THE REQUIREMENTS FOR G1 CHECKPOINT PROGRESSION OF TRYPANOSOMA-BRUCEI S-427 CLONE-1

Trypanosoma brucei S 427 clone 1 accumulated in G(1) when incubated un der growth-limiting conditions. Further incubation of the G(1)-restric ted organisms in medium containing 10% fetal bovine serum (FBS) and 2 mM hydroxyurea resulted in their reversible arrest after a G(1) checkp oint beyond which serum was not required for progress into and through S. Progress of the G(1)-restricted T. brucei through the G(1) checkpo int was linear and required continuous incubation with exogenous serum growth factors. These were principally low and high density lipoprote ins; both lipoproteins triggered G(1) progression in a dose- and time- dependent manner whilst their removal by immunoaffinity chromatography severely reduced the capacity of FBS to stimulate G(1) progression. S erum-induced progress of T. brucei through G(1) was Ca2+-independent, but required gene transcription, protein synthesis, and continuous kin ase activity that was inhibited by tyrphostin 51 and DAPH 1 which typi cally inhibit epidermal growth factor receptor protein tyrosine kinase activity. The tyrphostin 51-sensitive catalytic activity was not requ ired for T. brucei protein synthesis, glycolysis, or S phase progressi on but was required for tyrosine phosphorylation of several polypeptid es, none of which was specifically associated with serum-induced G(1) progression.