Donor substrate specificity of β3GalT activity of PgtA. A, GalT assay was conducted in the presence of 1.25 μm Gn-O-Skp1 and the indicated concentration of UDP-[3H]Gal for 10 min at 29 °C. (○) marks a data point for a reaction that included 5 μm GDP-Fuc. Incorporation into Skp1 was measured using the SDS-PAGE method. B, GalT assay was conducted in the presence of 7 μm GlcNAc-peptide-BSA conjugate, 10 μm UDP-[3H]Gal, and the indicated concentration of nucleotide diphosphate, for 2 h at 29 °C. Results are representative of two independent trials.