Resolution of glycoproteins by a lectin gel-shift assay

Advances in Strategies and Tools Available for Interrogation of Protein O-GlcNAcylation

The attachment of a single O-linked β-N-acetylglucosamine (O-GlcNAc) to serine and threonine residues of numerous proteins in the nucleus, cytoplasm, and mitochondria is a reversible post-translational modification (PTM) and plays an important role as a regulator of various cellular processes in both healthy and disease states. Advances in strategies and tools that allow for the detection of dynamic O-GlcNAcylation on cellular proteins have helped to enhance our initial and ongoing understanding of its dynamic effects on cellular stimuli and given insights into its link to the pathogenesis of several chronic diseases. Furthermore, chemical genetic strategies and related tools have been successfully applied to a myriad of biological systems with a new level of spatiotemporal and molecular precision. These strategies have started to be used in studying and controlling O-GlcNAcylation both in vivo and in vitro. In this minireview, overviews of recent advances in molecular tools being applied to the detection and identification of O-GlcNAcylation on cellular proteins as well as on individual proteins are provided. In addition, chemical genetic strategies that have already been applied or are potentially usable in O-GlcNAc functional are also discussed.

ARL15 modulates magnesium homeostasis through N-glycosylation of CNNMs

Cyclin M (CNNM1-4) proteins maintain cellular and body magnesium (Mg²⁺) homeostasis. Using various biochemical approaches, we have identified members of the CNNM family as direct interacting partners of ADP-ribosylation factor-like GTPase 15 (ARL15), a small GTP-binding protein. ARL15 interacts with CNNMs at their carboxyl-terminal conserved cystathionine-β-synthase (CBS) domains. In silico modeling of the interaction between CNNM2 and ARL15 supports that the small GTPase specifically binds the CBS1 and CNBH domains. Immunocytochemical experiments demonstrate that CNNM2 and ARL15 co-localize in the kidney, with both proteins showing subcellular localization in the endoplasmic reticulum, Golgi apparatus and the plasma membrane. Most importantly, we found that ARL15 is required for forming complex N-glycosylation of CNNMs. Overexpression of ARL15 promotes complex N-glycosylation of CNNM3. Mg²⁺ uptake experiments with a stable isotope demonstrate that there is a significant increase of ²⁵Mg²⁺ uptake upon knockdown of ARL15 in multiple kidney cancer cell lines. Altogether, our results establish ARL15 as a novel negative regulator of Mg²⁺ transport by promoting the complex N-glycosylation of CNNMs.

NS1 Recombinant Proteins Are Efficiently Produced in Pichia pastoris and Have Great Potential for Use in Diagnostic Kits for Dengue Virus Infections

Dengue is one of the major diseases causing global public health concerns. Despite technological advances in vaccine production against all its serotypes, it is estimated that the dengue virus is responsible for approximately 390 million infections per year. Laboratory diagnosis has been the key point for the correct treatment and prevention of this disease. Currently, the limiting factor in the manufacture of dengue diagnostic kits is the large-scale production of the non-structural 1 (NS1) antigen used in the capture of the antibody present in the infected patients' serum. In this work, we demonstrate the production of the non-structural 1 protein of dengue virus (DENV) serotypes 1-4 (NS1-DENV1, NS1-DENV2, NS1-DENV3, and NS1-DENV4) in the methylotrophic yeast Pichia pastoris KM71H. Secreted recombinant protein was purified by affinity chromatography and characterized by SDS-PAGE and ELISA. The objectives of this study were achieved, and the results showed that P. pastoris is a good heterologous host and worked well in the production of NS1DENV 1-4 recombinant proteins. Easy to grow and quick to obtain, this yeast secreted ready-to-use proteins, with a final yield estimated at 2.8-4.6 milligrams per liter of culture. We reached 85-91% sensitivity and 91-93% specificity using IgM as a target, and for anti-dengue IgG, 83-87% sensitivity and 81-93% specificity were achieved. In this work, we conclude that the NS1 recombinant proteins are efficiently produced in P. pastoris and have great potential for use in diagnostic kits for dengue virus infections. The transformed yeast obtained can be used for production in industrial-scale bioreactors.

