Two immunodominant regions of the human papillomavirus type 16 E7 protein are masked in the nuclei of monkey COS-1 cells.

The human papillomavirus type 16 E7 protein is a 98-amino acid (AA)-long nuclear oncoprotein. We established eight independent mouse hybridoma cell lines producing monoclonal antibodies (MAbs) specific for the E7 protein and characterized the MAbs by competitive binding analysis with the bacterially expressed, nonfusion E7 protein and synthesized oligopeptides. The MAbs were classified into two groups; three and five MAbs recognizing epitopes in probable immunodominant regions AAs 8 to 22 (region I) and AAs 39 to 54 (region II), respectively. These MAbs were capable of immunoprecipitating the E7 protein transiently expressed in monkey COS-1 cells. By immunofluorescence staining of the acetone-fixed E7-producing COS-1 cells, the MAbs for regions I and II detected no E7 protein and only cytoplasmic E7 protein, respectively, whereas a rabbit polyclonal antiserum (anti-lac-E7) detects both nuclear and cytoplasmic E7 proteins. Like anti-lac-E7, the MAbs of the two groups stained both nuclear and cytoplasmic E7 proteins in the COS-1 cells that were denatured with formaldehyde. The results show that two immunodominant regions of the HPV 16 E7 protein are masked in the COS-1 nuclei for binding with the corresponding MAbs. The masking of the intranuclear E7 protein may result from complex formation with cellular proteins (or structures), polymerization, or self-folding.