The human papillomavirus E7 protein is phosphorylated at the two serines in positions 31/32, which are part of a consensus sequence for casein kinase II (CKII). In this study, we have investigated the effect of CKII phosphorylation site mutations, all of which lead to unphosphorylated E7 proteins. The replacement of the two serines by uncharged alanine residues drastically reduced the ability of E7 to cotransform primary cells with ras, whereas negatively charged aspartic acid at the same positions produced only a slight effect. This difference was not reflected in the p105Rb binding or the E2 promoter transactivation capability of these two mutants. Mutations that changed the CKII consensus without altering the serine residues also resulted in a loss of phosphorylation and transformation. This indicated that negative charge at positions 31/32 provided either by phosphorylation or by a negatively charged amino acid is necessary for efficient transformation without significantly affecting p105Rb binding or transactivation.