LEISHMANIA-MAJOR PARASITES EXPRESS CYCLOPHILIN ISOFORMS WITH AN UNUSUAL INTERACTION WITH CALCINEURIN

The immunosuppressive effects of the fungal metabolite cyclosporin A ( CsA) are mediated primarily by binding to cyclophilins (Cyps). The res ulting CsA-Cyp complex inhibits the Ca2+- regulated protein phosphatas e calcineurin and down-regulates signal transduction events. Previousl y we reported that CsA is a potent inhibitor of infections transmitted by the human pathogenic protozoan parasite Leishmania major in vitro and in vivo, but does not effect the extracellular growth of L. major itself. It is unknown how L. major exerts this resistance to CsA. Here we report that a major Cyp, besides additional isoforms with the same N-terminal amino acid sequence, was expressed in L. major. The cloned and sequenced gene encodes a putative 174-residue protein called L. m ajor Cyp 19 (LmCyp19). The recombinant LmCyp19 exhibits peptidyl-proly l cis/trans isomerase activity with a substrate specificity and an inh ibition by CsA that are characteristic of other eukaryotic Cyps. To de termine whether calcineurin is involved in the discrimination of the e ffects of CsA we also examined the presence of a parasitic calcineurin and tested the interaction with Cyps. Despite the expression of funct ionally active calcineurin by L. major, neither LmCyp19 nor other L. m ajor Cyps bound to its own or mammalian calcineurin. The amino acid se quence of most Cyps includes an essential arginine residue around the calcineurin-docking side. In LmCyp19 this is replaced by an asparagine residue. This exchange and additional charged residues are apparently responsible for the lack of LmCypl9 interaction with calcineurin. The se observations indicate that resistance of L. major to CsA in vitro i s mediated by the lack of complex formation with calcineurin despite C sA binding by parasitic Cyp.