Question

Which of the following statements is NOT true of the pRb protein?

pRb is functionally inactive when it is hyperphosphorylated.

pRb is mostly unphosphorylated during the G0 phase of the cell cycle.

pRb is growth-promoting during early G1 phase.

pRb phosphorylation is initiated by D-type cyclins and CDK4/6.

pRb is functionally inactivated at the R-point.

Answer 1

Option 3 is the correct answer.

The statement that pRb is growth promoting during early G 1 phase is not true. It is a tumor supressor protein that is dysfunctional in sveral major cancers. When the cell is ready to divide, pRb is phosphorylated, becomes inactive and allows cell cycle progression. Active cyclin D-cdk complexes phosphorylate retinoblastoma protein (pRb) in the nucleus. Unphosphorylated Rb acts as an inhibitor of G₁ by preventing E2F-mediated transcription. E2Fpromotes its own activation, and also promotes the inhibition of its own inhibitor (pRb).