Handbook of Glycosyltransferases and Related Genes pp 1687-1691 | Cite as
Map 5: Biosynthetic Pathways of GPI-Anchor
The biosynthetic pathway of GPI-anchor and its transfer to protein composes at least 11 steps: transfer of GlcNAc to PI, de-N-acetylation, acylation of the inositol ring, transfer of three mannoses and three ethanolaminephosphates, and transamidation to protein (Fig. 151.1). Twenty-three genes involved in this pathway have been already identified and characterized in mammals and yeast. The intermediates of GPI-anchor in the first two steps are synthesized on the cytoplasmic leaflet of the endoplasmic reticulum (ER) membrane. GlcN-PI is thought to flip to the luminal leaflet of the ER mediated by a flippase that has not been identified. Because the functionally essential residues of inositol acyltransferase (yeast PIGW homologue Gwt1p) are located on the luminal side of the ER, the inositol ring of GlcN-PI would be acylated on the luminal side. The later steps of the GPI-anchor synthesis and transfer to proteins by the transamidation occur on the luminal side of the ER. The fatty-chain composition of PI moiety changes from diacyl to a mixture of 1-alkyl-2-acyl and diacyl in the third GPI intermediate, GlcN-(acyl)PI. The reaction mechanism of this lipid remodeling is yet to be determined. The putative 1-alkyl glycerol-containing donor substrate in the lipid remodeling reaction is dependent upon the peroxisome. A donor of all GPI-mannosyltransferases, dolichol-phosphate-mannose (Dol-P-Man), is synthesized from GDP-Man and Dol-P by a complex of Dol-P-Man synthase (DPM1, 2 and 3) in mammalian cells and Dpm1p in yeast, respectively. MPDU1 (Mannose-P-dolichol utilization defect 1) is required for utilization of Dol-P-Man in the synthesis of GPI-anchor. Whereas the biosynthetic pathway of GPI-anchor is basically conserved in various eukaryotes, the yeast pathway is different from the mammalian pathway in that an addition of fourth mannose is essential for proceeding to the next reaction in yeast. The transamidation is mediated by transamidase enzyme complex that consists of five components (PIGK, GAA1, PIGS, PIGT, and PIGU). Preassembled GPI is transferred to the carboxyl terminus of protein by replacing the GPI-attachment signal peptide. After the attachment of the GPI to protein, the several modifications occur on the glycan and lipid portions of the protein-bound GPI during the transport to the cell surface through the Golgi apparatus (Fig. 151.2). Before the GPI-anchored proteins exit the ER, the inositol-linked acyl chain and the ethanolaminephosphate at the second mannose are removed by PGAP1 (post-GPI attachment to proteins 1) and PGAP5, respectively, in mammalian cells (Fig. 151.2a). These changes act as signals for efficient export from the ER. In mammalian cells, further remodeling of the PI moiety, termed fatty-acid remodeling, occurs in the Golgi apparatus, which is critical for GPI-APs’ association with the detergent-resistant membrane at the cell surface. An unsaturated fatty acid at the sn-2 position is removed by PGAP3, generating a lyso-GPI-AP intermediate. The lyso-GPI-APs are then reacylated with saturated fatty acid (usually stearic acid in mammalian cells), in which PGAP2 is involved. The lipid remodeling in yeast occurs in the ER before the removal of ethanolaminephosphate at the second mannose (Fig. 151.2b). After the attachment of GPI to the proteins, the deacylation of inositol ring and the formation of the lyso-GPI-AP intermediate are mediated by Bst1p and Per1p, yeast homologues of PGAP1 and PGAP3, respectively. Long saturated fatty acid (C26: 0) is then transferred to yeast lyso-GPI-AP intermediate by Gup1p, a member of membrane-bound O-acyltransferase family. Furthermore, the lipid portion of GPI-APs may be exchanged from diacyl to ceramide-type of lipid chain, following the removal of ethanolaminephosphate at the second mannose. Many yeast GPI-APs are cleaved between the first mannose and GlcN and transferred to the cell wall β1-6glucan via their GPI glycan after their transport to the cell surface. Although the first mannose of some GPI-APs is modified by β1-4GalNAc or a glycan-containing β1-4GalNAc during their transport from the ER to the cell surface in mammalian cells, physiological roles of these modifications are still unknown.
KeywordsEndoplasmic Reticulum Golgi Apparatus Luminal Side Essential Residue Lipid Portion
A mixture of 1-alkyl-2-acyl glycerol and diacylglycerol
Mannose-P-dolichol utilization defect 1