4 The pRb protien is regulated by covalent modification Phos

4. The pRb protien is regulated by covalent modification. Phosphorylation of the pRb preents its association with E2F.

A. During which phase of the cell cycle is pRb first phosphorylated and by which cyclin-CDK complex?

B. What consequence does phosphorylation of pRb have on the cell cycle progression?

C. After the completion of S phase, E2F function is no longer needed. pRb must be dephosphorylated so that it can once again bind to E2F. Name the types of enzyme responsible for removing phosphate groups from protiens.

D. Knowing what you know about the cell cycle, explain why the RB gene would be considered a tumor suppressor gene.

Solution

A) the Prb protein is hypophosphorylated in the G1 phase of the cell cycle and phosphorylation increases during the cell cycle. The phosphorylation takes place by cyclic CYC and a cyclic CDK( Cyclic dependent kinase ).a combination of cyclin D1 and CDK4

B) E2F transcription factors are liberated by control of Rb protein and induce deregulation of the cell cycle.

c) Phosphatases remove phosphate group

D) Rb when hypophosphorylated bound to a E2F transcription factor , When Rb becomes hyperphosphorylated they releases E2F which inturn derives the cell cycle progression into the S phase If Rb were to be mutated E2F would be free to drive cell cycle progression all of the time so Rb is considered as tumor suppression gene