Apicomplexan parasites such as Toxoplasma gondii and Plasmodium species can cause serious diseases in humans and domestic animals. Because the parasites are eukaryotes and share many metabolic pathways with their hosts, it has proved difficult to develop safe and effective treatments but researchers at Washington University School of Medicine in St. Louis have now identified an essential kinase in T. gondii which is unlike human kinases and more closely resembles those found in plants. In a study published in Nature, the team used conditional suppression to show that T. gondii calcium-dependent protein kinase 1 (TgCDPK1) is essential for survival of the parasite. The enzyme controls the ability of T. gondii parasites to secrete microneme proteins which allow the parasites to control their movement and move in and out of host cells.
It should be possible to exploit the differences between the parasite kinase and human kinases to develop potent and selective inhibitors of the parasite enzyme and the team have already identified compounds that block CDPK1 signalling without affecting human cells. Pyrazolopyrimidine-derived compounds such as 3-MB-PPI were found to specifically inhibit TgCDPK1 and disrupt the parasite’s life cycle at stages dependent on microneme secretion. The disruption was dependent on TgCDPK1 inhibition since parasites expressing a mutant kinase not sensitive to the inhibitors.
Calcium-dependent protein kinases have a kinase domain similar to that of calmodulin-dependent kinase, regulated by a calcium-binding domain in the C terminus. X-ray structures of TgCDPK1, published in Nature Structural and Molecular Biology, showed that, in the auto-inhibited (apo) form, the C-terminal activation domain resembles a calmodulin protein with an unexpected long helix in the N terminus that inhibits the kinase domain in the same manner as calmodulin-dependent kinase II. Calcium binding triggers reorganization of the C-terminal activation domain into a highly intricate fold, leading to its relocation around the base of the kinase domain to a site remote from the substrate binding site. This large conformational change constitutes a distinct mechanism in calcium signal-transduction pathways.
CDPK1 may play a similar role in Plasmodium species which cause malaria, but the researchers predict that it may be harder to selectively inhibit the Plasmodium enzymes.