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From Biology-Online Dictionary
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One name for the covalent assembly of sugars linked to phosphatidyl inositol joined to the C terminal residue of many proteins by a modified ethanolamine residue. Also called a greasy foot. Another term for this modification is glypiation. The function of the pigtail is to act as the sole anchor of the protein to the external surface of the lipid bilayer. The moiety is added to the protein during co translational insertion into the endoplasmic reticulum membrane on the luminal side. The addition is synchronised with the removal of a large C terminal polypeptide sequence that is usually hydrophobic and could itself have formed a membrane anchor. The surface proteins of many unicellular protozoa very commonly have this modification, the best known being the variable surface glycoprotein of trypanosomes and of malaria parasites.

Examples are probably present in all eukaryotic plasma membranes.