Isolation, cDNA sequences, and biochemical characterization of the major cyclosporin-binding proteins of Toxoplasma gondii

The activities of the immunosuppressive, antifungal compounds cyclosporin A (CsA), FK-506, and rapamycin are dependent upon high affinity binding proteins collectively termed immunophilins. We report the isolation, biochemical characterization, and amino acid sequences of two major CsA- binding proteins, cyclophilins, from the pathogenic protozoan, Toxoplasma gondii. The 18.5- and 20-kDa molecular mass proteins exhibit peptidylproline cis-trans-isomerase activity, which is inhibitable by 10^-8 M CsA. The amino acid sequences of these two proteins, deduced from cDNA clones, reveal up to 70% amino acid identity to previously isolated cyclophilins. The 18.5-kDa protein appears to be synthesized as a precursor with a 15 amino acid signal peptide. The amino-terminal region of the mature 20-kDa protein has significant homology to the B subunit of the calmodulin-dependent phosphatase, calcineurin. The two T. gondii cyclophilins are products of different genes and appear to have different subcellular distributions.