THE SURFACE OF TOXOPLASMA TACHYZOITES IS DOMINATED BY A FAMILY OF GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED ANTIGENS RELATED TO SAG1

Toxoplasma gondii is an Apicomplexan parasite with a complex life cycl e that includes a rapidly dividing asexual stage known as the tachyzoi te, The tachyzoite surface has been reported to comprise five major an tigens, the most abundant of which is designated SAG1 (for surface ant igen), At least one of the other four (SAG3) and another recently desc ribed minor antigen (SRS1 [for SAG1-related sequence 1]) have previous ly been shown to be structurally related to SAG1, To determine if furt her SAG1 homologs exist, we searched a Toxoplasma expressed sequence t ag (EST) database and found numerous ESTs corresponding to at least th ree new genes related to SAG1, Like SAG1, these new SRS genes encode a pparently glycosylphosphatidylinositol-anchored proteins that share se veral motifs and a set of conserved cysteine residues. This family app ears to have arisen by divergence from a common ancestor under selecti on for the conservation of overall topology. The products of two of th ese new genes (SRS2 and SRS3) are shown to be expressed on the surface of Toxoplasma tachyzoites by immunofluorescence, We also identified s train-specific differences in relative expression levels. A total of 1 0 members of the SAG1 gene family have now been identified, which appa rently include three of the five major surface antigens previously des cribed and one antigen expressed only in bradyzoites, The function of this family may be to provide a redundant system of receptors for inte raction with host cells and/or to direct the immune responses that lim it acute T. gondii infections.