Magic-angle spinning (MAS) solid-state NMR becomes an increasingly important tool for the determination of structures of membrane proteins and amyloid fibrils. Extensive deuteration of the protein allows multidimensional experiments with exceptionally high sensitivity and resolution to be obtained. Here we present an experimental strategy to measure highly unambiguous spatial correlations for distances up to 13 Å. Two complementary three-dimensional experiments, or alternatively a four-dimensional experiment, yield highly unambiguous cross-peak assignments, which rely on four encoded chemical shift dimensions. Correlations to residual aliphatic protons are accessible via synchronous evolution of the (15)N and (13)C chemical shifts, which encode valuable amide-methyl distance restraints. On average, we obtain six restraints per residue. Importantly, 50% of all restraints correspond to long-range distances between residues i and j with |i - j| > 5, which are of particular importance in structure calculations. Using ARIA, we calculate a high-resolution structure for the microcrystalline 7.2 kDa α-spectrin SH3 domain with a backbone precision of ∼1.1 Å.

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Cholesterol is an important regulator of membrane protein function. However, the exact mechanisms involved in this process are still not fully understood. Here we study how the tertiary and quaternary structure of the mitochondrial translocator protein TSPO, which binds cholesterol with nanomolar affinity, is affected by this sterol. Residue-specific analysis of TSPO by solid-state NMR spectroscopy reveals a dynamic monomer–dimer equilibrium of TSPO in the membrane. Binding of cholesterol to TSPO's cholesterol-recognition motif leads to structural changes across the protein that shifts the dynamic equilibrium towards the translocator monomer. Consistent with an allosteric mechanism, a mutation within the oligomerization interface perturbs transmembrane regions located up to 35 Å away from the interface, reaching TSPO's cholesterol-binding motif. The lower structural stability of the intervening transmembrane regions provides a mechanistic basis for signal transmission. Our study thus reveals an allosteric signal pathway that connects membrane protein tertiary and quaternary structure with cholesterol binding.

We present a systematic study of the effect of the level of exchangeable protons on the observed amide proton linewidth obtained in perdeuterated proteins. Decreasing the amount of D 2 O employed in the crystallization buffer from 90 to 0%, we observe a fourfold increase in linewidth for both 1 H and 15 N resonances. At the same time, we find a gradual increase in the signal-to-noise ratio (SNR) for 1 H-15 N correlations in dipolar coupling based experiments for H 2 O concentrations of up to 40%. Beyond 40%, a significant reduction in SNR is observed. Scalarcoupling based 1 H-15 N correlation experiments yield a nearly constant SNR for samples prepared with B30% H 2 O. Samples in which more H 2 O is employed for crystallization show a significantly reduced NMR intensity. Calculation of the SNR by taking into account the reduction in 1 H T 1 in samples containing more protons (SNR per unit time), yields a maximum SNR for samples crystallized using 30 and 40% H 2 O for scalar and dipolar coupling based experiments, respectively. A sensitivity gain of 3.8 is obtained by increasing the H 2 O concentration from 10 to 40% in the CP based experiment, whereas the linewidth only becomes 1.5 times broader. In general, we find that CP is more favorable compared to INEPT based transfer when the number of possible 1 H, 1 H interactions increases. At low levels of deuteration (C60% H 2 O in the crystallization buffer), resonances from rigid residues are broadened beyond detection. All experiments are carried out at MAS frequency of 24 kHz employing perdeuterated samples of the chicken a-spectrin SH3 domain.