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A look at the MDM2 protein
A look at the MDM2 protein
MDM2 is a Ubiquitin Protein Ligase that is a potent inhibitor of p53 that binds to the transcriptional activation domain/ N-terminus of p53, blocking its ability to bind to p53 target genes and aid in their expression. Additionally, p53 also activates the expression of the MDM2 gene1 in an auto regulatory feedback loop . MDM2 has two Biological assemblies you can see here for reference.
Fact: The Human Homolog of MDM2 is usually referred to HDM2 as well.
A slightly more detailed look into MDM2 - p53 interaction
A slightly more detailed look into MDM2 - p53 interaction
Since MDM2 functions as an E3 ubiquitin ligase, it specifically binds to P53 to induce ubiquitination via its C-terminal RING domain, resulting in P53 degradation by proteasomes.
How we discovered that MDM2 Mediates Ubiquitination of p53
Based on prior research, Fang et al hypothesized that MDM2 could be a protein that ubiquitinates p53. To test this, the team immobilized GST-Mdm2 on glutathione-Sepharose beads (GS) followed by a ubiquitination reaction using recombinant 32P-labeled Ub in the presence of E1 and E2. By applying the recombinant GST-Mdm2 fusion protein to p53 in an in vitro p53 ubiquitination assay, they discovered that p53 was bound to GST-Mdm2 suggesting MDM2 aided in the ubiquitination of p53.
Some Extra Facts
Some Extra Facts
In their research,data indicated that chelation of zinc from the MDM2 RING resulted in loss of MDM2 activity. To verify this, Fang et al mutated each of the 10 potential zinc coordination residues and evaluating the effects on the E3 activity of Mdm2, they discovered that mutation of the following amino acids resulted in the complete loss of ubiquitination:
Cys-438, His-452, His-457, Cys-461, Cys-464, Cys-475, Cys-478 and Cys-441
When we analyze this result, we can confirm that these results clearly demonstrate that there are amino acid residues in MDM2 that contribute to the ubiquitination of p53.
So, How does MDM2 affect Lung Cancer?
So, How does MDM2 affect Lung Cancer?
So far, we have seen that MDM2 contributes to rapid p53 degradation. Considering what we've learned so far about p53 we know:
a) In normal circumstances, the p53 pathway remains dormant
b) MDM2 is a negative feedback inhibitors of the p53 pathway
c) The p53 pathway is activated when DNA is damaged
Based on these observations, we see that when the p53 pathway is required to be activated, concentrations of MDM2 is decreased.
Therefore, when we put 2 and 2 together, we come to the conclusion:
In conditions where MDM2 concentrations are up-regulated despite DNA damage being present, the p53 pathway remains dormant and cells develop to become cancerous.
There are other pathways that affect p53-MDM2 interaction that results in cancer. We will explore the TRIM65 inhibition of p53 next.
Animesh Dali | DePauw University
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