Peroxisome dependency of alkyl-containing GPI-anchor biosynthesis in the endoplasmic reticulum

Abstract

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 alkylacyl change occurring in the third GPI intermediate, glucosaminylinositolacylated-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.alkyl lipids ͉ glycosylphosphatidylinositol ͉ lipid remodeling

GPI-Anchor: Update for Biosynthesis and Remodeling

“…Since alkyl chains linked to glycerol have been reported to be synthesized only in the peroxisome, we tested whether alkyl-acyl type GPIs can be synthesized in mutant cells in which the peroxisome enzymes involved in the synthesis of alkyl-acyl glycerols are defective. The results showed that the peroxisome enzymes are indispensable for the synthesis of alkyl-acyl-type GPIs (62). Since enzymes directly catalyzing (56,57) (62) the replacement of an acyl chain with an alkyl one, except for the enzymes in peroxisome, are not known at present, it is speculated that this structural remodeling is executed by an enzyme with phospholipase C or D activity which replaces (phospho-)diacylglycerol with (phospho-)alkylacylglycerol.…”

“…The biological significance of alkyl-acyltype GPI-APs has not been clarified due to the absence of cells or animals defective in this step. Recent reports revealed that this remodeling takes place during GPI synthesis in the ER (60) and that the peroxisome is involved in the synthesis of alkyl chains utilized in this step (62). GPI intermediates accumulated in various mutant cells enabled us to analyze the composition of lipid chains in each GPI intermediate.…”

Potential Roles of GPI-Anchor Remodeling in Protein Trafficking and Raft Association in Mammalian Cells

“…In Chinese hamster ovary (CHO) cells, 1-alkyl-2-acylglycerol accounts for 90% or more of the lipid structures of GPI-anchored proteins (26). In contrast, the main intracellular species of PI are diacylglycerol-types, with only a small percentage being 1-alkyl-2-acyl-types (25).…”

“…In the biosynthetic pathway, the diacyl-type PIs are the major forms until GlcN-PI, whereas 1-alkyl-2-acyl-types become major components after GlcN-(acyl)PI (25). The 1-alkyl-2-acyl-type lipids utilized in the exchange step are derived from 1-alkyl-glycerone phosphate generated in the peroxisome (26). In mutant cells defective for GNPAT (glyceronephosphate O-acyltransferase), AGPS (alkylglycerone phosphate synthase) or PEX7, which participate in alkyl lipid biosynthesis in the peroxisome, GPI anchors do not become 1-alkyl-2-acyl-types, but remain diacyl-types (26,27).…”

“…The 1-alkyl-2-acyl-type lipids utilized in the exchange step are derived from 1-alkyl-glycerone phosphate generated in the peroxisome (26). In mutant cells defective for GNPAT (glyceronephosphate O-acyltransferase), AGPS (alkylglycerone phosphate synthase) or PEX7, which participate in alkyl lipid biosynthesis in the peroxisome, GPI anchors do not become 1-alkyl-2-acyl-types, but remain diacyl-types (26,27). It is known that mutations in GNPAT, AGPS and PEX7 cause rhizomelic chondrodysplasia punctata (RCDP), autosomal recessive diseases clinically characterized by severe symmetrical shortening of the proximal limb segments, stippled epiphyses and periepiphyseal calcification, facial dysmorphism, cataracts, and severe growth and mental retardation (28).…”