The challenging nature of studies of membrane proteins has made it difficult to determine the molecular mechanism of transmembrane signaling. For the bacterial chemoreceptor family, there are crystal structures of the internal and external domains, structural models of the transmembrane domain, and evidence for subtle ligand-induced conformational changes, but the signaling mechanism remains controversial. We have used a novel site-directed solid-state NMR distance measurement approach, using 13C19F REDOR, to measure a ligand-induced change of 1.0 ± 0.3 Å in the distance between helices α1 and α4 of the ligand-binding domain in the intact, membrane-bound serine receptor. This distance change is shown not to be due to motion of the side chain and thus is due to motion of either the α1 or the α4 helix. Additional distance measurements can be used to determine the type of backbone motion and to follow it to the cytoplasm, to test and refine current proposals for the mechanism of transmembrane signaling. This is a promising general method for high-resolution measurements of local structure in intact, membrane-bound proteins.
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