P130, P107, AND PRB ARE DIFFERENTIALLY REGULATED IN PROLIFERATING CELLS AND DURING CELL-CYCLE ARREST BY ALPHA-INTERFERON

We have determined how the phosphorylation of the retinoblastoma famil y (pRb, p107, and p130) is governed in individual cell cycle phases of Daudi B-cells during cell cycle exit triggered by alpha-interferon (a lpha-IFN). alpha-IFN causes dephosphorylation of pRb and loss of p130 phosphorylated Form 3. However, the change in p130 phosphorylation in response to alpha-IFN occurs before dephosphorylation of pRb is comple te because loss of p130 Form 3 occurs throughout the cell cycle prior to complete arrest in G(1), whereas pRb is dephosphorylated only in G( 1). In contrast, p107 is dephosphorylated and is then depleted from ce lls as they exit the cell cycle. p130, predominantly in Form 1, and hy pophosphorylated pRb bind an E2F DNA binding site; p130 complexes E2F- 4, whereas pRb binds both E2F-4 and E2F-1, The phosphorylated forms of E2F-4 that bind to the E2F DNA site are different from hyperphosphory lated E2F-4, which predominates in primary hemopoietic cells in G(1). We conclude that although cell cycle arrest induced by alpha-IFN may b e mediated in part by formation of a complex containing p130 and E2F-4 , alpha-IFN does not induce hyperphosphorylation of E2F-4, which chara cterizes primary hemopoietic cells in G(1).