9. Which of the following mechanisms may explain these results? (Choose TRUE or FALSE) [Go to the clickable image of cell cycle progression]

A. Elevated activity levels of CDK4-cyclin D.
B. Inhibition of CDK2-cyclin A activity.
C. Partial phosphorylation of pRb.
D. Inhibition of CDK2-cyclin E activity.

10. Which of the following experiments can test your hypothesis? (Choose between irrelevant, possible and recommended):

A. Analysis of substance X effect on CDK2 and CDK4 ability to phosphorylate pRb using  phosphopeptide mapping of pRb.
B. Analysis of substance X effect on CDK2 and CDK4 kinase activities.
C. Analysis of substance X effect on p21 and p27 levels in CDK2 containing complexes.
D. Analysis of substance X effect on T161 phosphorylation levels in CDK1.

You have decided to perform the following experiment: cells are synchronized to G0 by starvation. CS is than added for 16 hrs. Cells are exposed to PBS or substance X at t=0-8 or at t=8-16, but all cells are harvested at t=16. Anti CDK2 and anti CDK4 antibodies are used to immunoprecipitate CDK2 and CDK4 immunocomplexes, and the kinase activities of these complexes are assayed in-vitro using histone H1 and pRb respectively. In addition, you test by western blots the levels of CDK2 and CDK4 in the immunocomplexes.

Here are your results:

11. You may draw the following conclusions (true or false):

A. Substance X does not alter CDK4 levels in the immunocomplex but reduces its kinase activity.
B. Substance X does not affect CDK2 or CDK4 levels in the respective immunocomplexes.
C. Substance X reduces the kinase activity of the CDK2 containing immunocomplex at t=8-16.
D. CDK2 kinase activity in control cells is lower at t=8 than at t=16.