Expression and characterization of silkworm Bombyx mori β-1,2-N-acetylglucosaminyltransferase II, a key enzyme for complex-type N-glycan biosynthesis

Efficient production and characterization of soluble active human β-1,2-N-acetylglucosaminyltransferase II in bacteria

In humans, almost all the cell surface and secreted glycoproteins are modified with complex-type N-glycans. Thus, it is essential to obtain complex-type N-glycans to fully understand the biological properties of glycoproteins. Here, human β-1,2-N-acetylglucosaminyltransferase II (hGnT-II), a Golgi-localized enzyme integral to complex-type N-glycan biosynthesis, was cloned as a truncated transmembrane form (GnT-II-ΔTM) and heterologously overexpressed in Escherichia coli. Our results showed that hGnT-II could be overexpressed in its soluble form by fusing the truncated enzyme with a thioredoxin (Trx)-tag in the Rosetta-Gami 2 strain. Using the optimized induction conditions, the expression level of recombinant protein was enhanced to yield approximately 4 mg per liter culture after affinity purification. The enzyme exhibited appropriate glycosyltransferase activity, and the calculated Km value was 52.4 μM, similar to the protein expressed in mammalian cells. Furthermore, the effect of MGAT2-CDG mutations on enzyme activity was also measured. These results suggested that the E. coli expression system was capable of the large-scale production of bioactive hGnT-II, which can be used for functional study and effective synthesis of complex-type N-glycans.

Crystal structure and sugar-binding ability of the C-terminal domain of N-acetylglucosaminyltransferase IV establish a new carbohydrate-binding module family

N-glycans are modified by glycosyltransferases in the endoplasmic reticulum and Golgi apparatus. N-acetylglucosaminyltransferase IV (GnT-IV) is a Golgi-localized glycosyltransferase that synthesizes complex-type N-glycans in vertebrates. This enzyme attaches N-acetylglucosamine (GlcNAc) to the α-1,3-linked mannose branch of the N-glycan core structure via a β-1,4 linkage. Deficiency of this enzyme is known to cause abnormal cellular functions, making it a vital enzyme for living organisms. However, there has been no report on its 3-dimensional structure to date. Here, we demonstrated that the C-terminal regions (named CBML) of human GnT-IVa and Bombyx mori ortholog have the ability to bind β-N-acetylglucosamine. In addition, we determined the crystal structures of human CBML, B. mori CBML, and its complex with β-GlcNAc at 1.97, 1.47, and 1.15 Å resolutions, respectively, and showed that they adopt a β-sandwich fold, similar to carbohydrate-binding module family 32 (CBM32) proteins. The regions homologous to CBML (≥24% identity) were found in GnT-IV isozymes, GnT-IVb, and GnT-IVc (known as GnT-VI), and the structure of B. mori CBML in complex with β-GlcNAc indicated that the GlcNAc-binding residues were highly conserved among these isozymes. These residues are also conserved with the GlcNAc-binding CBM32 domain of β-N-acetylhexosaminidase NagH from Clostridium perfringens despite the low sequence identity (<20%). Taken together with the phylogenetic analysis, these findings indicate that these CBMLs may be novel CBM family proteins with GlcNAc-binding ability.

Mechanisms behind polyphagia in a pest insect: Responses of Spodoptera frugiperda (J.E. Smith) strains to preferential and alternative larval host plants assessed with gene regulatory networks

A dataset of gene expression from Spodoptera frugiperda, a highly generalist pest moth, was used to understand how gene regulation is related to larval host plant preference. Transcriptomic data of corn and rice strains of S. frugiperda larvae, reared on different diets, were analysed with three different approaches of gene network inference, namely co-expression, weighted co-expression and Bayesian networks, since each methodology provides a different visualization of the data. Using these approaches, it was possible to identify two loosely interconnected co-expression networks, one of them responsible for fast response to herbivory and anti-herbivory mechanisms and the other related to housekeeping genes, which present slower response to environmental variations. Integrating different levels of information such as gene expression patterns, gene assembly, transcriptomics, relationship among genes and phenotypes, functional relationships, among other information, enabled a wider visualization of S. frugiperda response to diet stimuli. The biological properties in the proposed networks are here described and discussed, as well as patterns of gene expression related to larval performance attributes.

