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Protein phosphatase 2A (PP2A) is a family of multimeric protein phosphatases containing distinct regulatory subunits. The regulatory subunits target the core enzyme to individual phosphoprotein substrates. The PR70 subunit is a member of the PPP2R3/PR72 family of PP2A regulatory subunits. PR70 interacts with the Cdc6 protein, which is involved in the initiation of DNA replication. PR70 contains two EF-hand calcium binding motifs. Calcium binding to PR70 enhances the interaction with the core enzyme. The interaction of PR70 with Cdc6 is not affected by calcium, but calcium causes enhanced recruitment of the core dimer to Cdc6. Cdc6 is stabilized during the G1 phase of the cell cycle by phosphorylation of its N-terminal domain by cyclin E-CDK2, which inhibits degradation mediated by the APC/Ccdh1 ubiquitin ligase. Overexpression and RNA interference studies showed that PR70 can regulate the stability of Cdc6 in a phosphorylation-dependent manner. The data suggest that targeting of PP2A by the PR70 subunit is involved in regulating the initiation of DNA replication.

The PPP2R5/B56 family of regulatory subunits is involved in targeting PP2A to a variety of signaling pathways. We have found that two members of the family, B56γ and B56δ, are involved in targeting PP2A to ribosomal protein S6 protein kinase (S6 kinase), which is an important downstream effector of the target of rapamycin (TOR) protein kinase. Co-immunoprecipitation experiments showed that the scaffold (A) and catalytic (C) subunits of PP2A interact with S6 kinase in a manner regulated by TOR activity. Exogenously expressed B56γ and B56&delta:, but not other members of the B56 family, also co-precipitate with S6 kinase and endogenous B56δ is present in S6 kinase complexes. RNA interference experiments showed that knockdown of B56δ, and to a lesser extent B56γ, resulted in loss of the A and C subunits from S6 kinase complexes. The results indicate that these subunits mediate an interaction of PP2A with S6 kinase and suggest that PP2A plays an important role in the TOR signaling pathway.