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

The origin licensing checkpoint protects cells from premature S phase entry that could lead to genome instability (Fig. 8 ). This checkpoint couples the activity of CDK to the status of MCM loading such that the G1 CDKs are not activated while MCM loading is still low. Cyclin E1 overproduction can bypass the checkpoint-induced delay and induce premature S phase entry while cells are still underlicensed (Fig. 3 ) (Ekholm-Reed et al., 2004; Matson et al., 2017) . Moreover, p53 loss clearly impairs the normal coupling of MCM loading and S phase entry whereas p53 activation lengthens G1 to give enough time to complete licensing ( Fig. 4 , 5, and 7). These observations fit Hartwell and Weinert's classic definition for a cell cycle checkpoint (Hartwell and Weinert, 1989) . A checkpoint enforces the dependence of one event on a previous event, but the dependency can be bypassed by mutation to occur out of order. The The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/558783 doi: bioRxiv preprint ability of mutations to induce inappropriate cell cycle progression indicates that in wildtype cells, progression could have occurred but was instead restrained by the checkpoint. This relationship is in contrast to a simple inability to progress. Of relevance to the origin licensing checkpoint, we induced S phase entry when the amount of loaded MCM was low, but not completely absent, since RNAi-mediated depletion of MCM loading factors does not prevent all MCM loading. If we could have entirely prevented MCM loading, then of course, there would have been no DNA synthesis even in p53 null or Cyclin E-overproducing cells because MCM is an essential component of the replicative helicase.