Solid-state O-17 NMR spectroscopy of a phospholemman transmembrane domain protein : Implications for the limits of detecting dilute 170 sites in biomaterials

The O-17-'diluted' glycine-14 sites in a phospholemman (PLM) transmembrane domain protein are characterized by solid-state O-17 NMR spectroscopy. The PLM transmembrane domain is an a-helical tetramer unit of four 28-residue peptides and is rigidly embedded in a bilayer where each a-helix has an average tilt of 7.3 degrees against the membrane normal. The PLM sample investigated here consists of a high lipid/peptide molar ratio (25: 1) with one glycine residue in each helix enriched to < 40% O-17; thus, this is a very dilute 17 -sample and is the most dilute O-17-membrane protein to date to be characterized by solid-state 17 0 NMR spectroscopy. Based on the spectral analysis of O-17 magic angle spinning (MAS) at 14.1 and 18.8T, the PLM transmembrane domain protein consists of multiple crystallographic gly14 sites, suggesting that the tetramer protein is an asymmetric unit with either C-2- or C-1-rotational symmetry along the bilayer normal.