Cell Cycle Arrest by Transforming Growth Factor β1 near G1/S Is Mediated by Acute Abrogation of Prereplication Complex Activation Involving an Rb-MCM Interaction

Understanding inhibitory mechanisms of transforming growth factor β1 (TGF-β1) has provided insight into cell cycle regulation and how TGF-β1 sensitivity is lost during tumorigenesis. We show here that TGF-β1 utilizes a previously unknown mechanism targeting the function of prereplication complexes (pre-RCs) to acutely block S-phase entry when added to cells in late G1 , after most G1 events have occurred. TGF-β1 treatment in early G1 suppresses Myc and CycE-Cdk2 and blocks pre-RC assembly. However, TGF-β1 treatment in late G1 acutely blocks S-phase entry by inhibiting activation of fully assembled pre-RCs, with arrest occurring prior to the helicase unwinding step at G1 /S. This acute block by TGF-β1 requires the function of Rb in late G1 but does not involve Myc/CycE-Cdk2 suppression or transcriptional control. Instead, Rb mediates TGF-β1 late-G1 arrest by targeting the MCM helicase. Rb binds the MCM complex during late G1 via a direct interaction with Mcm7, and TGF-β1 blocks their dissociation at G1 /S. Loss of Rb or overexpression of Mcm7 or its Rb-binding domain alone abrogates late-G1 arrest by TGF-β1. These results demonstrate that TGF-β1 acutely blocks entry into S phase by inhibiting pre-RC activation and suggest a novel role for Rb in mediating this effect of TGF-β1 through direct interaction with and control of the MCM helicase.