Phosphatidylinositol-glycan

Example of a phosphatidylinositol glycan membrane protein anchor. GlcNH2 = glucosamine. 

Proteins with long C-terminal hydrophobic signal sequences may become attached to phosphatidylinositol-glycan anchors embedded in the plasma membrane (Fig. 8-13). An example is a human alkaline phosphatase in which the a carboxyl of the terminal aspartate residue forms an amide linkage with the ethanolamine part of the anchor. Attachment may occur by a direct attack of the -NH2 group of the ethanolanine on a peptide linkage in a transacylation reaction that releases a 29-residue peptide from the C terminus.118119 (See Chapter 29). 

Anchors, membrane for proteins 402 diphytanylglyceryl 402 glycosylphosphatidylinositol (phosphatidylinositol-glycan) 403, 403s, 523... 

Pak, Y, Ryals, PE, Thompson, GA, Jr. Phosphatidylinositol glycan formation and utilization by the dliate Tetrahymena mimbres. J Biol Chem 266 15054-15059. 

R. Micanovic, L. D. Gerber, J. Berger, K. Kodukula S. Udenfriend. Selectivity of the cleavage/attachment site of phosphatidylinositol-glycan-anchored membrane proteins determined by site-specific mutagenesis at Asp-484 of placental alkaline phosphatase. Proc Natl Acad Sci USA, 1990, 87, 157-161. 

K. Kodukula, D. Cines, R. Amthauer, L. Gerber S. Udenfriend. Biosynthesis of phosphatidylinositol-glycan (PI-G)-anchored membrane proteins in cell-free systems cleavage of the nascent protein and addition of the Pl-G moiety depend on the size of the COOH-terminal signal peptide. Proc Natl Acad Sci USA, 1992, 89, 1350-1353. 

Biosynthesis of GPI anchors starts with the core structure assembly by sequential addition of UDP-GlcNAc (followed by iV-deacetylation), dolichol-phosphate-mannose, and phospho-ethanolamine to phosphatidylinositol and culminates in the en bloc transfer to protein shortly after the protein is synthesized. However, the biosynthetic pathways can differ strikingly between different organisms with respect to specific modifications and fatty acid remodeling occurring after completion of the core glycan. This also applies for the point when certain modifications are introduced, e. g. before or after the transfer of the GPI-moiety to the protein. GPI anchors can be cleaved at defined positions by an array of enzymatic and chemical methods, respectively (O Fig. 5). Thus, it becomes possible to identify GPI-anchored proteins and, moreover, analyze the structure and biosynthesis of GPI anchors [103]. 

W. J. Masterson, T. L. Doering, G. W. Hart, and P. T. Englund, A novel pathway for glycan assembly biosynthesis of the glycosyl-phosphatidylinositol anchor of the trypanosome variant surface glycoprotein, Cell., 56 (1989) 793-800. 

Fig. 6.1 Scheme representing the structure and hydrolysis of glycosyl-phosphatidylinositol (GPI) (adapted from Refs. [1] and [7]). Free GPI and GPI-anchored proteins are represented, showing their similarity in structure. The points for phospholipase C (PLC) and phospholipase D (PLD) hydrolysis are indicated. The glycan component of free GPI is... 

Davitz, M.A., Hereld, D., Shak, S., Krakow, J., Englund, P.T., and Nussen-zwEiG, V. A glycan-phosphatidylinositol-specific phospholipase D in human serum. Science, 1987, 238, 81-84. 

Misek, D.E. and Saltiel, A.R. An inositol phosphate glycan derived from a Trypanosoma brucei glycosyl-phosphatidylinositol mimics some of the metabolic actions of insulin. J. Biol. Chem., 1992, 267, 16266-16273. 

Fig. 10.7. The glycosylphosphatidylinositol glycan anchor (GPI). The carboxy terminus of the protein is attached to phosphoethanolamine, which is bound to a branched oligosaccharide that is attached to the inositol portion of phosphatidylinositol. The hydrophobic fatty acyl chains of the phosphatidylinositol portion are bound in the hydrophobic core of the membrane.

Schneider, R, Ralton, J.E., McConville, M.J., and Ferguson, M.A. 1993. Analysis of the neutral glycan fractions of glycosyl-phosphatidylinositols by thin-layer chromatography, AwaZ. Biochem., 210 106-112. 

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