Role of secondary structure in discrimination between constitutive and inducible activators

We have examined structural differences between the proto-oncogene c-Myb an d the cyclic AMP-responsive factor CREB that underlie their constitutive or signal-dependent activation properties. Both proteins stimulate gene expre ssion via activating regions that articulate with a shallow hydrophobic gro ove in the KIX domain of the coactivator CRFEB-binding protein (CBP). Three hydrophobic residues in c-Myb that are conserved in CREB function importan tly in cellular gene activation and in complex formation with KM. These hyd rophobic residues are assembled on one face of an amphipathic helix in both proteins, and mutations that disrupt c-Myb or CREB helicity in this region block interaction of either factor with KM. Binding of the helical c-Myb d omain to KIX is accompanied by a substantial increase in entropy that compe nsates for the comparatively low enthalpy of complex formation. By contrast , binding of CREB to KIX entails a large entropy cost due to a random coil- to-helix transition in CREB that accompanies complex formation. These resul ts indicate that the constitutive and inducible activation properties of c- Myb and CREB reflect secondary structural characteristics of their correspo nding activating regions that influence the thermodynamics of formation of a complex with CBP.