Golgi galactosyltransferase contains serine‐linked phosphate

In HeLa and HepG2 cells the Golgi complex enzyme galactosyltransferase became phosphorylated following incubation with ³²P_i. Analysis on sodium dodecyl sulphate/polyacrylamide gel electrophoresis revealed incorporation of ³²P into the mature 54‐kDa form. This phosphorylation was independent of protein synthesis. Serine was identified as the sole phosphorylated amino acid; no radioactive phosphate was detected on N‐linked oligosaccharide. The phosphate‐labelled galactosyltransferase has the same turnover as [³⁵S]methionine‐labelled polypeptides (t_1/2= 20 h). Soluble enzyme, released by the cells, contained very little phosphate relative to that which remained cell‐associated. Charge heterogeneity arising from phosphorylation contributes in part to the heterodispersed appearance of the enzyme on two‐dimensional gels, as the degree of radioactive phosphate differs among the different iso‐enzymes.