WGA-based lectin affinity gel electrophoresis: A novel method for the detection of O-GlcNAc-modified proteins

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc) occurs selectively on serine and/or threonine residues of cytoplasmic and nuclear proteins, and dynamically regulates their molecular functions. Since conventional strategies to evaluate the O-GlcNAcylation level of a specific protein require time-consuming steps, the development of a rapid and easy method for the detection and quantification of an O-GlcNAcylated protein has been a challenging issue. Here, we describe a novel method in which O-GlcNAcylated and non-O-GlcNAcylated forms of proteins are separated by lectin affinity gel electrophoresis using wheat germ agglutinin (WGA), which primarily binds to N-acetylglucosamine residues. Electrophoresis of cell lysates through a gel containing copolymerized WGA selectively induced retardation of the mobility of O-GlcNAcylated proteins, thereby allowing the simultaneous visualization of both the O-GlcNAcylated and the unmodified forms of proteins. This method is therefore useful for the quantitative detection of O-GlcNAcylated proteins.

N-glycosylation and topology of the human SLC26 family of anion transport membrane proteins

The human solute carrier (SLC26) family of anion transporters consists of 10 members (SLCA1-11, SLCA10 being a pseudogene) that encode membrane proteins containing ~12 transmembrane (TM) segments with putative N-glycosylation sites (-NXS/T-) in extracellular loops and a COOH-terminal cytosolic STAS domain. All 10 members of the human SLC26 family, FLAG-tagged at the NH2 terminus, were transiently expressed in HEK-293 cells. While most proteins were observed to contain both high-mannose and complex oligosaccharides, SLC26A2 was mainly in the complex form, SLC26A4 in the high-mannose form, and SLC26A8 was not N-glycosylated. Mutation of the putative N-glycosylation sites showed that most members contain multiple N-glycosylation sites in the second extracytosolic (EC) loop, except SLC26A11, which was N-glycosylated in EC loop 4. Immunofluorescence staining of permeabilized cells localized the proteins to the plasma membrane and the endoplasmic reticulum, with SLC26A2 highly localized to the plasma membrane. N-glycosylation was not a necessary requirement for cell surface expression as the localization of nonglycosylated proteins was similar to their wild-type counterparts, although a lower level of cell-surface biotinylation was observed. No immunostaining of intact cells was observed for any SLC26 members, demonstrating that the NH2-terminal FLAG tag was located in the cytosol. Topological models of the SLC26 proteins that contain an even number of transmembrane segments with both the NH2 and COOH termini located in the cytosol and utilized N-glycosylation sites defining the positions of two EC loops are presented.

Polyacrylamide Gel Affinity Electrophoresis for Separation of Enzyme Isoforms

Affinity electrophoresis of differently glycosylated isoforms of enzymes using lectin as affinity ligand has been reported on support media such as cellulose acetate membrane (CAM) or agarose gel. We report a method for affinity electrophoresis in polyacrylamide gel (PAG) using wheat germ agglutinin (WGA). WGA is added to acrylamide-Bis mixture and incubated for 10 minutes at room temperature. This causes WGA to react covalently with acrylamide and Bis. Polymerization is initiated with addition of N,N,N,N-tetramethylethylene diamine (TEMED) and ammonium persulphate to give polyacrylamide gel with immobilized lectin. This gel has been found to effectively separate differently glycosylated isoforms of alkaline phosphatase (ALP). Concanavalin - A, similarly immobilized, did not give effective separation of ALP isoforms. The immobilization of lectin on polyacrylamide as support media requires less amount of lectin in comparison to CAM and agarose. Additional advantage of affinity electrophoresis on PAG is separation of biomolecules according to size.

