The E7 proteins encoded by the human papillomaviruses (HPVs) associated with anogenital lesions share significant amino acid sequence homology. The E7 proteins of these different HPVs were assessed for their ability to form complexes with the retinoblastoma tumor suppressor gene product (p105‐RB). Similar to the E7 protein of HPV‐16, the E7 proteins of HPV‐18, HBV‐6b and HPV‐11 were found to associate with p105‐RB in vitro. The E7 proteins of HPV types associated with a high risk of malignant progression (HPV‐16 and HPV‐18) formed complexes with p105‐RB with equal affinities. The E7 proteins encoded by HPV types 6b and 11, which are associated with clinical lesions with a lower risk for progression, bound to p105‐RB with lower affinities. The E7 protein of the bovine papillomavirus type 1 (BPV‐1), which does not share structural similarity in the amino terminal region with the HPV E7 proteins, was unable to form a detectable complex with p105‐RB. The amino acid sequences of the HPV‐16 E7 protein involved in complex formation with p105‐RB in vitro have been mapped. Only a portion of the sequences that are conserved between the HPV E7 proteins and AdE1A were necessary for association with p105‐RB. Furthermore, the HPV‐16 E7‐p105‐RB complex was detected in an HPV‐16‐transformed human keratinocyte cell line.

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The early human papillomavirus type 16 genes that directly participate in the in vitro transformation of primary human keratinocytes have been defined. In the context of the full viral genome, mutations in either the E6 or E7 open reading frame completely abrogated transformation of these cells. Mutations in the El, E2, and E2-E4 open reading frames, on the other hand, had no effect. Thus, both the full-length E6 and E7 genes were required for the induction of keratinocyte immortalization and resistance to terminal differentiation. The E6 and E7 genes expressed together from the human ,-actin promoter were sufficient for this transformation; mutation of either gene in the context of this recombinant plasmid eliminated the ability to induce stable differentiation-resistant transformants.

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