The tick-transmitted protozoan parasites Theileria annulata and Theileria parva are important intracellular pathogens of domestic cattle in tropical and sub-tropical regions. Proliferative phases take place within both lymphocytes and erythrocytes. The lymphocyte is stimulated to enter the cell cycle by the parasite and the multinucleate parasite can establish a state in which karyokinesis and cytokinesis occur in phase with the host cell. The link between parasite nuclear division and cytokinesis is altered during the formation of merozoites (a non-dividing, invasive, extracellular stage). These features imply a high degree of control over parasite nuclear division and cytokinesis. Two different approaches have been used to identify clones from both species which are extremely highly conserved homologues. These encode a cdc2-related kinase which is > 60% identical to eukaryotic cyclin-dependent kinases of the p34cdc2/p32CDK2 subfamily. There is typical conservation of kinase domains, implying an in vivo protein kinase activity for the polypeptide. The PSTAIRE region, implicated in cyclin binding, is well conserved suggesting that ThCRK2 will bind cyclin molecules closely related to the eukaryotic A/B-type cyclins. However, there is divergence in certain key motifs potentially associated with binding of molecules that regulate the activity of the kinase. Expression patterns of RNA and protein indicate that ThCRK2 is likely to function in all dividing stages of the parasite and, taken together, the results point to a central role in the regulation of nuclear division.
