The N Terminus of Adenovirus Type 12 E1A Inhibits Major Histocompatibility Complex Class I Expression by Preventing Phosphorylation of NF-κB p65 Ser 276 through Direct Binding

The immune-escape strategy employed by human oncogenic adenovirus type 12 (Ad12) involves downregulation of major histocompatibility complex class I (MHC-I) transcription by disabling the transactivator NF-κB (p50/p65). This is accomplished by the Ad12 E1A protein (E1A-12), which prevents NF-κB from becoming phosphorylated by the protein kinase A catalytic subunit (PKAc). In this study, we examined the interactions between E1A-12 and NF-κB. Our data show that an E1A-12 mutant retaining the N-terminal 66 amino acids was as effective as the wild-type E1A-12 protein (266 amino acids) in binding p65, preventing phosphorylation of p65-Ser 276 , and inhibiting transactivation. In contrast, the nontumorigenic adenovirus type 5 E1A protein (E1A-5) and other E1A-12 mutants lacking the N-terminal regions were severely defective in these activities. Further studies revealed that an N-terminal peptide consisting of residues 1 to 40 of E1A-12 was able to associate directly with p65 in vitro and prevent PKAc from phosphorylating p65-Ser 276 . In the absence of the N terminus, there is an almost complete loss of E1A-12 binding to p65. These findings provide solid evidence for the role of the E1A-12 N terminus as an NF-κB binding domain. Significantly, this study indicates that the E1A-12 N terminus prevents PKAc from gaining access to p65 to account for Ser 276 hypophosphorylation. The E1A-12 N terminus interaction with p65 serves as a key explanation of how Ad12 downregulates MHC-I transcription and contributes to oncogenesis by escaping cytotoxic T lymphocytes.