What is the expected outcome regarding the RB protein function when it interacts with E2F?

• A. Induces cell cycle progression
• B. Inhibits transcription activation
• C. Enhances E2F activity
• D. Promotes apoptosis

Answer: (B)

The RB protein, or retinoblastoma protein, plays a crucial role in regulating the cell cycle, particularly by controlling the transition from the G1 phase to the S phase. When RB is active, it binds to the E2F transcription factor, which is essential for the expression of genes required for cell cycle progression.

When RB interacts with E2F, it inhibits the transcriptional activation function of E2F. This inhibition prevents the expression of genes necessary for S phase entry, thus effectively blocking the cell cycle progression. In this scenario, the RB protein functions as a tumor suppressor, maintaining control over cell division by preventing excessive proliferation of cells.

By binding to E2F, RB essentially acts as a gatekeeper, ensuring that cells do not proceed to replicate their DNA and divide unless conditions are favorable and the cell is properly prepared to do so. Thus, the interaction between RB and E2F is critical in tumor suppression and regulating normal cellular growth.