Conformational landscape of the transcription factor ATF4 is dominated by disordered-mediated inter-domain coupling

Transient intramolecular interactions between transactivation domain and DNA binding domain of transcription factors are known to play important functional roles, including modulation of DNA binding affinity and specificity. Similar type of inter-domain interactions has recently been reported for the transcription factor ATF4/CREB-2, a key regulator of the Integral Stress Response. In the case of ATF4, transient coupling between the transactivation and basic-leucine zipper (bZip) domains regulates the degree of phosphorylation of the disordered transactivation domain achievable by the casein kinase CK2. Despite the crucial importance of these inter-domain interactions, their structural and molecular basis remain ill-determined. In the present study, we use a combination of experimental and computational techniques to determine the precise nature of the long-range contacts established between the transactivation and bZip domains of ATF4 prior to its association with protein partners and DNA. Solution NMR spectroscopy experiments reveal that the isolated bZip domain of ATF4 is predominantly disordered and display evidence of conformational dynamics over a wide range of timescales. These experimental findings are supported by multi-microsecond timescale all-atom molecular simulations that unveil the molecular basis of the long-range interactions between the transactivation and bZip domains of ATF4. We found that inter-domain coupling is primarily driven by disorder-mediated interactions between a leucine-rich region of the transactivation domain and the leucine-zipper region of the bZip domain. This study uncovers the role played by structural disorder in facilitating the formation of long-range intramolecular interactions that shape the conformational ensemble of ATF4 in a critical manner.