Intrinsic checkpoint deficiency during cell cycle re-entry from quiescence

Jacob Peter Matson; Amy M. House; Gavin D. Grant; Huaitong Wu; Joanna Perez; Jeanette Gowen Cook

Finally, we predicted that since the second cell cycle has a robust origin licensing checkpoint, bypassing the checkpoint and artificially shortening the second G1 would induce a phenotype similar to the underlicensed first cell cycle. To test this prediction, we overproduced Cyclin E1 as cells approached the second cell cycle to shorten the second G1 phase and bypass the checkpoint. We then compared licensing in the second S phase in these Cyclin E1-overproducing cells to both control second S phase and first S phase cells (Fig. 7F , 7G, and Fig. S4G ). The second cell cycle with a bypassed licensing checkpoint strongly resembled the first cell cycle (compare green and orange lines, Fig. 7H , fold change in Fig. 7I , and percentage of underlicensed cells in Fig. 7J ). Taken together, we conclude that cells re-entering the cell cycle from G0 are routinely underlicensed, but that the second and subsequent cell cycles are fully licensed because of a robust p53-dependent checkpoint that ultimately controls the timing of Cyclin E/CDK2-mediated S phase entry.