LAUSR

Cell cycle progression and genome stability are regulated by a ubiquitin ligase, the anaphase‐promoting complex/cyclosome (APC/C). Cyclin‐dependent kinase 1 (Cdk1) has long been implicated in APC/C activation; however, the molecular mechanisms of governing this process in vivo are largely unknown. Recently, a Cdk1‐dependent phosphorylation relay within Apc3‐Apc1 subunits has been shown to alleviate Apc1‐mediated auto‐inhibition by which a mitotic APC/C co‐activator Cdc20 binds to and activates the APC/C. However, the underlying mechanism for dephosphorylation of Cdc20 and APC/C remains elusive. Here, we show that a disordered loop domain of Apc1 (Apc1‐loop500) directly binds the B56 regulatory subunit of protein phosphatase 2A (PP2A) and stimulates Cdc20 loading to the APC/C. Using the APC/C reconstitution system in Xenopus egg extracts, we demonstrate that mutations in Apc1‐loop500 that abolish B56 binding decrease Cdc20 loading and APC/C‐dependent ubiquitylation. Conversely, a non‐phosphorylatable mutant Cdc20 can efficiently bind the APC/C even when PP2A‐B56 binding is impeded. Furthermore, PP2A‐B56 preferentially dephosphorylates Cdc20 over the Apc1 inhibitory domain. These results indicate that Apc1‐loop500 plays a role in dephosphorylating Cdc20, promoting APC/C‐Cdc20 complex formation in mitosis.