Sequence-specific and stereospecific assignment of methyl groups using paramagnetic lanthanides
Pseudocontact shifts (PCSs) induced by a site-specifically bound paramagnetic lanthanide ion are shown to provide fast access to sequence-specific resonance assignments of methyl groups in proteins of known three-dimensional structure. Stereospecific assignments of Val and Leu methyls are obtained as well as resonance assignments of all other methyls, including Met epsilon CH3 groups. No prior assignments of the diamagnetic protein are required nor are experiments that transfer magnetization between the methyl groups and the protein backbone. Methyl C-z-exchange experiments were designed to provide convenient access to PCS measurements in situations where a paramagnetic lanthanide is in exchange with a diamagnetic lanthanide. In the absence of exchange, simultaneous C-13-HSQC assignments and PCS measurements are delivered by the newly developed program Possum. The approaches are demonstrated with the complex between the N-terminal domain of the subunit epsilon and the subunit theta of the Escherichia coli DNA polymerase III.