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Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
Reprint requests to: Hiroshi Matsuo, Department of Biochemistry, Molecular Biology and Biophysics, 321 Church Street, S.E. Jackson Hall 6-155, University of Minnesota, Minneapolis, MN 55455, USA; e-mail: matsu029{at}umn.edu; fax: (612) 625-2163.
(RECEIVED April 7, 2004; FINAL REVISION May 3, 2004; ACCEPTED May 3, 2004)
| Abstract |
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Keywords: NMR structure; oncogene product; ataxia-telangiectasia; acute myeloid leukemia; chromatin-associated protein; chromosomal translocation
Abbreviations: DEK(309375), deletion of the N-terminal 308 amino acids of the human DEK protein AML, acute myelogenous leukemia A-T, ataxia telangiectasia JRA, juvenile rheumatoid arthritis HSQC, hetero-nuclear single quantum coherence NOE, nuclear Overhauser effect RMSD, root mean square deviation.
Supplemental material: see www.proteinscience.org
Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.04797104.
| Introduction |
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In addition to these sequence-specific DNA-binding functions, the chromatin association of DEK has been linked to several important biological processes. DEK is capable of changing the topology of chromatin and is suggested to participate in chromatin remodeling (Alexiadis et al. 2000). It is also reported to substantially reduce the replication efficiency of chromatin but not of naked DNA templates (Alexiadis et al. 2000). The chromatin association of DEK is also important for maintaining the latency of Kaposis sarcoma associated herpesvirus (KSHV). DEK forms a complex with the viral protein LANA (Kaposis sarcoma herpes viral latency-associated nuclear antigen) and this interaction allows the viral DNA of KSHV to be tethered to the chromosome of host cells (Krithivas et al. 2002).
Here we describe the structure of the C-terminal region of DEK that extends amino acids 309375 [DEK(309375)]. Structural similarities between DEK(309375) and the DNA-binding domain of DP2, which is a member of the winged-helix DNA-binding proteins, is discussed. Using NMR chemical shift perturbation analysis, we have identified a surface of DEK(309375) that can interact with DNA.
| Results and Discussion |
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The structure of DEK(309375) is provided in Figure 2A
. The structured portion of this construct includes residues 320375 and consists of three
-helices that span residues 322333, 343353, and 361374. The two additional residues at the N terminus (MG) are unstructured, whereas the introduced LE residues in the C-terminal region form a helical turn. However, the amide protons of these residues are not protected from exchange with water in a deuterium exchange experiment (protected residues are shown in Fig. 1B
).
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The structure of DEK(309375) is related to the winged helix DNA-binding motif
We tested whether DEK(309375) shares structural homology with other protein structures by using the program DALI (Holm and Park 2000). The DALI search revealed 10 protein structures with a Z-score higher than 3.0. Inspection of these structures indicated that the DNA-binding domain of the DP2 protein complexed with its heterodimeric partner E2F4 shares significant similarity with DEK(309375), with a Z-score of 3.7. Residues 319375 of DEK(309375) superimpose onto the helical region of the DNA-binding domain of DP2(68133) with an RMSD of 2.3Å (Fig. 2B
). The overall fold of the DNA-binding domain of DP2 is very similar to that of DEK(309375). Both proteins contain three tightly packed, twisted
-helices (Fig. 2B
). In both structures, the middle helix is the shortest and the C-terminal helix is the longest. The DNA-binding domain of DP2 contains an additional
-sheet consisting of a short strand preceding
2 and two strands C-terminal to the
-helices (Fig. 2B
).
Several protein families have winged helix motifs, including the hepatocyte nuclear factor-3 (HNF-3) family (Costa et al. 1989; Sheng et al. 2002), Drosophila homeotic forkhead proteins (Weigel et al. 1989), forkhead box family (Anderson et al. 1998; Biggs et al. 2001; Weigelt et al. 2001; Carlsson and Mahlapuu 2002), E2F/DP family (Slansky and Farnham 1996), Genesis (Sutton et al. 1996), and regulatory factor X 1(RFX1; Siegrist et al. 1993). With the exception of Drosophila homeotic forkhead protein family, these protein families are transcription factors. Forkhead transcription factors including FOX proteins, HNF-3, genesis, and RFX1 bind double-strand DNA (dsDNA) as monomers, whereas E2F/DP binds dsDNA as a heterodimer.
