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Variable Control of Ets-1 DNA Binding by Multiple Phosphates in an Unstructured Region
Miles A. Pufall,1
Gregory M. Lee,2
Mary L. Nelson,1
Hyun-Seo Kang,2
Algirdas Velyvis,3
Lewis E. Kay,3
Lawrence P. McIntosh,2
Barbara J. Graves1*
Abstract:
Cell signaling that culminates in posttranslational modificationsdirects protein activity. Here we report how multiple Ca2+-dependentphosphorylation sites within the transcription activator Ets-1act additively to produce graded DNA binding affinity. Nuclearmagnetic resonance spectroscopic analyses show that phosphorylationshifts Ets-1 from a dynamic conformation poised to bind DNAto a well-folded inhibited state. These phosphates lie in anunstructured flexible region that functions as the allostericeffector of autoinhibition. Variable phosphorylation thus servesas a "rheostat" for cell signaling to fine-tune transcriptionat the level of DNA binding.
1 Huntsman Cancer Institute, Department of Oncological Sciences, University of Utah, Salt Lake City, UT 841125550, USA. 2 Department of Biochemistry and Molecular Biology, Department of Chemistry, and The Michael Smith Laboratory, University of British Columbia, Vancouver, British Columbia, V6T 1Z3, Canada. 3 Departments of Medical Genetics, Biochemistry, and Chemistry, University of Toronto, Toronto, Ontario, M5S 1A8, Canada.
* To whom correspondence should be addressed. E-mail: Barbara.Graves{at}hci.utah.edu
Proteins are activated or repressed by post-translational modificationsin response to extracellular cues. Phosphorylation, a typicalmodification, often accumulates at multiple sites until a thresholdlevel is reached. The outcome is described as a sharp on/offswitch of protein activity (1, 2). However, biological processesmay need more sensitive regulation. Despite this need, variableregulation of protein activity in response to multiple modificationshas not been documented. Here we show how the transcriptionfactor Ets-1 exhibits graded DNA binding activity with differentlevels of phosphorylation.
Fig. 1.. Differential phosphorylation of Ets-1 leads to variable DNA binding affinity. (A) NMR-derived structure of the partially active Ets-1 truncation N301, showing the ETS domain (red) and inhibitory helices (purple) (8). (B) Crystallographic structure of DNA-bound N280, showing inhibitory helix HI-1 unfolded (31). (C) Schematic secondary structure of N244 with SRR (yellow) and sites phosphorylated by CaMKII in vitro (labeled P) as determined by mass spectrometry and/or NMR for the wild type and mutants. Boxed phosphoserines are critical for the reinforcement of autoinhibition. Additional phosphates have negligible individual effects and vary with mutation pattern (32). (D) Relative phosphorylation-induced inhibition = KD(phosphorylated mutant)/KD(unmodified mutant) with the standard error of the ratio (n 4 replicas) (15). Single mutants are colored red and double mutants are colored yellow. The quadruple mutant (gray) (11) provides expected data for the disruption of three critical phosphoacceptor sites (251, 282, and 285). (E) Gobinding = RTln(relative induced inhibition, N244) + RT ln(relative induced inhibition, mutant), where R is the ideal gas constant and T is temperature, is plotted as a function of the number of phosphoacceptor mutations (wild type, exes; single, squares; double, triangles; triple, circle). The linear relationship (correlation coefficient r = 0.96) implicates additivity of mutational effects (33).
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Ets-1 is regulated by an autoinhibitory module composed of four helices (HI-1, HI-2, H4, and H5) that flank the DNA-bindingETS domain. In the native protein, these helices pack cooperativelyon a surface of the ETS domain that is opposite to the DNA bindinginterface (Fig. 1A), reducing the affinity of Ets-1 for DNA10-fold, compared with the affinity of the minimal ETS domain(35). This inhibitory module opposes the structural changethat accompanies DNA binding, the most striking feature of whichis the unfolding of inhibitory helix HI-1 (Fig. 1B) (3, 68).Recent nuclear magnetic resonance (NMR) spectroscopic studiesindicate that helix HI-1 is labile, even in the absence of DNA,and could serve as a control point for modulating DNA bindingaffinity (8). Cooperative DNA binding with runt-related protein1 (RUNX1) counteracts autoinhibition, activating the bindingof native Ets-1 10-fold (9), whereas Ca2+-dependent phosphorylationof Ets-1 at multiple sites reinforces autoinhibition by loweringDNA affinity 50-fold further to an overall inhibition of 500-to 1000-fold (fig. S1, A to D) (10, 11). The modulation of Ets-1DNA binding is correlated with transcriptional activity in invivo assays and is responsive to cellular cues (1214).For example, Ca2+ signaling in vivo disrupts Ets-1 DNA bindingand reduces the transcription of Ets-1driven reporters(13). We show that this multiple phosphorylation is not simplyan on/off switch, but rather an incremental rheostat-like controlfor Ets-1 binding activity.
