12. Which of the following mechanisms may explain these results ?

Choose wrong or possible.

A. Substrate X inhibits phosphorylation of Y15 on CDK2 by Wee1, and thus inhibits the activation of CDK2. WrongPhosphorylation of Y15 is one of the mechanisms which turn CDK2 inactive. Inhibition of this process cannot explain the reduction of kinase activity in CDK2 containing immunocomplexes.

B. Substrate X inhibits cyclin E binding to CDK2, and thus inhibits the activation of CDK2. Possible

C. Substrate X inhibits p27 binding to CDK2-cyclin E complex, and thus inhibits the activation of CDK2. Wrong p27 is an inhibitor of CDK2 activation. Inhibition of its binding to CDK2-cyclin E complex should allow CDK2 activation.

D. Substrate X inhibits the phosphorylation of T160 on CDK2 by cdc25A, and thus
inhibits the activation of CDK2. Wrong Inhibition of T160 phosphorylation should indeed inhibit CDK2 activation. However, this phosphorylation is carried out by CDK Activating Kinase (CAK).

E. Substrate X blocks CDK2  substrate binding site and serves as an inhibitor of the activity of the kinase itself. Possible