Multi-spin REDOR NMR measures the binding pocket structure and explores the topology of the membrane-bound serine chemoreceptor

Rotational Echo Double Resonance (REDOR) is a powerful solid-state NMR technique that allows the measurement of heteronuclear inter-atomic distances in complex systems such as membrane-bound proteins whose study has been hampered by the limits of X-ray diffraction and solution NMR techniques. We have applied REDOR NMR to explore the structure of the E. coli membrane-bound serine receptor, which is involved in the chemotaxis signaling pathway. 13C-15N and 13C-19F REDOR are used to measure ligand-to-protein distances that map the serine ligand site structure. The results confirm the proposed similarities between the ligand binding sites of the intact membrane-bound serine receptor and the aspartate receptor periplasmic fragment. Preliminary receptor-to-membrane 13C-31P REDOR distance measurements suggest a method for the study of receptor topology. Using REDOR to measure the number of sites that are close to the membrane gives a qualitative insight into the orientation of the protein with respect to the lipid bilayer. We also propose additional multispin REDOR experiments to measure the number of spins that are interacting with the unique ligand spin and thus refine the structure of the ligand site of the receptor. These approaches can be used to map the ligand binding sites and receptor topology in more complex and less understood systems such as human membrane receptors of medical relevance