Glycophosphatidylinositol

Glycosylphosphatidylinositol ( pronunciation ), or glycophosphatidylinositol, or GPI in short, is a glycolipid that can be attached to the C-terminus of a protein during posttranslational modification. Proteins containing a GPI anchor play key roles in a wide variety of biological processes. It is composed of a phosphatidylinositol group linked through a carbohydrate-containing linker (glucosamine and mannose glycosidically bound to the inositol residue) and via an ethanolamine phosphate (EtNP) bridge to the C-terminal amino acid of a mature protein. The two fatty acids within the hydrophobic phosphatidyl-inositol group anchor the protein to the cell membrane.

Glypiated (GPI-linked) proteins contain a signal sequence, thus directing them to the endoplasmic reticulum (ER). The protein is co-translationally inserted in the ER membrane via a translocon and is attached to the ER membrane by its hydrophobic C terminus; the majority of the protein extends into the ER lumen. The hydrophobic C-terminal sequence is then cleaved off and replaced by the GPI-anchor. As the protein processes through the secretory pathway, it is transferred via vesicles to the Golgi apparatus and finally to the plasma membrane where it remains attached to the a leaflet of the cell membrane. Since the glypiation is the sole means of attachment of such proteins to the membrane, cleavage of the group by phospholipases will result in controlled release of the protein from the membrane. The latter mechanism is used in vitro; i.e., the membrane proteins released from the membranes in the enzymatic assay are glypiated protein.

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