Published on Wed Sep 15 2021

Cdc6 is sequentially regulated by PP2A-Cdc55, Cdc14 and Sic1 for origin licensing in S. cerevisiae

Philip, J., Ord, M., Silva, A., Singh, S., Diffley, J. F. X., Remus, D., Loog, M., Ikui, A. E.

Cdc6, a subunit of the pre-replicative complex, contains multiple regulatory Cdk1 consensus sites, SP or TP motifs. Cdc6 accumulates in mitosis and is tightly bound by Clb2 through N-terminal phosphorylation in order to prevent

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Abstract

Cdc6, a subunit of the pre-replicative complex, contains multiple regulatory Cdk1 consensus sites, SP or TP motifs. In S. cerevisiae, Cdk1 phosphorylates Cdc6-T7 to recruit Cks1, the Cdk1 phospho-adaptor in S-phase, for subsequent multisite phosphorylation and protein degradation. Cdc6 accumulates in mitosis and is tightly bound by Clb2 through N-terminal phosphorylation in order to prevent premature origin licensing and degradation. It has been extensively studied how Cdc6 phosphorylation is regulated by the Cyclin-Cdk1 complex. However, a detailed mechanism on how Cdc6 phosphorylation is reversed by phosphatases has not been elucidated. Here, we show that PP2ACdc55 dephosphorylates Cdc6 N-terminal sites to release Clb2. Cdc14 dephosphorylates the C-terminal phospho-degron, leading to Cdc6 stabilization in mitosis. In addition, the Cdk1 inhibitor, Sic1, releases Clb2-Cdk1-Cks1 from Cdc6 to load Mcm2-7 on the chromatin upon mitotic exit. Thus, pre-RC assembly and origin licensing is promoted by the attenuation of distinct CDK-dependent Cdc6 inhibitory mechanisms.