A novel tool for probing membrane protein structure: Solid-state NMR with proton spin diffusion and X-nucleus detection

We present a new solid-state NMR approach, based on H-1 spin diffusion with X-nucleus (N-15, C-13, P-31) detection, for investigating the structure of membrane proteins. For any segment with a resolvable signal in the X-nucleus spectrum, the depth of insertion into the lipid bilayer can be determined. The technique represents the adaptation of the Goldman-Shen H-1 spin-diffusion experiment with X-nucleus detection to proteins in hydrated Lipid bilayers (>25% water by weight)in the gel state at 240 K. The experiments are demonstrated on the 21-kDa channel-forming domain of the toxin-like colicin E1 molecule incorporated into lipid vesicles. More than 32% of the protons in our sample are in mobile H2O molecules, which can be selected efficiently by the H-1 T-2 filter in the Goldman-Shen sequence. The transfer of H-1 magnetization from mobile H2O to the colicin E1 channel domain is 80% complete within only 5 ms. This transfer to the protein, probed by the amide N-15 signals, is faster than the transfer to the rigid protons on average, proving that most of the protein is preferentially located between the water and the lipid bilayer. From the spin-diffusion and dipolar-dephasing data, 60% of the 24 lysine side groups are shown to be highly mobile. Quantitative depth profiling is demonstrated using the P-31 in the Lipid phosphate head groups and the C-13 nuclei in the Lipid acyl chains as distance markers for the spin diffusion.