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 though t to be an important virulence factor and potential vaccine candidate again st invasive amebiasis. Here, we show that the E. histolytica lipophosphogly cans are in fact glycosylphosphatidylinositol (GPI)-anchored proteophosphog lycans (PPGs). These PPGs contain a highly acidic polypeptide component whi ch is rich in Asp, Glu and phosphoserine residues. This polypeptide compone nt is extensively modified with linear glycan chains having the general str ucture, [Glc alpha 1-6]nGlc beta 1-bGal (where n = 2-23). These glycan chai ns can be released after mild-acid hydrolysis with trifluoroacetic or hydro fluoric acid and are probably attached to phosphoserine residues in the pol ypeptide backbone. The PPGs are further modified with a GPI anchor which di ffers from all other eukaryotic GPI anchors so far characterized in contain ing a glycan core with the structure, Gal(1)Man(2)GlcN-myo-inositol, and in being heterogeneously modified with chains of a-galactose. Trophozoites of the pathogenic HM-1:IMSS strain synthesize two distinct classes of PPG whi ch 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 length s. In contrast, the non-pathogenic Rahman strain synthesizes one class of P PG which is only elaborated with short disaccharide side-chains (i.e. Glc b eta 1-6Gal). However, the PPG are abundant in all strains (8 x 10(7) copies per cell) and are likely to form a protective surface coat. (C) 2000 Acade mic Press.