Analvsis of localization of a glycosyltransferase and transfort of glycolipids by gene manipulation

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The amino acid sequence deduced from the cloned human cDNA of beta-1,4-N-acetyl-galactosaminyltransferase (GalNAc-T ; EC2.4.1.92) gene predicted three potential sites for N-linked glycosylation. Although many glycosyltransferases isolated contain from 2 to 6 N-glycosylation sites, their significance has not been adequately demonstrated. To clarify the roles of N-glycosylation in GalNAc-T function, we generated a series of mutant cDNAs, in which some or all of the glycosylation recognition sites were eliminated by polymerase chain reaction( PCR)-mediated site directed mutagenesis. Using transcription/translation in vitro, we confirmed the all potential N-glycosylation sites could be used. Although cell lines transfected with mutant cDNAs showed equivalent levels of GalNAcbeta1*4 (NeuAcalpha2*3) Galbeta1*4GlcCer (G_<M2>) to that of the wild-type, the extracts from mutant cDNA transfectants demonstrated lower enzyme activity than in the wild-type. The decrease in enzyme activity was more evident as the number of deglycosylated sites increased, with about 90% decrease in a totally deglycosylated mutant. The enzyme kinetics analysis revealed no significant change of K_m among wild-type and mutant cDNA products. The intracellular localization of GalNAc-T expressed in transfectants with wild-type or mutant cDNAs also showed a similar perinuclear pattern (Golgi pattern). These results suggest that N-linked carbohydrates on GalNAc-T are required for regulating the stability of the enzyme structure.