Induction of pRb degradation by the human papillomavirus type 16 E7 protein is essential to efficiently overcome p16INK4a-imposed G1 cell cycle arrest

It has previously been shown that the E7 protein from the cutaneous human papillomavirus type 1 (HPV1), which is associated with benign skin lesions, binds the product of the tumor suppressor gene retinoblastoma (pRb) with an efficiency similar to that of the E7 protein from the oncogenic HPV type 16. Despite this ability, HPV1 E7 does not display any activity in transforming primary cells. In addition, the two viral proteins differ in their mechanisms of targeting pRb. HPV16 E7 promotes pRb destabilization, while cells expressing HPV1 E7 do not show any decrease in pRb levels. In this study, we show that HPV1 E7, in contrast to HPV16 E7, has only a weak activity to neutralize the effect of cyclin-dependent kinase inhibitor p16(INK4a). By generation of HPV1/16 E7 chimeric proteins, we have identified a central motif in the two E7 proteins, which determines their different abilities to overcome the p16(INK4a)-mediated cell cycle arrest. This motif is located downstream of the pRb-binding domain and comprises only three amino acids in HPV16 E7. Swapping this central motif in the two viral proteins causes an exchange of their activities involved in circumventing the inhibitory function of p16(INK4a). Most importantly, our data show that the efficiency of the E7 proteins in neutralizing the inhibitory effect of p16(INK4a) correlates with their ability to promote pRb degradation.