REDOR solid-state NMR as a probe of the membrane locations of membrane-associated peptides and proteins

Rotational-echo double-resonance (REDOR) solid-state NMR is applied to probe the membrane locations of specific residues of membrane proteins. Couplings are measured between protein (CO)-C-13 nuclei and membrane lipid or cholesterol H-2 and P-31 nuclei. Specific (CO)-C-13 labeling is used to enable unambiguous assignment and H-2 labeling covers a small region of the lipid or cholesterol molecule. The (CO)-C-13-P-31 and (CO)-C-13-H-2 REDOR respectively probe proximity to the membrane headgroup region and proximity to specific insertion depths within the membrane hydrocarbon core. One strength of the REDOR approach is use of chemically-native proteins and membrane components. The conventional REDOR pulse sequence with 100 kHz H-2 pi pulses is robust with respect to the H-2 quadrupolar anisotropy. The 2H T-1's are comparable to the longer dephasing times (tau's) and this leads to exponential rather than sigmoidal REDOR buildups. The (CO)-C-13-H-2 buildups are well-fitted to A x (1 - e(-gamma tau)) where A and gamma are fitting parameters that are correlated as the fraction of molecules (A) with effective (CO)-C-13-H-2 coupling d = 3 gamma/2. The REDOR approach is applied to probe the membrane locations of the "fusion peptide" regions of the HIV gp41 and influenza virus hemagglutinin proteins which both catalyze joining of the viral and host cell membranes during initial infection of the cell. The HIV fusion peptide forms an intermolecular antiparallel beta sheet and the REDOR data support major deeply-inserted and minor shallowly-inserted molecular populations. A significant fraction of the influenza fusion peptide molecules form a tight hairpin with antiparallel N- and C-alpha helices and the REDOR data support a single peptide population with a deeply-inserted N-helix. The shared feature of deep insertion of the beta and alpha fusion peptide structures may be relevant for fusion catalysis via the resultant local perturbation of the membrane bilayer. Future applications of the REDOR approach may include samples that contain cell membrane extracts and use of lower temperatures and dynamic nuclear polarization to reduce data acquisition times. (C) 2015 Elsevier Inc. All rights reserved.