Biosynthesis and deficiencies of glycosylphosphatidylinositol

Abstract

At least 150 different human proteins are anchored to the outer leaflet of the plasma membrane via glycosylphosphatidylinositol (GPI). GPI preassembled in the endoplasmic reticulum is attached to the protein's carboxyl-terminus as a post-translational modification by GPI transamidase. Twenty-two PIG (for Phosphatidyl Inositol Glycan) genes are involved in the biosynthesis and protein-attachment of GPI. After attachment to proteins, both lipid and glycan moieties of GPI are structurally remodeled in the endoplasmic reticulum and Golgi apparatus. Four PGAP (for Post GPI Attachment to Proteins) genes are involved in the remodeling of GPI. GPI-anchor deficiencies caused by somatic and germline mutations in the PIG and PGAP genes have been found and characterized. The characteristics of the 26 PIG and PGAP genes and the GPI deficiencies caused by mutations in these genes are reviewed.

Figure 1.

Structure of mammalian GPI-APs. The structure of core backbone is EtNP-6Manα1-2Manα1-6Manα1-4GlcNα1-6myoInositol-phospholipid. The EtNP linked to Man3, the α1-2 linked Man, makes an amide-bond with the C-terminus. The EtNP side-branch linked to Man1, the α1-4 linked Man, is common among proteins whereas the Man side-branch that is α1-2 linked to Man3, and GalNAc side-branch that is β1-4 linked to Man1 are found in some of the GPI-APs. The GalNAc side-branch can be further modified by galactose and sialic acid.

Figure 2.

Biosynthesis and attachment to proteins of GPI. GPI precursor is biosynthesized in the ER by at least 11 stepwise reactions and en bloc transferred to the C-terminus of proteins. GlcN-PI flips from the cytoplasmic side to the luminal side (step 3). Lipid structure changes from diacyl PI to a mixture of 1-alkyl,2-acyl PI, major form, and diacyl PI, minor form in GlcN-(acyl)PI (step 5).

Figure 3.

GPI attachment to proteins by GPI transamidase. Precursor of GPI-APs have N-terminal signal peptide for translocation across the ER membrane and GPI attachment signal peptide at the C-terminus. After removal of the N-terminal signal peptide, GPI transamidase, consisting of PIG-K, GPAA1, PIG-S, PIG-T, and PIG-U, recognizes and cleaves the GPI attachment signal peptide between ω and ω + 1 site amino acids, generating protein-enzyme complex linked by a thioester bond between active site cysteine of PIG-K and carboxyl-group of the ω site amino acid. The thioester bond is attacked by the amino group of the terminal EtNP of GPI to complete transamidation.

Figure 4.

Remodeling of GPI in the ER and the Golgi. After attachment of GPI to proteins in the ER and before departure for the Golgi, the inositol-linked acyl chain and Man1-linked EtNP side branch are removed by PGAP1 and PGAP5, respectively. A complex of four p24 family proteins acts as a cargo receptor of GPI-APs for packing into COPII coated transport vesicles. In the Golgi, an unsaturated fatty acid at the sn2 position is removed by PGAP3, generating lyso-GPI-AP, which is reacylated with a saturated fatty acid by PGAP2. After this fatty acid remodeling, GPI-APs are expressed on the outer leaflet of the plasma membrane (PM).