PHOSPHORYLATION OF ADENOVIRUS-E1A PROTEINS BY THE P34(CDC2) PROTEIN-KINASE

Adenovirus early region 1 A (E1 A) products are phosphorylated nuclear oncoproteins which appear to derive transforming activity largely through interactions with cellular proteins including the tumor suppressor p1051Rb-1 and cyclin A (p60cycA), a regulatory subunit associated with p34cdc2 and the related protein kinase p33cdk2. We have identified several sites of phosphorylation on E1 A proteins previously and showed that phosphorylation at Ser-89 alters electrophoretic mobility significantly and affects E1 A-mediated transforming activity to some extent. We now report that both Ser-89 and Ser-219, the major EiA phosphorylation site, were phosphorylated in vitro by p34cycA purified from HeLa cells. We also found that E1 A proteins seemed to be phosphorylated at the highest levels in vivo in mitotic cells which express maximal levels of p34cycA kinase activity. Thus, in addition to forming complexes with p60cycA, a regulator of p34cycA and related kinases, and p105/Rb-1 which exhibits cell cycle-dependent phosphorylation, E1 A proteins seem to be substrates for p34cycA. These data suggested that a link could exist between phosphorylation, cell cycle progression, and the regulation of transforming activity of E1 A proteins. © 1992.