Purification and characterization of human papillomavirus type 16 E7 protein with preferential binding capacity to the underphosphorylated form of retinoblastoma gene product.

Human papillomavirus type 16 E7 is considered to be a major viral oncoprotein playing an important role(s) in cervical cancers. E7 protein was shown to bind to the protein product of the retinoblastoma gene (RB), while simian virus 40 large T and adenovirus E1A were also shown to possess binding activity to RB protein. The RB protein is a cell cycle regulator that is highly phosphorylated specifically in S, G2, and M, whereas it is underphosphorylated in G0 and G1. Recently, large T was demonstrated to bind preferentially to the underphosphorylated RB protein, which is considered to be an active form restricting cell proliferation. However, it is not known whether E7 can bind to phosphorylated RB protein. We successfully purified large quantities of unfused human papillomavirus type 16 E7 protein expressed in Escherichia coli by using a T7 promoter-T7 RNA polymerase system. The purified E7 protein was demonstrated to bind preferentially to the underphosphorylated RB protein.