Kanzawa Noriyuki, Maeda Yusuke, Ogiso Hideo, Murakami Yoshiko, Taguchi Ryo, Kinoshita Taroh
Research Institute for Microbial Diseases, Osaka University, Suita-city, Osaka 565-0871, Japan.
Proc Natl Acad Sci U S A. 2009 Oct 20;106(42):17711-6. doi: 10.1073/pnas.0904762106. Epub 2009 Oct 7.
Glycosylphosphatidylinositol-anchored proteins (GPI-APs) play various roles in cell-cell and cell-environment interactions. GPI is synthesized in the endoplasmic reticulum (ER) from phosphatidylinositol (PI) through step-wise reactions including transfers of monosaccharides and preassembled GPI is transferred en bloc to proteins. Cellular PI contains mostly diacyl glycerol and unsaturated fatty acid in the sn-2 position, whereas mammalian GPI-APs have mainly 1-alkyl-2-acyl PI and almost exclusively stearic acid, a saturated chain, at the sn-2 position. The latter characteristic is the result of fatty acid remodeling occurring in the Golgi, generating GPI-anchors compatible with raft membrane. The former characteristic is the result of diacyl to alkyl-acyl change occurring in the third GPI intermediate, glucosaminyl-inositolacylated-PI (GlcN-acyl-PI). Here we investigated the origin of the sn-1 alkyl-chain in GPI-APs. Using cell lines defective in the peroxisomal alkyl-phospholipid biosynthetic pathway, we demonstrated that generation of alkyl-containing GPI is dependent upon the peroxisomal pathway. We further demonstrated that in cells defective in the peroxisome pathway, the chain composition of the diacyl glycerol moiety in GlcN-acyl-PI is different from those in the first intermediate N-acetylglucosaminyl-PI and cellular PI, indicating that not only diacyl to alkyl-acyl change but also diacyl to diacyl change occurs in GlcN-acyl-PI. We therefore propose a biosynthetic step within GlcN-acyl-PI in which the diacyl glycerol (or diacyl phosphatidic acid) part is replaced by diradyl glycerol (or diradyl phosphatidic acid). These results highlight cooperation of three organelles, the ER, the Golgi, and the peroxisome, in the generation of the lipid portion of GPI-APs.
糖基磷脂酰肌醇锚定蛋白（GPI-APs）在细胞间和细胞与环境的相互作用中发挥着多种作用。GPI在内质网（ER）中由磷脂酰肌醇（PI）通过逐步反应合成，包括单糖的转移，并且预先组装好的GPI会整体转移到蛋白质上。细胞中的PI在sn-2位主要含有二酰甘油和不饱和脂肪酸，而哺乳动物的GPI-APs主要具有1-烷基-2-酰基PI，并且在sn-2位几乎只含有硬脂酸（一种饱和链脂肪酸）。后一特征是高尔基体中发生的脂肪酸重塑的结果，产生了与筏膜兼容的GPI锚定物。前一特征是在第三个GPI中间体N-乙酰葡糖胺酰基肌醇酰化PI（GlcN-酰基-PI）中发生的二酰基向烷基-酰基转变的结果。在此，我们研究了GPI-APs中sn-1烷基链的来源。利用过氧化物酶体烷基磷脂生物合成途径存在缺陷的细胞系，我们证明含烷基GPI的生成依赖于过氧化物酶体途径。我们进一步证明，在过氧化物酶体途径存在缺陷的细胞中，GlcN-酰基-PI中二酰甘油部分的链组成与第一个中间体N-乙酰葡糖胺酰基PI和细胞PI中的不同，这表明在GlcN-酰基-PI中不仅发生了二酰基向烷基-酰基的转变，还发生了二酰基向二酰基的转变。因此，我们提出了GlcN-酰基-PI中的一个生物合成步骤，其中二酰甘油（或二酰基磷脂酸）部分被双自由基甘油（或双自由基磷脂酸）取代。这些结果突出了内质网、高尔基体和过氧化物酶体这三种细胞器在GPI-APs脂质部分生成过程中的协同作用。
