The malaria parasite Plasmodium falciparum undergoes distinct morphologic changes during its 48-h life cycle inside human red blood cells. Parasite proteinases appear to play important roles at all stages of the erythrocytic cycle of human malaria. Proteases involved in erythrocyte rupture and invasion are possibly required to breakdown erythrocyte membrane skeleton. To identify such proteases, soluble cytosolic extract of isolated trophozoites/schizonts was incubated with erythrocyte membrane ghosts or spectrin-actin depleted inside-out vesicles, which were then analyzed by SDS-PAGE. In both cases, a new protein band of 155 kDa was detected. The N-terminal peptide sequencing established that the 155 kDa band represents truncated ankyrin. Immunoblot analysis using defined monoclonal antibodies confirmed that ankyrin was cleaved at the C-terminus. While the enzyme preferentially cleaved ankyrin, degradation of protein 4.1 was also observed at high concentrations of the enzyme. The optimal activity of the purified enzyme, using ankyrin as substrate, was observed at pH 7.0-7.5, and the activity was strongly inhibited by standard inhibitors of cysteine proteinases (cystatin, NEM, leupeptin, E-64 and MDL 28 170), but not by inhibitors of aspartic (pepstatin) or serine (PMSF, DFP) proteinases. Furthermore, we demonstrate that protease digestion of ankyrin substantially reduces its interaction with ankyrin-depleted membrane vesicles. Ektacytometric measurements showed a dramatic increase in the rate of fragmentation of ghosts after treatment with the protease. Although the role of ankyrin cleavage in vivo remains to be determined, based on our findings we postulate that the parasite-derived cysteine protease activity cleaves host ankyrin thus weakening the ankyrin-band 3 binding interactions and destabilizing the erythrocyte membrane skeleton, which, in turn, facilitates parasite release. Further characterization of the enzyme may lead to the development of novel antimalarial drugs.