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

Our findings raised a larger question about the relationship between origin licensing and S phase entry. The consistently high amount of loaded MCM we observed prior to G1/S in the second and subsequent proliferating cell cycles is consistent with an active origin licensing checkpoint (e.g. Fig. 1A arrow) . The origin licensing checkpoint is thought to delay S phase entry when the amount of loaded MCM is still low in untransformed cells by delaying the activation of G1 Cyclin Dependent Kinases (CDK) (McIntosh and Blow, 2012) . Evidence for this checkpoint is low CDK activity and lengthening of G1 in proliferating cells treated to reduce MCM loading by RNAimediated depletion of MCM loading factors or by overproduction of the MCM loading inhibitor, Geminin (Shreeram et al., 2002; Liu et al., 2009; Nevis et al., 2009) . The origin licensing checkpoint ensures that S phase begins with abundant licensed dormant origins to tolerate replication stress. Because we observed routinely underlicensed cells in the first S phase after G0 and consistently higher licensing in the second cell cycle, we hypothesized that the origin licensing checkpoint is primarily active in only the second and subsequent cell cycles.