Leucine residues in conserved region of 33K protein of bovine adenovirus - 3 are important for binding to major late promoter and activation of late gene expression

- Anti-33K sera specific for BAdV-3 33K protein detects protein(s) in the cells transfected with plasmid DNAs containing BAdV-3 22K.
- Anti-22K serum detects protein only in the cells transfected with plasmid DNA expressing splice-site variant of BAdV-3 22K.
- Unlike BAdV-3 22K, BAdV-3 33K stimulate the transcription from adenovirus major late promoter (MLP).
- Amino acids 201-240 of C-terminus conserved region of BAdV-3 33K are required for the activation of MLP.
- Leucine residues (217, 224, 232 and 240) of the conserved leucine zipper appear to be required for the binding of BAdV-3 33K to MLP.

The L6 region of bovine adenovirus 3 (BAdV-3) encode 33K (spliced) and 22K (unspliced) proteins. Earlier, anti-33K serum detected five major and three minor proteins in BAdV-3 infected cells. Here, we demonstrate that anti-sera raised against L6-22K protein detected two proteins of 42 and 37 kDa in BAdV-3 infected cells and one protein of 42 kDa in transfected cells expressing splice-site variant 22K protein (pC.22K containing substituted splice acceptor/donor sequence). Unlike 22K, 33K stimulated the transcription from the major late promoter (MLP) by binding to the downstream sequence elements (DE). Analysis of the variant proteins demonstrated that amino acids 201-240 of the conserved C-terminus of 33K containing the potential leucine zipper and RS repeat are required for the activation of MLP. Furthermore, amino acid substitution analysis demonstrated that unlike arginine residues of RS repeat, the leucine residues (217, 224, 232 and 240) of the conserved leucine zipper appear required for the binding of 33K to the MLP.