The function of hypoxia-inducible factor (HIF) is independent of the endoplasmic reticulum protein OS-9

The protein “amplified in osteosarcoma-9” (OS-9) has been shown previously to interact with the prolyl hydroxylases PHD2 and PHD3. These enzymes initiate oxygen-dependent degradation of the α-subunit of hypoxia-inducible factor (HIF), a transcription factor that adapts cells to insufficient oxygen supply (hypoxia). A new model has been proposed where OS-9 triggers PHD dependent degradation of HIF-α. It was the aim of our study to define the molecular mode of action of OS-9 in the regulation of PHD and HIF activity. Although initial co-immunoprecipitation experiments confirmed physical interaction between OS-9 and PHD2, neither overexpression nor lentiviral inhibition of OS-9 expression affected HIF regulation. Subcellular localization experiments revealed a distinct reticular staining pattern for OS-9 while PHD2 was mainly localized in the cytoplasm. Further cell fractionation experiments and glycosylation tests indicated that OS-9 is a luminal ER protein. In vivo protein interaction analysis by fluorescence resonance energy transfer (FRET) showed no significant physical interaction of overexpressed PHD2-CFP and OS-9-YFP. We conclude that OS-9 plays no direct functional role in HIF degradation since physical interaction of OS-9 with oxygen sensing HIF prolyl hydroxylases cannot occur in vivo due to their different subcellular localization.

Beta-amyloid controls altered Reelin expression and processing in Alzheimer's disease

Reelin is a glycoprotein that modulates synaptic function and plasticity in the mature brain, thereby favouring memory formation. We recently reported altered cerebral Reelin expression in Alzheimer's disease (AD). Here we demonstrate pronounced Reelin changes at protein and mRNA levels in the frontal cortex in adult Down's syndrome (DS), where the extra copy of chromosome 21 leads to overexpression of beta-amyloid. In cortical extracts of fetal DS samples we detected increased levels of the full-length Reelin and the 310-kDa fragment. Overexpression of mutant human amyloid precursor protein also led to an increase in levels of Reelin fragments in Tg2576 transgenic mice for human beta-amyloid. Finally, in vitro Abeta42 treatment of SH-SY5Y neuroblastoma cells led to increased Reelin levels. An altered pattern of Reelin glycosylation was detected in extracts from the frontal cortex of AD patients and in Abeta42-treated SH-SY5Y cells, supporting the notion that beta-amyloid triggers altered Reelin processing. These results provide evidence that Reelin expression and processing is altered in several amyloid conditions.

Microcontact printing of Concanavalin A and its effect on mammalian cell morphology

In this study a major lectin called Concanavalin A (ConA) has been micropatterned on a glass substrate by microcontact printing and the patterns have been characterized with fluorescent and atomic force microscope for their uniformity. Interaction of the patterns with mammalian cells has been investigated by culturing L929 mouse fibroblast cells on the ConA printed glass surface. Cell culture results obtained from the microcontact printed patterns have also been compared and benchmarked with another patterning technique named micromolding in capillaries (MIMIC). It has been revealed that in spite of molecular level heterogeneity and agglomeration of protein molecules in microcontact printed form, they can still interact with cell surface glycoproteins, impede the mobility of membrane receptor which results in altered morphology of the fibroblast cells.

