Adenovirus structural protein IIIa is involved in the serotype specificity of viral DNA packaging

The packaging of the adenovirus (Ad) genome into a capsid displays serotype specificity. This specificity has been attributed to viral packaging proteins, the IVa2 protein and the L1-52/55K protein. We previously found that the Ad17L1-52/55K protein was not able to complement the growth of an Ad5L1-52/55K mutant virus, whereas two other Ad17 packaging proteins, IVa2 and L4-22K, could complement the growth of Ad5 viruses with mutations in the respective genes. In this report, we investigated why the Ad17L1-52/55K protein was not able to complement the Ad5L1-52/55K mutant virus. We demonstrate that the Ad17L1-52/55K protein binds to the Ad5 IVa2 protein in vitro and the Ad5 packaging domain in vivo, activities previously associated with packaging function. The Ad17L1-52/55K protein also associates with empty Ad5 capsids. Interestingly, we find that the Ad17L1-52/55K protein is able to complement the growth of an Ad5L1-52/55K mutant virus in conjunction with the Ad17 structural protein IIIa. The same result was found with the L1-52/55K and IIIa proteins of several other Ad serotypes, including Ad3 and Ad4. The Ad17 IIIa protein associates with empty Ad5 capsids. Consistent with the complementation results, we find that the IIIa protein interacts with the L1-52/55K protein in vitro and associates with the viral packaging domain in vivo. These results underscore the complex nature of virus assembly and genome encapsidation and provide a new model for how the viral genome may tether to the empty capsid during the encapsidation process.