The major surface antigens of Entamoeba histolytica trophozoites are GPI-anchored proteophosphoglycans

Trophozoites of the parasitic protozoa, Entamoeba histolytica, synthesize a cell surface lipoglycoconjugate, termed lipophosphoglycan, which is thought to be an important virulence factor and potential vaccine candidate against invasive amebiasis. Here, we show that the E. histolytica lipophosphoglycans are in fact glycosylphosphatidylinositol (GPI)-anchored proteophosphoglycans (PPGs). These PPGs contain a highly acidic polypeptide component which is rich in Asp, Glu and phosphoserine residues. This polypeptide component is extensively modified with linear glycan chains having the general structure, [Glcalpha1-6](n)Glcbeta1-6Gal (where n=2-23). These glycan chains can be released after mild-acid hydrolysis with trifluoroacetic or hydrofluoric acid and are probably attached to phosphoserine residues in the polypeptide backbone. The PPGs are further modified with a GPI anchor which differs from all other eukaryotic GPI anchors so far characterized in containing a glycan core with the structure, Gal(1)Man(2)GlcN-myo-inositol, and in being heterogeneously modified with chains of alpha-galactose. Trophozoites of the pathogenic HM-1:IMSS strain synthesize two distinct classes of PPG which have polydisperse molecular masses of 50-180 kDa (PPG-1) and 35-60 kDa (PPG-2) and are modified with glucan side-chains of different average lengths. In contrast, the non-pathogenic Rahman strain synthesizes one class of PPG which is only elaborated with short disaccharide side-chains (i.e. Glcbeta1-6Gal). However, the PPGs are abundant in all strains (8x10(7) copies per cell) and are likely to form a protective surface coat.