The topology of the winged helix motif consists of three
helices (
1,
2, and
3), three
strands (
1,
2, and
3), and two loops ("wings," W1 and W2), arranged as
1
1
2
3
2-W1
3-W2. E2F and DP proteins lack the W2 wing. Many winged helix proteins, including HNF-3, E2F/DP, and genesis, bind the major groove of dsDNA by using polar residues within
3 (Clark et al. 1993; Jin et al. 1999; Zheng et al. 1999). The DP and E2F protein families comprise closely related transcription factors and form E2F/DP heterodimers to bind their target DNA sequences (for reviews, see Slansky and Farnham 1996; Helin 1998). These proteins bind dsDNA using an RRxYD motif, where x is an aliphatic residue (Slansky and Farnham 1996; Helin 1998). In DP2,
3 is situated in the DNA major groove and contains an RRVYD sequence with each arginine forming hydrogen bonds to guanine bases of sense- and antisense-strands (Zheng et al. 1999). Interestingly, DEK(309375) contains a similar amino acid sequence in its
2; KKVYE. We therefore tested whether DEK can bind the E2F/DP consensus DNA sequence through contacts with
2.
DEK(309375) interacts with dsDNA through
2
We monitored the chemical shift perturbation of the amide nitrogen and proton atoms of DEK(309375) in [1H-15N] HSQC spectra on addition of the E2F/DP consensus DNA sequence (ds5'-TTTCGCGCG-3'; Fig. 3A
). Although DEK(309375) appears to interact with the E2F/DP consensus sequence, the binding is weak, as the chemical shift perturbations were not saturated at a 1:4 molar ratio, and further chemical shift changes and line broadening are observed at a 1:8 molar ratio (data not shown). The residues that experienced chemical shift perturbations include those in the KKVYE sequence (K348, K349, Y351) as well as those in surrounding residues within
2 (M343, K344, I346, C347; colored red in Fig. 1B
). Residues within the loop between
1 and
2 (N337, E339, V341) and two lysines of
3 (K367, K371) also experienced chemical shift perturbations (colored red in Fig. 1B
). All of the residues that experienced chemical shift perturbations on addition of the dsDNA were mapped onto a surface representation of DEK(309375; Fig. 3C,D
). It is clear that the surface shown in Figure 3C
is the DNA-interacting surface of DEK(309375) and that the surface shown in Figure 3D
does not interact with dsDNA. The electrostatic properties of these surfaces are illustrated in Figure 3, E and F
, respectively. The DNA-interacting surface has a positively charged region that includes the five lysines that experienced chemical shift perturbation on addition of the dsDNA. In contrast, the opposite side has exposed hydrophobic residues, including L333 and A334 of
1, L338 of the loop between
1 and
2, and L373 from
3.
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In this work, we reveal that the C-terminal region of DEK [DEK(309375)] forms a putative dsDNA-binding domain. This domain is structurally related to the winged-helix motif (Gajiwala and Burley 2000). Despite the structural similarity with winged helix proteins, the DNA-interaction mechanism of DEK(309375) appears different from that of winged helix proteins, as it uses the second helix to interact with DNA, whereas other winged helix proteins use the third helix to interact with DNA. It is possible that DEK(309375) requires another protein to form a stable proteinDNA complex. Further experiments are required to reveal the functional significance of this C-terminal regions DNA-binding properties.
| Materials and methods |
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Sedimentation velocity analysis
The association status of DEK(309375) was analyzed by sedimentation velocity measurements performed at the National Analytical Ultracentrifugation Facility (University of Connecticut Bio-technology Center). The sample solutions were prepared at three protein concentrations, including 0.1, 0.3, and 0.7 mg/mL, and subjected to a sedimentation velocity run, using the 8-hole rotor at 20°C and 50,000 rpm. The data for each loading concentration were analyzed by the program Sedfit2 (version 8.7) using the model of a single, noninteracting, discrete species.
