GPI anchor
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A GPI anchor (phosphatidyl-inositol glycane) is a common modification of the C-terminus of membrane-attached proteins. It is composed of a hydrorophobic phosphatidyl inositol group linked through a carbohydrate containing linker (glucosamine and mannose linked to phosphoryl ethanolamine residue) to the C-terminal amino acid of a mature protein. The two fatty acids within the hydrophobic inositol group anchor the protein to the membrane.
Glypiated proteins contain a signal peptide, thus directing them into the endoplasmic reticulum (ER). The C-terminus is composed of hydrophobic amino acids which stay inserted in the ER membrane. The hydrophobic end is then cleaved off and replaced by the GPI-anchor. As the protein processes through the secretory route, it is transferred via vesicles to the Golgi and finally to the extracellular space where it remains attached to the exterior 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.
Defects of GPI anchors occur in the rare disease paroxysmal nocturnal hemoglobinuria.