Molecular and biochemical characterization of a Plasmodium falciparum cyclophilin containing a cleavable signal sequence.

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.

Isolation of a novel Plasmodium falciparum gene encoding a protein homologous to the Tat-binding protein family.

We have cloned a Plasmodium falciparum gene that belongs to the nuclear Tat-binding protein (TBP) gene family. This gene, PfTBP, is (A + T)-rich and encodes a 49.5-kDa protein. The predicted protein encoded by this gene has highest similarity to the slime mold protein DdTBP10 (86%) and to the yeast protein SUG1 (81.8%), both of which belong to the Tat-binding protein family. In agreement with the characteristics of this family, PfTBP contains a highly conserved domain of approximately 200 amino acids, in which are found the motifs A and B of ATPases, and amino acid sequences characteristic of a large family of RNA or DNA helicases, suggesting a role in RNA or DNA unwinding. Like DdTBP10, the PfTBP protein has a heptad repeat of four leucine residues, reminiscent of a leucine zipper motif known to mediate dimerization. We have further characterized PfTBP gene expression by Northern-blot analysis. This gene is expressed in a stage-specific manner, with higher expression in the late trophozoite stage. The recombinant PfTBP gene has been expressed in Escherichia coli and a polyclonal antiserum has been raised in rabbits against the recombinant protein. This antibody has been used to study the protein in the parasite. The gene product is expressed in a stage-specific manner with higher expression in the late trophozoite and schizont stages, and is localized in the nucleus of the erythrocytic stage parasite. Thus the protein might have a function in DNA synthesis and/or in transcription, as is the case for other Tat-binding proteins.