Toxoplasma gondii: Identification of a developmentally regulated family ofgenes related to SAG2

Previous studies have shown the surface of Toxoplasma gondii to be dominate d by a family of proteins closely related to SAG1. In this study, we report the existence of a second family of genes defined by homology to SAG2. The predicted amino acid sequences of these three new proteins suggests that t hey are all glycosylphosphatidylinositol-linked surface antigens. All three also contain N-linked glycosylation sites, although their use has yet to b e demonstrated. One of these SAG2-related antigens, SAG2B, is expressed in tachyzoites with an apparent size of 23 kDa. It is distinct, however, from the previously identified P23. In contrast to SAG2B, SAG2C and SAG2D appear to be expressed exclusively on the surface of bradyzoites. Analysis of the SAG2 family shows it to have weak but significant, homology to the SAG1 fa mily. Thus, all of the sequenced surface antigens of tachyzoites and many o f those of bradyzoites fall into one large superfamily that can be divided into two subgroups defined by the prototypic and highly immunogenic SAG1 an d SAG2, respectively. (C) 2000 Academic Press.