Structure and backbone dynamics of Apo-CBFbeta in solution.

Runx proteins constitute a family of mammalian transcription factors that interact with DNA through their evolutionarily conserved Runt domain. CBFbeta, alternatively denoted PEBP2beta, is the non-DNA-binding heterodimer partner and acts to enhance the DNA binding affinity of Runx proteins. Runx proteins and CBFbeta are associated with a variety of biological functions and human diseases; they are, for example, together the most frequent targets for chromosomal rearrangements in acute human leukemias. We have determined the solution structure and characterized the backbone dynamics of C-terminally truncated fragments containing residues 1-141 of CBFbeta. The present apo-CBFbeta structure is very similar to that seen in a Runt-CBFbeta complex. An evaluation of backbone (15)N NMR relaxation parameters shows that CBFbeta is a rigid molecule with high order parameters throughout the backbone; the only regions displaying significant dynamics are a long loop and the C-terminal alpha-helix. A few residues display relaxation behavior indicating conformational exchange on microsecond to millisecond time scales, but only one of these is located at the Runt binding surface. Our structure and dynamics analysis of CBFbeta therefore suggests that the protein binds to Runt without large conformational changes or induced folding ("lock-and-key" interaction). The apo-CBFbeta structure presented here exhibits several significant differences with two other published NMR ensembles of very similar protein fragments. The differences are located in four regions outside of the central beta-barrel, whereas the beta-barrel itself is almost identical in the three NMR structures. The comparison illustrates that independently determined NMR structures may display rather large differences in backbone conformation in regions that appear to be well-defined in each of the calculated NMR ensembles.

Chloride binding by the AML1/Runx1 transcription factor studied by NMR.

It is known that the DNA binding Runt domain of the AML1/Runx1 transcription factor coordinates Cl(-) ions. In this paper we have determined Cl(-) binding affinities of AML1 by (35)Cl nuclear magnetic resonance (NMR) linewidth analysis. The Runt domain binds Cl(-) with a dissociation constant (K(d,Cl)) of 34 mM. If CBFbeta is added to form a 1:1 complex, the K(d,Cl) value increases to 56 mM. Homology modeling suggests that a high occupancy Cl(-) binding site overlaps with the DNA binding surface. NMR data show that DNA displaces this Cl(-) ion. Possible biological roles of Cl(-) binding are discussed based on these findings.

Crystallization and preliminary studies of the DNA-binding runt domain of AML1.

The acute myeloid leukaemia 1 (AML1) protein belongs to the Runx family of transcription factors and is crucial for haematopoietic development. The genes encoding Runx1 and its associated factor CBF beta are the most frequent targets for chromosomal rearrangements in acute human leukaemias. In addition, point mutations of Runx1 in acute leukaemias and in the familial platelet disorder FPD/AML cluster within the evolutionary conserved runt domain that binds both DNA and CBF beta. Here, the crystallization of the Runx1 runt domain is reported. Crystals belong to space groups C2 and R32 and diffract to 1.7 and 2.0 A resolution, respectively.

Solution properties of the free and DNA-bound Runt domain of AML1.

The Runt domain is responsible for specific DNA and protein-protein interactions in a family of transcription factors which includes human AML1. Structural data on the Runt domain has not yet become available, possibly due to solubility and stability problems with expressed protein fragments. Here we describe the optimization and characterization of a 140-residue fragment, containing the Runt domain of AML1, which is suitable for structural studies. The fragment of AML1 including amino acids 46-185 [AML1 Dm(46-185)] contains a double cysteine-->serine mutation which does not affect Runt domain structure or DNA-binding affinity. Purified AML1 Dm(46-185) is soluble and optimally stable in a buffer containing 200 mm MgSO4 and 20 mm sodium phosphate at pH 6.0. Nuclear magnetic resonance and circular dichroism spectroscopy indicate that the Runt domain contains beta-sheet, but little or no alpha-helical secondary structure elements. The 45 N-terminal residues of AML1 are unstructured and removal of the N-terminal enhances sequence-specific DNA binding. The NMR spectrum of AML1 Dm(46-185) displays a favorable chemical shift dispersion and resolved NOE connectivities are readily identified, suggesting that a structure determination of this Runt domain fragment is feasible. A titration of 15N-labelled AML1 Dm(46-185) with a 14-bp cognate DNA duplex results in changes in the 15N NMR heteronuclear single quantum coherence spectrum which indicate the formation of a specific complex and structural changes in the Runt domain upon DNA binding.

Structure and dynamics of the glucocorticoid receptor DNA-binding domain: comparison of wild type and a mutant with altered specificity.

Nuclear magnetic resonance was used to compare parameters reflecting solution structure and dynamics of the glucocorticoid receptor DNA-binding domain (GRDBD), which binds specifically to a GRE binding site on DNA, and a triple mutant (GRDBDEGA), which binds to an ERE site. The studies were prompted by an earlier observation that the cooperativity for dimeric DNA-binding is 10 times higher for the GRDBDEGA-ERE association than for the GRDBD-GRE association (Lundbäck et al., 1994). The higher binding cooperativity of the mutant was unexpected since the triple mutation (G458E, S459G, and V462A) is made in the recognition helix and distant from the dimerization surface which is formed by residues in the fragment A477-N491. Sequential and long-range NOE connectivities and measured 3JHNHalpha coupling constants indicate that the overall structures of the two proteins are very similar, possibly with a less well-defined structure of the fragment K486-N491 in GRDBDEGA. However, chemical shift changes, line broadening, and increased amide proton exchange rates are observed for several residues at, or close to, the dimerization surface of the mutant. These observations are interpreted as a lower stability and/or several slowly interconverting folded conformations of this region of GRDBDEGA. The effects are likely to be due to the loss of a hydrogen bond which links S459 to the dimerization region in GRDBD. Different mechanisms for the increased binding cooperativity of the mutant are discussed, and it is noted that the properties of the GRDBDEGA dimerization region are reminiscent of those reported for the estrogen receptor DBD, which also binds to an ERE site.