Which one of the following is true concerning retinoblastoma protein and the transcription factor E2F?

a. E2F phosphorylates Rb

b. E2F binds phosphorylated Rb

c. E2F is phosphorylated by Rb

d. E2F binds unphosphorylated Rb

d. E2F binds unphosphorylated Rb

Explanation:

Rb (Retinoblastoma) is a protein which suppresses tumor. During G1 phase of cell cycle it regulates cell cycle progression.

When Rb is unphosphorylated, E2F is bound to it. But the growth factors in G1 phase cause synthesis of cyclin D. Cyclin D then interacts with a serine - threonine kinase named as CDK-4 which phosphorylates Rb. As soon as Rb gets phosphorylated it gets degraded and E2F becomes free to induce expression of cyclin A and cyclin E which leads to progression of cell cycle from G1 phase to S phase.