ABSTRACT
Glycosylphosphatidylinositols (GPIs) are functionally important constituents in the outer leaflet of the plasma membrane of many eukaryotic cells (reviewed by Low, 1989; Cross, 1990; Thomas et al., 1990). These glycolipids can be defined as having the structure glycan-inositolphospholipid where the glycan moiety contains the core sequence Mana1-4GlcNa1-6myoinositol and where the inositolphospholipid may be either a glycerolipid or a sphingolipid. GPIs are minor membrane components in higher eukaryotes and generally function as membrane anchors for a diverse group of surface proteins. Free GPIs have been reported, but not characterized, in some mammalian cells and are proposed to act as mediators of some of the actions of insulin (reviewed by Low, 1989). By contrast, GPI glycolipids are often major components in lower eukaryotes. In parasitic protozoa belonging to the Trypanosomatidae, which include the African trypanosomes, Trypanosoma cruzi and Leishmania spp., they may constitute the major glycolipid class. Interest in these glycolipids has been stimulated by the finding that GPI-anchored molecules and related structures are involved in parasite infectivity and survival. They are also important antigens on the parasite surface and may be involved in host immunity. Three types of GPI glycolipid have been identified in these parasites:
1. those that are covalently linked to protein; 2. those that are covalently linked to polysaccharide (the ‘lipophosphoglycans’);
and 3. free GPI glycolipids that are not linked to either protein or polysaccharide.