NMR spectroscopy and collection of structure restraints
NMR spectra were recorded at 20°C, using 1 mM DEK(309375) in 50 mM sodium phosphate (pH 6.5), 100 mM KCl, and 95% H2O/5% 2H2O or 100% 2H2O at 1H NMR frequencies of 600 and 800 MHz on Varian Unity INOVA NMR spectrometers equipped with triple resonance probes and pulsed field gradients. Sequential resonance assignments were obtained from standard three-dimensional CBCANH (Grzesiek and Bax 1992b), CBCA(CO)NH (Grzesiek and Bax 1992a), total correlated spectroscopy (TOCSY)-15N-HSQC (Marion et al. 1989a) and HCCHTOCSY experiments (Bax et al. 1990). NOE restraints were obtained from 1H-1H two-dimensional NOESY and 13C and 15N edited three-dimensional NOESY experiments (Solomon 1955; Fesik et al. 1989; Marion et al. 1989a, b) with 150 msec mixing times and total recording times of 24, 92, and 87 h, respectively. Slowly exchanging protons were identified after exchanging the H2O buffer to a 2H2O buffer. Dihedral angle restraints were calculated from chemical shifts using the program TALOS (Cornilescu et al. 1999). The protection of amide protons from exchange with water was observed by the deuterium exchange experiment. DEK(309375) is lyophilized in 50 mM NaPO4, and 100 mM KCl, 100% H2O and dissolved in 100% 2H2O. 1H-15N HSQC spectra were recorded every 5 min after being dissolved in 2H2O.
Structure calculations
Spectra were processed using the NMRPipe software (Delaglio et al. 1995) on SGI workstations. Cross-peaks in the NOESY spectra were assigned and integrated using the program XEASY (Bartels et al. 1995). The NMR structure was calculated using a simulated annealing protocol in the program XPLOR (Brunger 1993). Eighty random conformers were annealed in 60,000 steps using Cartesian dynamics. All simulated annealed conformers were energy-minimized using the refinement protocol in the XPLOR (Brunger 1993). Table 1
shows an overview of the restraints used and structural statistics. The backbone dihedral angles were analyzed by Ramachandran plot using PROCHECK-NMR (Laskowski et al. 1996). No cis-peptide bonds were observed and no residues were found with a positive
angle. No residue was found in the disallowed regions. Secondary structure elements and RMSD values were calculated using the program MOLMOL (Koradi et al. 1996), which was also used to generate Figure 2
.
Mapping DNA-interacting surfaces of DEK(309375)
DNA-interacting surfaces of DEK(309375) were studied using chemical shift perturbation analysis of the amide nitrogen and proton signals in 1H-15N HSQC spectra on addition of DNA. We used chemically synthesized dsDNA that has the consensus E2F/ DP binding site (ds5'-TTTCGCGCG-3'), a nonspecific sequence (ds5'-CCCATATAT-3'), the HIV-2 peri-ets sequence (ds5'-GCT ATACTTGGTCAGG-3'), and the DQA1 Y-box sequence (ds5'-CTAATTGGCC-3'). Each dsDNA was titrated into 15N-labeled DEK(309375) at molar ratios of 1:1, 1:2, 1:4, and 1:8. Perturbed chemical shifts were followed through the DNA titration in order to assign signals at each molar ratio. Reference spectra were taken at the same protein concentration in the same buffer conditions. In addition, the chemical shift perturbation on addition of an RNA (5'-UUGAUUGGUUGUCAA-3') or a single-strand DNA (5'-GCTATACTTGGTCAGG-3') was observed.
Data bank accession codes
The coordinates of the 20 energy-refined XPLOR conformers of DEK(309375) have been deposited in the PDB with the accession code 1Q1V. The NMR chemical shifts will be deposited in the BioMagResBank (BMRB).
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| Acknowledgments |
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The publication costs of this article were defrayed in part by payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.
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