Human papillomavirus type 16 E1≈E4 contributes to multiple facets of the papillomavirus life cycle

The life cycle of human papillomaviruses (HPVs) is tightly linked to the differentiation program of the host's stratified epithelia that it infects. E1<^>E4 is a viral protein that has been ascribed multiple biochemical properties of potential biological relevance to the viral life cycle. To identify the role(s) of the viral E1<^>E4 protein in the HPV life cycle, we characterized the properties of HPV type 16 (HPV16) genomes harboring mutations in the E4 gene in NIKS cells, a spontaneously immortalized keratinocyte cell line that when grown in organotypic raft cultures supports the HPV life cycle. We learned that E1<^>E4 contributes to the replication of the viral plasmid genome as a nuclear plasmid in basal cells, in which we also found E1<^>E4 protein to be expressed at low levels. In the suprabasal compartment of organotypic raft cultures harboring E1<^>E4 mutant HPV16 genomes there were alterations in the frequency of suprabasal cells supporting DNA synthesis, the levels of viral DNA amplification, and the degree to which the virus perturbs differentiation. Interestingly, the comparison of the phenotypes of various mutations in E4 indicated that the E1<^>E4 protein-encoding requirements for these various processes differed. These data support the hypothesis that E1<^>E4 is a multifunctional protein and that the different properties of E1<^>E4 contribute to different processes in both the early and late stages of the virus life cycle.