Degradation of human RAP80 is cell cycle regulated by Cdc20 and Cdh1 ubiquitin ligases

Receptor-associated protein 80 (RAP80) is a component of the BRCA1-A complex that recruits BRCA1 to DNA damage sites in the DNA damage-induced ubiquitin signaling pathway. RAP80-depleted cells showed defective G ₂-Mphase checkpoint control. In this study, we show that RAP80 protein levels fluctuate during the cell cycle. Its expression level peaked in the G₂ phase and declined during mitosis and progression into the G₁ phase. Also, RAP80 is polyubiquitinated and degraded by the anaphase-promoting complex (APC/C)^Cdc20 or (APC/C)^Cdh1. Consistent with this, knockdown of Cdc20 or Cdh1 expression by transfecting with small interfering RNAs blocked RAP80 degradation during mitosis or the G ₁ phase, respectively. A conserved destruction box (D box) in RAP80 affected its stability and ubiquitination, which was dependent on APC/cyclosome^Cdc20 (C^Cdc20) or APC/cyclosome ^Cdh1(C^Cdh1). In addition, overexpression of RAP80 destruction box1 deletion mutant attenuated mitotic progression. Thus, APC/C^Cdc20 or APC/C^Cdh1 complexes regulate RAP80 stability during mitosis to the G₁ phase, and these events are critical for a novel function of RAP80 in mitotic progression.