N- and O-glycans are not directly involved in the oligomerization and apical sorting of GPI proteins

Oligomerization of glycosyl-phosphatidylinositol-anchored proteins (GPI-APs) into high molecular weight complexes is an essential step for their apical sorting in polarized epithelial cells. However, the mechanism by which apical GPI-APs oligomerize is still unclear. To investigate the possible role of N- and O-glycosylation, we have analysed the behaviour of two glycosylated GPI-anchored apical proteins, p75GPI and placental alkaline phosphatase (PLAP), and their glycosylation mutants. We found that both the N- and O-glycosylation mutants are apically sorted, associate to detergent-resistant microdomains and are able to oligomerize, like the wild-type proteins, suggesting that glycosylation does not have a direct role in GPI-AP oligomerization and apical sorting. Interestingly, when cells are depleted of cholesterol and treated with tunicamycin, treatments that by themselves do not affect PLAP sorting, PLAP is not able to oligomerize and is missorted to the basolateral surface, thus supporting an indirect role of N-glycosylation, possibly mediated by a raft-associated glycosylated interactor.