Effective surface immobilization is a prerequisite for numerous carbohydrate-related studies including carbohydrate-biomolecule interactions. In the present work, we report a simple and rapid modification technique for diverse carbohydrate types in which direct oriented immobilization onto a gold surface is accomplished by coupling the amine group of a thiol group-bearing aminophenyl disulfide as a new coupling reagent with an aldehyde group of the terminal reducing sugar in the carbohydrate. To demonstrate the generality of this proposed reductive amination method, we examined its use for three types of carbohydrates: glucose (monosaccharide), lactose (disaccharide), and GM1 pentasaccharide. Through successful mass identifications of the modified carbohydrates, direct binding assays on gold surface using surface plasmon resonance and electrochemical methods, and a terminal galactose-binding lectin assay using atomic force microscopy, we confirmed several advantages including direct and rapid one-step immobilization onto a gold surface and exposure of functional carbohydrate moieties through oriented modification of the terminal reducing sugar. Therefore, this facile modification and immobilization method can be successfully used for diverse biomimetic studies of carbohydrates, including carbohydrate-biomolecule interactions and carbohydrate sensor or array development for diagnosis and screening.
Most insect cells have a simple N-glycosylation process and consequently paucimannosidic or simple core glycans predominate. Previously, we have shown that paucimannosidic N-glycan structures are dominant in Drosophila S2 cells. It has been proposed that beta-N-acetylglucosaminidase (GlcNAcase), a hexosaminidase in the Golgi membrane which removes a terminal N-acetylglucosamine (GlcNAc), might contribute to simple N-glycosylation in several insects and insect-derived cells except S2 cells. In the present work, we investigated the substantial effects of GlcNAcase on N-glycan patterns in Drosophila S2 cells using two GlcNAcase suppression strategies: an mRNA-targeting approach using RNA interference (RNAi) and a protein-targeting approach using the specific chemical inhibitor 2-acetamido-1,2-dideoxynojirimycin (2-ADN). Using high-performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analyses, we found that the N-glycosylation patterns of human erythropoietin (hEPO) secreted by stably transfected S2 cells were more complex following GlcNAcase suppression, which generated N-glycan structures with a terminal GlcNAc and/or galactose. These data demonstrate that GlcNAcase may be an important factor in the formation of paucimannosidic core N-glycans in Drosophila S2 cells and suggest that it may be possible to express complex glycoproteins in engineered Drosophila S2 cells by suppressing GlcNAcase in the N-glycosylation pathway.
Mussel adhesive proteins (MAPs) have been suggested as promising bioadhesives for diverse application fields, including medical uses. Previously, we successfully constructed and produced a new type of functional recombinant MAP, fp-151, in a prokaryotic Escherichia coli expression system. Even though the E. coli-derived MAP showed several excellent features, such as high production yield and efficient purification, in vitro enzymatic modification is required to convert tyrosine residues to l-3,4-dihydroxyphenyl alanine (dopa) molecules for its adhesive ability, due to the intrinsic inability of E. coli to undergo post-translational modification. In this work, we produced a soluble recombinant MAP in insect Sf9 cells, which are widely used as an effective and convenient eukaryotic expression system for eukaryotic foreign proteins. Importantly, we found that insect-derived MAP contained converted dopa residues by in vivo post-translational modification. In addition, insect-derived MAP also had other post-translational modifications including phosphorylation of serine and hydroxylation of proline that originally occurred in some natural MAPs. To our knowledge, this is the first report on in vivo post-translational modifications of MAP containing dopa and other modified amino acid residues.
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