The Orientation and Location of the C-terminal Helix of Surfactant Protein B in Lipid Bilayers as Studied by Solid State NMR

The lung surfactant protein B (SP-B) is essential for life and plays a critical role in reducing surface tension in the lungs during breathing. An amphipathic helical fragment of SP-B composed of the 16 C-terminal residues, termed SP-BCTERM or SP-B63-78, has a charge of +3 and retains significant bioactivity compared to native SP-B. We have used 31P-, 2H-, 13C-, and 15N- solid state NMR to investigate the insertion of SP-BCTERM into lipid bilayers composed of model lipids, as well as bovine lung extracted surfactant (BLES). When the external magnetic field is parallel to the normal of the oriented lipid bilayers, the 15N chemical shift of the peptide backbone is at ∼ 98 ppm, which corresponds to a helical axis alignment of approximately 70° relative to the lipid bilayer normal. The depth of peptide insertion in vesicles was investigated by 13C{31P} REDOR, as well as by measuring the 13C relaxation time in the absence and presence of the paramagnetic probe Mn2+.