Degradation of the retinoblastoma tumor suppressor by the human papillomavirus type 16 E7 oncoprotein is important for functional inactivation and isseparable from proteasomal degradation of E7

The steady-state level and metabolic half-life of retinoblastoma tumor supp ressor protein pRB are decreased in cells that express high-risk human papi llomavirus (HPV) E7 proteins. Here we show that pRB degradation is a direct activity of E7 and does not reflect a property of cell lines acquired duri ng the selection process for E7 expression. An amino-terminal domain of E7 that does not directly contribute to pRB binding but is required for transf ormation is also necessary for E7-mediated pRB degradation. Treatment with inhibitors of the 26S proteasome not only blocks E7-mediated pRB degradatio n but also causes the stabilization of E7. Mutagenic analyses, however, rev eal that the processes of proteasomal degradation of E7 and pRB are not lin ked processes. HPV type 16 E7 also targets the pRB-related proteins p107 an d p130 for destabilization by a proteasome-dependent mechanism. Using the S AOS2 flat-cell assay as a biological indicator for pRB function, we demonst rate that pRB degradation, not solely binding, is important for the E7-indu ced inactivation of pRB.