We used a minimal fragment, N244, which displays the DNA bindingproperties of full-length Ets-1 (fig. S1, A to D) (11, 15).N2445P, a form which is homogenously phosphorylated by CaMKII,contains five specific phosphoserines in the serine-rich region(SRR, residues 244 to 300) (Fig. 1C) and appears to undergothe same structural transition upon DNA binding as do unmodifiedN244 (fig. S1E) and other Ets-1 species (3, 7, 11).
In cells, Ets-1 is variably phosphorylated in response to Ca2+release (10). To gauge the effect of differential phosphorylationon activity, we mimicked in vivo phosphorylation by mutatingphosphoacceptor serines to alanines singly and in pairs (Fig. 1C),and we then measured the DNA binding affinity of phosphorylatedand unmodified forms. Mutation of three (251, 282, and 285)of the five serines, but not two others (270 and 273), significantlyreduced inhibition. Furthermore, mutation of one, two, or allthree critical sites (11) yielded a graded reduction in phosphorylation-dependentinhibition (Fig. 1D), with each phosphate contributing additivelyto the change in the free energy of binding (Fig. 1E). Thisfinding shows that the number and context of sites, and notsimply a buildup of charge, affects inhibition. Thus, in contrastto proteins such as Sic1 (2) and NFAT1 (nuclear factor of activatedT cells) (1, 16), for which a threshold level of phosphorylationserves as a binary switch, multiple sites within Ets-1 regulateDNA binding in a graded manner across a wide range of affinities,which is consistent with observed variable regulation in vivo(13).
The phosphorylated SRR was found to be predominantly unstructuredand highly flexible. Based on main-chain 1H and 13C chemicalshifts, NMR spectra revealed no predominant secondary structurewithin this region (17) (Fig. 2A), and the amide hydrogen exchange(HX) rates were comparable to those of a random coil polypeptide(Fig. 2B). Further, NMR relaxation experiments demonstrateda high degree of backbone conformational mobility on nanosecondto picosecond time scales (Fig. 2C). Thus, the dynamic phosphorylatedSRR may impart variable regulation of DNA binding by makingtransient interactions with the remainder of the protein.
Fig. 2.. The phosphorylated SRR is predominantly unstructured and flexible. Panels present data for residues 244 to 320 of N2445P (15). Circles and P denote the five phosphoserines. Missing data correspond to prolines, or residues with spectral overlap and/or weak signals. (A) In contrast to HI-1, differences between the observed and random coil (13C-13Cß) chemical shifts indicated an absence of any detectable secondary structure in the SRR (17). (B) Small protection factors (kpred/kex) demonstrate that backbone SRR amides undergo HX at rates (kex) similar to those predicted for a random coil polypeptide (kpred). Amides within HI-1 do not show measurable HX by the CLEANEX method (kex < 0.5 s1, protection factors > 80). (C) Small or negative heteronuclear 1H{15N} nuclear Overhauser enhancement values indicate a high degree of flexibility. The horizontal line at 0.4 marks an approximate boundary between flexible (below) and rigid (above) amides on a subnanosecond time scale (34).
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Despite this flexibility, the SRR has a profound impact on thestructure, stability, and dynamics of the DNA binding domainand on the inhibitory helices. We compared three Ets-1 fragmentswith increasing levels of inhibition: partially activated N301;N280, a minimal fragment that recapitulates unmodified autoinhibitedEts-1 binding; and N2445P (dissociation constants KD1011,1010, and 108 M, respectively) (fig. S1, A, B,and D) (6, 8, 18). The comparison of 1H-15N heteronuclear singlequantum correlation (HSQC) spectra detected significant changesin backbone amide chemical shifts for 25 residues common tothe three species, indicative of structural perturbations (figs.S2 and S3 and table S1). The labile inhibitory helix HI-1 andhelices H1 and H3 of the DNA binding domain were most widelyaffected. The perturbed residues, many of which are nonpolar,form a hydrophobic network connecting the inhibitory elementsand the DNA binding interface (Fig. 3A). Mutation of leucine-429to alanine within this network reduced autoinhibition and impairedphosphorylation reinforcement (fig. S1C) (5, 11), indicatingthat the integrity of this hydrophobic network is required forregulation.
