Calcium-dependent protein phosphorylation in Babesia bovis and its role in growth regulation.

Intracellular growth of protozoan parasite Babesia bovis has been followed to study the effect of some chemical agents on growth regulation. Using an in vitro parasite culture system we present evidence that the normal growth of the parasite is dependent upon available calcium and a Ca2(+)-binding protein, calmodulin, because sequestration of either of these 2 components from the culture medium causes inhibition of parasitic growth. Further studies demonstrate that the parasite contains a protein kinase that can phosphorylate a 40-kDa parasitic protein and its activity is regulated by calcium and calmodulin. Both the enzyme and its substrate are present in the membrane of the parasite. In addition, the parasite also contains a highly active protein kinase C activity that is documented by phosphorylating histone, a known substrate for protein kinase C. These findings suggest a possible correlation between the growth of parasite and calcium/calmodulin-dependent protein phosphorylation activity.