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

We were surprised to find that the first G1 phase after cell cycle re-entry from quiescence (G0) is routinely followed by an underlicensed S phase in multiple cell lines. To our knowledge, cell cycle re-entry from G0 is the first known naturally-occurring underlicensed cell cycle; previous investigations required siRNA or mutations to artificially force underlicensing (McIntosh and Blow, 2012) . Clearly the extra time in G1 after G0 was not enough to lead to full licensing. Our investigation leads us to conclude that the routine underlicensing in cells re-entering the cycle is the consequence of a combination of slow MCM loading plus an ineffective licensing checkpoint. If MCM loading were fast enough in the first cell cycle, then their checkpoint deficiency wouldn't matter and they would not be underlicensed in the first S phase. Although it is not yet technically possible to monitor MCM loading in real time in individual cells, we used the transition from Cdc6-negative to Cdc6-positive as a proxy for the maximum amount of time available for MCM loading (Fig. 6 ). Since cells in the first G1 phase had more time with abundant nuclear Cdc6 than cells in the second G1 phase, yet began S phase with less MCM loaded, we are confident in our assertion that MCM loading is particularly slow in the first G1 phase.