Apical transport of osteopontin is independent of N-glycosylation and sialylation

Studies of how epithelial surface polarity into apical and basolateral doma ins is generated and maintained have proposed that carbohydrate modificatio ns serve as apical targeting signals for proteins by interacting with lecti n sorters. However, the experimental evidence in support of N-glycans, O-gl ycans and sialic acids mediating apical transport is still very controversi al. This could be partly due to the fact that in most studies exogenously e xpressed proteins were analysed. One has, therefore, examined the role of c arbohydrate moieties in apical targeting of the endogenous secretory protei n osteopontin in MDCK cells. It was found, however, that sorting of osteopo ntin does not require N-glycosylation of the protein itself nor that of oth er factors involved in the sorting process. Incubation of cells with the in hibitor of O-glycosylation benzyl-alpha -GalNAc reduced the molecular weigh t of osteopontin by blocking sialic acid addition to O-glycans. Interesting ly, also impairment of sialylation had no effect on polar secretion of the protein. Thus, the results show that both N-glycans and sialic acids are no t essential sorting signals, suggesting that inner core carbohydrates and/o r a proteinaceous signal mediate apical targeting of osteopontin.