Genetic analysis of a potential zinc-binding domain of the adenovirus E4 34k protein

E4 34k, the product of adenovirus early region 4 (E4) open reading frame 6, modulates viral late gene expression, viral DNA replication, apoptosis, double strand break repair, and transformation through multiple interactions with components in infected and transformed cells. Conservation of several cysteine and histidine residues among E4 34k sequences from a variety of adenovirus serotypes suggests the presence of a zinc binding domain important for function. Consistent with the hypothesis that E4 34k is a zinc metalloprotein, zinc binding by baculovirus-expressed E4 34k protein was demonstrated in a zinc blotting assay. To investigate the relationship between the potential zinc-binding region and E4 34k function, a series of mutant genes containing single amino acid substitutions at each of the conserved cysteine and histidine residues in E4 34k were constructed. The mutant proteins were examined for the ability to complement the late protein synthetic defect of an E4 deletion mutant, to physically interact with the viral E1b 55-kDa protein (E1b 55k) and cellular p53 protein, to relocalize E1b 55k, and to destabilize the p53 protein. These analyses identified a subset of cysteine and histidine residues required for stimulation of late gene expression, physical interaction with E1b 55k, and p53 destabilization. These data suggest that a zinc-binding domain participates in the formation of the E4 34k-E1b 55k physical complex and that the complex is required in late gene expression and for p53 destabilization.     

Human papillomavirus type 18 E7 protein requires intact Cys-X-X-Cys motifs for zinc binding, dimerization, and transformation but not for Rb binding.

Human papillomavirus type 18 (HPV-18) E7 proteins bind zinc through Cys-X-X-Cys repeats located at the C terminus of the protein. In order to examine the role of these cysteine motifs in E7 function, we expressed the HPV-18 E7 protein in bacteria and found that purified E7 forms a dimer through interactions with zinc. Mutants with single mutations within the Cys-X-X-Cys motifs bound a reduced level of zinc in a zinc blot assay, while a double mutant lost all zinc-binding activity. When expressed in vivo, none of the mutants cooperated with an activated ras oncogene to transform primary rat embryo fibroblasts, but all mutants retained nearly wild-type Rb-binding activity. The results indicate that the cysteine motifs play an important role in transformation by HPV-18 E7 but do not contribute to Rb binding.