Sensitive Cu2+-Cu2+ Distance Measurements in a Protein-DNA Complex by Double-Quantum Coherence ESR
Double Quantum Coherence (DQC) ESR spectroscopy is applied to measure the Cu2+-Cu2+ distance in the EcoRI-DNA complex. A simple method is proposed to reduce the contribution of nuclear hyperfine and quadrupole interactions to such data. The effects of such interactions between the electron spin of Cu2+ and neighboring nuclei on the DQC data, make it difficult to measure the nanometer range interspin distance. The DQC data is in good agreement with results obtained by Double Electron Electron Resonance (DEER) spectroscopy. At the same time, the signal to noise ratio per shot in DQC is high. Taken together, these results provide impetus for further development of paramagnetic metal ion-based DQC techniques.
Introduction<!>Enzyme expression and purification<!>Cu2+-EcoRI-DNA sample preparation<!>Electron Spin Resonance Spectroscopy<!>Results and Discussion<!>Conclusions
<p>Herein, we demonstrate a simple way to measure paramagnetic metal ion based nanoscale distances in proteins. The advent of Double Electron Electron Resonance (DEER)1,2 and Double Quantum Coherence (DQC)3,4 methods that measure nanometer range interspin distances have had a profound impact on the application of ESR in biological research.5–12 Most of these distance measurements are based on the methanethiolsulfonate nitroxide spin-label, which is chemically attached to the cysteine residue in proteins.13 The extension of DEER and DQC distance measurements to paramagnetic metal ions can potentially generalize the technique to many proteins that contain endogenous metal ion binding sites. However, DEER and DQC with metal ions as spin probe are challenging because of large ESR spectral width and low signal to noise ratio (SNR). Nevertheless, our group14,15 and others16–26 have made tangible progress. In principle DQC can have a high SNR per shot. This provides impetus for further research of DQC. On the other hand, the DQC signal has substantial contributions from electron-nuclear interactions between the electron spin and neighboring 14N as well as 1H nuclear spins that are present in the amino-acid coordination environment. These nuclear modulation27 effects swamp the modulation due to the electron-electron dipolar (EED) interactions and make it difficult to measure the interspin distance.</p><p>EcoRI is a 62 kDa homodimeric protein28,29 that recognizes and binds to the 5'-GAATTC-3'DNA sequence with high specificity30–32, even in the absence of metal cofactors. In the presence of Mg2+, EcoRI catalyzes cleavage of both DNA strands at this site. When Mg2+ is replaced by other metal ions the cleavage rates decrease according to the series Mg2+ ≈ Mn2+ > Co2+>> Zn2+ >> Cd2+ > Ni2+.31 Interestingly, Cu2+ is a powerful inhibitor of EcoRI catalysis.33 In order to shed light on the molecular mechanism of catalytic inhibition by Cu2+, we have recently exploited DEER based distance measurements to determine that Cu2+ binds to His114 in each subunit of EcoRI,33 at points 13 Å from the Mg2+ positions in the catalytic sites. The positions of the His114 sidechains are consequently altered, and the resulting disruption of critical protein-DNA interactions and water molecules in the catalytic centers leads to inhibition of catalysis.33</p><p>In this work we present a simple way to minimize the low frequency nuclear peaks in the DQC spectrum and resolve the dipolar interaction between two paramagnetic probes Cu2+ with high sensitivity in the complex of restriction endonuclease EcoRI with its cognate DNA.</p><!><p>The EcoRI protein was expressed from a maltose-binding protein-EcoRI (MBP-EcoRI) fusion construct. Details for generation of the fusion gene and expression of the fusion protein are given in supporting information for reference 33. The complete EcoRI protein without extra amino acids was isolated, purified, and characterized as described in reference 34.</p><!><p>A solution of EcoRI (5 μM) in the presence of fivefold molar excess of TCGCGAATTCGCG was exchanged into 30 mM N-ethylmorpholine (NEM) buffer, which contains 0.3 M NH4Cl, 10% dioxane, 30% deuterated glycerol (d8), 65% D2O (pH 8.0) and concentrated. The final concentrations of EcoRI and DNA were 380 μM and 1.5 mM, respectively. Isotopically enriched 63CuCl2 (Cambridge Isotope Labs, Inc) was added at a 4:1 molar ratio (Cu2+ : protein dimer). The sample was stored at −80°C and flash-frozen before each ESR experiment.</p><!