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19F Paramagnetic Relaxation-Based NMR for Quaternary Structural Restraints of Ion Channels.

Quaternary distance restraints are essential to define the three-dimensional structures of protein assemblies. These distances often fall within a range of 10-18 Å, which challenges the high and low measurement limits of conventional nuclear magnetic resonance (NMR) spectroscopy and double electron-electron resonance electron spin resonance spectroscopy. Here, we report the use of 19F paramagnetic relaxation enhancement (PRE) NMR in combination with 19F/paramagnetic labeling to equivalent sites in different subunits of a protein complex in micelles to determine inter-subunit distances. Feasibility of this strategy was evaluated on a pentameric ligand-gated ion channel, for which we found excellent agreement of the 19F PRE NMR results with previous structural information. The study suggests that 19F PRE NMR is a viable tool in extracting distance restraints to define quaternary structures.