Ubiquitous 9-O-acetylation of sialoglycoproteins restricted to the Golgi complex

9-O-Acetylation of sialic acid is known as a cell type-specific modification of secretory and plasma membrane glycoconjugates of higher vertebrates with important functions in modulating cell-cell recognition. Using a recombinant probe derived from influenza C virus hemagglutinin, we discovered 9-O-acetylated protein in the Golgi complex of various cell lines, most of which did not display 9-O-acetylated sialic acid on the cell surface. All cell lines expressed a sulfated glycoprotein of 50 kDa (sgp50) carrying 9-O-acetylated sialic acids, which was used as a model substrate. Like gp40, the major receptor for influenza C virus of Madin-Darby canine kidney I cells, sgp50 is 9-O-acetylated on O-linked glycans. However, gp40 was not 9-O-acetylated when expressed in Madin-Darby canine kidney II or COS-7 cells. The results demonstrate the existence of two 9-O-acetylation machineries for O-glycosylated proteins with distinct substrate specificities. The widespread occurrence of 9-O-acetylated protein in the Golgi furthermore suggests an additional intracellular role for this modification.