Fig. 1.

N-glycan processing pathways. Relevant steps in the protein N-glycosylation pathways of insect and mammalian cell systems emphasizing differences between the two. Both pathways include the initial transfer of a preassembled N-glycan to a nascent protein, which is not shown, followed by enzymatic removal of terminal sugars (trimming steps), which produce an intermediate common to both systems (GlcNAcMan₃GlcNAc₂). The major processed N-glycans in insect systems are produced by FDL, which removes the terminal N-acetylglucosamine residue from the common intermediate (Geisler et al., 2008; Leonard et al., 2006) to form paucimannosidic end products (e.g. Man₃GlcNAc₂). In contrast, the major processed N-glycans in mammalian systems are produced by various glycosyltransferases and other machinery, which elongate the common intermediate to form complex, terminally galactosylated (Gal₂GlcNAc₂Man₃GlcNAc₂) or even more extensively processed end products.