Two opposing enzymatic reactions control the activity of the retinoblastoma tumour suppressor protein, pRB. Phosphorylation inactivates pRB's ability to sequester miscellaneous cellular proteins, mostly involved in regulating gene transcription, whereas pRB dephosphorylation restores this ability. For some time now it has been suspected that members of the cyclin/cyclin-dependent kinase (cyclin/cdk) family mediate pRB inactivation. Recent results indicate that pRB phosphorylation is not executed by a single kinase but by a combination of cyclin/cdks, each one phosphorylating a subset of pRB's phosphorylation sites. The different kinases appear to be activated by growth factors through distinct signal transduction pathways. This lends itself to an attractive model whereby pRB phosphorylation may constitute an integration point for these signalling pathways, perhaps allowing cell cycle progression only when concurrent activation of these signalling pathways has been achieved.
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