The acceptor substrate specificity of porcine submaxillary UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase is dependent on the amino acid sequences adjacent to serine and threonine residues.
The acceptor substrate specificity of a pure polypeptide N-acetylgalactosaminyltransferase has been examined with synthetic polypeptides with sequences identical, or similar to those found in porcine mucin or human erythropoietin. The sequences adjacent to either threonine or serine markedly influence the formation of GalNAc-O-Thr and GalNAc-O-Ser. Examination of the mucin-like peptide VLGXXAV, where X is Thr, Ser, or Ala, shows only Thr-containing peptides to be acceptors. The best substrate is formed when XX is TT. Peptides with XX as either AT or TA are less effective and those with XX as either ST or TS are much less effective acceptors. The amino acids adjacent to serine in the peptide formed by residues 121-131 in human erythropoietin, PPDAASAAPLR, also markedly influence the formation of GalNAc-O-Ser. Thus, PPDASSSAPLR and PPDVVSVVPLR are about 5- and 30-fold, respectively, less active than the erythropoietin peptide. The peptide PPDGGSGGPLR is inactive. The shorter peptide DAASAAPL is also about 5-fold less active than the full-length peptide, but the peptide AASAA is inactive. These studies indicate that one transferase can form both GalNAc-O-Ser and GalNAc-O-Thr residues when the sequences adjacent to the glycosylated residue are of the proper kind. Thus, in contrast to earlier suggestions, there is no evidence that different transferases form GalNAc-O-Ser and GalNAc-O-Thr. Examination of tissue homogenates from various tissues confirms this conclusion.