In proliferating cells, certain pRb conformations (when RxL motif if bound by protein phosphatase 1 or when it is acetylated or methylated) are resistant to CDK phosphorylation and retain other function throughout cell cycle progression, suggesting not all pRb in the cell are devoted to guarding the G1/S transition.
Studies have also demonstrated that hyperphosphorylated pRb can specifically bind E2F1 and form stable complexes throughout the cell cycle to carry out unique unexplored functions, a surprising contrast from the classical view of pRb releasing E2F factors upon phosphorylation.
In summary, many new findings about pRb's resistance to CDK phosphorylation are emerging in pRb research and shedding light on novel roles of pRb beyond cell cycle regulation.