Induction of pRb degradation by the human papillomavirus type 16 E7 protein is essential to efficiently overcome p16(INK4a)-imposed G(1) cell cycle arrest

It has previously been shown that the E7 protein from the cutaneous human p apillomavirus 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 1 6. Despite this ability, HPV1 E7 does not display any activity in transform ing primary cells. In addition, the two viral proteins differ in their mech anisms 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 g eneration of HPV1/16 E7 chimeric proteins, we have identified a central mot if in the two E7 proteins, which determines their different abilities to ov ercome the p16(INK4a)-mediated cell cycle arrest. This motif is located dow nstream of the pRb-binding domain and comprises only three amino acids in H PV16 E7. Swapping this central motif in the two viral proteins causes an ex change of their activities involved in circumventing the inhibitory functio n 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.