Equilibrium dissociation and unfolding of human papillomavirus E2 transactivation domain

Papillomavirus E2 protein that performs essential functions such as viral oncogene expression and replication represents specific target for therapeutic intervention. DNA-binding activity is associated with its C-terminal DNA-binding domain (DBD), while the N-terminal transactivation domain (TAD) is responsible for replication and transactivation functions. Although both demonstrate large dependence on dimerization for mediating their functions, KD for N-terminal dimerization is significantly high suggesting more dynamic role of this domain. However, unlike DBD, very little information is available on TAD dimerization, its folding and stability. Therefore, with an aim at delineating the regulatory switch of its dimerization, we have characterized high-risk HPV18 E2 TAD. Our studies demonstrate that E2 TAD is a weak but thermodynamically stable dimer (KD ∼ 1.8 μM, ΔGH2O = 18.8 kcal mol−1) with α2-α3 helices forming the interface. It follows a three-state folding pathway, in which unfolding involves dissociation of a dimeric intermediate. Interestingly, 90% of the conformational free energy is associated with dimer dissociation (16.9 of 18.8 kcal mol−1) suggesting dimerization significantly contributes to its overall thermodynamic stability. These revelations might be important toward designing inhibitors for targeting dimerization or folding intermediates and hence multiple functions that E2 performs.