The structure of a CREB bZIP·somatostatin CRE complex reveals the basis for selective dimerization and divalent cation-enhanced DNA binding

The cAMP responsive element-binding protein (CREB) is central to second messenger regulated transcription. To elucidate the structured mechanisms of DNA binding and selective dimerization of CREB, we determined to 3.0 Å resolution, the structure of the CREB bZIP (residues 283-341) bound to a 21-base pair deoxynucleotide that encompasses the canonical 8-base pair somatostatin cAMP response element (SSCRE). The CREB dimer is stabilized in part by ionic interactions from Arg³¹⁴ to Glu³¹⁹' and Glu³²⁸ to Lys³³³' as well as a hydrogen bond network that links the carboxamide side chains of Gln³²²'-Asn³²¹-Asn³²¹'-Gln³²². Critical to family selective dimerization are intersubunit hydrogen bonds between basic region residue Tyr³⁰⁷ and leucine zipper residue Glu³¹², which are conserved in all CREB/CREM/ATF-1 family members. Strikingly, the structure reveals a hexahydrated Mg²⁺ ion bound in the cavity between the basic region and SSCRE that makes a water-mediated DNA contact. DNA binding studies demonstrate that Mg²⁺ ions enhance CREB bZIP:SSCRE binding by more than 25-fold and suggest a possible physiological role for this ion in somatostatin cAMP response element and potentially other CRE-mediated gene expression.