| The steady-state level and metabolic half-life of the retinoblastoma tumor suppressor protein, pRB, are decreased in cells that express high-risk human papillomavirus E7 proteins. My work shows that pRB degradation is a direct activity of E7 and does not reflect a property of cell lines acquired during the selection process for E7 expression. An amino terminal domain of E7 that does not directly contribute to pRB binding, but is required for transformation, is also necessary for E7-mediated pRB degradation. Treatment with inhibitors of the 26S proteasome not only blocks E7-mediated pRB degradation, but also causes stabilization of E7. Mutagenic analyses, however, reveal that proteasomal degradation of E7 and pRB are not linked processes. HPV-16 E7 also targets the pRB-related proteins, p107 and p130, for destabilization by a proteasome-dependent mechanism. Using the SAOS2 flat cell assay as a biological read-out for pRB function, I demonstrate that pRB degradation, not solely binding, is important for E7-induced inactivation of pRB. Lastly, I have begun characterization of a novel interaction between HPV-16 E7 and p62, a ubiquitin binding protein. Establishing the biological implications of this interaction may be key to understanding how HPV-16 E7 targets pRB for degradation. |
