Protein Anchoring to Membrane by Glycosylphosphatidylinositol. Determination of the COOH-Terminal Signal Peptide Sequence and GPI-Attachment Site in Bovine Liver 5′-Nucleotidase

Abstract

Since 1985, more than seventy proteins in the cells of eukaryotes including protozoa, yeast, insects and vertebrates, have been shown to be anchored to the membranes by glycosylphosphatidylinositol (GPI), a glycolipid consisting mainly of phosphatidylinositol (PI) and oligosaccharide (Ferguson et al., 1985). Attachment of GPI anchor to these proteins is now considered to be a post-translational event. After synthesis of the polypeptide chain of each protein in ER-attached ribosomes, a hydrophobic peptide was cleaved from the COOH-terminus of the protein and replaced by the GPI precursor formed in the ER membrane (Bangs et al., 1986; Ferguson et al., 1986; Low, 1987). The whole scheme of this modification was originally proposed for variant surface glycoprotein (VSG) of Trypanosoma brucei and supported by the findings on other GPI-anchored proteins such as Thy- 1 (Low and Kincade, 1985) and decay-accelerating factor, DAF (Caras et al., 1987). The replacement of the COOH-terminal peptide by the GPI precursor inevitably requires a certain recognition process; this hydrophobic peptide in the nascent protein was found to act as a signal for GPI attachment. The COOH-terminal signal peptide is usually composed of 17–31 amino acid residues. Also, the site for attachment of GPI is important.