Damaged DNA binding protein 1 ( DDB 1 ) interacts with Cdh 1 and modulates the function of APC /

APC/C plays a key role in mitotic exit and has essential targets in the G1 phase; however, these mechanisms are poorly understood. In this report, we provide evidence that DDB1 (Damaged DNA binding protein 1) is capable of binding the WD40 domains of Cdh1, but not of Cdc20, through its BPA and BPC domains. Moreover, cells lacking DDB1 exhibit markedly elevated levels of the protein substrates of APC/C. Depletion of DDB1 in mitotic cells significantly delays mitotic exit, which demonstrates that the interaction between DDB1 and Cdh1 plays a critical role in regulating APC/C activity. However, cells depleted of Cdh1 demonstrated no change in the UV-induced degradation of Cdt1, the main function of DDB1 as an E3 ligase. Strikingly, the APC/C substrate levels are normal in cell knockdowns of Cul4A and Cul4B, which, along with DDB1, form an E3 ligase complex. This finding indicates that DDB1 modulates the function of APC/C in a manner independent on the Cul4-DDB1 complex. Our results suggest that DDB1 may functionally regulate mitotic exit by modulating APC/C activity. This study reveals that there may be crosstalk among DDB1, Cdh1, and Skp2 in the control of cell cycle division. INTRODUCTION The APC/C (anaphase promoting complex/cyclosome) (1,2), a ubiquitin E3 ligase complex, plays crucial roles in late mitosis and during G1 phase by degrading cell cycle related proteins, such as cyclins, Plk1, and Skp2. The temporal cell cycle specificity of APC/C is achieved by activation of two closely related activators, Cdc20 and Cdh1, which facilitate the recruitment of substrates and ubiquitination by the core complex (3-5). Cdc20 and Cdh1 are members of the Cdc20 protein family and contain seven WD40 repeats in their C terminal domains (6). These repeats form a seven-blade propeller structure that mediates protein-protein interactions. In early mitosis, APC/C binding to Cdc20 leads to the initiation of anaphase. In contrast, association with Cdh1 in late http://www.jbc.org/cgi/doi/10.1074/jbc.M109.094144 The latest version is at JBC Papers in Press. Published on April 15, 2010 as Manuscript M109.094144 Copyright 2010 by The American Society for Biochemistry and Molecular Biology, Inc. by gest on Sptem er 1, 2017 hp://w w w .jb.org/ D ow nladed from