The anomalous electrophoretic behavior of the human papillomavirus type 16 E7 protein is due to the high content of acidic amino acid residues.

In the presence of sodium dodecyl sulfate and 2-mercaptoethanol, the human papillomavirus 16 E7 protein migrates as a 17 kD protein during polyacrylamide gel electrophoresis. However, the theoretical molecular mass of this protein is approximately 11 kD. Substitution of 2 basic amino acids for 2 acidic residues in the amino terminus of the protein restored normal electrophoretic mobility. Furthermore, neutralization of negative charge through chemical modification of the wild type protein normalized migration. These results indicate that the substantial net negative charge of the wild type E7 protein is responsible for its anomalous electrophoretic behavior.