infection up-regulates FBXO2 levels. In summary, our findings highlight the significance of high-mannose modification of gB and reveal a novel host defense mechanism involving glycoprotein homeostasis regulation.

Cytosolic N-Glycans: Triggers for Ubiquitination Directing Proteasomal and Autophagic Degradation: Molecular Systems for Monitoring Cytosolic N-Glycans as Signals for Unwanted Proteins and Organelles

Proteins on the cell surface and secreted proteins are modified with sugar chains that generate and modulate biological complexity and diversity. Sugar chains not only contribute physically to the conformation and solubility of proteins, but also exert various functions via sugar-binding proteins (lectins) that reside on the cell surface or in organelles of the secretory pathway. However, some glycosidases and lectins are found in the cytosol or nucleus. Recent studies of cytosolic sugar-related molecules have revealed that sugar chains on proteins in the cytosol act as signals of adverse cellular conditions. In this review, we summarize recent reports that cytosolic sugar chains can trigger ubiquitination, followed by proteasomal and autophagic degradation to maintain cellular homeostasis. In addition, we discuss the functions of sugar-binding proteins revealed to date, along with possibilities not yet explored.

An engineered high affinity Fbs1 carbohydrate binding protein for selective capture of N-glycans and N-glycopeptides

A method for selective and comprehensive enrichment of N-linked glycopeptides was developed to facilitate detection of micro-heterogeneity of N-glycosylation. The method takes advantage of the inherent properties of Fbs1, which functions within the ubiquitin-mediated degradation system to recognize the common core pentasaccharide motif (Man3GlcNAc2) of N-linked glycoproteins. We show that Fbs1 is able to bind diverse types of N-linked glycomolecules; however, wild-type Fbs1 preferentially binds high-mannose-containing glycans. We identified Fbs1 variants through mutagenesis and plasmid display selection, which possess higher affinity and improved recovery of complex N-glycomolecules. In particular, we demonstrate that the Fbs1 GYR variant may be employed for substantially unbiased enrichment of N-linked glycopeptides from human serum. Most importantly, this highly efficient N-glycopeptide enrichment method enables the simultaneous determination of N-glycan composition and N-glycosites with a deeper coverage (compared to lectin enrichment) and improves large-scale N-glycoproteomics studies due to greatly reduced sample complexity.

Protein glycosylation is an essential post-translational modification which analysis is complicated by the diversity of glycan composition and heterogeneity at individual attachment sites. Here the authors describe a method to selectively enrich N-linked glycopeptides to facilitate the detection of micro-heterogeneity in N-glycosylation.

A DsbA-Deficient Periplasm Enables Functional Display of a Protein with Redox-Sensitive Folding on M13 Phage

The requirements for target protein folding in M13 phage display are largely underappreciated. Here we chose Fbs1, a carbohydrate binding protein, as a model to address this issue. Importantly, folding of Fbs1 is impaired in an oxidative environment. Fbs1 can be displayed on M13 phage using the SRP or Sec pathway. However, the displayed Fbs1 protein is properly folded only when Fbs1 is translocated via the SRP pathway and displayed using Escherichia coli cells with a DsbA-negative periplasm. This study indicates M13 phage display may be improved using a system specifically designed according to the folding requirements of each target protein.

Type 1 diabetes in mice and men: gene expression profiling to investigate disease pathogenesis

Type 1 diabetes (T1D) is a complex polygenic disease that is triggered by various environmental factors in genetically susceptible individuals. The emphasis placed on genome-wide association studies to explain the genetics of T1D has failed to advance our understanding of T1D pathogenesis or identify biomarkers of disease progression or therapeutic targets. Using the nonobese diabetic (NOD) mouse model of T1D and the non-disease prone congenic NOD.B10 mice, our laboratory demonstrated striking tissue-specific and age-dependent changes in gene expression during disease progression. We established a "roadmap" of differential gene expression and used this to identify candidate genes in mice (and human orthologs) that play a role in disease pathology. Here, we describe two genes, Deformed epidermal autoregulatory factor 1 (Deaf1) and Adenosine A1 receptor (Adora1), that are differentially expressed and alternatively spliced in the pancreatic lymph nodes or islets of NOD mice and T1D patients to form dominant-negative non-functional isoforms. Loss of Deaf1 function leads to reduced peripheral tissue antigen expression in lymph node stromal cells and may contribute to a breakdown in peripheral tolerance, while reduced Adora1 function results in an early intrinsic alpha cell defect that may explain the hyperglucagonemia and resulting beta cell stress observed prior to the onset of diabetes. Remarkably, both genes were also alternatively spliced in the same tissues of auto-antibody positive prediabetic patients, and these splicing events resulted in similar downstream effects as those seen in NOD mice. These findings demonstrate the value of gene expression profiling in studying disease pathogenesis in T1D.

