Identification of antigenic differences between the phosphorylated and nonphosphorylated forms of the E7 protein of human papillomavirus type 16

To analyze the antigenic properties of the human papillomavirus type 16 E7 oncoprotein, two monoclonal antibodies, VD6 and IB10, that have different reactivities to the E7 protein were generated. While the VD6 antibody reacted strongly with E7 protein in CaSki cell extracts, the other antibody, IB10, showed much weaker reactivity with E7. This reactivity increased in a dose-dependent manner in the presence of the casein kinase II-specific inhibitor DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole). Antigenic site estimation and an in vitro phosphorylation assay, using bacterially expressed E7 protein, demonstrated that the weak reactivity of IB10 was related to the phosphorylation status of the E7 protein. Phosphorylation of E7 reduced considerably the reactivity of IB10 but did not affect the reactivity of VD6, which reacts with the N-terminal portion of E7. In immunoprecipitation (IP) assays, IB10 precipitated weakly the E7 protein from CaSki cell extracts. Together, these data suggest that unphosphorylated E7 protein shows distinct antigenic character compared to its phosphorylated form under denaturing conditions; however, under native conditions, the phosphorylated and nonphosphorylated E7 proteins have some antigenic cross-reactivity.