The immunosuppressive drug cyclosporin A (CsA) inhibits the growth of malaria parasites in vitro and in vivo. Cyclosporin A exerts its immunosuppressive effect in T lymphocytes by binding to cyclophilin (CyP), a peptidylprolyl cis-trans isomerase (PPIase). It is believed that the cyclosporin/cyclophilin complex inhibits a Ca(2+)-activated protein phosphatase, calcineurin, involved in T-cell activation. A cDNA encoding a cyclophilin of the human malaria parasite Plasmodium falciparum has been isolated as a step in the elucidation of the mechanism of antimalarial action of CsA. This cDNA, termed PfCyP, encodes a protein of 195 amino acids which has highest similarity with the Candida albicans (73.1%) and the Drosophila melanogaster (73.1%) cytoplasmic cyclophilins. A Northern blot reveals an approximately 900-bp nucleotide transcript that is consistent with the predicted size of the encoded polypeptide. The predicted PfCyP protein has a putative endoplasmic-reticulum-directed signal sequence at its N-terminus and two potential N-linked glycosylation sites. Expression of PfCyP RNA in an in vitro translation/translocation system reveals that the PfCyP protein is translocated across microsomes, that the signal peptide is cleaved and that the PfCyP protein is glycosylated at two sites. The PfCyP cDNA open reading frame coding for the predicted mature protein has been expressed in Escherichia coli. The purified recombinant protein is an active PPIase (kcat/Km = 2.3 x 10(6) s-1 M-1); this enzymic activity is inhibited by CsA (IC50 = 10 nM). The PfCyP protein has thus the same sensitivity to CsA as the PPIase activity associated with P. falciparum extracts [Bell, A. et al. (1994) Biochem. Pharmacol. 48, 495-503] suggesting that PfCyP may be responsible for the PPIase activity in those extracts. If different cyclophilins exist in P. falciparum, we conclude that either the PfCyP protein is the major cyclophilin detected in the parasite or that there are other cyclophilins with similar susceptibilities to CsA.