SIMILARITIES BETWEEN THE PRIMARY STRUCTURES OF 2 DISTINCT MAJOR SURFACE-PROTEINS OF TOXOPLASMA-GONDII

Toxoplasma gondii possesses a 43-kDa surface protein (SAG3) that is ex pressed by all invasive stages. We have cloned and sequenced cDNAs enc oding SAG3, with the longest one encoding a primary product of 385 ami no acid residues. The deduced amino acid sequence contains a putative NH2-terminal signal sequence, as well as a glycosylphosphatidylinosito l anchor attachment site. It is characterized by 12 cysteine residues whose distribution suggests a tandem duplication of a single ancestral motif containing 6 cysteine residues. Although no DNA sequence analog ies were found, comparative amino acid sequence analysis detected a re semblance to SAG1, which is the major surface antigen specifically exp ressed by the proliferative tachyzoite stage. Despite a low degree of identity between the two amino acid sequences (24%), the conservative distribution of the cysteine and tryptophan residues, as well as of re peated motifs, together with oligopeptide identities suggest similar f olding and;possibly similar function for both proteins.