The glycosylphosphatidylinositol anchor: a complex membrane-anchoring structure for proteins

Abstract

Positioned at the C-terminus of many eukaryotic proteins, the glycosylphosphatidylinositol (GPI) anchor is a posttranslational modification that anchors the modified protein in the outer leaflet of the cell membrane. The GPI anchor is a complex structure comprising a phosphoethanolamine linker, glycan core, and phospholipid tail. GPI-anchored proteins are structurally and functionally diverse and play vital roles in numerous biological processes. While several GPI-anchored proteins have been characterized, the biological functions of the GPI anchor have yet to be elucidated at a molecular level. This review discusses the structural diversity of the GPI anchor and its putative cellular functions, including involvement in lipid raft partitioning, signal transduction, targeting to the apical membrane, and prion disease pathogenesis. We specifically highlight studies in which chemically synthesized GPI anchors and analogues have been employed to study the roles of this unique posttranslational modification.

Figure 1

Structure of the GPI anchor from human erythrocyte acetylcholinesterase (16). The three domains of the GPI anchor are (i) a phosphoethanolamine linker (red), (ii) the conserved glycan core (black), and (iii) a phospholipid tail (blue). Appendages in blue (including the lipids of the lipid tail) are variable.

Figure 2

Membrane-associated proteins in a lipid bilayer containing lipid raft domains. GPI-anchored proteins and other lipidated proteins are believed to associate with lipid rafts.

Figure 3

A proposed model for the role of the GPI anchor in the conversion of PrP^C to PrP^Sc and progression to clinical disease (9). (A) When exposed to PrP^Sc, GPI-anchored PrP^C is converted into aggregates of PrP^Sc. The aggregates may interfere with the normal signaling events involving PrP^C, leading to neuron death. (B) Transgenic mice expressing PrP^C lacking a GPI anchor still form PrP^Sc aggregates upon infection with exogenous PrP^Sc. However, these aggregates may be unable to disrupt signal transduction pathways due to the lack of a GPI anchor. Figure adapted from ref (57). Copyright 2005 AAAS.

Figure 4

Structures of peptides/proteins attached to GPI anchor substitutes (–70). The GPI anchor replacement structures were chemically synthesized and coupled to either expressed proteins or chemically synthesized peptides or proteins. Single letter abbreviations are used for amino acids (2 and 7). PrP106 is the 106-amino acid truncated mouse prion protein (64), PrP(23−231) is the truncated mouse (2) or hamster (4) prion protein (65,68), PrP(214−231) is a fragment of the human prion protein (66), PrP(S230C) is the truncated mouse prion protein with a serine-to-cysteine mutation at residue 230 (69), GFP is enhanced green fluorescent protein (67), and PrP(90−232) is the truncated mouse prion protein (70).

Figure 5

Structures of GPI-protein analogues bearing systematic deletions in the glycan core (71). The GPI anchor analogues possess no monosaccharides (8), one mannosyl unit (9), or two mannosyl units (10) and were coupled to GFP using native chemical ligation. These GPI-protein analogues were used to investigate the functional significance of the GPI anchor glycan core (72).