α-L-Fucosidase from Bombyx mori has broad substrate specificity and hydrolyzes core fucosylated N-glycans

N-glycans play a role in physiological functions, including glycoprotein conformation, signal transduction, and antigenicity. Insects display both α-1,6- and α-1,3-linked fucose residues bound to the innermost N-acetylglucosamine of N-glycans whereas core α-1,3-fucosylated N-glycans are not found in mammals. Functions of insect core-fucosylated glycans are not clear, and no α-L-fucosidase related to the N-glycan degradation has been identified. In the genome of the domestic silkworm, Bombyx mori, a gene for a protein, BmFucA, belonging to the glycoside hydrolase family 29 is a candidate for an α-L-fucosidase gene. In this study, BmFucA was cloned and recombinantly expressed as a glutathione-S-transferase tagged protein (GST-BmFucA). Recombinant GST-BmFucA exhibited broad substrate specificity and hydrolyzed p-nitrophenyl α-L-fucopyranoside, 2'-fucosyllactose, 3-fucosyllactose, 3-fucosyl-N,N'-diacetylchitobiose, and 6-fucosyl-N,N'-diacetylchitobiose. Further, GST-BmFucA released fucose from both pyridylaminated complex-type and paucimannose-type glycans that were core-α-1,6-fucosylated. GST-BmFucA also shows hydrolysis activity for core-fucosylated glycans attached to phospholipase A₂ from bee venom. BmFucA may be involved in the catabolism of core-fucosylated N-glycans in B. mori.

Characterization of Bombyx mori N-acetylglucosaminyltransferase II splicing variants

N-Acetylglucosaminyltransferase II (GNTII), which catalyzes the transfer of N-acetylglucosamine to N-glycans, plays an essential role in the biosynthesis of branched and complex-type N-glycans. Some characteristics of the GNTIIs from various species have been identified, but not all features have been revealed because some insects have GNTII redundancies due to the possession of splicing variants. In this study, we focused on four splicing variants of silkworm Bombyx mori GNTII (BmGNTII) that differ only in the absence or presence of Exon 2, Exon 9 or both, and we characterized the spatiotemporal transcript levels and enzymatic properties of each. Two of the splicing variants, BmGNTII-B and BmGNTII-D, lack Exon 9, and were expressed more highly in silk glands than any other organs. With respect to the enzymatic properties, optimal temperature and pH were similar among the recombinant BmGNTIIs, but the specific activities and temperature stabilities differed according to the presence or absence of Exon 9 in the splicing variants. These results demonstrate that the B. mori genome encodes splicing variants of GNTII with different enzymatic properties.

How glycosylation affects glycosylation: the role of N-glycans in glycosyltransferase activity

N-glycosylation is one of the most important posttranslational modifications of proteins. It plays important roles in the biogenesis and functions of proteins by influencing their folding, intracellular localization, stability and solubility. N-glycans are synthesized by glycosyltransferases, a complex group of ubiquitous enzymes that occur in most kingdoms of life. A growing body of evidence shows that N-glycans may influence processing and functions of glycosyltransferases, including their secretion, stability and substrate/acceptor affinity. Changes in these properties may have a profound impact on glycosyltransferase activity. Indeed, some glycosyltransferases have to be glycosylated themselves for full activity. N-glycans and glycosyltransferases play roles in the pathogenesis of many diseases (including cancers), so studies on glycosyltransferases may contribute to the development of new therapy methods and novel glycoengineered enzymes with improved properties. In this review, we focus on the role of N-glycosylation in the activity of glycosyltransferases and attempt to summarize all available data about this phenomenon.

Biochemical characterization and mutational analysis of silkworm Bombyx mori β-1,4-N-acetylgalactosaminyltransferase and insight into the substrate specificity of β-1,4-galactosyltransferase family enzymes

Silkworm Bombyx mori is one of the insect hosts for recombinant protein production at academic and industrial levels. B. mori and other insect cells can produce mammalian proteins with proper posttranslational modifications, such as N-glycosylation, but the structures of N-glycans in B. mori are mainly high mannose- and paucimannose-type, while mammals also produce hybrid- and complex-type glycans. Recently, complex-type N-glycans whose structures are different from mammalian ones have been identified in some insect cell N-glycomes at very low levels compared with levels of high mannose- and paucimannose-type glycans. However, their functions and the enzymes involved in the biosynthesis of insect complex-type N-glycans are not clear, and complex-type N-glycans, except for N-acetylglucosamine-terminated glycans, are still not identified in the B. mori N-glycome. Here, we focused on the β-1,4-galactosyltransferase family (also known as glycosyltransferase family 7, GT7) that contains mammalian β-1,4-galactosyltransferase and insect β-1,4-N-acetylgalactosaminyltransferase. A gene for a GT7 protein (BmGalNAcT) from B. mori was cloned, expressed in a soluble form using a silkworm expression system, and the gene product showed strict β-1,4-N-acetylgalactosaminyltransferase activity but not β-1,4-galactosyltransferase activity. A mutation in Ile298 or Ile310, which are predicted to be located in the active site, reduced its glycosyltransferase activity, suggesting that these residues and the corresponding residues are responsible for substrate specificity of GT7. These results suggested that BmGalNAcT may be involved in the complex-type N-glycans, and moreover, bioinformatics analysis revealed that B. mori might have an extra gene for a GT7 enzyme with different specificity in addition to the known insect GT7 glycosyltransferases.