Growth regulation in the protozoan parasite Trypanosoma brucei: Evidence of tyrosine-specific protein kinase activity

Phosphorylation of proteins at tyrosine is an important mechanism for regulating cell growth and proliferation in metazoan organisms. This phosphorylation is conducted by a tyrosine-specific protein kinase. The activation of tyrosine kinases is an important step in transducing extracellular signals to a cell's interior. Until recently, protozoan organisms were thought not to possess tyrosine-specific protein kinases. This investigation demonstrates that Trypanosoma brucei, a protozoan parasite, has a tyrosine-specific protein kinase that plays a role in regulation of proliferation of this protozoan. Genistein, a tyrosine kinase inhibitor, prevented multiplication of the parasite and caused the trypanosomes to accumulate in the G2 or M phase of the cell cycle. An in vitro kinase assay demonstrated the presence of a kinase capable of phosphorylating an exogenous substrate at tyrosine, and genistein was able to reduce trypanosome-mediated phosphorylation of this substrate. Two-dimensional, thin-layer electrophoresis of alkali-hydrolyzed ($\sp{32}$P) -labeled total trypanosome protein revealed the presence of phosphotyrosine. An immunoblot of trypanosome proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and incubated with an anti-phosphotyrosine monoclonal antibody revealed the presence of a tyrosine phosphoprotein. An alkali digestion of ($\sp{32}$P) -labeled trypanosome proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated several additional proteins that may be phosphorylated at tyrosine. The phosphorylation of some of these proteins was sensitive to the inhibitor genistein. These results indicate that T. brucei has a tyrosine kinase that is involved in proliferation or growth regulation of the parasite and provide further evidence for the possibility of growth factor regulation and signal transduction in trypanosomes.U of I OnlyETDs are only available to UIUC Users without author permissio