ERAD and how viruses exploit it

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a universally important process among eukaryotic cells. ERAD is necessary to preserve cell integrity since the accumulation of defective proteins results in diseases associated with neurological dysfunction, cancer, and infections. This process involves recognition of misfolded or misassembled proteins that have been translated in association with ER membranes. Recognition of ERAD substrates leads to their extraction through the ER membrane (retrotranslocation or dislocation), ubiquitination, and destruction by cytosolic proteasomes. This review focuses on ERAD and its components as well as how viruses use this process to promote their replication and to avoid the immune response.

A new dual-specific incompatibility allele revealed by absence of glycosylation in the conserved C2 site of a Solanum chacoense S-RNase

The stylar determinant of gametophytic self-incompatibility (GSI) in Solanaceae, Rosaceae, and Plantaginaceae is an S-RNase encoded by a multiallelic S-locus. The primary structure of S-RNases shows five conserved (C) and two hypervariable (HV) regions, the latter forming a domain implicated in S-haplotype-specific recognition of the pollen determinant to SI. All S-RNases are glycosylated at a conserved site in the C2 region, although previous studies have shown that N-linked glycans at this position are not required for S-haplotype-specific recognition and pollen rejection. Here the incompatibility phenotype of three constructs derived from an originally monoglycosylated S11-RNase of Solanum chacoense, that were designed to explore the role of the HV domain in determining pollen recognition and the role of the N-linked glycan in the C2 region, is reported. In one series of experiments, a second glycosylation site was introduced in the HVa region to test for inhibition of pollen-specific recognition. This modification does not impede pollen rejection, although analysis shows incomplete glycosylation at the new site in the HVa region. A second construct, designed to permit complete glycosylation at the HVa site by suppression of the conserved site in the C2 region, did increase the degree of site occupancy, but, again, glycosylation was incomplete. Plants expressing this construct rejected S 11 pollen and, surprisingly, also rejected S 13 pollen, thus displaying an unusual dual specificity phenotype. This construct differs from the first by the absence of the conserved C2 glycosylation site, and thus the dual specificity is observed only in the absence of the C2 glycan. A third construct, completely lacking glycosylation sites, conferred an ability to reject only S 11 pollen, disproving the hypothesis that lack of a conserved glycan would confer a universal pollen rejection phenotype to the plant.

Proteomic identification of common SCF ubiquitin ligase FBXO6-interacting glycoproteins in three kinds of cells

FBOX6 ubiquitin ligase complex is involved in the endoplasmic reticulum-associated degradation pathway by mediating the ubiquitination of glycoproteins. FBXO6 interacts with the chitobiose in unfolded N-glycoprotein, pointing glycoproteins toward E2 for ubiquitination. Although the glycoprotein-recognizing mechanism of FBXO6 is well documented, its bona fide interacting glycoproteins are largely unknown. Here we utilized a protein purification approach combined with LC-MS to systematically identify the FBXO6-interacting glycoproteins. Following identification of 39 proteins that specifically interact with FBXO6 in all three different cell lines, 293T, HeLa and Jurkat cells, we compared the protein complex organization between wild-type FBXO6 and its mutant, which fails to recognize glycoproteins. Combining these databases, 29 highly confident glycoproteins that interact with FBXO6 in an N-glycan dependent manner are identified. Our data provide valuable information for the discovery of the interacting glycoproteins of FBXO6 and also demonstrate the potential of these approaches as general platforms for the global discovery of interacting glycoproteins of other FBAs (F-box associated regions) containing F-box proteins.

Characterization of non-specific cytotoxic cell receptor protein 1: a new member of the lectin-type subfamily of F-box proteins

