Dynamic p21-dependency during quiescence arrest unveiled by a rapid p21 depletion system

p21 inhibits CDK2 activity to induce quiescence in response to stress or developmental stimulation. It is currently unclear whether p21 exhibits an equal functional importance across different stages and states of the quiescence arrest. Here employing a rapid p21 degradation system, we evaluate the contribution of p21 across heterogeneous quiescence arrest states during quiescence progression. Our findings reveal that cells exhibit a dynamics dependency on p21 during quiescence arrest. At low levels of p21, quiescence is exclusively dependent on p21-mediated inhibition of CDK2 activity to prevent cell cycle progression. In contrast, when p21 accumulates to higher levels, quiescence transitions into an "auto-maintenance" state where p21 becomes less essential. Mechanistically, we found an active attenuation of the KRAS/ERK signalling pathway as a driver of reduced proliferation potential in this "auto-maintenance" state. This attenuation reinforces the robustness of quiescence through a mechanism that is independent of p21. Our results thus support a dynamic, adaptive mechanism for quiescence regulation that synchronizes the anti- and pro-proliferation signals. This mechanism is applicable over various stress or developmental quiescence context, offering a basis for cells to explore distinct quiescence states to achieve different degrees of robustness in cell cycle arrest.