The Adenovirus L4-22K Protein Has Distinct Functions in the Posttranscriptional Regulation of Gene Expression and Encapsidation of the Viral Genome

The adenovirus L4-22K protein is multifunctional and critical for different aspects of viral infection. Packaging of the viral genome into an empty capsid absolutely requires the L4-22K protein to bind to packaging sequences in cooperation with other viral proteins. Additionally, the L4-22K protein is important for the temporal switch from the early to late phase of infection by regulating both early and late gene expression. To better understand the molecular mechanisms of these key functions of the L4-22K protein, we focused our studies on the role of conserved pairs of cysteine and histidine residues in the C-terminal region of L4-22K. We found that mutation of the cysteine residues affected the production of infectious progeny virus but did not interfere with the ability of the L4-22K protein to regulate viral gene expression. These results demonstrate that these two functions of L4-22K may be uncoupled. Mutation of the histidine residues resulted in a mutant with a similar phenotype as a virus deficient in the L4-22K protein, where both viral genome packaging and viral gene expression patterns were disrupted. Interestingly, both mutant L4-22K proteins bound to adenovirus packaging sequences, indicating that the paired cysteine and histidine residues do not function as a zinc finger DNA binding motif. Our results reveal that the L4-22K protein controls viral gene expression at the posttranscriptional level and regulates the accumulation of the L4-33K protein, another critical viral regulator, at the level of alternative pre-mRNA splicing.