Our previous microarray study showed that the non-specific cytotoxic cell receptor protein 1 (Nccrp1) transcript is significantly upregulated in the gastric mucosa of carbonic anhydrase IX (CA IX)-deficient (Car9^−/−) mice. In this paper, we aimed to characterize human NCCRP1 and to elucidate its relationship to CA IX. Recombinant NCCRP1 protein was expressed in Escherichia coli, and a novel polyclonal antiserum was raised against the purified full-length protein. Immunocytochemistry showed that NCCRP1 is expressed intracellularly, even though it has previously been described as a transmembrane protein. Using bioinformatic analyses, we identified orthologs of NCCRP1 in 35 vertebrate genomes, and up to five paralogs per genome. These paralogs are FBXO genes whose protein products are components of the E3 ubiquitin ligase complexes. NCCRP1 proteins have no signal peptides or transmembrane domains. NCCRP1 has mainly been studied in fish and was thought to be responsible for the cytolytic function of nonspecific cytotoxic cells (NCCs). Our analyses showed that in humans, NCCRP1 mRNA is expressed in tissues containing squamous epithelium, whereas it shows a more ubiquitous tissue expression pattern in mice. Neither human nor mouse NCCRP1 expression is specific to immune tissues. Silencing CA9 using siRNAs did not affect NCCRP1 levels, indicating that its expression is not directly regulated by CA9. Interestingly, silencing NCCRP1 caused a statistically significant decrease in the growth of HeLa cells. These studies provide ample evidence that the current name, “non-specific cytotoxic cell receptor protein 1,” is not appropriate. We therefore propose that the gene name be changed to FBXO50.

Redox control of the ubiquitin-proteasome system: from molecular mechanisms to functional significance

In their natural environments, cells are regularly exposed to oxidizing conditions that may lead to protein misfolding. If such misfolded proteins are allowed to linger, they may form insoluble aggregates and pose a serious threat to the cell. Accumulation of misfolded, oxidatively damaged proteins is characteristic of many diseases and during aging. To counter the adverse effects of oxidative stress, cells can initiate an antioxidative response in an attempt to repair the damage, or rapidly channel the damaged proteins for degradation by the ubiquitin-proteasome system (UPS). Recent studies have shown that elements of the oxidative stress response and the UPS are linked on many levels. To manage the extra burden of misfolded proteins, the UPS is induced by oxidative stress, and special proteasome subtypes protect cells against oxidative damage. In addition, the proteasome is directly associated with a thioredoxin and other cofactors that may adjust the particle's response during an oxidative challenge. Here, we give an overview of the UPS and a detailed description of the degradation of oxidized proteins and of the crosstalk between oxidative stress and protein degradation in health and disease.

Mannose trimming is required for delivery of a glycoprotein from EDEM1 to XTP3-B and to late endoplasmic reticulum-associated degradation steps

Although the trimming of α1,2-mannose residues from precursor N-linked oligosaccharides is an essential step in the delivery of misfolded glycoproteins to endoplasmic reticulum (ER)-associated degradation (ERAD), the exact role of this trimming is unclear. EDEM1 was initially suggested to bind N-glycans after mannose trimming, a role presently ascribed to the lectins OS9 and XTP3-B, because of their in vitro affinities for trimmed oligosaccharides. We have shown before that ER mannosidase I (ERManI) is required for the trimming and concentrates together with the ERAD substrate and ERAD machinery in the pericentriolar ER-derived quality control compartment (ERQC). Inhibition of mannose trimming prevents substrate accumulation in the ERQC. Here, we show that the mannosidase inhibitor kifunensine or ERManI knockdown do not affect binding of an ERAD substrate glycoprotein to EDEM1. In contrast, substrate association with XTP3-B and with the E3 ubiquitin ligases HRD1 and SCF(Fbs2) was inhibited. Consistently, whereas the ERAD substrate partially colocalized upon proteasomal inhibition with EDEM1, HRD1, and Fbs2 at the ERQC, colocalization was repressed by mannosidase inhibition in the case of the E3 ligases but not for EDEM1. Interestingly, association and colocalization of the substrate with Derlin-1 was independent of mannose trimming. The HRD1 adaptor protein SEL1L had been suggested to play a role in N-glycan-dependent substrate delivery to OS9 and XTP3-B. However, substrate association with XTP3-B was still dependent on mannose trimming upon SEL1L knockdown. Our results suggest that mannose trimming enables delivery of a substrate glycoprotein from EDEM1 to late ERAD steps through association with XTP3-B.

Crystallization and preliminary X-ray characterization of the Skp1-Fbg3 complex

F-box proteins are the substrate-recognition components of Skp1-Cullin1-F-box protein-Rbx1 (SCF) ubiquitin ligase complexes. Fbs1, an F-box protein, binds specifically to proteins modified with high-mannose oligosaccharides. Fbg3, another F-box protein, has 51% sequence identity to Fbs1. Although the residues that are necessary for binding to oligosaccharides are conserved between Fbs1 and Fbg3, Fbg3 does not bind glycoproteins. Skp1 and Fbg3 were co-expressed in Escherichia coli and their complex was purified to homogeneity and crystallized. Microseeding combined with the sandwiched hanging-drop technique improved the quality of the resulting crystals. The plate-shaped crystals belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 34.1, b = 76.6, c = 193.9 A and one molecule per asymmetric unit.

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.

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

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of "glycobiology in the cytosol" which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.