A transcriptionally inactive E2F-1 targets the MDM family of proteins for proteolytic degradation

E2F-1-activated transcription promotes cell cycle progression and apoptosis . These functions are regulated by several factors including the E2F-1-bind ing protein MDM2 and the retinoblastoma protein pRb. Using a yeast two-hybr id screen we have identified the MDM2-related protein, MDMX as an E2F-1-bin ding protein. In these studies we find that coexpression of MDMX with E2F-1 results in degradation of the MDMX protein. Although this proteolytic degr adation can be blocked by the protease inhibitors bafilomycin A(1), N-acety l-Leu-Leu-Norleu-AL, and N-acetyl-Leu-Leu-Met-AL, MDMX degradation is not i nhibited by lactacystin, suggesting that degradation occurs by a proteasome -independent mechanism. Using an E2F-1 deletion mutant (E2F-1(180-437)) we show that E2F-1-targeted degradation of MDMX does not require the E2F-1 DNA binding domain and therefore is independent of E2F-1-driven transcription. We also find that this transcriptionally inactive E2F-1 mutant is capable of degrading the MDMX-related protein MDM2 and the MDMX isoform MDMX-S. Map ping of the E2F-1 C terminus reveals that neither a previously characterize d C-terminal MDM2 binding domain nor the pRb binding domain on E2F-1 is req uired for MDMX and MDM2 degradation.