The E7 protein of human papillomavirus type 16 is phosphorylated by casein kinase II.
The E7 protein of human papillomavirus type 16 (HPV16) transforms cultured cells and cooperates with the ras or fos oncogenes in the transformation of primary cells. In this study we have investigated the phosphorylation of E7. When we immunoprecipitated E7 from CaSki cells with a rabbit polyclonal antiserum to a bacterial fusion protein (trpE-E7), we found that E7 was phosphorylated at serine residues contained in five characteristic thermolysin peptides. Immunoprecipitated E7, and fusion proteins harboring the E7 protein from various HPV types, could all be specifically phosphorylated in vitro by the ubiquitous, growth factor-activated casein kinase II (CKII). Comparative peptide mapping showed that the sites of in vivo and in vitro phosphorylation are the same. CKII was shown previously to specifically phosphorylate serine or threonine residues within a cluster of acidic amino acids. The E7 protein contains such a sequence between amino acids 30 and 37. When a synthetic peptide corresponding to this region of E7 was phosphorylated by CKII in vitro, its thermolysin digestion products were the same as those in the phosphorylated E7 protein. We conclude that E7 is phosphorylated in vivo only at serines within the predicted CKII site and that CKII, or a CKII-like enzyme, participates in the reaction. Both the E1A and SV40 large T proteins contain similar CKII consensus sites proximal to the regions required for their associations with the retinoblastoma gene product (p105Rb). Thus it is conceivable that CKII phosphorylation can modulate the interaction between the transforming proteins and the retinoblastoma gene product.
Firzlaff
J M
JM
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98104.
Galloway
D A
DA
Eisenman
R N
RN
Lüscher
B
B
D000595:Amino Acid Sequence; D047388:Casein Kinases; D006493:Heparin / Q000494:pharmacology; D008969:Molecular Sequence Data; D009856:Oncogene Proteins, Viral / Q000378:metabolism*; D027383:Papillomaviridae / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D050725:Papillomavirus E7 Proteins; D010446:Peptide Fragments / Q000378:metabolism; D010449:Peptide Mapping; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012689:Sequence Homology, Nucleic Acid; D012694:Serine; D013912:Threonine
D009856:Oncogene Proteins, Viral; D050725:Papillomavirus E7 Proteins; D010446:Peptide Fragments; D047428:Protein Kinase Inhibitors; D011993:Recombinant Fusion Proteins; C059731:oncogene protein E7, Human papillomavirus type 16; D013912:Threonine; D012694:Serine; D006493:Heparin; D011494:Protein Kinases; D047388:Casein Kinases