ABSTRACT
Mucin-type O-linked oligosaccharides have been identified in a large num ber of glycoproteins of diverse origin and function [1,2]. The complexity of these structures ranges from that of a single monosaccharide-Afacetylgalactosamine (GalNAc)—to that of large, branched structures com posed of more than 20 saccharide units [2-4]. Likewise, the number of glycosylated amino acid residues in any given glycoprotein ranges from one, for example, in the human transferrin receptor [5,6], to the several hundred commonly found in many mucins [3,4,7]. The assembly of mucintype oligosaccharides occurs by sequential addition of monosaccharide units to the growing oligosaccharide chain, a process invariably initiated by the transfer of an iV-acetylgalactosamine unit to a serine or threonine resi due on the acceptor glycoprotein [8]. Hence, the exact location of mucintype oligosaccharides on the acceptor polypeptide is largely determined by the specificity of the enzyme catalyzing the transfer of this monosaccha ride, namely UDP-GalNAc:polypeptide iV-acetylgalactosaminyl transferase (E.C. 2.4.1.41), commonly called GalNAc-transferase.
