The Human Adenovirus Type 5 E1B 55-Kilodalton Protein Is Phosphorylated by Protein Kinase CK2

The human adenovirus type 5 (HAdV5) early region 1B 55-kDa protein (E1B-55K) is a multifunctional phosphoprotein playing several critical roles during adenoviral productive infection, e.g., degradation of host cell proteins, viral late mRNA export, and inhibition of p53-mediated transcription. Many of these functions are apparently regulated at least in part by the phosphorylation of E1B-55K occurring at a stretch of amino acids resembling a potential CK2 consensus phosphorylation motif. We therefore investigated the potential role of CK2 phosphorylation upon E1B-55K during adenoviral infection. A phosphonegative E1B-55K mutant showed severely reduced virus progeny production, although viral early, late, and structural protein levels and viral DNA replication were not obviously affected. Binding studies revealed an interaction between the CK2 alpha catalytic subunit and wild-type E1B-55K, which is severely impaired in the phosphonegative El B mutant. In addition, in situ the alpha-catalytic subunit is redistributed into ring-like structures surrounding E1B-55K nuclear areas and distinct cytoplasmic accumulations, where a significant amount of CK2 alpha colocalizes with E1B-55K. Furthermore, in in vitro phosphorylation assays, wild-type E1B-55K glutathione S-transferase fusion proteins were readily phosphorylated by the CK2 alpha subunit but inefficiently phosphorylated by the CK2 holoenzyme. Addition of the CK2-specific inhibitors TBB (4,5,6,7-tetrabromobenzotriazole) and DMAT (2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole) to infected cells confirmed that CK2 alpha binding to E1B-55K is necessary for efficient phosphorylation of E1B-55K. In summary, our data show that CK2 alpha interacts with and phosphorylates HAdV5 E1B-55K at residues S490/491 and T495 and that these posttranslational modifications are essential for E1B-55K lytic functions.