O-linked glycans mediate apical sorting of human intestinal sucrase-isomaltase through association with lipid rafts.

The plasma membrane of polarised epithelial cells is characterised by two structurally and functionally different domains, the apical and basolateral domains. These domains contain distinct protein and lipid constituents that are sorted by specific signals to the correct surface domain [1]. The best characterised apical sorting signal is that of glycophosphatidylinositol (GPI) membrane anchors [2], although N-linked glycans on some secreted proteins [3] and O-linked glycans [4] also function as apical sorting signals. In the latter cases, however, the underlying sorting mechanisms remain obscure. Here, we have analysed the role of O-glycosylation in the apical sorting of sucrase-isomaltase (SI), a highly polarised N- and O-glycosylated intestinal enzyme, and the mechanisms underlying this process. Inhibition of O-glycosylation by benzyl-N-acetyl-alpha-D-galactosaminide (benzyl-GalNAc) was accompanied by a dramatic shift in the sorting of SI from the apical membrane to both membranes. The sorting mechanism of SI involves its association with sphingolipid- and cholesterol-rich membrane rafts because this association was eliminated when O-glycosylation was inhibited by benzyl-GaINAc. The results demonstrate for the first time that O-linked glycans mediate apical sorting through association with lipid rafts.