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 (HPVs). 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 indicated that it can bind RbAB as a monomer. The 
low-risk HPV11 E7 protein bound 2.0 kcal.mol(-1) more weakly than the high-risk 
HPV16 and HPV18 type counterparts, but the modularity and binding mode were 
conserved. Phosphorylation at a conserved casein kinase II site in the natively 
unfolded N-terminal domain of E7 affected the local conformation by increasing 
the polyproline II content and stabilizing an extended conformation, which 
allowed for a tighter interaction with the Rb protein. 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 discussed a mechanistic 
model of the interaction of the Rb protein with a viral target in solution, 
integrated with structural data and the analysis of other cellular and viral 
proteins, which provided information about the balance of interactions involving 
the Rb protein and how these determine the progression into either the normal 
cell cycle or transformation.