Biochemical characterization and mutational analysis of silkworm Bombyx mori ß-1,4-N-acetylgalactosaminyltransferase and insight into the substrate specificity of ß-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 ß-1,4-galactosyltransferase family (also known as glycosyltransferase family 7, GT7) that contains mammalian ß-1,4-galactosyltransferase and insect ß-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 ß-1,4-N-acetylgalactosaminyltransferase activity but not ß-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.

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

N-glycans are involved in various physiological functions and their structures diverge among different phyla and kingdoms. Insect cells mainly produce high mannose-type and paucimannose-type glycans but very few mammalian-like complex-type glycans. However, many insects possess genes for proteins homologous to the enzymes involved in complex-type N-glycan synthesis in mammalian cells, and their N-glycosylation pathway is incompletely understood compared with that of mammals. Here, we cloned a candidate gene for ß-1,2-N-acetylglucosaminyltransferase II (GnTII), which is a Golgi-localized enzyme involved in a key step in the conversion to complex-type N-glycans, from silkworm Bombyx mori, and the gene was found to be expressed ubiquitously in the larval and pupal stages. In addition, recombinant B. mori GnTII was expressed as a soluble form using a silkworm-B. mori nucleopolyhedrovirus bacmid expression system. The recombinant enzyme exhibited similar pH and temperature dependency and the same substrate specificity as human GnTII, but deglycosylation with peptide:N-glycanase F did not affect its enzymatic activity. Compared with the structure of human GnTII, the amino acid residues involved in catalytic activity and substrate recognition are almost fully conserved in B. mori GnTII, which is consistent with its enzymatic properties. These results raised the possibility of mammalian-like complex-type N-glycan synthesis using the GnTII ortholog in silkworm.