Plasmodium falciparum glycogen synthase kinase-3: Molecular model, expression, intracellular localisation and selective inhibitors

Worldwide increasing resistance of Plasmodium falciparum to common anti-malaria agents calls for the urgent identification of new drugs. Glycogen synthase kinase-3 (GSK-3) represents a potential screening target for the identification of such new compounds. We have cloned PfGSK-3, the P. falciparum gene homologue of GSK-3β. It encodes a 452-amino-acid, 53-kDa protein with an unusual N-terminal extension but a well-conserved catalytic domain. A PfGSK-3 tridimensional homology model was generated on the basis of the recently crystallised human GSK-3β. It illustrates how the regions involved in the active site, in substrate binding (P+4 phosphate binding domain) and in activity regulation are highly conserved. Recombinant PfGSK-3 phosphorylates GS-1, a GSK-3-specific peptide substrate, glycogen synthase, recombinant axin and the microtubule-binding protein tau. Neither native nor recombinant PfGSK-3 binds to axin. Expression and intracellular localisation of PfGSK-3 were investigated in the erythrocytic stages. Although PfGSK-3 mRNA is present in similar amounts at all stages, the PfGSK-3 protein is predominantly expressed at the early trophozoite stage. Once synthesized, PfGSK-3 is rapidly transported to the erythrocyte cytoplasm where it associates with vesicle-like structures. The physiological functions of PfGSK-3 for the parasite remain to be elucidated. A series of GSK-3β inhibitors were tested on both PfGSK-3 and mammalian GSK-3β. Remarkably these enzymes show a partially divergent sensitivity to the compounds, suggesting that PfGSK-3 selective compounds might be identified.