GPI anchors attachment

Fig. 2 Three-dimensional structure of FLAP. Left panet. Monomer A is shown in ribbon representabon and in yellow, while monomer B is shown in surface representation in blue. The relative location of the active site, N-terminus, and GPI anchoring site are indicated. Right panel. Modeled structure of the GPI anchor attached to the 3D structure of PLAP. N-linked complex glycan was also modeled at N122 and at N249. This figure was kindly produced and contributed by Dr. Mark R. Wormald, Oxford Glycobiology Institute, University of Oxford, Oxford, UK...

Fig. I Structure of the human prion protein, (a) The mature human prion protein as it can be found on the outer cell membrane. The GPI-anchor (attached to Ser230) and typical glycans (attached to Asnl81 and Asnl97) are shown in sticks. Structure obtained from MD simulation (Van der Kamp, Koldsp and Daggett, unpublished results), (b) The structured part of the recombinant human prion protein, as obtained by protein NMR [39]. Secondary structure elements are labeled...

Two peptide sequences are necessary for GPI anchor addition, an N-terminal signal peptide directing the nascent protein into the endoplasmic reticulum and a C-terminal signal peptide directing GPI anchor attachment. The signal for GPI anchor attachment has been localized to the C-terminal region of GPI-linked proteins [101-103]. Yet, comparison of cDNA deduced amino acid sequences of C-terminal regions from GPI-anchored proteins does not identify a clear consensus sequence [5,104—106]. 

Fig. 1. Structure and posttranslational processing of PrP. (Upper) Structure of the primary translation product of mammalian PrP. The five proline/glycine-rich repeats in mouse PrP have the sequence P(Q/H)GG(T/G/S)WGQ. (Lower) Structure of the mature protein. The GPI anchor attaches the polypeptide chain to the membrane. (See Fig. 4B for a schematic of the core anchor structure.) Arrows A and B indicate the positions of cleavage sites in PrP, and arrow C a cleavage site in PrP. Site A lies within the GPI anchor, between the glycerolipid moiety and the ethanolamine residue that is attached to the C-terminal amino acid. Site B lies near position HO, and site C near position 89. (Reprinted with permission from Harris, 1999).

GPI anchors not only exist in T. brucei but are ubiquitous in eukaryotic cells [100]. Their principal function is to attach proteins to the plasma membrane. Various proteins have been found to be GPI anchored and their role in biological recognition processes has attracted a great deal of attention [101]. GPI anchors attach proteins to membranes via a phosphoethanolamine unit linked to a trimannose—glucosamine—inositol backbone and a hydrophobic Upid that anchors the system to the membrane [102,103]. The carbohydrate backbone is conserved in all GPI anchors described to date. Nevertheless, various species specific carbohydrate side chains are observed alongside additional phosphoethanolamine units and variations in the lipid unit. 

M. Benghezal, A. Benachour, S. Rusconi, M. Aebi A. Conzelmann. Yeast GpiSp is essential for GPI anchor attachment onto proteins. EMBO J, 1996, 15, 6575-6583. 

Paroxysmal nocturnal hemoglobinuria (MIM 311770) Mutation resulting in deficient attachment of the GPI anchor to certain proteins of the red cell membrane... 

The cell wall of S. cerevisiae is composed of glucan, mannoproteins, and chitin (Klis, 1994 Cid et al., 1995). Of the mannoproteins, some of them are first synthesized as GPI-anchored and mannosylated proteins. Subsequently, they are incorporated from the plasma membrane to the cell wall and are covalently linked to the glucan there. Therefore, some signals should exist for dictating the cell wall incorporation. One found at a short N-terminal region near the GPI-attached asparagine (the co-site) is important (Hamada et al., 1998b). Namely, a plasma membrane GPI-anchored protein was localized to the cell wall if the (V/I)... 

PNH is caused by a mutation in certain types of adult blood cells. Because of this mutation, certain types of proteins, including complement inhibitors, are unable to attach to the surface of the cell, as is normally the case. More specifically, the PNH mutation prevents the assembly of a fatty tail, known as a glycosyl-phosphatidylinositol (GPI) anchor, a necessary step in surface attachment of some proteins. 

Bacteria also contain a very rich variety of glycolipids with unusual structures. Lipid A13 is the site of attachement of the 0-specific chain of Gram (-) bacteria, which constitutes the antigenic lipopolysaccharide [87]. Other members of this family can be quoted, for example glycosyl glycerophospholipids in which the carbohydrate and glycerol moieties are linked by a phosphodiester bond (e.g. GPI anchor 14) [88] or carbohydrate esters (e.g. cord-factor of mycobacteria 15). 

Beyond merely anchoring a protein to the membrane, the attached lipid may have a specific role. In the plasma membrane, proteins with GPI anchors are exclusively on the outer face and are confined within clusters, as we shall see below, whereas other types of lipid-linked proteins (with farnesyl or geranylgeranyl groups attached Fig. 11-14) are exclusively on the inner face. Attachment of a specific lipid to a newly synthesized membrane protein therefore has a targeting function, directing the protein to its correct membrane location. 

Specific proteins can be covalently attached via a carbohydrate bridge to membrane-bound PI (glycosylphosphatidylinositol, or GPI). This allows GPI-anchored proteins rapid lateral mobility on the surface of the plasma membrane. A deficiency in the synthesis of GPI in hematopoietic cells results in a hemolytic disease, paroxysmal nocturnal hemoglobinuria. 

Many proteins synthesized by the ribosomes of the RER contain short chains of carbohydrates (oligosaccharides) and are called glycoproteins. The oligosaccharides are of two main types O-linked (to the OH side chain of Ser or Thr) and N-linked (to the NH2 side chain of Asn). Some proteins are attached to the plasma membrane by a third type of carbohydrate structure called a glycosyl phosphatidylinositol (GPI) anchor. 

In addition, several proteins are now known to be attached to the plasma membrane via a specific structure that involves carbohydrate, namely a glycosyl phosphatidylinositol (GPI) anchor. This is covered in Topic E2. 

Commonly attached at 03 of Man-I of the Tryp. brucei variant surface glycoprotein GPI anchor is a digalactose branch, which may further carry additional D-galactose (Gal) units [13], In 1991, the first total synthesis of the common region of the GPI anchor of Tryp. brucei was accomplished by the Ogawa group [14, 32], The Gal... 

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