Solution- NMR Characterization of Outer- Membrane Protein A from E. coli in Lipid Bilayer Nanodiscs and Detergent Micelles

X-ray crystallography and solution NMR of detergent-reconstituted OmpA (outer membrane protein A from E. coli) had shown that this protein forms an eight-stranded transmembrane -barrel, but only limited information was obtained for the extracellular loops. In NMR studies of OmpA in two different detergent micelles, NMR-invisible amino acid residues in-between the extracellular loops and the -barrel prevented complete structural characterization. Here, we show that this NMR-invisible ring around the -barrel of OmpA is also present in lipid bilayer nanodiscs and in mixed micelles with a third detergent, thus suggesting that the implicated rate processes have a functional role rather than representing an artifact of the protein reconstitution. In addition to sequence-specific NMR assignments for OmpA in the nanodiscs, the present results are based on a protocol of micro-coil TROSY- and CRINEPT-type NMR diffusion measurements for studying the hydrodynamic properties and the foldedness of [H-2,N-15]-labeled membrane proteins in nanodiscs. This protocol can be applied under conditions closely similar to those used for NMR structure determinations or crystallization trials.