E1A blocks hyperphosphorylation of p130 and p107 without affecting the phosphorylation status of the retinoblastoma protein

The phosphorylation status of the pRB family of growth suppressor proteins is regulated in a cell cycle entry-, progression-, and exit-dependent manne r in normal cells, We have shown previously that p130, a member of this fam ily, exhibits patterns of phosphorylated forms associated with various cell growth and differentiation stages. However, human 293 cells, which are tra nsformed cells that express the adenoviral oncoproteins E1A and E1B, exhibi t an abnormal pattern of p130 phosphorylated forms. Here we report that, un like pRB, the phosphorylation status of both p130 and p107 is not modulated during the cell cycle in 293 cells as it is in other cells. Conditional ov erexpression of individual G(1)/S cyclins in 293 cells does not alter the p hosphorylation status of p130, suggesting that the expression of ELA and/or E1B blocks hyperphosphorylation of p130. In agreement with these observati ons, transient cotransfection of vectors expressing E1A 12S, but not E1B, i n combination with pocket proteins into U-2 OS cells blocks hyperphosphoryl ation of both p130 and p107. However, the phosphorylation status of pRB is not altered by cotransfection of E1A 12S vectors. Moreover, MC3T3-E1 preost eoblasts stably expressing E1A 12S also exhibit a block in hyperphosphoryla tion of endogenous p130 and p107, Direct binding of EIA to p130 and p107 is not required for the phosphorylation block since EIA 12S mutants defective in binding to the PRE family also block hyperphosphorylation of p130 and p 107, Our data reported here identify a novel function of EIA, which affects p130 and p107 but does not affect pRB, Since EIA does not bind the hyperph osphorylated forms of p130, this function of EIA might prevent the existenc e of "free" hyperphosphorylated p130, which could act as a CDK inhibitor.