Figure 1.

 APC activity at different stages of cell cycle. HeLa cells were synchronized at prometaphase by a thymidine–nocodazole block. Cells were collected immediately after nocodazole treatment (lane 3), or released into fresh medium for 0.5, 1, 1.5, 4, and 10 hr (lanes 4–8). Asynchronous cells (lane 1) and cells arrested at G₁/S boundary by a double thymidine block (lane 2) were included as controls. (A) Cell cycle stage was determined by FACS analysis. (Open bars) G₁; (hatched bars) S; (solid bars) G₂/M. (B) Cdc2 kinase was immunopurified from cell lysates and assayed for histone H1 kinase activity. (C) APC was purified from the cell lysates with anti-CDC27/protein A beads and analyzed for its ability to ubiquitinate a ¹²⁵I-labeled amino-terminal fragment of Xenopus cyclin B1.