title: The RUNX1 Runt domain at 1.25A resolution: a structural switch and specifically bound chloride ions modulate DNA binding.
abstract: The evolutionarily conserved Runt homology domain is characteristic of the RUNX family of heterodimeric eukaryotic transcription factors, including RUNX1, RUNX2 and RUNX3. The genes for RUNX1, also termed acute myeloid leukemia protein 1, AML1, and its dimerization partner core-binding factor beta, CBFbeta, are essential for hematopoietic development and are together the most common targets for gene rearrangements in acute human leukemias. Here, we describe the crystal structure of the uncomplexed RUNX1 Runt domain at 1.25A resolution and compare its conformation to previously published structures in complex with DNA, CBFbeta or both. We find that complex formation induces significant structural rearrangements in this immunoglobulin (Ig)-like DNA-binding domain. Most pronounced is the movement of loop L11, which changes from a closed conformation in the free Runt structure to an open conformation in the CBFbeta-bound and DNA-bound forms. This transition, which we refer to as the S-switch, and accompanying structural movements that affect other parts of the Runt domain are crucial for sustained DNA binding. The closed to open transition can be induced by CBFbeta alone; suggesting that one role of CBFbeta is to trigger the S-switch and to stabilize the Runt domain in a conformation enhanced for DNA binding.A feature of the Runt domain hitherto unobserved in any Ig-like DNA-binding domain is the presence of two specifically bound chloride ions. One chloride ion is coordinated by amino acid residues that make direct DNA contact. In a series of electrophoretic mobility-shift analyses, we demonstrate a chloride ion concentration-dependent stimulation of the DNA-binding activity of Runt in the physiological range. A comparable DNA-binding stimulation was observed for negatively charged amino acid residues. This suggests a regulatory mechanism of RUNX proteins through acidic amino acid residues provided by activation domains during cooperative interaction with other transcription factors.
authors.affiliation: Biocrystallography Group, Umeå Centre for Molecular Pathogenesis (UCMP), Umeå University, SE-901 87 Umeå, Sweden.
authors.forename: Stefan
authors.initials: S
authors.lastname: Bäckström
authors.forename: Magnus
authors.initials: M
authors.lastname: Wolf-Watz
authors.forename: Christine
authors.initials: C
authors.lastname: Grundström
authors.forename: Torleif
authors.initials: T
authors.lastname: Härd
authors.forename: Thomas
authors.initials: T
authors.lastname: Grundström
authors.forename: Uwe H
authors.initials: UH
authors.lastname: Sauer
mesh_terms: D000208:Acute Disease; D000595:Amino Acid Sequence; D000818:Animals; D002712:Chlorides / Q000378:metabolism*; D050659:Core Binding Factor Alpha 1 Subunit; D050676:Core Binding Factor Alpha 2 Subunit; D018360:Crystallography, X-Ray; D004247:DNA / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007951:Leukemia, Myeloid / Q000235:genetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009068:Movement; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011518:Proto-Oncogene Proteins; D016415:Sequence Alignment; D055672:Static Electricity; D013329:Structure-Activity Relationship; D050656:Transcription Factor AP-2; D014157:Transcription Factors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D002712:Chlorides; D050659:Core Binding Factor Alpha 1 Subunit; D050676:Core Binding Factor Alpha 2 Subunit; D004268:DNA-Binding Proteins; D011518:Proto-Oncogene Proteins; C493728:RUNX1 protein, human; C493709:RUNX2 protein, human; D050656:Transcription Factor AP-2; D014157:Transcription Factors; D004247:DNA