Characterization of the Varicella-zoster virus ORF25 gene product: pORF25 interacts with multiple DNA encapsidation proteins

The Herpesviridae contain a group of highly conserved proteins designated the Herpes UL33 Superfamily (pfam03581). The Varicella-zoster virus (VZV) homolog, encoded by the ORF25 gene, was used to generate a GST-ORF25 fusion protein. Purified GST-ORF25 was used to generate a polyclonal rabbit antiserum that detected the 17.5 kDa ORF25 protein (pORF25) in VZV infected cells. In pull-down assays, GST-ORF25 interacted with a number of encapsidation proteins including ORF30, ORF42 (the second exon of ORF45/42) and itself. The self-interaction was confirmed via a yeast two-hybrid assay. Additionally, pORF25 and pORF30 were shown to co-immunoprecipitate from VZV infected cells. Our results suggest that pORF25 is part of the trimeric terminase complex for VZV. However, combined with data from previous studies on HSV-1 and Kaposi's sarcoma associated herpesvirus (KSVH), we hypothesize that VZV pORF25 and the Herpes UL33 Superfamily homologs are not encapsidation proteins per se but instead work to bring viral proteins together to form functional complexes.