5. p53 is activated under cellular stress, and turns on genes involved in stress response. In the absence of stress its levels are kept low by proteasomal degradation through interactions with MDM2 ubiquitin ligase that marks it for degradation. (17 pts) A. Identify at least three hydrogen bonding interactions that stabilize the p53-MDM2 complex? (3 pts)/nB. Use the secondary structure prediction web server Proteus (http://www.proteus2.ca/proteus2/) and underline the residues that will participate in helix formation (2 pts): SQETFSDLWKLLPEN SQETFSDLWKLLAEN C. Based on Borcherds et al, which residue in the peptide can be mutated to increase the helical structure of the peptide? Is this in line with the prediction from (2) above (2 pts)? D. How does the Kd between p53 and MDM2 change upon mutating Pro27 to Ala (3 pts)?/nE. What assay was used by Boercherd's et al to determine the effect of Pro27Ala mutation on DNA replication post DNA damage (3 pts)? F. What is the effect of Pro27Ala mutant on cell cycle progression post DNA damage? Based on this result what is the mechanism by which p53 regulates cell fate after DNA damage? (4 pts)

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