CDC6 as A Key Inhibitory Regulator of CDK1 Activation Dynamics and The Timing of Mitotic Entry and Progression

Abstract

Timely mitosis is critically important for the early embryo development. It is regulated by the activity of the ubiquitously conserved CDK1 kinase. The dynamics of CDK1 activation must be precisely controlled to assure physiologic and timely entry into mitosis. Recently, a known S-phase regulator CDC6 emerged as a key player in the mitotic CDK1 activation cascade in early embryonic divisions, operating together with Xic1 as a CDK1 inhibitor upstream of the Aurora A- and PLK1 kinases, both CDK1 activators. Herein we review the molecular mechanisms that underlie the control of the mitotic timing, with special emphasis on how CDC6/Xic1 function impacts CDK1 regulatory network. We focus on the presence of two independent mechanisms inhibiting the dynamics of CDK1 activation: Wee1/Myt1- and CDC6/Xic1-dependent, and how they cooperate with CDK1 activating mechanisms. As a result we propose a comprehensive model integrating CDC6/Xic1-dependent inhibition into the CDK1-activation cascade. The physiological dynamics of CDK1 activation appear to be tuned by the system of multiple inhibitors and activators and their integrated modulation ensures concomitantly both the robustness and certain flexibility of the control of this process. Identification of multiplied activators and inhibitors of CDK1 activation upon M-phase entry allows a better understanding of why cells divide at a specific time, and how the pathways involved in the timely regulation of the cell division are all integrated to precisely tune the control of mitotic events.