Crystal structure of the CCAAT box/enhancer-binding protein beta activating transcription factor-4 basic leucine zipper heterodimer in the absence ofDNA

dThe crystal structure of the heterodimer formed by the basic leucine zippe r (bZIP) domains of activating transcription factor-4 (ATF4) and CCAAT box/ enhancer-binding protein beta (C/EBP beta), from two different bZIP transcr iption factor families, has been determined and refined to 2.6 A. The struc ture shows that the heterodimer forms an asymmetric coiled-coil. Even in th e absence of DNA, the basic region of ATF4 forms a continuous a-helix, but the basic region of C/EBP beta is disordered. Proteolysis, electrophoretic mobility shift assay, circular dichroism, and NMR analyses indicated that ( i) the bZIP domain of ATF4 is a disordered monomer and forms a homodimer up on binding to the DNA target; (ii) the bZIP domain of ATF4 forms a heterodi mer with the bZIP domain of C/EBP beta that binds the cAMP response element , but not CCAAT box DNA, with high affinity; and (iii) the basic region of ATF4 has a higher alpha -helical propensity than that of C/EBP beta. These results suggest that the degree of ordering of the basic region and the for k and the dimerization properties of the leucine zipper combine to distingu ish the structurally similar bZIP domains of ATF I: and C/EBP beta with res pect to DNA target sequence. This study provides insight into the mechanism by which dimeric bZIP transcription factors discriminate between closely r elated but distinct DNA targets.