Processing of asparagine-linked oligosaccharides in insect cells. N-Acetylglucosaminyltransferase I and II activities in cultured lepidopteran cells

Abstract

The levels of β1,2-N-acetylglucosaminyltransferase (GlcNAc-T) I and II activities in cultured cells from Bombyxmori (Bm-N), Mamestra brassicae (IZD-Mb-0503) and Spodoptera frugiperda (Sf-9 and Sf-21) were investigated. Apart from initial experiments with Manα-3(Manα1–6)-Manβ1-O(CH2)8COOH3 and 3H-labelled UDP-GlcNAc as substrates, GlcNAc-T I activity was measured with a non-radioactive HPLC method using pyridylaminated Man3-GlcNAc2 and Man5GlcNAc2 as acceptor oligosaccharides. It was shown by reversed-phase HPLC, exoglycosidase digestion and methylation analysis that the product obtained with Man3GlcNAc2 contained a terminal GlcNAc residue linked β1,2 to the α1,3 arm of the acceptor. Compared to the enzyme from the human hepatoma cell line HepG2, insect cell GlcNAc-T I exhibited a much higher preference for the Man5 substrate. The GlcNAc-T I from Mb-0503 cells had apparent Km and Vmax values for pyridylaminated Man3- and Man5GlcNAc2 of 2.15 and 0.21 mM, and of 3.4 and 11.4 nmol/h/mg of cell protein, respectively. When Man5GlcNAc2 was used as the acceptor substrate, the levels of GlcNAc-T I activity in the four insect cell lines ranged between 7.5 and 14.7 nmol/h/mg of cell protein, and thus were comparable to that of HepG2 cells. Evidence is presented for the dependence of lepidopteran fucosyltransferase on the presence of terminal N-acetylglucosamine. GlcNAc-T II activity could be demonstrated by HPLC using GlcNAcβ1–2Manα1–3(Manα1–6)Manβ1–4GlcNAcβ1–4GlcNAc-pyridylamine as the acceptor in the presence of 6-acetamido-6-deoxycastanospermine as an inhibitor of β-N-acetylglucos-aminidase. However, the insect cells exhibited specific activities of GlcNAc-T II of only 0.02–0.11 nmol/h/mg of cell protein, much less than HepG2 cells.