Glycosylphosphatidylinositol (GPI) is a generic term for compounds containing the structural motif: D-GlcN-(α1–6)-D-myo-inositol-1-PO3H-lipid, where the lipid is either a glycerolipid (sn-1,2-diacylglycerol, sn-1-acylglycerol, sn-1-alkyl-2-acylglycerol) or a ceramide. They can be further divided into three types, depending on the glycosyl substituents attached to the glucosamine (GlcN) residue. Those containing Man-(α1–6)-Man-(α1–4)-GlcN are type-1 GPIs, those containing Man-(α1–3)-Man-(α1–4)-GlcN are type-2 GPIs, and those containing Man-(α1–3)-[Man-(α1–6)-]Man-(α1–4)-GlcN are hybrid GPIs.
The type-1 GPIs are the most widely distributed, and they include the protein-linked GPI membrane anchors that, with a few protist exceptions, appear to be ubiquitous among the eukaryotes. Mature protein-linked GPI membrane anchors have a common core structure of NH2CH2CH2PO3H–6-Man-(α1–2)-Man-(α1–6)-Man-(α1–4)-GlcN-(α1–6)-...