Biosynthesis and biology of mammalian GPI-anchored proteins

Abstract

At least 150 human proteins are glycosylphosphatidylinositol-anchored proteins (GPI-APs). The protein moiety of GPI-APs lacking transmembrane domains is anchored to the plasma membrane with GPI covalently attached to the C-terminus. The GPI consists of the conserved core glycan, phosphatidylinositol and glycan side chains. The entire GPI-AP is anchored to the outer leaflet of the lipid bilayer by insertion of fatty chains of phosphatidylinositol. Because of GPI-dependent membrane anchoring, GPI-APs have some unique characteristics. The most prominent feature of GPI-APs is their association with membrane microdomains or membrane rafts. In the polarized cells such as epithelial cells, many GPI-APs are exclusively expressed in the apical surfaces, whereas some GPI-APs are preferentially expressed in the basolateral surfaces. Several GPI-APs act as transcytotic transporters carrying their ligands from one compartment to another. Some GPI-APs are shed from the membrane after cleavage within the GPI by a GPI-specific phospholipase or a glycosidase. In this review, I will summarize the current understanding of GPI-AP biosynthesis in mammalian cells and discuss examples of GPI-dependent functions of mammalian GPI-APs.

Keywords: GPI deficiency; biosynthetic pathway; glycosylphosphatidylinositol; post-translational modification; protein shedding.

Figure 1.

Mammalian GPI-APs. The conserved core glycan of mammalian GPI, which consists of EtNP attached to the protein, three Mans, EtNP attached to Man1, and GlcN, is linked to the lipid moiety, which is PI. In some GPI-APs, the core glycan is modified by Man4 and/or GalNAc side chains. The GalNAc side chain can be elongated by Gal and Sia. The entire GPI-AP is anchored to the outer leaflet of PM only by hydrocarbon chains of PI.

Figure 2.

Biosynthesis of mammalian GPI in the ER. The complete GPI precursor competent for attachment to proteins is synthesized from PI by stepwise reactions (1)–(11). The Man4 side chain is attached in the ER to some GPI (step (12)). The preassembled GPI is en bloc transferred to proteins (step (13)). Genes involved in these reaction steps are shown below step numbers.

Figure 3.

Maturation of mammalian GPI-APs during ER–PM transport. Nascent GPI-APs generated by the transfer of GPIs to proteins (step 12) undergo two reactions, inositol-deacylation (step 14) and removal of the EtNP side chain from Man2 (step 15) in the ER. The ER–Golgi transport of GPI-APs is mediated by COPII-coated vesicles (step 16). In the Golgi apparatus, GPI-APs undergo fatty acid remodelling (steps 17 and 18). Some GPI-APs is modified by the GalNAc side chain (steps 19–21). The mature GPI-APs are transported to the PM where they are associated with raft microdomains. Genes involved in these reaction steps are shown below step numbers.

Figure 4.

Steps in biogenesis of GPI-APs. Preproprotein of GPI-AP has the N-terminal signal peptide for ER localization (brown box) and the C-terminal signal peptide for attachment of GPI (yellow box). The ω site is the amino acid residue, to which GPI is attached. Upon translocation into the ER, the N-terminal signal peptide is cleaved off, generating proprotein. Preassembled GPI is attached to the ω site by replacing the C-terminal signal peptide by GPI transamidase, generating nascent GPI-AP. Nascent GPI-AP undergoes maturation reactions to become mature GPI-AP.