Toxoplasma gondii attachment to host cells is regulated by a calmodulin-like domain protein kinase

The role of calcium-dependent protein kinases in the invasion of Toxoplasma gondii into its animal host cells was analyzed. KT5926, an inhibitor of ca lcium-dependent protein kinases in other systems, is known to block. the mo tility of Toxoplasma tachyzoites and their attachment to host cells. lit vi vo, KT5926 blocks the phosphorylation of only three parasite proteins, and in parasite extracts only a single KT5926-sensitive protein kinase activity was detected. This activity was calcium-dependent but did not require calm odulin, In a search for calcium-dependent protein kinases in Toxoplasma, tw o members of the class of calmodulin-like domain protein kinases (CDPKs) we re detected. TgCDPK2 was only expressed at the mRNA level in tachyzoites, b ut no protein was detected, TgCDPK1 protein was expressed in Toxoplasma tac hyzoites and cofractionated precisely with the peak of KT5926-sensitive pro tein kinase activity. TgCDPK1 kinase activity was calcium-dependent but did not require calmodulin or phospholipids. TgCDPK1 was found to be inhibited effectively by KT5926 at concentrations that block parasite attachment to host cells. In vitro, TgCDPK1 phosphorylated three parasite proteins that m igrated identical to the three KT5926-sensitive phosphoproteins detected in vivo, Based on these observations, a central role is suggested for TgCDPK1 in regulating Toxoplasma motility and host cell invasion.