Abstract
Dysregulation of cyclin-dependent kinases (CDKs) impacts cell proliferation, driving cancer. Here, we ask why the cyclin-D/CDK4 complex governs cell cycle progression through the longer G1 phase, whereas cyclin-E/CDK2 regulates the shorter G1/S phase transition. We consider available experimental cellular and structural data including cyclin-E's high-level burst, sustained duration of elevated cyclin-D expression, and explicit solvent molecular dynamics simulations of the inactive monomeric and complexed states, to establish the conformational tendencies along the landscape of the distinct activation scenarios of cyclin-D/CDK4 and cyclin-E/CDK2 in the G1 phase and G1/S transition of the cell cycle, respectively. These lead us to propose slower activation of cyclin-D/CDK4 and rapid activation of cyclin-E/CDK2. We provide the mechanisms through which this occurs, offering innovative CDK4 drug design considerations. Our insightful mechanistic work addresses a compelling cell cycle regulation question and illuminates the distinct activation speeds between the G1 and the G1/S phases, which are crucial for function.