Fig. 3.. Autoinhibition and phosphorylation regulate binding by shifting the conformational equilibrium of a hydrophobic network. Comparison of 1H to 15N HSQC spectra of N301 (green), N280 (black), and N2445P (red) revealed colinear chemical shift perturbations among 25 common residues in the three species (table S1 and fig. S3) (15). (A) Shifted residues depicted on the NMR-derived structure of N301 by van der Waals surfaces (8). (B) Spectral overlays highlight the progressive shift changes (black exes and dashed lines) for G302, L341, and Y395 (20). (C) Peaks from six phosphorylated phosphoacceptor mutants follow the same colinear chemical shift pattern (between N280 and N2445P peaks on dashed lines). (D) Color key and consensus relative peak positions.
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A striking linear pattern of change was observed from N301 toN280 to N2445P in the amide 1H and 15N chemical shifts of almostall of the affected residues (Fig. 3B and table S1). This progressivecolinear pattern is a signature of an allosterically regulatedmolecule that is in conformational equilibrium between at leasttwo states, with the intermediate chemical shifts representinga population-weighted average of these states (19). Based onthe correlation between the linear pattern of amide chemicalshifts with DNA binding affinity (20), we propose that freeEts-1 exists in equilibrium between an active state (representedmost closely by N301), which is poised to bind DNA, and an inactivestate (represented by N2445P). According to this allostericmodel, Ets-1 DNA binding affinity reflects the balance of thesetwo states.
A comparison of the backbone amide chemical shifts of phosphorylatedN244 with single or double serine-to-alanine mutations alsorevealed a colinear shifting pattern between N280 and N2445Pin at least 16 of the 25 residues within the aforementionedhydrophobic network. The position of the chemical shifts foreach species, phosphorylated at the remaining CaMKII sites,roughly correlated with affinity. This finding emphasizes thecontext dependence of each phosphate and suggests that eachhas a distinct role in modulating the conformational equilibrium(Fig. 3, C and D, and table S1). These NMR studies show thatdifferential phosphorylation regulates DNA binding by fine-tuningthe balance between the active and inactive states of Ets-1.
Amide HX experiments helped characterize the dynamics of theactive and inactive states (Fig. 4, A to C, and fig. S4). Consistentwith previous studies (8), backbone amides within the inhibitoryhelices HI-1 and HI-2, as well as the DNA binding helix H3 ofthe active N301, exhibited limited protection from exchange,indicating that these helices sample locally unfolded conformationseven in the absence of DNA. In contrast, the same amides exhibitedelevated HX protection within N280, and further in N2445P, suggestiveof a progressively less dynamic and more stably folded species.The dynamic active state and more stable inactive state areconsistent with previous studies, showing that affinity forDNA decreases as the propensity of helix HI-1 to unfold decreases(3, 7, 11).
Fig. 4.. Concerted motion of the hydrophobic network is dampened by autoinhibition and phosphorylation. N301 (A and D), N280 (B and E), and N2445P (C and F) were analyzed by amide HX and backbone and side-chain relaxation dispersion (15). Relative values (figs. S4 to S6) are highlighted on the NMR-derived structure of N301 (8). [(A), (B), and (C)] For backbone HX, amides exhibiting measurable exchange (kex > 0.5 s1) are depicted by green balls with diameters scaled according to increasing kex. [(D), (E), and (F)] For relaxation dispersion, backbone and Trp side-chain nitrogens (red balls) and 13CmethylsofIle1, Val, and Leu (blueballs) with Rex 2 s1 are shown, where the diameter of the balls increases in proportion to Rex at 800 MHz (the largest balls correspond to Rex 15 s1). Pink balls represent atoms whose NMR signals are broadened beyond detection. Ile, Leu, and Val side chains are shown as green sticks.
