This review examines the functions of the adenovirus (Ad) E4 gene, which encodes multiple proteins with critical roles in viral replication and host cell interactions. The E4 region generates at least 18 distinct mRNAs through alternative splicing, encoding seven different polypeptides. E4 Orf3 and Orf6 are essential for efficient viral gene expression and DNA replication, facilitating nuclear RNA transport and stability, particularly for late viral transcripts. Orf6 forms a complex with the E1B 55K protein to enhance mRNA export while suppressing cellular mRNA export. Additionally, Orf3 directs reorganization of nuclear POD structures containing transcription and replication factors. E4 Orf6 independently inactivates the tumor suppressor p53, complementing E1B 55K function. Other E4 proteins include Orf6/7, which modulates E2F transcription factors; Orf4, which regulates phosphorylation via protein phosphatase 2A; and Orf1, a transforming protein in certain serotypes. The review also discusses E4 Orf6's role in supporting adeno-associated virus (AAV) replication and addresses implications of E4 deletion in developing adenoviral vectors for gene therapy, where E4 status affects transgene persistence in vivo.

Key findings

• E4 Orf3 and Orf6 proteins possess complementary but distinct functions in promoting viral late gene expression and DNA replication through RNA transport and nuclear stabilization mechanisms
• E4 Orf6 independently inactivates p53 in addition to cooperating with E1B 55K, providing redundant tumor suppressor inhibition
• E4 Orf3 directs reorganization of nuclear POD structures containing essential transcription and replication factors during infection
• E4 Orf6 promotes AAV helper function by facilitating AAV second-strand DNA synthesis through mechanisms potentially related to DNA repair
• E4 deletion in adenoviral vectors produces conflicting effects on transgene persistence in vivo, suggesting context-dependent roles in maintaining transgene expression