Construction of ST6Gal I expression plasmids and their effect on α2, 6 sialylation at the cell surface. A. Expression plasmids for full-length WT ST6Gal I (WT), double-mutant ST6Gal I (L37A/K40A) and N-terminal ST6Gal I deletion mutant (ΔN, containing amino acids 43-403) were constructed. WT and L37A/K40A were N-terminally tagged with Flag; ΔN was tagged with HA. Expression of each construct was confirmed by immunoblotting. SW480 cells were transfected with WT, L37A/K40A, or ΔN. Then, ST6Gal I enzymatic activity was measured by ELISA (B), and cell surface sialylation was detected by FACS (C). Data are presented as means ± SDs of three replicates (*p < 0.05 vs. the corresponding control).