The ubiquitin E3 ligase Mdm2 (murine double minute 2; human homolog, Hdm2) is well known for its oncogenic activities and as the master regulator of the powerful tumor suppressor p531,2. Moreover, Mdm2 may function as an oncogenic protein independent of p533. Mdm2 is able to inhibit p53-mediated gene expression through two pathways: inhibition of transcriptional activity by direct binding and ubiquitin-mediated degradation via its E3 ligase activity4; however, the effectiveness of p53 inhibition by direct Mdm2 binding has been questioned5.
Under normal, non-stress conditions, Mdm2 maintains p53 expression and activity at a minimal level to tightly regulate its apoptotic/cell death transcriptional activities. Under conditions of cellular stress, Mdm2-mediated ubiquitination of p53 ceases, allowing p53 to activate transcription of apoptotic genes and those involved in inhibiting cell growth. Much research concludes this is due to Mdm2's auto-inhibition by self-ubiquitination1-3. However, this story appears to be more complex than originally thought5 and involves multiple post-translational modifications (PTMs). Here, we discuss Mdm2's regulation by ubiquitination, SUMOylation, phosphorylation, and acetylation6-8 (Fig. 1).
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