Characterization of protein Ser Thr phosphatases of the malaria parasite, Plasmodium falciparum: inhibition of the parasitic calcineurin by cyclophilin-cyclosporin complex

Two major protein phosphatase (PP) activities were purified from cytosolic extracts of the erythrocytic stage of the malaria parasite, Plasmodium falc iparum. Both enzymes were specific for phosphoserine and phosphothreonine r esidues with very little activity against phosphotyrosine residues. The bio chemical properties of the enzymes suggested their strong similarity with e ukaryotic PP2A and PP2B protein phosphatases. Both enzymes preferentially d ephosphorylated the a subunit of phosphorylase kinase, and were resistant t o inhibitor-1. The PP2A-like enzyme required Mn2+ for activity and was inhi bited by nanomolar concentrations of okadaic acid (OA). The cDNA sequence o f the PP2A-like enzyme was identified through a match of its predicted amin o acid sequence with the N-terminal sequence of the catalytic subunit. The PP2B-like (calcineurin) enzyme was stimulated by calmodulin and Ca2+ or Ni2 +, but was resistant to OA. Malarial calcineurin was strongly and specifica lly inhibited by cyclosporin A (CsA) only in the presence of wild type P. f alciparum cyclophilin but not a mutant cyclophilin. The inhibition was nonc ompetitive, and provides a potential explanation for the cyclosporin-sensit ivity of the parasite. There was no significant quantitative difference in the total protein Ser/Thr phosphatase activity among the ring, trophozoite, and schizont stages. (C) 1999 Elsevier Science B.V. All rights reserved.