EARLY PHOSPHORYLATION OF THE RETINOBLASTOMA GENE-PRODUCT REGULATES PROTEIN-BINDING TO THE C-FOS RETINOBLASTOMA CONTROL ELEMENT DURING T-CELL ACTIVATION

Function of the retinoblastoma tumor suppressor protein [pRb] is regul ated by phosphorylation during the G1 and S phases of the cell cycle. pRb regulates transcription of several genes, including c-Sos. However , since c-fos is regulated during exit from G0, it has remained unclea r how pRb participates in c-Sos regulation. We have identified a prote in complex, the retinoblastoma control factor A [RCF-A] which binds to the c-fos retinoblastoma control element [RCE] and is regulated by pR b within 10 min after T cell activation. We demonstrate that pRb contr ol of RCF-A is dependent upon the state of phosphorylation of pRb. pRb becomes hyperphosphorylated on specific peptides al 10 min after mito genic stimulation and pRb is dephosphorylated by 30 min. This time cou rse coincides with RCF-A DNA binding. RCF-A binds RCE DNA longer when cells are treated with okadaic acid, and okadaic acid prevents pRb dep hosphorylation. Dephosphorylated pRb inhibits RCF-A binding in vitro b ut phosphorylated pRb does not. Thus, in addition to the described G1/ S regulation of pRb, transient inactivation by phosphorylation of pRb in T cells may also be important as resting cells leave G0. (C) 1997 E lsevier Science Ltd.