Targeting mechanism of the retinoblastoma tumor suppressor by a prototypical viral oncoprotein

Abstract

DNA tumor viruses ensure genome amplification by hijacking the cellular replication machinery and forcing infected cells to enter the S phase. The retinoblastoma (Rb) protein controls the G1/S checkpoint, and is targeted by several viral oncoproteins, among these the E7 protein from human papillomaviruses. A quantitative investigation of the interaction mechanism between the HPV16 E7 protein and the RbAB domain in solution revealed that 90% of the binding energy is determined by the LxCxE motif, with an additional binding determinant (1.0 kcal·mol-1) located in the C-terminal domain of E7, establishing a dual-contact mode. The stoichiometry and subnanomolar affinity of E7 indicate that it can bind RbAB as a monomer. The low-risk HPV11 E7 protein binds 2.0 kcal·mol-1 weaker than the high-risk 16 and 18 type counterparts, but the modularity and binding mode are conserved. Phosphorylation at a conserved casein kinase II site in the natively unfolded N-terminal domain of E7 affects the local conformation by increasing the polyproline II content and stabilizing an extended conformation, which allows for a tighter interaction with Rb. Thus, the E7-RbAB interaction involves multiple motifs within the N-terminal domain of E7 and at least two conserved interaction surfaces in RbAB. We discuss a mechanistic model of the interaction of Rb with a viral target in solution, integrated with structural data and the analysis of other cellular and viral proteins, which provides information about the balance of interactions involving Rb and how these determine the progression into either normal cell cycle or transformation.