Glycophosphatidylinositol
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A GPI anchor or glycosylphosphatidylinositol is a glycolipid that can be attached to the C-terminus of a protein during posttranslational modification. It is composed of a hydrophobic 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 phosphatidyl-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 pathway, it is transferred via vesicles to the Golgi apparatus 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.
Phospholipase C (PLC) is an enzyme that is known to cleave the phospho-glycerol bond found in GPI-anchored proteins. Treatment with PLC will cause release of GPI-linked proteins from the outer cell membrane. The T-cell marker Thy-1, acetylcholinesterase, as well as both intestinal and placental alkaline phosphatase are known to be GPI-linked and are released by treatment with PLC. GPI-linked proteins are thought to be preferentially located in lipid rafts, suggesting a high level of organization within microdomains plasma membrane.
Defects of GPI anchors occur in the rare disease paroxysmal nocturnal hemoglobinuria.
