DNA replication specificity and functional E2 interaction of the E1 proteins of human papillomavirus types 1a and 18 are determined by their carboxyl-terminal halves

Replication of most papillomaviruses (PVs) requires the viral-encoded E1 and E2 proteins that bind to the origin of replication (ori) containing the E1- and E2-binding sites and help recruit host replication factors during the initiation of DNA replication. We studied the ability of heterologous E1 and E2 proteins to interact in vivo and support replication, using the human papillomavirus (HPV) types 1a and 18 as model systems. The E1 protein of HPV- 1a in combination with HPV-18 E2 supported high-level replication of various ori plasmids. In contrast, the HPV-18 E1 protein interacted weakly with HPV- 1a E2 during the replication of ori plasmids. We have previously shown that the E1 protein of HPV-1a alone is sufficient for replication of HPV-1a ori plasmids, whereas HPV-18 replication requires both the E1 and E2 proteins. However, in the latter case, E2-binding sites alone in the absence of the E1- binding site can function as the minimal ori. Based on the above observations, we generated hybrids between HPV-1a and HPV-18 E1 proteins in an effort to identify their 'replication specificity' domains using a transient replication assay. These hybrids were also used to localize the domains in the E1 proteins that are involved in their functional interaction with the E2 protein during replication. Our results suggest that the 'replication specificity' and functional E2 interaction domains of the HPV-1a and HPV-18 E1 proteins are located in their carboxyl-terminal halves.