Final answer:
The function of retinoblastoma protein (Rb) is to inhibit transcription factor E2F, preventing cell cycle progression. Rb binds to E2F in its active state and blocks the production of proteins necessary for cell division. As the cell grows, Rb becomes inactivated and releases E2F to promote cell cycle progression.
Step-by-step explanation:
The function of retinoblastoma protein (Rb) is to inhibit E2F, which is a transcription factor that turns on genes that lead to cell proliferation. In its active, dephosphorylated state, Rb binds to E2F and blocks the production of proteins necessary for the G₁/S transition, thus preventing cell cycle progression. As the cell increases in size, Rb is phosphorylated and becomes inactivated, releasing E2F to turn on the gene that produces the transition protein. This is an important regulatory mechanism to ensure proper cell growth and division.