Plasmodium falciparum is one of four species known to cause malaria in humans and is the species that is associated with the most virulent form of the disease. Malaria causes nearly two million deaths each year, many of these occurring among children in under-developed countries of the world. One reason for this is the prevalence of drug resistant strains of malaria that mitigate the efficacy of existing drugs. Hence, the identification of a new generation of pharmacological agents for malaria is extremely urgent. The recent identification of a group of novel protein kinases within the Plasmodium falciparum genome has provided researchers with a basis for what many hope to be new potential drug targets for malaria. Identified within the Plasmodium genome and a few select apicomplexans, these novel proteins have been predicted to be protein kinases based solely on certain sequence features shared with other eukaryotic protein kinases (ePKs). However, to date, no significant studies to determine the function of these novel kinases have been performed. Termed FIKKs, these proteins all possess a non-conserved N-terminal sequence that contains a Plasmodium export element (Pexel) which may target the proteins for export from the parasite and a conserved C-terminal catalytic domain containing a FIKK sequence common to all twenty members of this family. We analyzed the localization of one of the FIKK proteins, FIKK11, encoded by the PF11_0510 locus, during intraerythrocyte differentiation of P. falciparum by Western blot analysis and indirect immunofluorescence assay. Western blot analysis demonstrated that FIKK 11 is expressed within the parasite at all stages of its erythrocytic life cycle with its highest expression occurring during the schizont stage. Immunofluorescence assays showed that this protein is exported from the Plasmodium parasite into the host erythrocyte cytosol which is consistent with studies on other Plasmodium proteins that also have the Pexel motif. To determine the enzymatic activity of FIKK11, we overexpressed the recombinant protein in E. coli and then purified it. However, no protein kinase activity was detected using several commonly used protein kinase substrates including histone H1, myelin basic protein, or dephosphorylated casein. We also did not detect any kinase activity of the native enzyme using pull-down assays of the Plasmodium falciparum cell extract against those same substrates. In addition, kinase substrate peptide array analysis of FIKK11 showed no evidence of protein kinase activity either for FIKK11. Interestingly, however, we were able to detect some kinase activity using the recombinant protein alone with no substrate. The lack of the glycine triad within subdomain I of these FIKK kinases as compared with most traditional eukaryotic protein kinases may explain why we were unable to find any interactions between FIKK11 and other commonly protein kinase substrates. Of interest was the observation that the protein reproducibly exhibited what appeared to be an autophosphorylation activity when using the standard protein kinase assay. Further analyses, however, showed that FIKK11 actually possesses diacylglycerol kinase activity utilizing 1-Stearoyl-2-arachidonoyl-sn-glycerol as a substrate. This is the first evidence of diacylglycerol kinase activity in Plasmodium falciparum. Because FIKK11 is exported into the host cell and is localized on the erythrocyte membrane, its enzymatic activity may potentially have relevance in the pathophysiology of the disease.

Malaria is a major public health problem that affects millions of lives globally. The increased burden of malaria requires new interventions that will address the eradication of the disease. Current interventions include vector control by using insecticide-treated bed nets and indoor residual spraying, and antimalarial drugs to control the parasite. Parasite resistance has been reported for the currently used effective antimalarial drugs. To pre-empt the impact of parasite resistance a continued development of new antimalarial drugs that have novel mechanisms of action should be pursued. Antimalarial drug discovery requires that potential antimalarial drugs should have different drug targets to those already targeted, to lower the chances of resistance. Potential antimalarial drugs should preferably provide a single radical cure to prevent reproduction at all life cycle stages. This study tested the effects of in silico designed compounds targeting plasmodial Ca2+- dependent protein kinases (CDPK) 1 & 4, FIKK kinases and bromodomain proteins on the Plasmodium parasite. These enzymes are involved in gene regulation and are important factors during gene transcription. In P. falciparum the gatekeeper kinases contain small hydrophobic pockets near the ATP-binding site. These hydrophobic pockets allow for selective inhibition of these proteins at the ATP-binding site. The compounds were tested in vitro to determine their antiplasmodial activity. These compounds are shown to be potential inhibitors of the intra-erythrocytic P. falciparum parasites as three of the compounds showed selective cytotoxic activity at less than 1 μM against the chloroquine sensitive laboratory strains (3D7 and NF54). Even though the proteins targeted by these compounds have been previously indicated to play a role at specific stages during the parasite’s life cycle, the compounds tested here were not able to target the sexual gametocyte stages of the Plasmodium parasite. Further optimisation of these compounds should be performed to improve activity against both the asexual and sexual stages of the parasites. / Dissertation (MSc)--University of Pretoria, 2017. / Pharmacology / MSc / Unrestricted