THE SAG2 ANTIGEN OF TOXOPLASMA-GONDII AND THE 31-KDA SURFACE-ANTIGEN OF SARCOCYSTIS-MURIS SHARE SIMILAR SEQUENCE FEATURES

In the search for similar cysteine distribution patterns among surface antigens of members of the phylum of Apicomplexa, we found a resemblance between TgSAG2, a 22-kDa surface antigen of Toxoplasma gondii, and the C-terminal half of a 31-kDa surface antigen of Sarcocystis muris, SmSAG1. The overall degree of similarity is low, corresponding to 28% of identity in the 146 amino acids aligned. However, the conservative spacing between the cysteine residues suggest that this similarity might have functional implications, The described amino acid similarity provides a new clue to the molecular relationship between two closely related coccidian parasites.