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 structural 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.

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