GLYCOSYLATION IN LEPIDOPTERAN INSECT CELLS - IDENTIFICATION OF A BETA-1-]4-N-ACETYLGALACTOSAMINYLTRANSFERASE INVOLVED IN THE SYNTHESIS OF COMPLEX-TYPE OLIGOSACCHARIDE CHAINS

The choice for a heterologous expression system to produce glycoprotei n therapeutics highly depends on its potential to perform mammalian-li ke posttranslational modifications such as glycosylation. To gain more insight into the glycosylation potential of the baculovirus mediated insect cell expression system, we have studied the expression of glyco syltransferases involved in complex-type N-glycosylation, Lepidopteran insect cell lines derived from Trichoplusia ni, Spodoptera frugiperda , and Mamestra brassicae were found to express a beta 1 --> 4-N-acetyl galactosaminyltransferase (beta 4-GalNAcT) that catalyzes the transfer of GalNAc from UDP-GalNAc to oligosaccharides and glycoproteins carry ing a terminal beta-linked GlcNAc residue, These results suggest that Lepidopteran insect cells are capable of synthesizing complex-type car bohydrate chains containing GalNAc beta 1 --> 4GlcNAc (LacdiNAc) units , Baculovirus infection of the cells, however, resulted in a decrease in the activity of beta 4-GalNAcT from 80 to <1 pmol . min(-1) mg(-1) protein within 48 h post infection, Furthermore, considerable beta-N-a cetylgalactosaminidase and beta-N-acetylglucosaminidase activity was o bserved in insect cells, whether or not infected with baculovirus, as well as in the culture medium, These enzyme activities could be respon sible for degradation of complex-type oligosaccharide chains containin g LacdiNAc units, Our findings provide ain enzymatic basis for the obs ervation that most recombinant glycoproteins produced by baculovirus i nfected insect cells carry oligomannosidic-type N-linked glycans, in s pite of the fact that uninfected insect cells have the potential for t he synthesis of mammalian-like complex-type glycans.