Regulation of the stability and activity of CDC25A and CDC25B by protein phosphatase PP2A and 14-3-3 binding.

Cyclin-dependent kinase (CDK)-activating phosphatases, CDC25A and CDC25B, are labile proteins, and their levels vary in a cell cycle-dependent manner. Immediate-early response IER5 protein negatively regulates the cellular CDC25B levels, and stress-induced IER5 expression potentiates G2/M arrest. IER5 binds to protein phosphatase PP2A and regulates the PP2A substrate specificity. We show that IER5 binds to CDC25B and assists PP2A to convert CDC25B to hypophosphorylated forms. Hypophosphorylation at Ser323 results in the dissociation of CDC25B from 14-3-3 phospho-binding proteins. In IER5 expressing cells, CDC25B dissociated from 14-3-3 is unstable but slightly activated, because 14-3-3 inhibits CDC25B polyubiquitination and CDC25B binding to CDK1. The 14-3-3 binding to CDC25A also impedes CDC25A degradation and CDC25A-CDK2 interaction. We propose that 14-3-3 is an important regulator of CDC25A and CDC25B and that PP2A/IER5 controls the stability and activity of CDC25B through regulating the interaction of CDC25B and 14-3-3.