Mucin CoreO-Glycosylation Is Modulated by Neighboring Residue Glycosylation Status
The influence of peptide sequence and environment on the initiation and elongation of mucinO-glycosylation is not well understood. Thein vivoglycosylation pattern of the porcine submaxillary gland mucin (PSM) tandem repeat containing 31O-glycosylation sites (Gerken, T. A., Gilmore, M., and Zhang, J. (2002)J. Biol. Chem.277, 7736–7751) reveals a weak inverse correlation with hydroxyamino acid density (and by inference the density of glycosylation) with the extent of GalNAc glycosylation and core-1 substitution. We now report the time course of thein vitroglycosylation of the apoPSM tandem repeat by recombinant UDP-GalNAc:polypeptide α-GalNAc transferases (ppGalNAc transferase) T1 and T2 that confirm these findings. A wide range of glycosylation rates are found, with several residues showing apparent plateaus in glycosylation. An adjustable kinetic model that reduces the first-order rate constants proportional to neighboring glycosylation status, plus or minus three residues of the site of glycosylation, was found to reasonably reproduce the experimental rate data for both transferases, including apparent plateaus in glycosylation. The unique, transferase-specific, positional weighting constants reveal information on the peptide/glycopeptide recognition site for each transferase. Both transferases displayed high sensitivities to neighboring Ser/Thr glycosylation, whereas ppGalNAc T2 displayed additional high sensitivities to the presence of nonglycosylated Ser/Thr residues. This is the first demonstration of the ability to model mucinO-glycosylation kinetics, confirming that under the appropriate conditions neighboring glycosylation status can be a significant factor modulating the first step of mucinO-glycan biosynthesis.