Phosphorylation at Ser-133 of the kinase inducible domain of CREB (KID) triggers its binding to the KIX domain of CBP via a concomitant coil-to-helix transition. The exact role of this key event is still puzzling: it does not switch between disordered and ordered states, nor its direct interactions fully account for selectivity. Hence, we reasoned that phosphorylation may shift the conformational preferences of KID towards a binding-competent state. To this end we investigated the intrinsic structural properties of the unbound KID in phosphorylated and unphosphorylated forms by simulated annealing and molecular dynamics simulations. Although helical populations show subtle differences, phosphorylation reduces the flexibility of the turn segment connecting the two helices in the complexed structure and induces a transient structural element that corresponds to its bound conformation. It is stabilized by the pSer-133-Arg-131 interaction, which is absent from the unphosphorylated KID. Diminishing this coupling decreases the 3.1 kcal/mol contribution of pSer-133 to the binding free energy (DeltaGbind) of the phosphorylated KID to KIX by 1.1 kcal/mol, as computed in reference to Ser-133. In a binding competent form of the S133E KID mutant, the contribution of Glu-133 to DeltaGbind is by 1.5 kcal/mol smaller than that of pSer, suggesting that altered structural properties due to pSer --> Glu replacement impair the binding affinity. Thus, we propose that phoshorylation contributes to selectivity not merely by the direct interactions of the phosphate group with KIX, but also by promoting the formation of a transient structural element in the highly conserved turn segment.
