Dimer Organization of Membrane‐Associated NS5A of Hepatitis C Virus as Determined by Highly Sensitive 1 H‐Detected Solid‐State NMR

Divide and Conquer: A Tailored Solid‐state NMR Approach to Study Large Membrane Protein Complexes

Membrane proteins are known to exert many essential biological functions by forming complexes in cell membranes. An example refers to the β‐barrel assembly machinery (BAM), a 200 kDa pentameric complex containing BAM proteins A–E that catalyzes the essential process of protein insertion into the outer membrane of gram‐negative bacteria. While progress has been made in capturing three‐dimensional structural snapshots of the BAM complex, the role of the lipoprotein BamC in the complex assembly in functional lipid bilayers has remained unclear. We have devised a component‐selective preparation scheme to directly study BamC as part of the entire BAM complex in lipid bilayers. Combination with proton‐detected solid‐state NMR methods allowed us to probe the structure, dynamics, and supramolecular topology of full‐length BamC embedded in the entire complex in lipid bilayers. Our approach may help decipher how individual proteins contribute to the dynamic formation and functioning of membrane protein complexes in membranes.

Optimisation of 1H PMLG homonuclear decoupling at 60 kHz MAS to enable 15N-1H through-bond heteronuclear correlation solid-state NMR spectroscopy

The Lee–Goldburg condition for homonuclear decoupling in ¹H magic-angle spinning (MAS) solid-state NMR sets the angle θ, corresponding to arctan of the ratio of the rf nutation frequency, ν₁, to the rf offset, to be the magic angle, θ_m, equal to tan⁻¹(√2) = 54.7°. At 60 kHz MAS, we report enhanced decoupling compared to MAS alone in a ¹H spectrum of ¹⁵N-glycine with at θ = 30° for a ν₁ of ∼100 kHz at a ¹H Larmor frequency, ν₀, of 500 MHz and 1 GHz, corresponding to a high chemical shift scaling factor (λ_CS) of 0.82. At 1 GHz, we also demonstrate enhanced decoupling compared to 60 kHz MAS alone for a lower ν₁ of 51 kHz, i.e., a case where the nutation frequency is less than the MAS frequency, with θ = 18°, λ_CS = 0.92. The ratio of the rotor period to the decoupling cycle time, Ψ = τ_r/τ_c, is in the range 0.53 to 0.61. Windowed decoupling using the optimised parameters for a ν₁ of ∼100 kHz also gives good performance in a ¹H spin-echo experiment, enabling implementation in a ¹H-detected ¹⁵N–¹H cross polarisation (CP)-refocused INEPT heteronuclear correlation NMR experiment. Specifically, initial ¹⁵N transverse magnetisation as generated by ¹H–¹⁵N CP is transferred back to ¹H using a refocused INEPT pulse sequence employing windowed ¹H decoupling. Such an approach ensures the observation of through-bond N–H connectivities. For ¹⁵N-glycine, while the CP-refocused INEPT experiment has a lower sensitivity (∼50%) as compared to a double CP experiment (with a 200 μs ¹⁵N to ¹H CP contact time), there is selectivity for the directly bonded NH₃⁺ moiety, while intensity is observed for the CH₂¹H resonances in the double CP experiment. Two-dimensional ¹⁵N–¹H correlation MAS NMR spectra are presented for the dipeptide β-AspAla and the pharmaceutical cimetidine at 60 kHz MAS, both at natural isotopic abundance. For the dipeptide β-AspAla, different build-up dependence on the first spin-echo duration is observed for the NH and NH₃⁺ moieties demonstrating that the experiment could be used to distinguish resonances for different NH_x groups.

¹⁵N–¹H heteronuclear NMR correlation at natural abundance in the solid state via J couplings is enabled by optimisation of phase-modulated Lee–Goldburg (PMLG) ¹H homonuclear decoupling during the spin echoes, far from the ideal magic-angle condition.