N-glycosylation in insects revisited

The interest in insect glycosylation has been heightened by the biotechnological prospects of the insect cell/baculovirus system for the production of recombinant glycoproteins. It is evident that insects produce, in addition to the ubiquitous oligomannosidic N-glycans, unique structures not found in mammalian or plant glycoproteins. However, there are structural similarities to plant glycoproteins, such as alpha 1,3-fucosylation of the asparagine-bound GlcNAc residue, leading to immunological cross-reactions between plant and insect glycoproteins. The first steps of N-glycan biosynthesis appear to be highly conserved throughout eukaryotic cells, insects not being an exception. Thus, transfer of Glc(3)Man(9)GlcNAc(2) from dolichol to protein in the endoplasmic reticulum is followed by deglucosylation and transient re-glucosylation by a glucosyltransferase acting only towards incorrectly folded glycoproteins. Mannosidase trimming then leads to oligomannosidic structures and eventually permits the action of GlcNAc-transferase I. This step turns the oligosaccharide into a substrate for alpha-mannosidase II. Depending on the cell line or tissue, alpha 1,3- and/or alpha 1,6-fucosyltransferases or GlcNAc-transferase II may now enter the scene. At least in honeybees there are additional transferases generating a GalNAc beta 1-->4(Fuc alpha 1-->3)GlcNAc antenna. In a locust, the non-sugar substituent 2-aminoethylphosphonate was found. The action of sialyltransferases or galactosyl-transferase acting on N-glycans has hitherto not been confirmed. There is now good evidence that in most insects and cell lines, the GlcNAc provided by GlcNAc-transferase I is finally removed by a membrane-bound and branch specific beta-N-acetylglucosaminidase.