Truncation of the human adenovirus type 5 L4 33-kDa protein: Evidence for an essential role of the carboxy-terminus in the viral infectious cycle

The subgroup C human adenovirus L4 33-kDa protein is a nuclear phosphoprotein that plays a direct, but dispensable, role in virion assembly. The r-strand open reading frame (ORF) for this protein lies opposite to the 5′ end of the I-strand E2 early (E2E) transcription units. To facilitate studies of regulation of E2E transcription, we wished to construct a mutant virus in which the 33-kDa ORF was truncated to serve as a background into which specific E2E mutations could be introduced without also altering the 33-kDa protein. We constructed viral DNA (vDNA) containing within the 33-kDa ORF two tandem, premature stop codons that should prevent translation of the C-terminal 47 amino acids of the protein (Δ47). We report here the unanticipated lethality of such truncation of the L4 33-kDa protein. Viral DNA harboring the Δ47 mutations did not produce infectious virus when transfected into cultured cells. In contrast, infectious virus was recovered upon transfection of revertant vDNA, indicating that the Δ47 mutations were responsible for the observed phenotype. The Δ47 mutations did not affect E2E transcription or production of the E2 DNA-binding protein. Transfected Δ47 vDNA was replicated and directed the production of early and late viral proteins, including hexon protein in the trimer conformation. However, no virus particles of any kind were produced. We propose that truncation of the adenovirus 33-kDa protein results in a lethal, late block in the infectious cycle during the assembly of progeny virions and discuss the implications of this phenotype for the mechanism of virion assembly.