糖基化磷脂酰肌醇(GPI)锚定蛋白是真核细胞中普遍存在的翻译后修饰。GPI锚定蛋白(GPI-AP)在酶促、信令,监管,和粘附过程。超过20种酶参与GPI合成,附着在客户蛋白质上,依恋后的重塑。GPI转酰胺酶(GPI-T),一个位于内质网膜的大型复合体,通过用GPI替换前蛋白的C末端信号肽来催化附着步骤。在过去的三十年里,对转酰胺化反应的机理进行了广泛的研究,GPI-T复合体的组成部分,每个亚基的作用,和底物特异性。最近的两项研究报道了GPI-T的三维结构,它们代表了路径的第一个结构。这些结构提供了详细的组装机制,使先前的生化结果和亚基依赖性稳定性数据合理化。虽然结构数据证实了PIGK的催化作用,它可能使用胱天蛋白酶样机制来切割前蛋白,他们认为与以前提出的不同,GPAA1不是催化亚基。该结构还揭示了GPI结合的共享腔。有点出乎意料,PIGT,一种单程膜蛋白,在GPI识别中起着至关重要的作用。与组装机制和活动站点体系结构一致,大多数疾病突变发生在活性位点或亚基界面附近。最后,催化对偶位于距离GPI结合位点的膜界面约22埃外,并且这种结构可以通过底物和细长活性位点之间的拓扑匹配来赋予底物特异性。到目前为止进行的研究揭示了GPI锚定所涉及的复杂过程,并为GPI-T的进一步机理研究铺平了道路。
Glycosylphosphatidylinositol (GPI) anchoring of proteins is a ubiquitous posttranslational modification in eukaryotic cells. GPI-anchored proteins (GPI-APs) play critical roles in enzymatic, signaling, regulatory, and adhesion processes. Over 20 enzymes are involved in GPI synthesis, attachment to client proteins, and remodeling after attachment. The GPI transamidase (GPI-T), a large complex located in the endoplasmic reticulum membrane, catalyzes the attachment step by replacing a C-terminal signal peptide of proproteins with GPI. In the last three decades, extensive research has been conducted on the mechanism of the transamidation reaction, the components of the GPI-T complex, the role of each subunit, and the substrate specificity. Two recent studies have reported the three-dimensional architecture of GPI-T, which represent the first structures of the pathway. The structures provide detailed mechanisms for assembly that rationalizes previous biochemical results and subunit-dependent stability data. While the structural data confirm the catalytic role of PIGK, which likely uses a caspase-like mechanism to cleave the proproteins, they suggest that unlike previously proposed, GPAA1 is not a catalytic subunit. The structures also reveal a shared cavity for GPI binding. Somewhat unexpectedly, PIGT, a single-pass membrane protein, plays a crucial role in GPI recognition. Consistent with the assembly mechanisms and the active site architecture, most of the disease mutations occur near the active site or the subunit interfaces. Finally, the catalytic dyad is located ~22 Å away from the membrane interface of the GPI-binding site, and this architecture may confer substrate specificity through topological matching between the substrates and the elongated active site. The research conducted thus far sheds light on the intricate processes involved in GPI anchoring and paves the way for further mechanistic studies of GPI-T.