No abstract

Biosynthesis of mucin-type O-glycans is initiated by a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases, which contain several conserved cysteine residues among the isozymes. We found that a cysteine-specific reagent, p-chloromercuriphenylsulfonic acid (PCMPS), irreversibly inhibited one of the isozymes (GalNAc-T1). Presence of either UDP-GalNAc or UDP during PCMPS treatment protected GalNAc-T1 from inactivation, to the same extent. This suggests that GalNAc-T1 contains free cysteine residues interacting with the UDP moiety of the sugar donor. For the functional analysis of the cysteine residues, several conserved cysteine residues in GalNAc-T1 were mutated individually to alanine. All of the mutations except one resulted in complete inactivation or a drastic decrease in the activity, of the enzyme. We identified only Cys212 and Cys214, among the conserved cysteine residues in GalNAc-T1, as free cysteine residues, by cysteine-specific labeling of GalNAc-T1. To investigate the role of these two cysteine residues, we generated cysteine to serine mutants (C212S and C214S). The serine mutants were more active than the corresponding alanine mutants (C212A and C214A). Kinetic analysis demonstrated that the affinity of the serine-mutants for UDP-GalNAc was decreased, as compared to the wild type enzyme. The affinity for the acceptor apomucin, on the other hand, was essentially unaffected. The functional importance of the introduced serine residues was further demonstrated by the inhibition of all serine mutant enzymes with diisopropyl fluorophosphate. In addition, the serine mutants were more resistant to modification by PCMPS. Our results indicate that Cys212 and Cys214 are sites of PCMPS modification, and that these cysteine residues are involved in the interaction with the UDP moiety of UDP-GalNAc.

No abstract

No abstract

Polypeptide N-acetylgalactosaminyltransferases (GalNAc-transferases) catalyze the initial reaction of mucin-type O-glycosylation. Here, we report the first biochemical characterization of one of the Drosophila GalNAc-transferases, dGalNAc-T3. This enzyme retains conserved motifs essential for the catalytic activity, but is a novel isozyme in that it has several inserted sequences in its lectin-like domain. Northern hybridization analysis of this isozyme identified a 2.5-kb mRNA in Drosophila larva. Biochemical characterization was carried out using the recombinant soluble dGalNAc-T3 expressed in COS7 cells. dGalNAc-T3, which required Mn 2ϩ for the activity, had a pH optimum ranging from pH 7.5 to 8.5, and glycosylated most effectively at 29-33°C. Its K m for UDP-GalNAc was 10.7 m mM, which is as low as that of mammalian isozymes. dGalNAc-T3 glycosylated the peptides containing a sequence of XTPXP or TTAAP most efficiently. The enzyme was irreversibly inhibited by p-chloromercuriphenylsulphonic acid, indicating the presence of essential Cys residues for the activity.