><p>All of the pulsed ESR experiments were performed on a Bruker Elexsys 580 spectrometer at 20 K, with a MD5 resonator. The six pulse DQC-ESR was using a π/2-tp+dt-π-tp+dt-π/2-t1-π-t1-π/2-t2-π-echo sequence. A 64 step phase cycle was carried out to select the correct coherence pathway and the DQC-echo after the sixth pulse was integrated.14,35 The π/2 pulse and π pulse lengths were 8 ns (or 12 ns, 16 ns and 20 ns) and 16 ns (or 24 ns, 32 ns and 40 ns), respectively. The interval tp was incremented from 80 ns with a stepsize of 8 ns, for a total of 360 points. The interval t1 was 80 ns and t2 was 200 ns. The shot repetition time was 5 ms and all the DQC time domain data were averaged for 24 scans. Each DQC experiment was done at 3350 G where the maximum echo signal was observed. Details of the DEER experiment are given in supporting information and reference 33.</p><!><p>Figure 1 shows the Cu2+-DQC data obtained at the g⊥ position of the Cu2+ ESR absorption spectrum. The lengths of the π pulse were 16 ns (black solid line) and 40 ns (grey solid line), respectively.</p><p>The two spectra in Figure 1b with different length of π pulse clearly contain peaks that can be attributed to the 14N-nuclear modulations in the 0–5 MHz region14 as well as the 1H-nuclear peaks at ~14 MHz. For the purpose of nanoscale distance measurements the 0–5 MHz 14N-nuclear peaks are particularly problematic, since the EED interaction is in this region. Interestingly, the intensity of the peak at ~1.46 MHz in the DQC spectrum (Figure 1b) with 40 ns π pulse is much lower compared to the spectrum with 16 ns π pulse. With shorter π pulse, e. g., 16 ns, the spectral excitation probability of the DQC EED interaction is about 0.16.35 With the longer π pulse of 40 ns, the spectral excitation probability of the DQC EED interaction is about 0.04.35 It is very clear that longer π pulse cannot substantially populate the double quantum coherence of EED interaction. However, the nuclear hyperfine and quadrupole transitions are weakly sensitive to the length of π pulse.27 This suggests that 1.46 MHz peak might originate from the Cu2+-EED interaction.</p><p>Since the experimental DQC signal is a multiplication of the EED and the nuclear signals, division of the π=16 ns DQC data by the π=40 ns DQC data may suppress the nuclear signals and resolve the EED signal.14,35
Figure 2a shows the comparison of the DQC time domain data, after dividing the π=16 ns signal by the π=40 ns signal (black solid line), with the DEER data after subtraction of the homogeneous background (grey solid line).33 The modulation periods of the two time domain traces are comparable. In this analysis, the DQC signal with 40 ns π pulse was empirically chosen for division – the data with other choices are shown in the Supporting Information. The data acquisition time of DQC (including two traces) and DEER are ~50 hrs and ~43 hrs, respectively. The data collection time in DQC can be reduced by implementation of on-board phase cycling.</p><p>The DQC spectrum after division, shown in Figure 2b (black solid line), clearly resolves the major peak at ~1.46 MHz, which is consistent with the DEER data (Figure 2b, grey solid line). Nevertheless, the DQC spectrum is narrower than the DEER spectrum. In the experimental DQC data, there might be some small residual peaks from electron-14N interactions that occur at ~3 MHz.</p><p>Figure 2 also shows the simulated DQC data (black dotted line). Based on the best fitting of the DQC time domain data shown in Figure 2, we obtained a most probable Cu2+-Cu2+ distance of ~35 Å with a standard deviation of 1 Å (Figure 2b, black solid line inset). This result is consistent with our observation from DEER experiments (Figure 2b, red dotted line inset). The simulated DQC spectrum is in accord with the dominant peak obtained in the experiment, although the DQC data is narrower.33 Details of the simulations are provided in the Supporting Information. It is likely that in DQC a narrower range of orientations is selected because the microwave pulses have a finite coverage. Despite this, the signal to noise ratio per shot in DQC is high (cf. figure caption in Figure 2 and Supporting Information).</p><!><p>We have clearly shown here that metal ion based nanometer range distance can be measured by DQC. DEER has a significant advantage due to the ease of implementation on commercial instrument. Nevertheless, the DQC data have high SNR per shot. Further improvements can be made by the use of narrower pulse lengths35 and by understanding the role of orientational effects in such DQC measurements.15,36</p>