Mutagenesis of human papillomavirus types 6 and 16 E7 open reading frames alters the electrophoretic mobility of the expressed proteins.

The E7 open reading frames of human papillomavirus type 6 (HPV-6) and HPV-16 encode proteins consisting of 98 amino acids that are quite similar in sequence yet different in electrophoretic mobility. Moreover, these proteins vary strikingly in oncogenicity. To investigate the molecular basis of the differences in structure and function, site-directed mutagenesis was used to exchange non-conserved amino acid residues between the two proteins. The mutated coding regions were expressed as fusion proteins in Escherichia coli and identified by Western blotting. Comparative analysis of the affinity-purified mutated E7 fusion proteins in polyacrylamide slab mini-gels in the presence of SDS and 2-mercaptoethanol revealed altered electrophoretic mobilities. This analysis suggests that the aspartic acid at residue 4 (Asp 4) contributes to the characteristic aberrant migration of the HPV-16 E7 protein in SDS-polyacrylamide gels.