Docking of Protein−Protein Complexes on the Basis of Highly Ambiguous Intermolecular Distance Restraints Derived from1HN/15N Chemical Shift Mapping and Backbone15N−1H Residual Dipolar Couplings Using Conjoined Rigid Body/Torsion Angle Dynamics

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Abstract
A simple and reliable method for docking protein−protein complexes using 1HN/15N chemical shift mapping and backbone 15N−1H residual dipolar couplings is presented and illustrated with three complexes (EIN-HPr, IIAGlc-HPr, and IIAMtl-HPr) of known structure. The 1HN/15N chemical shift mapping data are transformed into a set of highly ambiguous, intermolecular distance restraints (comprising between 400 and 3000 individual distances) with translational and some degree of orientational information content, while the dipolar couplings provide information on relative protein−protein orientation. The optimization protocol employs conjoined rigid body/torsion angle dynamics in simulated annealing calculations. The target function also comprises three nonbonded interactions terms: a van der Waals repulsion term to prevent atomic overlap, a radius of gyration term (Ergyr) to avoid expansion at the protein−protein interface, and a torsion angle database potential of mean force to bias interfacial side chain conformations toward physically allowed rotamers. For the EIN-HPr and IIAGlc-HPr complexes, all structures satisfying the experimental restraints (i.e., both the ambiguous intermolecular distance restraints and the dipolar couplings) converge to a single cluster with mean backbone coordinate accuracies of 0.7−1.5 Å. For the IIAMtl-HPr complex, twofold degeneracy remains, and the structures cluster into two distinct solutions differing by a 180° rotation about the z axis of the alignment tensor. The correct and incorrect solutions which have mean backbone coordinate accuracies of ∼0.5 and ∼10.5 Å, respectively, can readily be distinguished using a variety of criteria: (a) examination of the overall 1HN/15N chemical shift perturbation map (because the incorrect cluster predicts the presence of residues at the interface that experience only minimal chemical shift perturbations; this information is readily incorporated into the calculations in the form of ambiguous intermolecular repulsion restraints); (b) back-calculation of dipolar couplings on the basis of molecular shape; or (c) the Ergyr distribution which, because of its global nature, directly reflects the interfacial packing quality. This methodology should be particularly useful for high throughput, NMR-based, structural proteomics.

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