CDC25 dual-specificity phosphatases are essential regulators that activate cyclin-dependent kinases (CDKs) at critical stages of the cell cycle. In human cells, CDC25A and C are involved in the control of G1/S and G2/M respectively, whereas CDC25B is proposed to act both in S phase and G2/M. Evidence for an interaction between CDC25 phosphatases and members of the 14-3-3 protein family has been obtained in vitro and in vivo in several organisms. On the basis of the work performed with CDC25C, it has been proposed that phosphorylation is required to mediate the interaction with 14-3-3. Here we have examined the molecular basis of the interaction between CDC25B phosphatases and 14-3-3 proteins. We show that in the two-hybrid assay all three splice variants of CDC25B interact similarly and strongly with 14-3-3eta, beta and zeta proteins, but poorly with epsilon and Theta. In vitro, CDC25B interacts at a low level with 14-3-3beta, epsilon, zeta, eta, and Theta isoforms. This interaction is not increased upon phosphorylation of CDC25B by CHK1 and is not abolished by dephosphorylation. In contrast, a specific, strong interaction between CDC25B and 14-3-3zeta and eta isoforms is revealed by a deletion of 288 residues in the amino-terminal region of CDC25B. This interaction requires the integrity of Ser 323, although it is independent of phosphorylation. Thus, interaction between 14-3-3 proteins and CDC25B is regulated in a manner that is different from that with CDC25C. We propose that, in addition to a low affinity binding site that is available for all 14-3-3 isoforms, post-translational modification of CDC25B in vivo exposes a high-affinity binding site that is specific for the zeta and eta14-3-3 isoforms.
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