Fig. 5. Cell cycle-dependent PCTAIRE-1 kinase activity in HeLa cells. HeLa cells were synchronized by a double thymidine block. Cell cycle progression was monitored by flow cytometric analysis (A) and by the pattern of cyclin A (B) and cyclin B1 (C) expression. D, protein kinase activity was determined upon immunoprecipitation with anti-PCTAIRE-1 antibody as described in "Materials and Methods." G1 (12 h after release), Lane 1; G1-S phase (0 h), Lane 2; S phase (4 h after release), Lane 3; G2 (8 h after release), Lane 4. S-phase extracts immunoprecipitated without an antibody (Lane 5) or with an anti-Cdk2 antibody (Lane 6) were used as negative and positive controls, respectively.