Glycosylphosphatidylinositol (GPI)-anchored proteins.

From the numerous studies developed at the last quarter of the 20th century, glycosylphosphatidylinositol (GPI) anchor has been established as a unique mode of protein binding to the plasma membrane. The core structure of this anchor consists of ethanolamine phosphate, trimannoside, glucosamine and inositol phospholipid in this order. The anchor is combined with the carboxyl-terminal of protein by the ethanolamine head. GPI-anchored proteins are ubiquitously distributed among Eucarya from vertebrates to protozoa, and also shown to be present in some of Archaeobacteria such as Sulfolobus. There is no evidence for the presence of GPI-anchored protein in Eubacteria. In the eucaryotic cells, both biosynthesis of GPI precursors and posttranslational protein modification with GPI proceed in the endoplasmic reticulum. On GPI modification, the carboxyl-terminal signal peptide is split off from the protein and the resulting new carboxyl-terminal is then combined with the amino group of ethanolamine residue in the GPI precursors. The whole process of cleavage and GPI attachment is catalyzed by GPI-transamidase complex. Many genes concerning GPI biosynthesis and protein modification have been cloned and sequenced. The carboxyl-terminal signal peptide containing hydrophobic tail is characterized by genetic analysis and shown to be essential for GPI modification of protein. Recent computational analysis further clarified the detailed requirement of the carboxyl-terminal regions for GPI-anchoring. GPI-anchored proteins are assumed to be transported from Golgi to the plasma membrane in the form of "lipid rafts", and expressed as the clusters in the cell surface.