Solution NMR of membrane proteins in bilayer mimics: small is beautiful, but sometimes bigger is better

Abstract

Considerable progress has been made recently on solution NMR studies of multi-transmembrane helix membrane protein systems of increasing size. Careful correlation of structure with function has validated the physiological relevance of these studies in detergent micelles. However, larger micelle and bicelle systems are sometimes required to stabilize the active forms of dynamic membrane proteins, such as the bacterial small multidrug resistance transporters. Even in these systems with aggregate molecular weights well over 100 kDa, solution NMR structural studies are feasible-but challenging.

Figure 1

Ligand binding activity of Smr in different sample conditions. The relative fluorescence change upon addition of ligand is plotted, with the solid lines representing the best fit of the binding function to the data. Solid squares and diamonds represent TPP binding to Smr in bicelles at pH 8.0 and 6.0, respectively. Solid circles denote methyl viologen binding to Smr in bicelles at pH 8.0. Open diamonds and crosses denote the fluorescence changes upon addition of TPP to Smr in β-nonylglucoside and LPPG, respectively.

Figure 2

¹H-¹⁵N TROSY spectra of Smr in different detergents and bicelles. The left four spectra are recorded in different detergents and bicelles as indicated. The two right panels show overlays of the spectrum in bicelles with spectra in either DM or LPPG.

Figure 3

Representative sequential strips from the NOESY-TROSY (top panel) and TROSY-HNCA (bottom panel) experiments. In the NOESY-TROSY strips, the connectivities from each amide peak to its i±1 NOE cross-peaks are shown. In the TROSY-HNCA, the connectivities from i to i-1 peak to i peak in the preceding residue’s strip are indicated.

Figure 4

Using the ¹³C-detect CACO experiment for making sequential backbone assignments. Starting from the i Cα chemical shift determined in the trHNCA experiment, the i CαC’ cross-peak in the CACO experiment is found, which then allows identification of the i+1 cross-peak in the trHNCO experiment through correlation of the C’ chemical shifts.