The E7 protein of human papillomavirus 8 Is a nonphosphorylated protein of 17 kDa and can be generated by two different mechanisms

The E7 protein of human papillomavirus (HPV) 8 shows no in vitro transforming activity, in contrast to E7 of the genital HPVs, but seems to be involved in the control of viral DNA replication. To determine whether functional differences between the E7 proteins of HPV16 and HPV8 were reflected in differences in their biochemical properties, we characterized the E7 protein of HPV8 expressed from the late simian virus 40 promoter in COS 7 cells. An E7-specific antiserum was obtained by immunizing rabbits with a β-gal-E7 fusion protein containing all of the E7 polypeptide. This antiserum specifically precipitated from [ 35S]cysteine but not from 32PO 4-labeled transiently transfected COS 7 cells a protein with a low mobility of 17 kDa in SDS-PAGE. The high sedimentation rate in nondenaturing glycerol gradients pointed to an interaction with other proteins or to the existence of E7 oligomers. An association with the retinoblastoma protein could, however, be excluded. The Fends of HPV8 transcripts derived from the E6–E7 region in G418 selected rodent cells, which were mapped by nuclease S1 digestion, are located upstream of ORFs E6 and E7, respectively. No evidence for an E6∗I-like mRNA was found. In COS 7 cells transfected with a plasmid expressing the E6–E7 region of HPV8 under the control of the late SV40 promoter, however, the E7 protein was translated from a polycistronic mRNA potentially encoding E6 and E7. These data indicate that the E7 protein of HPV8 may be expressed in two ways: (i) through translation of an E7-specific mRNA, as with HPV6 and HPV11, or (ii) through internal initiation of translation at the ATG of E7.