Glycosyl Transferases of Baby‐Hamster‐Kidney (BHK) Cells and Ricin‐Resistant Mutants

Abstract

Five cell lines of ricin-resistant BHK cells have been assayed for gross carbohydrate analysis of cellular glycoproteins, for the activities of several glycosidases and of specific glycosyl transferases active in assembly of N-glycans of glycoproteins. The latter enzymes include sialyl transferase using asialofetuin as glycosyl acceptor, fucosyl transferases using asialofetuin and asialoagalactofetuin acceptors, galactosyl transferases using ovalbumin, ovomucoid and N-acetylglucosamine as acceptors and N-acetylglucosaminyl transferases wing ovalbumin and glycopeptides as acceptors. Cell line Ric^R14, binding less ricin than normal BHK cells, contains reduced amounts of sialic acid, galactose and N-acetylglucosamine in cellular glycoproteins and lacks almost completely N-acetylglucosamine transferase I, an essential enzyme in assembly of ricin-binding carbohydrate sequences of N-glycans. These cells also contain reduced levels of N-acetylglucosamine transferase II active on a product of N-acetylglucosamine transferase I action. Sialyl transferase activity is severely depressed while fucose-(α1→6)-N-acetylglucosamine fucosyl transferase activity is increased. Cell lines Ric^R15, 17, 19 and 21 showed partial deficiencies in galactosyl and N-acetylalucosaminyl transferases. A hypothesis is put forward to account for the different carbohydrate compositions and ricin binding properties of glycoproteins synthesised by these cells in terms of the determined enzyme defects, the normal level of sialyl transferases detected in Ric^R15 and Ric^R21 cells and the elevated levels of sialyl and fucosyl transferases detected in Ric^R17 and 19 cells. None of the above changes in glycosyl transfer reactions in the Ric^R cell lines are due to enhanced glycosidase or sugar nucleotidase activities in the mutant cells.