Membrane integration and topology of the first transmembrane segment in normal and Southeast Asian ovalocytosis human erythrocyte anion exchanger 1

Anion exchanger 1 (AE1, or Band 3) is an integral membrane glycoprotein found in erythrocytes, responsible for the electroneutral exchange of chloride and bicarbonate ions across the plasma membrane. Southeast Asian ovalocytosis (SAO) results from a nine-amino acid deletion in the first transmembrane segment (TM) of the AE1 protein that abolishes its transport function. The effects of the SAO deletion on: (1) the efficiency of integration of TM1 into the membrane, and (2) the precise positioning of TM1 relative to the membrane were investigated using scanning N-glycosylation mutagenesis in a cell-free transcription/translation system and in transfected HEK293 cells. AE1 or SAO constructs containing either the endogenous N-glycosylation site at Asn642 in extracellular loop 4 (EC4) or single N-glycosylation sites engineered into an expanded extracellular loop 1 (EC1) were used. N-glycosylation efficiency of EC1 in the SAO construct was significantly lower than that of the AE1 construct, indicating that the SAO deletion impairs membrane integration of TM1 and the translocation of EC1 across the membrane. Scanning N-glycosylation mapping of EC1 in the cell-free system and in transfected cells showed that the C-terminus of both AE1 and SAO TM1 were at the same position relative to the membrane. Thus, the SAO deletion is likely to cause a pulling-in of the polar amino acid sequence immediately N-terminal to the deletion into the lipid bilayer, allowing SAO TM1 that was inserted to assume a transmembrane disposition.

Topology of transmembrane segments 1-4 in the human chloride/bicarbonate anion exchanger 1 (AE1) by scanning N-glycosylation mutagenesis

Human AE1 (anion exchanger 1), or Band 3, is an abundant membrane glycoprotein found in the plasma membrane of erythrocytes. The physiological role of the protein is to carry out chloride/bicarbonate exchange across the plasma membrane, a process that increases the carbon-dioxide-carrying capacity of blood. To study the topology of TMs (transmembrane segments) 1-4, a series of scanning N-glycosylation mutants were created spanning the region from EC (extracellular loop) 1 to EC2 in full-length AE1. These constructs were expressed in HEK-293 (human embryonic kidney) cells, and their N-glycosylation efficiencies were determined. Unexpectedly, positions within putative TMs 2 and 3 could be efficiently glycosylated. In contrast, the same positions were very poorly glycosylated when present in mutant AE1 with the SAO (Southeast Asian ovalocytosis) deletion (DeltaA400-A408) in TM1. These results suggest that the TM2-3 region of AE1 may become transiently exposed to the endoplasmic reticulum lumen during biosynthesis, and that there is a competition between proper folding of the region into the membrane and N-glycosylation at introduced sites. The SAO deletion disrupts the proper integration of TMs 1-2, probably leaving the region exposed to the cytosol. As a result, engineered N-glycosylation acceptor sites in TM2-3 could not be utilized by the oligosaccharyltransferase in this mutant form of AE1. The properties of TM2-3 suggest that these segments form a re-entrant loop in human AE1.

Expression and characterization of the anion transporter homologue YNL275w in Saccharomyces cerevisiae

A search of the yeast Saccharomyces cerevisiae genome has revealed an open reading frame, YNL275w, which encodes a 576-amino acid protein that shows sequence similarity to the family of mammalian Cl-/HCO3- anion exchangers and Na+/HCO3- cotransporters. This yeast protein also has a very similar hydropathy profile to the mammalian HCO3- transporters, indicating a similar membrane topology and structure. A V5 epitope and His6-tagged version of Ynl275wp was expressed in yeast and was localized to the plasma membrane by subcellular fractionation and immunofluorescence labeling. The protein was purified by nickel affinity chromatography and was found not to be N-glycosylated. The protein's mobility on SDS-PAGE gels was not altered by treatment with N-glycanase F, alpha-mannosidase, or by mutation of each of the five consensus N-glycosylation sites. The protein did not bind to concanavalin A by lectin blotting or lectin affinity chromatography. The expressed protein bound specifically to a stilbene disulfonate inhibitor resin (SITS-Affi-Gel), and this binding could be competed by certain anions (HCO3-, Cl-, NO3-, and I-) but not by others (SO4(2-) and PO4(3-)). These results suggest that the yeast gene YNL275w encodes a nonglycosylated anion transport protein, localized to the plasma membrane.