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. 2009 Nov;45(3):265-81.
doi: 10.1007/s10858-009-9366-3. Epub 2009 Aug 27.

High-resolution protein structure determination starting with a global fold calculated from exact solutions to the RDC equations

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High-resolution protein structure determination starting with a global fold calculated from exact solutions to the RDC equations

Jianyang Zeng et al. J Biomol NMR. 2009 Nov.

Abstract

We present a novel structure determination approach that exploits the global orientational restraints from RDCs to resolve ambiguous NOE assignments. Unlike traditional approaches that bootstrap the initial fold from ambiguous NOE assignments, we start by using RDCs to compute accurate secondary structure element (SSE) backbones at the beginning of structure calculation. Our structure determination package, called RDC-PANDA: (RDC-based SSE PAcking with NOEs for Structure Determination and NOE Assignment), consists of three modules: (1) RDC-EXACT: ; (2) PACKER: ; and (3) HANA: (HAusdorff-based NOE Assignment). RDC-EXACT: computes the global optimal solution of backbone dihedral angles for each secondary structure element by exactly solving a system of quartic RDC equations derived by Wang and Donald (Proceedings of the IEEE computational systems bioinformatics conference (CSB), Stanford, CA, 2004a; J Biomol NMR 29(3):223-242, 2004b), and systematically searching over the roots, each of which is a backbone dihedral varphi- or psi-angle consistent with the RDC data. Using a small number of unambiguous inter-SSE NOEs extracted using only chemical shift information, PACKER: performs a systematic search for the core structure, including all SSE backbone conformations. HANA: uses a Hausdorff-based scoring function to measure the similarity between the experimental spectra and the back-computed NOE pattern for each side-chain from a statistically-diverse rotamer library, and drives the selection of optimal position-specific rotamers for filtering ambiguous NOE assignments. Finally, a local minimization approach is used to compute the loops and refine side-chain conformations by fixing the core structure as a rigid body while allowing movement of loops and side-chains. RDC-PANDA: was applied to NMR data for the FF Domain 2 of human transcription elongation factor CA150 (RNA polymerase II C-terminal domain interacting protein), human ubiquitin, the ubiquitin-binding zinc finger domain of the human Y-family DNA polymerase Eta (pol eta UBZ), and the human Set2-Rpb1 interacting domain (hSRI). These results demonstrated the efficiency and accuracy of our algorithm, and show that RDC-PANDA: can be successfully applied for high-resolution protein structure determination using only a limited set of NMR data by first computing RDC-defined backbones.

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Figures

Figure 1
Figure 1
Schematic illustration of RDC-PANDA. Panel A: RDC-EXACT. Panel B: PACKER. Panel C: HANA. Panel D: the local minimization approach for loops.
Figure 2
Figure 2
Back-computed vs. experimental RDCs. Panels 1A, 1B, 1C and 1D: CH, NH, CαC’ and NC’ RDCs for FF2. All RDCs are scaled to the NH RDCs; a window of 2.0 Hz is shown as the error bars for the experimental RDCs. The plots of back-computed vs. experimental RDCs for ubiquitin, hSRI and pol η UBZ are shown in SM Fig. S2.
Figure 3
Figure 3
The SSE backbones of core structures computed by PACKER. Column 1: ensemble of WPS structures. Column 2: structure overlay of the mean WPS structure (blue) vs. the NMR reference structure (green). Column 3: structure overlay of the mean WPS structure (blue) vs. the X-ray structure (magenta).
Figure 4
Figure 4
Evaluation of packed structures computed by PACKER. Here we show results for FF2. The results for ubiquitin, hSRI and pol η UBZ are shown in SM Fig. S3. Panel 1A: NOE satisfaction score vs. packing score for all structures in the ensemble (structures with vdW energies larger than 80 and NOE score larger than 10 were truncated from the plot). Panel 1B: histogram of backbone RMSD to the reference structure for all packed structures. Panel 1C: histogram of backbone RMSD to the reference structures for WPS structures. The magenta lines show the cutoffs of NOE satisfaction score (horizontal) and packing score (vertical) for computing the WPS structures.
Figure 5
Figure 5
Comparison of aromatic rotamers vs. side-chain conformations in the reference structures. Blue: rotamers computed by HANA. Magenta: X-ray side-chains. Green: side-chains in the NMR reference structure.
Figure 6
Figure 6
The NMR structures of ubiquitin, hSRI, pol η UBZ and FF2 computed using our automatically-assigned NOEs. Panels A, B, C and D: the structures of ubiquitin. Panels E, F and G: the structures of hSRI. Panels H, I and J: the structures of pol η UBZ. Panels K, L and M: the structures of FF2. Panels A, E, H and K: the ensemble of 20 best NMR structures with the minimum energies. Panels B, F, I and L: ribbon view of the mean structures. Panel D: backbone overlay of the mean structures (blue) of ubiquitin vs. its X-ray reference structures (Vijay-Kumar et al. 1987) (magenta). Panels C, G, J and M: backbone overlay of the mean structures (blue) vs. corresponding NMR reference structures (green) (PDB ID of ubiquitin (Cornilescu et al. 1998): 1D3Z; PDB ID of FF2: 2E71; PDB ID of hSRI (Li et al. 2005): 2A7O; PDB ID of pol η UBZ (Bomar et al. 2007): 2I5O).

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