Published by Blackwell Pub. on behalf of the Federation of European Biochemical Societies
FEBS Journal vol:280 pages:627-643
The family of Retinoblastoma/pocket proteins is one of the master regulators of the eukaryotic cell cycle. It includes the retinoblastoma protein (Rb) and the related p107 and p130 proteins. The importance of the Rb pathway for homeostasis and tumor suppression is evident from the fact that inactivating mutations in Rb are frequently associated with many cancers. Retinoblastoma proteins regulate the cell cycle by controlling activity of the E2F family of transcription factors. The activity of Rb proteins themselves is modulated by their phosphorylation status at several Ser/Thr residues: phosphorylation by cyclin-dependent kinases (CDKs) inactivates Rb proteins and positively influences the transcription of genes necessary for cell cycle progression. While the mechanisms of CDK-mediated inactivation of Rb proteins are understood in great detail, our knowledge of the process that counteracts Rb phosphorylation is still quite limited. This review focuses on the Ser/Thr phosphatases that are responsible for dephosphorylation and thus activation of Rb proteins. Two major scenarios are considered: 1) when pocket proteins are dephosphorylated during regular cell cycle progression and 2) when rapid dephosphorylation is dictated by external stress or growth inhibitory conditions, such as oxidative stress, UV radiation or other DNA-damaging stimuli, and cell differentiation factors. It turns out that both PP1 and PP2A phosphatases can efficiently modulate pocket protein activity in a highly context-dependent manner and are tightly regulated by the presence of different regulatory subunits or interacting proteins.