Alfalah M, Jacob R, Preuss U, Zimmer K P, Naim H, Naim H Y
Department of Physiological Chemistry School of Veterinary Medicine D-30559, Hannover, Germany.
Curr Biol. 1999 Jun 3;9(11):593-6. doi: 10.1016/s0960-9822(99)80263-2.
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.
极化上皮细胞的质膜具有两个结构和功能不同的结构域，即顶端结构域和基底外侧结构域。这些结构域含有不同的蛋白质和脂质成分，它们通过特定信号被分选到正确的表面结构域[1]。最具特征的顶端分选信号是糖基磷脂酰肌醇（GPI）膜锚定信号[2]，尽管一些分泌蛋白上的N-连接聚糖[3]和O-连接聚糖[4]也可作为顶端分选信号。然而，在后一种情况下，潜在的分选机制仍不清楚。在这里，我们分析了O-糖基化在蔗糖酶-异麦芽糖酶（SI）顶端分选中的作用，SI是一种高度极化的N-和O-糖基化肠道酶，以及这一过程的潜在机制。用苄基-N-乙酰-α-D-半乳糖胺（苄基-GalNAc）抑制O-糖基化会伴随着SI分选从顶端膜向两个膜的显著转变。SI的分选机制涉及其与富含鞘脂和胆固醇的膜筏的结合，因为当苄基-GalNAc抑制O-糖基化时，这种结合就会消失。结果首次证明O-连接聚糖通过与脂筏结合介导顶端分选。
