Fig. (4) Phosphorylation of HPV-16 E7 protein. CaSki cells were starved for 16 h, labeled with [³²P]-ortophosphate and harvested at different periods of time. Cell extracts were immunoprecipitated with the anti-E7 polyclonal C89 antibody or with control rabbit serum. The precipitates were separated in a 15% acrylamide gel and visualized by autoradiography. The arrow shows the phosphorylated E7 protein of 17.5 kDa.