Study on the topology of the GPI anchor assembly

糖磷脂酰肌醇（GPI）锚被预先组装并转移到ER中相应新生蛋白的C-末端。组装开始于ER的腔侧，并在ER的胞质侧和腔侧之间进行。因此，GPI锚的前体在组装期间必须在脂质双层之间翻转。这种触发器的机制尚未得到很好的表征，也没有确定的触发器的线索。已知一些细胞系上的GPI锚蛋白是酰化的，而大多数GPI蛋白是不酰化的。这意味着酰化作为一种线索参与了这种翻转。我们已经调查了触发器的时间与红白血病细胞上前体的酰化的关系。GPI锚包括其肌醇残基被酰化的前体，其抗GPI特异性的磷脂酶C消化。因此，这一发现表明，酰化不参与触发器。

The glycophosphatidylinositol (GPI) anchor is preassembled and transferred to the c-terminus of the corresponding nascent proteins in the ER. The assembly starts in the luminal side of the ER and proceeds between the cytosolic and luminal sides of the ER. Thus, precursors of GPI anchor have to flip-flop between the lipid bilayer during the assembly. The mechanisms underlying this flip-flop has not been well characterized, nor identified are cues for the flip-flop. The GPI anchor proteins on some cell lines are known to be acylated while most of the GPI proteins are not acylated. This implied that acylation involved in the flip-flop as a cue. We have investigated the timing of the flip-flop in the relation, with the acylation of the precursors on erythroleukemia cells. The GPI anchor including the precursors of which inositol residue is acylated are resistant GPI specific pshopholipase C digestion.The deacylation took after the transfer of the preassembled GPI to the target nascent proteins. This findings thus suggested that the acylation is not involved in the flip-flop.