GPIanchoring

GPI anchoring, or glycosylphosphatidylinositol anchoring, is a post-translational modification that tethers certain proteins to the outer surface of the cell membrane via a GPI anchor. This mode of membrane attachment is widespread among eukaryotes and influences the localization, stability, and function of the attached proteins.

The GPI anchor is a glycolipid that links to the protein through a phosphodiester bond between a terminal ethanolamine phosphate on the anchor and the protein’s C-terminus. The core anchor comprises phosphatidylinositol linked to a glucosamine, which is connected to a glycan chain built from three mannose residues. Additional ethanolamine phosphate groups and lipid remodeling can modulate anchoring and interactions with other membrane components.

Biosynthesis and attachment occur in the endoplasmic reticulum. A complex of GPI biosynthetic enzymes forms the GPI precursor, which is attached to a nascent protein at a specific omega site by a GPI transamidase, replacing the protein’s C-terminal signal sequence. The GPI-anchored protein is then trafficked through the secretory pathway to the plasma membrane, where the anchor may be remodeled and the lipid portion adjusted or exchanged.

Functions and significance of GPI-anchored proteins are diverse. They are often enriched in lipid rafts and participate in signaling, enzymatic activity, and cell–cell or extracellular matrix interactions. Well-known examples include CD55 (decay-accelerating factor) and CD59 (protectin), as well as urokinase receptor (uPAR) and the prion protein PrP.

Clinical relevance is notable. Defects in GPI anchor biosynthesis underlie congenital disorders, and acquired mutations in PIGA and related genes can cause paroxysmal nocturnal hemoglobinuria, reflecting the essential role of GPI-anchored proteins in normal physiology.