Figure S9. Model; PIP box of Cdt2 promotes rapid clearance of substrate Cdt1 on PCNA^{on DNA}.

Both Cdt1 and CRL4^Cdt2 can bind independently to PCNA^{on DNA} through their PIP boxes, when DNA synthesis starts. Based on the fact that the binding of Cdt1 onto PCNA is transient in cells, whereas Cdt2 interactions are more stable, a model is shown, illustrating that Cdt2 PIP box promotes ubiquitination of Cdt1 on PCNA trimer through multiple steps. CRL4^Cdt2 binds to PCNA^{on DNA} tightly and independently of Cdt1, and catches and localizes Cdt1 when it is recruited to PCNA, forming a stable complex on PCNA trimer for ubiquitination. After poly-ubiquitinated Cdt1 was released, CRL4^Cdt2 catches next Cdt1, creating an efficient ubiquitination cycle.