mRNA splicing regulates human papillomavirus type 11 E1 protein production and DNA replication

The papillomavirus replicative helicase E1 and the origin recognition protein E2 are required for efficient viral DNA replication. We fused the green fluorescent protein (GFP) to the human papillomavirus type 11 E1 protein either in a plasmid with the E1 coding region alone (nucleotides [nt] 832 to 2781) (pGFP-11E1) or in a plasmid containing both the E1 and E2 regions (nt 2723 to 3826) and the viral origin of replication (ori) (p11Rc). The former supported transient replication of an ori plasmid, whereas the latter was a self-contained replicon. Unexpectedly, these plasmids produced predominantly a cytoplasmic variant GFP or a GFP-E1 E4 protein, respectively. The majority of the mRNAs had an intragenic or intergenic splice from nt 847 to nt 2622 or from nt 847 to nt 3325, corresponding to the E2 or E1 E4 messages. pGFP-11E1dm and p11Rc-E1dm, mutated at the splice donor site, abolished these splices and increased GFP-E1 protein expression. Three novel, alternatively spliced, putative E2 mRNAs were generated in higher abundance from the mutated replicon than from the wild type. Relative to pGFP-11E1, low levels of pGFP-11E1dm supported more efficient replication, but high levels had a negative effect. In contrast, elevated E2 levels always increased replication. Despite abundant GFP-E1 protein, p11Rc-E1dm replicated less efficiently than the wild type. Collectively, these observations show that the E1/E2 ratio is as important as the E1 and E2 concentrations in determining the replication efficiency. These findings suggest that alternative mRNA splicing could provide a mechanism to regulate E1 and E2 protein expression and DNA replication during different stages of the virus life cycle.