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A dynamic connection between the autoinhibitory module and theDNA binding interface was demonstrated by NMR backbone amideand side-chain methyl relaxation dispersion measurements (Fig. 4, D to F,and figs. S5 and S6) (21). The contribution of millisecondto microsecond time scale motions to linewidth broadening, denotedRex, is indicative of conformational switching. Consistent withprevious studies (8), the backbone amides of N301 exhibitedmobility on this time scale (exchange lifetimes ex0.3 ms) forresidues that shift colinearly between N301, N280, and N2445P.These motions were dampened in N280, and further in N2445P (Fig. 4,red balls). 13C methyl relaxation dispersion of Leu, Ile,and Val side chains within the hydrophobic network of N301 showedmotions on the same time scale (ex0.3 ms) that again were progressivelyreduced in N280 and N2445P (Fig. 4, blue balls). These dataindicate that the core hydrophobic packing of Ets-1 is dynamicin the active state and that increasing inhibition attenuatesthese motions. Localized collective motions on a similar timescale within both the backbone and side chains support our proposalthat the inhibitory module, DNA binding interface, and hydrophobiccore form a concerted unit that is linked as a dynamic hydrophobicnetwork. Furthermore, the HX and Rex data are consistent witha conformational equilibrium within this concerted unit betweenat least two states: one stable and inactive, another more dynamicand active.
The dynamic character of Ets-1 is likely essential for sequence-specificDNA binding but also provides an opportunity for regulation(22, 23). NMR studies of the Escherichia coli lactose repressorprotein suggest that fluctuations on a millisecond to microsecondtime scale are necessary for facilitated diffusion on nonspecificDNA, as well as facile adoption of a specifically bound conformation(24). The active state of Ets-1 is similarly poised to adopta high-affinity interaction with DNA. In this case, signal-dependentphosphorylation employs these motions to vary activity in agraded manner.
The surprising use of a highly flexible segment in the gradedregulation of Ets-1 DNA binding adds to the growing recognitionof the role of unstructured protein regions in biology (25).These flexible elements often display posttranslational modifications[for example, MAPK Ets-1 phosphorylation (26), histone tails(27), and SH2 domain targets (28)], tether independent domains[for example, protein kinase A (PKA) (29)], or require foldingfor activity [for example, the cyclic adenosine 3',5'-monophosphateresponse element binding protein (CREB) and the CREB bindingprotein (30)]. The highly mobile SRR demonstrates a role forunstructured protein segments in integrating signals that directthe variable regulation of protein activity.
18"> Limited solubility of N244 prevented assignment of resonances within the molecule. However, the HSQC spectra of N244 and N280 were essentially superimposable.
19"> B. F. Volkman, D. Lipson, D. E. Wemmer, D. Kern, Science291, 2429 (2001).[Abstract/Free Full Text]
20"> The series does not adhere strictly to a two-state model because the position of N280 does not scale consistently with activity at every residue. Nevertheless, the mean relative change in chemical shift [|(N301 N280)|]/[|(N301 N2445P|)] is 67 ± 16% for 25 residues, consistent with 80% estimated from affinity (15).
32"> Single-letter abbreviations for the amino acid residues are as follows: A, Ala; C, Cys; D, Asp; E, Glu; F, Phe; G, Gly; H, His; I, Ile; K, Lys; L, Leu; M, Met; N, Asn; P, Pro; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val; W, Trp; and Y, Tyr.
34"> L. E. Kay, D. A. Torchia, A. Bax, Biochemistry28, 8972 (1989).[CrossRef][Medline]
We acknowledge support from NIH grant R01 GM38663 (B.J.G.); NIH grants T32-CA93247 and -GM08537 (M.A.P.); P01-CA24014 (Huntsman Cancer Institute) and the U.S. Department of Energy and the Huntsman Cancer Foundation (B.J.G.); the National Cancer Institute of Canada with funds from the Canadian Cancer Society (L.P.M.); the Government of Canada's Network of Centres of Excellence Program supported by the Canadian Institutes of Health Research (CIHR); the Natural Sciences and Engineering Research Council through the Protein Engineering Network of Centres of Excellence (L.P.M. and L.E.K.); and the CIHR for a Scientist Award (L.P.M.) and funding (L.E.K.). L.E.K. holds a Canada Research Chair in Biochemistry.
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